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Survivorship Bias
What is Survivorship Bias?
âTwenty years ago,â says the inspirational speaker, âI was sitting in that audience where you are now. And look at me today. Thatâs proof that you can make it too if you donât give up hope.â
This speaker is either suffering from survivorship bias, or cynically exploiting it in their audience. Presumably there were thousands of other people sitting in the audience twenty years ago, and only one of them is standing on the stage today. For any given person in the audience today, it is more likely that they will end up like one of the forgotten thousands than like the one success story.
Survivorship bias means considering only the examples which made it through some kind of filter or selection process, and ignoring the ones which didnât, and then drawing unjustified conclusions or generalizations from the âsurvivorsâ.
Examples of Survivorship Bias
Success in Business or Entertainment
If you want to start a billion-dollar business, itâs not enough to look at Google or Amazon and copy what they did. Amazon did certain things and succeeded big-time, but many other companies youâve never heard of did the same things and failed, or perhaps achieved very modest success. Thatâs why youâve never heard of them. Youâre getting a false picture if you only consider the success stories. Similarly, if you want to be a star in music, movies or sport, itâs not enough to read the biography of your favorite star and copy what they did.
Music, Movies and Books
The curmudgeon who says the music, movies or books of today arenât as good as the ones of the distant past is exhibiting survivorship bias. We see all the media of today, good and bad; but we only see the best of the media of yesterday, because the lower-quality examples have been justly forgotten.
Military Aircraft
Military researchers in World War II studied the patterns of damage on aircraft caused by enemy fire, with a view to adding extra reinforcement to the areas of aircraft that were typically damaged. But a statistician called Abraham Wald pointed out that the patterns of damage they were seeing indicated the places where an aircraft could take damage and still survive. The aircraft that got hit in more critical areas never made it home to be studied. The original researchers were exhibiting survivorship bias, by only considering the aircraft that survived combat. Wald suggested that, instead, aircraft should be reinforced in the areas where returning aircraft tended not to be damaged. Assuming that enemy fire was approximately likely to hit an aircraft anywhere, Wald was able to infer that the places where returning aircraft didnât show damage were the places where an aircraft would be destroyed if hit, so those were the places which needed extra reinforcement.
Stock Market Scammers
Here is a strategy some scammers use. Pick a stock, and email half of a large mailing list with a prediction that the stock will go up tomorrow, and the other half with a prediction that it will go down tomorrow. If it goes up, stop emailing the second half of the list, and split the first half of the list in half again, and email one half with a prediction that a stock will go up and the other half with a prediction that it will go down. Repeat every day for a week.
By the end of a week, the scammer will have a group of people, 1/128 the size of the original mailing list, who have received seven correct predictions in a row by pure chance. It doesnât matter that the other 127/128 of the list have received at least one incorrect prediction; they have ceased to be the target of the scam. The âluckyâ 1/128 who have received seven correct predictions, if they are not savvy, will exhibit survivorship bias by not realizing they are part of a small group that has survived seven binary filters, and will think the series of seven correct predictions means that the scammer has some special and valuable insight into the stock market. Then they may be persuaded to give the scammer lots of money to subscribe to future updates or buy his book.
Survivorship Bias as a Logical Fallacy
Survivorship bias can arise from the logical fallacy of affirming the consequent.
The probability of event A given event B isnât the same as the probability of event B given event A. If 10% of CEOs are women, that doesnât mean 10% of women are CEOs. To make this more obvious, consider men: maybe 90% of CEOs are men, but that definitely doesnât mean 90% of men are CEOs!
Similarly, maybe youâve noticed that 90% of world-famous guitarists practiced for two hours every day, so you think practicing for two hours every day is a good route to becoming a famous guitarist. But the converse doesnât hold: itâs not true that 90% of aspiring guitarists who practice for two hours every day go on to become world-famous. Maybe only 1% do, or even fewer.
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Plate Boundaries
What are Plate Boundaries?
Plate boundaries are made of lithosphere which is the outer portion of the Earthâs surface, on which humans and all species live. This rigid, brittle layer is ~100 km thick and is made of the Earthâs crust and the uppermost mantle.
As shown in the image below, the lithosphere is broken into 12 major tectonic plates which fit together like a puzzle. The places where the plates meet are called plate boundaries which can be one of three main types depending on the relative movement of the plates on either side of the plate boundary.
As shown above, if the plates are moving away from each other the boundary is called divergent, if they are moving toward each other itâs a convergent boundary, and if they are sliding past each other horizontally, in parallel but opposite directions, then itâs a transform plate boundary.
Most of the Earthâs earthquakes occur along plate boundaries as the brittle plates slide and grind past each other as they move. Convergent plate boundaries are home to the deepest and largest earthquakes on the planet.
Movement of the plates is driven by heat generated from the radioactive decay of minerals in the earth. This heat fuels convection in the ductile mantle which brings magma toward the surface at divergent plate boundaries and forces plates to subduct at convergent plate boundaries.
Scientists and explorers began theorizing about plate boundaries and tectonics as early as the 1500âs when they observed that 1) the shape of some continents seemed to fit together like a puzzle and 2) the same plant and animal fossils could be found across these continents, as shown in the image below.
These theories were tested and confirmed by scientists and geologists from the early 1900âs to the mid 1960âs including Alfred Wegenerâs theory of continental drift, Harry Hessâs seafloor spreading hypothesis and the Frederick John Vine, Drummond Hoyle Matthews and Lawrence W. Morley hypothesis of remnant magnetization. Collectively, and with the help of modern technology, these scientists help set the stage for the theory of plate tectonics and the foundation of geology as we know it today.
Types of Plate Boundaries
Convergent Plate Boundaries
Convergent plate boundaries can be one of three main types depending on whether the crust on either side is made of thin, dense oceanic crust or thick, buoyant continental crust. These three types include: ocean-continent, ocean-ocean, or continent-continent convergent plate boundaries.
At both ocean-ocean and ocean-continent convergent plate boundaries, the denser, thinner oceanic plate will subduct beneath the thicker more buoyant plate. At ~100 km depth metamorphic reactions in the subducting slab release water into the overlying mantle wedge which lowers the melting point of the surrounding rocks. Like a hot air balloon rising through the sky, this process creates magma which either makes its way to the surface in the form of lava and volcanoes or freezes before it reaches the surface as granitic plutons and batholiths.
Because both plates at continent-continent convergent boundaries are made of thick, buoyant continental crust, neither want to subduct. Instead immense collisional forces and large thrust faults form in response uplifting large mountain ranges like the Himalaya. Due to the lack of subduction, few volcanoes are found at continental-continental plate boundaries.
Convergent boundaries are home to some of the largest geologic features on the planet including mountain ranges, volcanoes, oceanic trenches and island arcs. Some examples of include the Andes Mountains, the âRing of Fireâ, and the ancient Sierra Nevada batholith.
Divergent Plate Boundaries
Divergent plate boundaries, also called mid-ocean ridges, oceanic spreading centers or continental rifts, occur where the Earthâs tectonic plates move away from each other. Here, fresh, hot, basaltic magma is brought toward the surface forming new, rigid lithosphere as it cools. As the magma reaches the surface, the plates get pushed apart to make room for the incoming material.
Due to the non-uniform rate of magma being brought toward the surface along divergent boundaries, the plates move apart at different rates. To accommodate this, transform boundaries or strike-slip faults, shown in the first few images above, break through the lithosphere allowing pieces of the earthâs lithosphere to slide past each other horizontally at different rates.
Divergent plate boundaries form unique geological and chemical features like ophiolites and hydrothermal vents (aka black smokers, pictured above) which are home to extreme biological environments. Some examples of divergent plate boundaries include the mid-Atlantic ridge, the east Pacific rise and the east African rift.
Transform Plate Boundaries
Transform boundaries occur where the Earthâs tectonic plates slide past each other horizontally along transform or strike-slip faults. These boundaries and can be dextral (right-lateral) or sinistral (left-lateral) depending on which way the plates move. As shown below, a person standing on one side of a right-lateral strike-slip fault will watch a tree on the opposite side move right as the fault slips. Similarly, if the same situation occurred and the fault was left-lateral, the tree would appear to move left.
As mentioned above, transform boundaries are common along divergent plate boundaries where they connect sections of oceanic spreading centers or mid-ocean ridges, helping create some of the longest topographic features on the planet. Transform boundaries are also found on larger scales on land like the Alpine fault in New Zealand and the San Andreas fault in Western North America pictured below.
Mountains, basins and unique topography can form along transform boundaries depending on the faultâs geometry, the rock type and how parallel the motion of the plates are to the strike of the fault. In particular, bends along strike-slip faults can form both basins and mountains like those shown in the image above.
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Loss Aversion
What is Loss Aversion?
Loss aversion is the tendency for people to perceive a loss as more significant than an equivalent gain â to feel that the negative utility or âbadnessâ of losing something outweighs the positive utility or âgoodnessâ or of gaining it. Some studies have suggested that the psychological impact of a loss is twice as much as that of an equivalent gain.
There may be an evolutionary explanation for loss aversion: if an animal or proto-human has enough food to survive but no surplus (which is a common situation in nature, because itâs often inefficient to expend energy gathering surplus), gaining some extra food wonât make much difference to them, but losing an equivalent amount of food may make the difference between life and death. So the species evolves to fear losses more than it values gains, because the individuals who fear losses and act to prevent them will out-compete the individuals who put their energy into pursuing gains instead, and will pass on more of their genes to later generations.
Loss aversion has also been observed in non-human animals, including great apes and capuchin monkeys, which lends support to an evolutionary origin for the phenomenon.
Many modern humans across the world who live close to the poverty line are in a similar situation to the ancestral animal described above: gaining some money would be nice for them, but losing the same amount of money could be catastrophic. For them, loss aversion is often a rational behavior, even if it isnât rational for people in a rich consumer society, as we will see below.
Marketing
Marketers exploit the irrational side of loss aversion in various ways. They give customers a service for a free trial period, and then the customer must sign up for a payment contract if they wish to continue with the service, otherwise it will be cancelled. Loss aversion means that this strategy makes people more likely to sign up, because they are reluctant to lose the service theyâve been enjoying during the trial period. Whereas if there were no trial period, and they were simply asked âdo you want to pay for this service?â they would be more likely to say no, because they undervalue gaining a service they donât currently use, but overvalue the impact of losing a service theyâve been using. For the same reason, marketers using free trial periods can increase their prices for the same service, because people are willing to pay more to avoid losing the service theyâve been trialing than they would be willing to pay to start using it.
As with free trial periods for services, so with money-back guarantees for goods. Some companies will sell you a big-ticket item like a premium mattress or sofa with an unrestricted 30-day money-back guarantee. If you donât like the item for any reason, no matter how silly or personal, you can return it for a complete refund. That sounds like a really good deal: you can try the item with no risk, and you can even effectively ârentâ it for a month for free. But the company selling the item knows that at the end of the month it will be a lot harder to give up the item than it would have been to resist buying it in the first place, because of loss aversion. The company knows that many customers who bought the item intending to return it will end up keeping it.
Another marketing tactic is to frame changes in prices differently, as qualifying for a discount versus avoiding a surcharge. One is a gain, and the other is avoiding a loss, and consumers react differently to them even if the financial effect is the same.
Insurance and Extended Warranties
Some forms of insurance are necessary, or even legally mandated. If there is a risk of a negative event which would ruin you financially if you had to absorb the costs yourself, like your whole house burning down, then it is worth paying for insurance against that.
But there are salespeople out there whose job it is to persuade you to pay for insurance you donât need, and they often rely on loss aversion as a sales tactic.
Do you really want to pay $5 to cover your $50 food processor against accidental damage? If you did that for ten appliances, youâd have spent as much as another appliance on warranties. It is vanishingly unlikely that all ten of them will break. Perhaps one of the ten will break, in which case you can buy a new one with the $50 you saved by not buying warranties for any of them. In any case, you know that the cost of the warranty is more than the company expects to have to pay out â otherwise they wouldnât offer it, or theyâd charge more for it. So itâs not in your interests to buy it.
Salespeople use loss aversion to try to persuade you to pay for insurance or warranties against your interests. They emphasize the loss of the item in question, and remind you how much you would hate to lose it. This is especially relevant when youâre buying a replacement or upgrade for an item you already own: you already have a smartphone or a dishwasher or whatever it is, so youâre not really gaining anything, but you can imagine and dread the loss represented by not having one anymore.
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Hedonic Treadmill
What is the Hedonic Treadmill?
The hedonic treadmill refers to the idea that peopleâs subjective happiness levels stay around a given set point throughout their life, and are not permanently affected by major positive or negative life events.
This is unintuitive, because people tend to think that winning the lottery would make them permanently happier, or becoming severely disabled would make them permanently more miserable. However, research shows that lottery winners usually experience a temporary boost in happiness and then revert to their original happiness levels. Similarly, people who suffer debilitating accidents go through a period of grieving, but then often return to their original levels.
âHedonicâ refers to pleasure or happiness (from the Greek hedone, âpleasureâ). The same Greek word gives us âhedonismâ, the philosophy or lifestyle of pursuing pleasure. The âtreadmillâ description refers to the idea of having to keep moving just to stay in the same place. Many peopleâs life paths are characterized by working hard in order to progress in their careers, grow in wealth and prestige, and get a bigger house and a better standard of living; and yet their subjective happiness will often remain the same despite these improvements.
The hedonic treadmill applies to groups of people as well as individuals: research shows that as a countryâs wealth increases, its citizens do not report any increase in their subjective well-being.
Habituation
The hedonic treadmill is related to the idea of habituation: that people become accustomed to an ongoing stimulus and stop consciously noticing it. For example, you might walk into a building that smells strongly, but after spending some time in the building, you stop noticing the smell. If you spend long enough in the smelly building, you might even grow so used to the smell that you think the outside smells strange when you eventually leave! Similarly, many people can become accustomed to background noises such as traffic or ticking clocks, and stop consciously noticing them. (There is an apocryphal story about someone who lived near a clock which loudly chimed the hour, and he slept through the sound of the chimes every night, but one night the 3am chime failed to sound, and he leapt out of bed shouting âWhat was that?â)
It is more difficult for people on the autistic spectrum or those with sensory processing difficulties to âtune outâ stimuli such as smells, background noises and flashing lights. Neurotypical peopleâs brains can adjust to the idea that the stimulus is ongoing and is not new information, and so stop presenting it to the conscious mind; but some peopleâs brains are less able to do that, so they keep noticing the background noise or flashing light, and may become annoyed or fatigued by it more quickly.
Habituation is also illustrated by the old story about boiling a frog: if you put a frog into boiling water, it will leap out, but if you put it into comfortably warm water and gradually raise the temperature, it will stay put and allow itself to be boiled alive. In the same way, people will notice if they are suddenly thrust into an intolerable situation, and will take steps to change it, but if their situation gradually gets worse, they may not notice and may just put up with it. Each little change for the worse is small enough to be ignored, and habituation means that the new slightly-worse circumstances become the new normal, and you become accustomed to them. If this continues, a person may end up putting up with a really bad life situation, and the hedonic treadmill theory suggests that their subjective happiness might not be any lower than before the start of the decline.
However, the hedonic treadmill theory also implies that the opposite can happen: a personâs circumstances can get gradually better, while they habituate to the changes and remain unaffected by them, so their subjective happiness level remains at the level it was before the improvements began.
The Hedonic Treadmill and Decision Making
Having the hedonic treadmill in your conceptual toolbox can guide you toward more rational decision-making. For example, it can free you to step back from the rat race and not chase increases in wealth and status at all costs, because you understand that the happiness they bring will not last, and you will end up feeling much the same on an income of $80k as on an income of $40k, so it is probably not worth giving up other good things in your life to achieve it.
Secondly, you need not live in fear of adversity. You may dread becoming poor or disabled, or even just growing old; but if and when those things happen, it will not destroy your life forever. It will make you unhappy for a while, and then you will habituate and it will become the new normal.
Thirdly, the concept of the hedonic treadmill can help you develop virtues like consistency and faithfulness, by teaching you that habituating to a good thing doesnât mean it stopped being good. Sometimes people leave good marriages or good careers because they no longer feel the same thrill they felt at the beginning. But understanding the hedonic treadmill means understanding that the subjective good feelings may dwindle even if the marriage or the work is objectively as good as it ever was, and so the fading of excitement should not be taken as in indication that it is no longer any good, or as a reason to leave.
Criticisms of the Hedonic Treadmill
Happiness is subjective, and therefore hard to measure. If you ask someone to rate their happiness on a numerical scale, how is that scale defined? It might be up to individual interpretation. Consider two people in a hospital ward recovering from an injury, who are both asked to rate their happiness on a scale of 1 to 10. One of them grew up in a war zone, so rates their current situation as an 8, because they are well cared for, with warmth and shelter and three meals a day, and are not in danger of death. The other has had a more privileged life so far, and rates their current situation a 2, because they are confined to a bed, uncomfortable and bored. Even between people who have had similar life experiences so far, discrepancies in happiness ratings may also simply reflect individual temperament: whether someone is an optimist or a pessimist, whether they focus on the positives or negatives in their situation.
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Rate Law
Rate Law Definition
The rate law is the relationship between the concentrations of reactants and their various reaction rates. Reactions rates are often determined by the concentration of some, all, or none of the reactants present, and determines which reaction order the reaction falls into.
Rate law is a measurement which helps scientists understand the kinetics of a reaction, or the energy, speed, and mechanisms of a reaction. Using the rate law, scientists can understand how long a reaction will take to go to completion, the energy required to stimulate a reaction,
Rate Law Equation
For the reaction:
aA + bB â cC
The rate law equation would be the following:
Rate = k[A]Y[B]Z
This equation describes several different aspects of the rate law. The first is the rate constant or âkâ, which is specific to every reaction at a specific temperature. This rate constant can change with the temperature, as the temperature will affect the overall speed of the reaction.. [A] is the concentration of substance A, while [B] is the concentration of substance B. The exponents Y and Z are not related to âaâ and âbâ, or the reactant coefficients. Instead, Y and Z are determined experimentally and are called reaction orders. The sum of these reaction orders determines the overall reaction order.
How to Determine Rate Law
The rate law is most commonly determined by the initial rates method, which measures the initial rates of reactions, the concentration of reactants, and their effects on the overall reaction. Letâs consider the simplest possible example to determine how this works. Consider the following reaction:
A â B + C
In this reaction, reactant A is the only reactant. To determine the rate law, a series of experiments must be done which vary the concentration of the reactant and observe the initial rate. Suppose that you tested the above reaction, and got the following data:
Initial Concentration A Reaction Rate 1 mole/L 5 moles/sec 2 mole/L 10 moles/sec 4 mole/L 20 moles/sec
Given this âexperimental dataâ, we can easily calculate the rate law for this reaction. After we have our experimental data, we can simply input these different values into the rate equation to find the reaction orders of each reactant. Here is the general rate law equation for the reaction:
Rate = [A]Y
Thus, if we are comparing two experiments, we can put them into the same equation to find which exponents will complete the equation. For example:
Rate1/Rate2 = [A1]Y/[A2]Y
If we plug the experimental results into this equation, we find:
5/10 = 1Y/2Y
Rearranged and simplified, this leaves us with the equation:
0.5 = (1/2)Y
Clearly, the exponent in this case must be 1, making the reaction order 1 for substance A. However, with more complex equations you might need to use algebra to solve for Y. If substance A is the only reactant or product which influences the rate of the reaction, the overall reaction order will also be 1. These reaction orders within the rate law describe the change to the rate if changes in the concentration of reactants or products are made.
Rate Law Examples
Creating Nitric Acid
The following reaction describes a step in the production of nitric acid from oxygen and nitrogen monoxide:
O2 + 2NO â 2NO2
In this reaction, the oxygen molecule is split and one oxygen atom is added to the nitrogen monoxide, creating the acidic species nitrogen dioxide. Scientists have determined experimentally that the rate law for this reaction is:
Rate = k[O2][NO]2
But what does this mean for the reaction itself? Well, the rate law tells us too things about the reaction. First, if you notice that the concentration of oxygen does not have an exponent, we must realize this means â1â. Therefore, oxygen has a first order rate compared to its concentration. Simply put, this means that if you double the amount of oxygen present, the rate will also double.
Nitrogen monoxide, on the other hand, has a second order rate. This means that if you double the amount of NO, you will quadruple the rate. In this way, rate law can be used to determine the outcomes of changing different reaction conditions, especially concentration. Scientists can use this to determine things like the efficiency of their reactions, how they can increase or decrease the rate of a reaction, and can even allow them to conduct analyses of how profitable or efficient their process may be.
Formation of Ozone
The formation of ozone is a reaction that takes place high within the atmosphere. Here, gaseous oxygen (O2) turn into ozone (O3), which is an important molecule for blocking dangerous UV radiation from the sun. Below is the general equation:
2O3 â 3O2
Under experimental conditions, scientists have determined that the rate law equation for this reaction is:
Rate = k[O3]2[O2]-1
This rate law tells us some very important things about the rate and type of reaction. Here, the exponent on ozone tells us that every time the concentration of ozone doubles, the reaction rate quadruples. The negative exponent on oxygen tells us that if the concentration of oxygen doubles, the rate will actually be divided by that concentration, reducing the rate of reaction by half. If we consider both of these statements, we can see the true nature of the reaction. This rate law tells us that there is a delicate balance between the reactant and product, which slows to equilibrium as the concentration of ozone drops and is replaced by oxygen.
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Reaction Rate
Reaction Rate Definition
The reaction rate, sometimes called the rate of reaction, is the speed at which chemical substances react with one another. The reaction rate can be calculated as the instantaneous reaction rate or the average reaction rate, though it is more common to determine the average rate because it is easier to measure.
As a matter of designation, reaction rate values are always displayed as positive. This is true whether it is a reactant or product being measured. If the reactant is being measured, make sure you understand that in the forward reaction the reactant will be decreasing by the reaction rate, where the product will be increasing by the reaction rate.
Reaction Rate Equation
The equation for the average reaction rate is simple, but you must have an understanding of basic chemistry in order to use it. The reaction rate equation can be seen below:
Reaction rate = Moles of a substance used or produced / Number of seconds taken to complete
In this simple equation, it can be seen that the reaction rate is the number of moles of a substance used as a reactant or created as a product divided by how long it took to make or use that amount of substance. This is considered the average reaction rate.
In order to calculate the reaction rate of any given substance within a chemical reaction, you simply need to know how many moles of a substance have been used or created within a given timeframe. However, this is not always straightforward or easy to calculate. Some reactions happen in a fraction of a second while others take centuries to complete. Further, in order to accurately measure the reaction rate you must accurately measure and weigh the products or reactants of a reaction which can be complicated by losses during the experimental procedure.
What Determines the Reaction Rate?
Many things determine the reaction rate, such as temperature, acidity within the solution, or other environmental modifiers. These modifications of the environment prevent or enhance reactions between individual chemical species, which in turn affects how fast the overall reaction takes place. Further, many reaction rate measurements will change as the reaction takes place due to the effects of changing concentrations of products and reactants.
For example, the breakdown of sucrose, or table sugar, is a natural process. Sucrose breaks down into glucose and fructose, two smaller sugar molecules. Typically, however, the reaction rate for this process is abysmally low. In fact, it would take thousands of years for this reaction to take place naturally. Luckily for living organisms, there is an enzyme known as sucrase. This enzyme has the ability to lower the energy needed to break sucrose, which changes the reaction rate from thousands of years to a few seconds.
In non-biological reactions, other chemicals within the solution can change the reaction rate of an individual species by changing the way that those molecules react. For instance, metal ions are used in a number of reactions to help facilitate the reaction rate and increase the conversion of reactants to products.
Reaction Rate Examples
Burning Methane
Methane is a flammable gas with the chemical formula CH4. When methane burns it is oxidized, releasing many of the hydrogen atoms. The full equation for the reaction is as follows:
CH4 + 2O2 â CO2 + H2O
Therefore, as you can see by the balanced equation above, it takes two moles of oxygen for every mole of methane burned. So, to calculate the reaction rate for any of the above species, you would simply need to measure how many moles of each species are used during a specified time period.
For example, let us pretend that you started with 4 moles of methane. After 1 minute, all 4 moles of methane are gone. To determine the reaction rate of methane, simply divide 4 moles by 60 seconds.
4 moles of methane / 60 seconds = 0.067 moles/second
This reaction rate will be the same for all of the other species found in the reaction, besides oxygen. The other species (carbon dioxide and water) are produced in equivalent amounts to the amount of methane introduced. However, in order to combust the methane, two moles of oxygen are needed for every one mole of methane. Thus, 8 moles of oxygen were used in the 1 minute the reaction took place. Therefore, the reaction rate of oxygen would be:
8 moles of oxygen / 60 seconds = 0.134 moles/second
The rate is twice that of methane, because two molecules of oxygen are being used for every one mole of methane.
Measuring the Instantaneous Rate
Though the average reaction rate is commonly used to describe how quickly common reactions take place, the instantaneous reaction rate is used to determine how quickly the reaction is proceeding at various concentrations.
The instantaneous reaction rate is often calculated graphically. By plotting the amount of product and reactant over time, scientists can produce a graph which shows exactly how much change there is between every measurement. The tangent line, or a line that runs parallel to the graph at any point, will show the instantaneous change at that point.
The slope of this line is the instantaneous reaction rate. The instantaneous reaction rate can change depending on the concentration of products and reactants, and generally slows as the reaction nears completion. On the other hand, the instantaneous rate is often higher than the average at the beginning of the reaction because of the high concentration of reactants.
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Reaction Rate
Reaction Rate Definition
The reaction rate, sometimes called the rate of reaction, is the speed at which chemical substances react with one another. The reaction rate can be calculated as the instantaneous reaction rate or the average reaction rate, though it is more common to determine the average rate because it is easier to measure.
As a matter of designation, reaction rate values are always displayed as positive. This is true whether it is a reactant or product being measured. If the reactant is being measured, make sure you understand that in the forward reaction the reactant will be decreasing by the reaction rate, where the product will be increasing by the reaction rate.
Reaction Rate Equation
The equation for the average reaction rate is simple, but you must have an understanding of basic chemistry in order to use it. The reaction rate equation can be seen below:
Reaction rate = Moles of a substance used or produced / Number of seconds taken to complete
In this simple equation, it can be seen that the reaction rate is the number of moles of a substance used as a reactant or created as a product divided by how long it took to make or use that amount of substance. This is considered the average reaction rate.
In order to calculate the reaction rate of any given substance within a chemical reaction, you simply need to know how many moles of a substance have been used or created within a given timeframe. However, this is not always straightforward or easy to calculate. Some reactions happen in a fraction of a second while others take centuries to complete. Further, in order to accurately measure the reaction rate you must accurately measure and weigh the products or reactants of a reaction which can be complicated by losses during the experimental procedure.
What Determines the Reaction Rate?
Many things determine the reaction rate, such as temperature, acidity within the solution, or other environmental modifiers. These modifications of the environment prevent or enhance reactions between individual chemical species, which in turn affects how fast the overall reaction takes place. Further, many reaction rate measurements will change as the reaction takes place due to the effects of changing concentrations of products and reactants.
For example, the breakdown of sucrose, or table sugar, is a natural process. Sucrose breaks down into glucose and fructose, two smaller sugar molecules. Typically, however, the reaction rate for this process is abysmally low. In fact, it would take thousands of years for this reaction to take place naturally. Luckily for living organisms, there is an enzyme known as sucrase. This enzyme has the ability to lower the energy needed to break sucrose, which changes the reaction rate from thousands of years to a few seconds.
In non-biological reactions, other chemicals within the solution can change the reaction rate of an individual species by changing the way that those molecules react. For instance, metal ions are used in a number of reactions to help facilitate the reaction rate and increase the conversion of reactants to products.
Reaction Rate Examples
Burning Methane
Methane is a flammable gas with the chemical formula CH4. When methane burns it is oxidized, releasing many of the hydrogen atoms. The full equation for the reaction is as follows:
CH4 + 2O2 â CO2 + H2O
Therefore, as you can see by the balanced equation above, it takes two moles of oxygen for every mole of methane burned. So, to calculate the reaction rate for any of the above species, you would simply need to measure how many moles of each species are used during a specified time period.
For example, let us pretend that you started with 4 moles of methane. After 1 minute, all 4 moles of methane are gone. To determine the reaction rate of methane, simply divide 4 moles by 60 seconds.
4 moles of methane / 60 seconds = 0.067 moles/second
This reaction rate will be the same for all of the other species found in the reaction, besides oxygen. The other species (carbon dioxide and water) are produced in equivalent amounts to the amount of methane introduced. However, in order to combust the methane, two moles of oxygen are needed for every one mole of methane. Thus, 8 moles of oxygen were used in the 1 minute the reaction took place. Therefore, the reaction rate of oxygen would be:
8 moles of oxygen / 60 seconds = 0.134 moles/second
The rate is twice that of methane, because two molecules of oxygen are being used for every one mole of methane.
Measuring the Instantaneous Rate
Though the average reaction rate is commonly used to describe how quickly common reactions take place, the instantaneous reaction rate is used to determine how quickly the reaction is proceeding at various concentrations.
The instantaneous reaction rate is often calculated graphically. By plotting the amount of product and reactant over time, scientists can produce a graph which shows exactly how much change there is between every measurement. The tangent line, or a line that runs parallel to the graph at any point, will show the instantaneous change at that point.
The slope of this line is the instantaneous reaction rate. The instantaneous reaction rate can change depending on the concentration of products and reactants, and generally slows as the reaction nears completion. On the other hand, the instantaneous rate is often higher than the average at the beginning of the reaction because of the high concentration of reactants.
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Melting Point
Melting Point Definition
The melting point of any given substance is the temperature at which the substance experiences melting, or the moment that the substance changes from a solid into a liquid. Some materials, like metals, glass, and other substances which are solid at room temperature have a high melting point. In other words, they require a lot of energy between the atoms in order for the substance to become a liquid.
By contrast, most substance that we experience normally as gases, such as oxygen and carbon dioxide, have a relatively low melting point. Carbon dioxide, in its solid form, is commonly known as dry ice. This material has a melting point of -56 degrees Celsius (-69 degrees Fahrenheit). When exposed to room temperature air, the drastic change that much past the melting point causes sublimation, or direct conversion of carbon dioxide into a gaseous vapor.
The exact melting point of a substance is determined by many factors, which determine how the atoms within a molecule interact and bond with each other. Because of the differences in atoms and how they interact, one substance can have a different melting point compared to a similar molecule with atoms in a different arrangement.
Interactions that Determine Melting Point
There are several interactions studied in chemistry which determine a substanceâs melting point. The first of those is temperature, which directly influence the amount of free energy available to all atoms and molecules within a system. Temperature, as we experience it, seems to be heat sources such as the sun or a fire. While heat does come from these sources, overall temperature is really a measurement of how quickly molecules are moving and the energy they have to repel each other.
Consider a thermometer, the device used for measuring the temperature. In side of the thermometer is mercury, as seen in the image below. This chamber of mercury is measured to a specific size, and filled with a certain volume of mercury. The small chamber extends up the thermometer as a narrow tube. As the temperature rises, the mercury gets warmer and has more energy between individual molecules of mercury. As the molecules bounce off of each other, they expand the liquid and it travels up the thermometer.
Mercury makes a perfect metal for thermometers because it has an unusually low melting point for a metal. This means that at most temperatures experienced, mercury will remain a liquid and can accurately measure the temperature. However, mercury has a melting point of -38 degrees Celsius, or around -38 degrees Fahrenheit.
Melting point is not only affected by temperature because temperature is not the only thing that affects the movement and energy of molecules and atoms. Melting point is also heavily affected by pressure, which pushes the molecules together. Thus, if you increase the pressure the melting point also increases. Using this method of increasing the pressure, scientists have been able to solidify typically gaseous substances.
A third thing that influences melting point is the structure of the substances themselves. When you consider substances like water, the melting point is largely determined by the interactions between the molecules themselves. Water is a polar molecule, meaning that different areas of an individual molecule have different electrical attractions. In a solution of water, these molecules interact with each other to create hydrogen bonds, or weak bonds between the hydrogen of one water molecule and the oxygen of the next. This series of interactions stabilizes the entire solution, and lowers the melting point of the solution in general.
Scientists have found many other molecular interactions which influence melting point, such as van der Waals forces, molecular symmetry, and atomic forces with determine the interaction and stability of different substances. All of these small, seemingly minute interactions are important for determining how much energy it takes to get molecules to move from a solid to liquid form. Differences in melting point is the basic reason different substances are found in different phases (solid, liquid, gas) at the same temperature.
Chart of Common Melting Points
Substance Melting Point (In Degrees Celsius) Aluminum 660 Copper 1084 Steel 1425 Gold 1063 Lead 328 Water 0 Benzoic Acid 122 Brass 1000 Iron 1150 Silver 961 Mercury -39 Silicon 1411 Coconut Oil 24
How is Melting Point Determined?
A simple experimental setup for determining melting point can be set up with a beaker or crucible, a thermometer, and heat source such as a Bunsen burner. A sample of the material being studied is place in the beaker or crucible, heat is applied, and the temperature is observed. At the point when the substance begins to liquefy, the temperature is noted. This procedure should be repeated several times for accuracy.
However, there are much more advanced methods of determining melting point, such as the device below. This device works on the same basic principle, but can more accurately measure the temperature and energy input into the system. Further, scientists can now use computer applications to simulate the melting point of unknown substances, though they are not always correct.
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Secondary Succession
Secondary Succession Definition
Secondary succession is a type of ecological succession in which the natural succession has been disrupted and must restart with a smaller number of species. As opposed to primary succession, secondary succession happens after a basic ecosystem and nutrient-rich soils have been established, but some accident has wiped many species out. As is often the case after natural disasters, only a basis for an ecosystem and some small survivors remain in place.
Secondary succession proceeds as these smaller ecosystems build a basis for larger, more diverse organisms. Secondary succession is largely predictable by previously observed patterns, and often happens in identifiable steps, depending on the ecosystem. There are many factors which can affect the rate and progression of secondary succession, which are discussed below.
Factors of Secondary Succession
Secondary succession is a process that largely depends on the types of organisms which existed before the ecological disaster. Their remains will provide food for organisms in the soil. Whatever organisms remain in the soil are the organisms which will serve as a basis for what life can establish itself on the surface. Due to the high exposure to sunlight and limited nutrient content of the soil, this is often small plants like grasses and shrubs.
These plants reproduce, and their seeds are distributed across the fresh ground. With limited competition they grow rapidly and after several generations have greatly added to the nutrient content of the soil. The seeds of larger plants then have the ability to sprout and thrive, and soon grow tall on the nutrients from the grasses. These larger plants then drastically change the conditions of the ground below them, increasing the shade. In deciduous forests, this can lead to shade-tolerant trees again sprouting. As they grow above the faster-growing trees, these die off and the entire ecosystem again changes. This basic scenario is shown in the image below.
Which species end up occupying the final community is largely determined by the effects of the species which came before them. A plant which needs a lot of sunlight is unlikely to do well in an area blocked by taller species. This may seem like a change in one plant to another, but the implications are much deeper. Certain animal species and even bacteria in the soil depend on certain plant species through the food web.
If the plant disappears, it alters the whole structure of the ecosystem as it can literally change which heterotrophic organisms can survive there. These types of chain reactions are common in secondary succession and can lead to vastly different ecosystems in places that are similar.
Primary or Secondary Succession?
Primary succession is classified as the introduction to life in a barren environment. This slow progression of basic microscopic ecosystems must proceed before a climax community can be reached. This progression is considered primary succession. But, most ecosystems do not reach a climax before they are befallen by some sort of disaster. Among these are hurricanes, disease, fire, and other things which wipe out a majority of the species, but not the basic foundation of the ecosystem.
Secondary succession takes over when the ecosystem must recover from one of these disasters. It is different than primary succession because, with primary succession, there is no basis on which to start. Secondary succession is less of a blank canvas, and more like a paint-by-color. Though the environment can ultimately have a lot of different colors (or species) there is a precise foundation from which it must start. The conditions present naturally favor species present in the area, and they are the first to take advantage of the open, rich soil.
Secondary Succession Examples
In a Forest
In a forest, secondary succession takes place in 3 basic, well-documented steps. After a fire or other disaster destroys the trees and other plants, the community is reduced to the small organisms and insects present in the soil. These organisms continue to break down detritis in the soil, leaving nutrients deposited for future plants. The first step of secondary succession takes place when early opportunists jump on the open patch of ground, as seen in the image below.
These plants establish a new dynamic in the soil, creating more opportunities and niches for organisms like insects, mice, and other small animals to thrive. Birds often visit these short grass patches to collect seeds and insects. These animals often help disperse other seeds onto the ground, such as pinecones and seeds from berry-producing shrubs. These intermediate species are seen in the second stage of secondary succession.
These plants also change the basic forms of life that exist, blocking out sunlight for grasses below. This may also expel some small animals which depended on the grasses. But it also brings in other occupants, such as squirrels and birds which are dependent on the taller plants for food and shelter. In time, the slow-growing deciduous trees eventually outgrow the fast growers. This advanced stage of secondary succession is known a climax community. However, this community could easily be disrupted again and secondary succession would start over.
In a Kelp Forest
The climax community, in a kelp forest, consists of many different and biologically dependent species. A simple act of disrupting any of these species can cause massive effects on the entire community. Four of the most important species are otters, starfish, sea urchins, and the kelp itself. Kelp is a plant-like alga which anchors itself to the seafloor. Many species of fish reproduce and live in kelp forests. Sea urchins like to eat the base of the kelp. This can easily wipe out a kelp forest, but the urchins are held in check by their predators: the otters and the starfish.
Throughout the 1800s and into the 1900s otters were hunted to near extinction for their pelts. This disruption slightly upset the balance of kelp forests, and the kelp was thinned. Without the otters, the starfish increased in number feeding on the abundance of sea urchins. However, this abundant density of starfish led to a terrible starfish virus that wiped out a majority of their population. As such, many kelp forests are currently being destroyed by sea urchins. âUrchin barrensâ, as they are called, have no more kelp and therefore are like underwater deserts when it comes to other marine life.
These ecosystems have undergone a massive disruption and now must go through secondary succession to again reach a climax community. Luckily for the kelp forests, otters are returning in some numbers. They will be aided by the fact urchins are abundant, and eventually, the starfish will return. The kelp will be able to reestablish itself on the seafloor, and a wealth of life will come back to the underwater ecosystem.
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Electron Transport Chain
Electron Transport Chain Definition
The electron transport chain is a crucial step in oxidative phosphorylation in which electrons are transferred from electron carriers, into the proteins of the electron transport chain which then deposit the electrons onto oxygen atoms and consequently transport protons across the mitochondrial membrane. This excess of protons drives the protein complex ATP synthase, which is the final step in oxidative phosphorylation and creates ATP.
Electron Transport Chain Location
The electron transport chain is located within mitochondria, and the proteins of the electron transport chain span the inner mitochondrial membrane. This can be seen in the image below.
The electron transport chain consists of 4 main protein complexes. Each complex has a different role in the chain, some accepting electrons from carriers and some which serve to transfer electrons between the different complexes. The basic function of the electron transport chain is to move protons into the intermembrane space.
ATP synthase, which is not part of the process, is also located on the mitochondrial inner membrane. This complex will use the electrochemical gradient of the protons to essentially extract energy from the pressure of the protons wanting to cross the membrane to the mitochondrial matrix. This energy is then used to add a phosphate group to an ADP molecule, forming ATP. The electron transport chain must first extract the energy it needs to pump the hydrogen ions from electron carriers.
Electron Transport Chain Steps
Step One: Electron Carriers
Electron carriers get their energy (and electrons) from reactions during glycolysis and the Krebs cycle. These reactions release energy from molecules like glucose by breaking the molecules in smaller pieces and storing the excess energy in the bonds of the recyclable electron carriers.
Step Two: Hydrogen Ion Pump
These carriers are then transported to the inner mitochondrial membrane, where they can interact with the proteins of the electron transport chain. These carriers dump their electrons and stored energy in complexes I and II. These protein units relieve the electron carriers of excess hydrogen atoms. The electrons stay with the proteins, while the hydrogen atoms are left in the matrix. The electrons from these bonds pass through complexes I and II, through coenzyme Q. This specialized protein functions solely in passing electrons from these complexes to complex III.
Complex III serves as a hydrogen ion pump. It actively takes the energy from the electrons and uses it to pump the hydrogen ions against their natural gradient. Because the ions cannot easily travel through the membrane, they build up in the intermembrane space between the inner membrane and the outer membrane. This allows for the establishment of a proton-motive force, which will later be used by ATP synthase to store energy in molecules which can be used by other proteins as a source of energy.
Step Three: Disposing of the Electrons
The final step of the electron transport chain is to remove the electrons with lower energy out of the system. This allows for new electrons to be added, part of the reason the process is called a chain. Cytochrome C is the complex which transfers the electrons to the final protein in the electron transport chain. Complex IV has a unique function both pumping hydrogen ions as well as depositing the electrons on a final electron acceptor.
In the case of aerobic organisms, this acceptor is oxygen. Found in the form of dissolved gas in the blood, complex IV donates the electrons to two free hydrogens and one oxygen atom. The complex catalyzes the reaction, creating water. This allows the electron transport chain to release the electrons, freeing up a new spot in complex IV. This spot is filled by electrons from complex III, and so on all the way back up the electron transport chain.
Electron Transport Chain Products
During the course of the electron transport chain, only two things are really created. First, water is created as the electron transport chain deposits spent electrons into new water molecules. These water molecules can be reabsorbed by the body for use elsewhere or can be dispelled in the urine. Second, while the electron transport chain does not create ATP it does create the proper conditions for ATP to be produced. This is called the proton-motive force and is a product of the electron transport chain transporting hydrogen ions to one side of the inner mitochondrial membrane.
Stopping the Electron Transport Chain
One of the best ways to understand the function and purpose is to understand what happens if the electron transport chain stops. This can happen from two basic scenarios. The electron transport chain can stop because it does not have a source of electrons, or it can stop because it can no longer pass electrons on.
The first scenario would be caused by something like starvation. Without a source of glucose or other energy-rich molecules, cells would not be able to collect electrons on electron carriers. Without anything to transfer, the chain would simply stop pumping hydrogen ions. In turn, ATP synthase would stop functioning and the entire cell would soon run out of energy and deteriorate.
The second scenario is somewhat more common and happens when cells run out of oxygen. Organisms which are facultative anaerobes are able to use different processes when there is no oxygen for oxidative phosphorylation. In some organisms the process of fermentation allows glycolysis to continue, producing only a small amount of ATP. Without the electron transport chain, the cell still needs to recycle electron carriers. In the case of alcohol fermentation, the electron carriers dump their electrons in a reaction which creates ethanol as a final product. This allows glycolysis to continue producing ATP, allowing the cells to live through periods of low oxygen content.
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RNA Polymerase
RNA Polymerase Definition
RNA polymerase is the protein which synthesizes new RNA strands by transcribing the DNA sequence into RNA. This RNA molecule is then processed and read by a ribosome to produce a protein. RNA polymerase is found in all living organisms because of its importance to the processes of life. Slight differences are found between different types of RNA polymerase, and eukaryotes even have several different versions which process different parts of the DNA.
RNA Polymerase Function
RNA polymerase is the most important enzyme in the process of transcription. Remember that transcription is the process of copying the double-stranded DNA into a single strand of RNA. They may look slightly different, but they are both written in the language of nucleotides. In order for RNA polymerase to begin its work, it must first find an appropriate promoter region. This region can be targeted by transcription factors in eukaryotes. These proteins bind to the site, creating a suitable arrangement for RNA polymerase to bind and start transcription. In bacteria, there is only a single promoter, the sigma initiation factor. This process can be seen with the generalized RNA polymerize molecule shown below transcribing DNA. The transcription factors are not shown.
As RNA polymerase binds to the DNA, it changes conformation, or shape. This starts the enzymatic chain reaction which grows a new chain of nucleotides into an RNA molecule based off of the template presented. After RNA polymerase has created this new molecule, the RNA must be processed and released from the nucleus. Now called messenger RNA, or mRNA, it will encounter a ribosome which has the appropriate mechanisms for the process of translation.
Much like translating English to Spanish, the ribosome must âreadâ the sequence of the RNA and convert the message into the language of amino acids. These small molecules form long chains, which fold into intricate shapes to become biochemically active proteins. These proteins, in turn, create and maintain parts of the DNA, as well as replicate the DNA. Parts of the DNA store the genetic information for the RNA polymerase protein itself, which is first decoded by an RNA polymerase molecule. This situation is really the basis of the chicken and the egg if you think about it.
RNA Polymerase Structure
In Humans and Eukaryotes
Humans share a similar RNA polymerase structure with the rest of the eukaryotes with membrane-bound organelles. Though there are several different types, the overall structure is similar in each.
Like many eukaryotes, humans carry 3 different versions of RNA polymerase, identified by Roman numerals. RNA polymerase I specifically encodes the majority of the RNA used within ribosomes for them to attach to and decode RNA sequences. RNA polymerase II synthesizes most mRNAs and is responsible for transcribing the majority of the genetic code. It is also the largest, containing 12 subunits. RNA polymerase III synthesizes transfer RNA, which are small segments attached to free-floating amino acids to help the ribosome recognize them when they are bound into a protein.
Plants have two other RNA polymerase enzymes, with functions related to producing RNA which suppresses other genes. This is a form of gene regulation not seen in other eukaryotes, and which depends on a special form of RNA polymerase, but it gives plants the ability for more gene regulation.
In Other Organisms
RNA polymerase is also found in bacteria, archaea, and even some viruses. In bacteria, the enzyme consists of several subunits, and there is only one version. This version is initiated by a single transcription factor, called sigma. This protein first binds to the RNA polymerase, which is then more likely to bind to the correct promoter region. This method is slightly less efficient than the eukaryote method, in which there are many ways and proteins to inhibit or enhance gene expression.
Archaea have an RNA polymerase enzyme which is similar to both bacteria and eukaryotes, suggesting they are deeply related to both groups. While they only have 1 RNA polymerase type, it strongly represents both RNA polymerase II in eukaryotes as well as bacterial RNA.
Viruses, though they are not strictly considered living, also code for RNA polymerase, and include several unique types. Like living organisms, viruses have a need to express their DNA. Some viruses even have an RNA-dependent RNA polymerase. This is a confusing way of saying that the enzyme reads RNA to produce RNA. Unlike most organisms, some viruses store their genetic information as RNA, rather than DNA. These viruses must produce their own RNA polymerase because eukaryotic RNA polymerase would not be able to read RNA or express the viral genes. Some of these RNA polymerase enzymes are only a single subunit, being built from the smallest amount of genetic information possible.
RNA Polymerase and Evolution
This enzyme is strikingly similar among all the domains of life and is one of the reasons scientists suggest that all life originated from a common ancestor. RNA polymerase, as noted in the above section, creates a common debate among evolutionary scientists. Did genetic information start as RNA or DNA?
While it is simple to say that it must have been DNA because most life is now based on DNA, thinking about it may give you a headache. If DNA stores the information for RNA polymerase, but must be first decoded by RNA polymerase, how would the first DNA molecule have been replicated?
Without RNA polymerase, there is no mRNA and no proteins like polymerase itself. To fix this conundrum, scientists came up with the RNA world hypothesis. In this hypothesis, life started as simple strands of RNA. These strands of nucleotides would associate and dissociate at random, but certain combinations led to more stable patterns. Over time these chains gained the ability to be enzymes involved in binding amino acids, called ribozymes. These patterns of RNA are still seen in living organisms, as proof this is possible.
Eventually, as it is theorized, these ribozymes eventually evolved into fully housed protein complexes, what we would now call ribosomes. These structures have several pieces of RNA housed in a protein sheath. These complexes evolved over time to be extremely efficient at reproducing DNA and may have even created the first protocell. This fully housed RNA molecule system would be self-reproducing and may have been the first living cell.
The problem with the current theory, according to some scientists, is that it does not accurately reflect how DNA came into the picture. DNA, though still based on nucleotides, uses one different nucleotide and is a partially different system. However, these critics have a hard time explaining how DNA replicated itself. Some scientists suggest that DNA is a more structurally sound molecule, with a double-helix structure which protects it from damage. This could mean that some RNA organisms evolved DNA as a way of ensuring their genetic information would not be damaged. Others suggest that viruses may have introduced DNA into the RNA world.
Either way, RNA polymerase would have arisen as a natural means of decoding the DNA back into protein-producing RNA.
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Homologous Chromosomes
Homologous Chromosomes Definition
Homologous chromosomes are a pair of DNA molecules which contain information for the same genes, though each homologous chromosome may carry different alleles. A diploid organism (like a human) carries two copies of each gene, one on each part of this pair of homologous chromosomes.
Homologous chromosomes provide a number of functions to organisms, mainly in providing genetic variety. This genetic variety generally increases a speciesâ chance of surviving and adapting to changing environmental conditions. Homologous chromosomes typically have the same structure, because they contain genes coding for the same protein products. While the alleles for the genes may differ slightly by a few nucleotides, they are typically around the same length and form the same shape when condensed into a chromosome.
Homologous Chromosomes Function
Homologous chromosomes have the same function as any chromosome, they contain codons which specify the sequence of amino acids within a protein. Each codon consists of a three-nucleotide sequence, which can be matched to an amino acid. To create large and complex proteins, genes must contain many thousands of nucleotides. Each protein then has a unique and specific function, which contributes to the actions of the entire cell.
Diploid organisms carry two copies of each gene for every protein they create. This allows for a number of different possibilities depending on the function of the protein. Some alleles create normally functioning proteins, some create non-functioning proteins, and others produce novel or more efficient proteins. With homologous chromosomes, two versions of each protein are created. This means that an organism can have many levels of protein functionality, from totally non-functional to hyper-functional. In combination with the many thousands of proteins an organism uses and the environment, this creates enormous variety in animals with homologous chromosomes.
Further, homologous chromosomes may engage in the process of homologous recombination during the process of meiosis. This random process swaps two parts of a pair of homologous chromosomes which contain the same genes. This process adds even more variety, as it recombines the arrangement of genes you received from your parents.
Homologous Chromosomes Structure
Below is a picture of a karyotype, which is a graphic of sorted images showing every chromosome within a cell. Like the one below, most karyotypes show the homologous chromosomes next to each other. Realistically, unless they are lined up during cell division, these homologous chromosomes are randomly distributed throughout the nucleus.
If you look at the image above, you will see 22 pairs of homologous chromosomes as well as the sex chromosomes X and Y. If you look at any one of the pairs, you will notice the homologous chromosomes are always the same basic length and size. Look closer and you will notice that the dark banding on the homologous chromosomes seems to match. This is because the chromosomes have the same basic structure, changing only at a few nucleotides. When the dark dye binds to the chromosome it prefers certain locations based on the molecular structure. Each of the homologous chromosomes is the same, so the dye stains them the same.
Homologous chromosomes can only be identified this way when they are condensed. As chromatin, the individual molecules unwind into a loose structure that cannot be separated. Though some of the homologous chromosomes look like slightly different shapes in the above karyotype, they were likely being bent and pushed around by the cell membrane or other cellular components.
Homologous Chromosomes vs Sister Chromatids
Homologous chromosomes are often confused with the similar term, sister chromatids. Sister chromatids are formed when DNA is copied. DNA is made of two complimentary strands, which wrap around each other in a helix formation. As proteins replicate the DNA they split the strands apart, and match new nucleotides to each side. The new strands grow, until two newly formed strands of DNA are completed. These sister chromatids will be fully separated during mitosis.
In contrast, new homologous chromosomes are created during meiosis when duplicated chromosomes are created and separated into individual gametes. When two gametes fuse together, these homologous chromosomes will contribute the maternal and paternal alleles for each gene. When this organism matures and undergoes meiosis to create gametes, these homologous chromosomes will be rearranged, recombined, and repackaged into unique genetic combinations.
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Phospholipid
Phospholipid Definition
A phospholipid is an amphiphilic molecule consisting of a polar head region, a unit of glycerol, and two or more non-polar fatty acid tails, typically found in a cell membrane. A bilayer of phospholipid molecules forms a plasma membrane.
When the phospholipid molecules are joined by other lipids and integral proteins, the surface can function as a cellular membrane. This semi-permeable membrane blocks the flow of polar substances, allowing the cell to control the concentration of various substances through the use of protein channels.
Phospholipid Structure
A phospholipid consists of two basic parts: the head and the tail. The hydrophilic head consists of a glycerol molecule bound to a phosphate group. These groups are polar and are attracted to water. The second group, the hydrophobic tail, consists of two fatty acid chains. Some species use three fatty acid chains, but two is most common. The fatty acid chains can be saturated, or unsaturated. Unsaturated fatty acid chains have less hydrogen, forcing the molecule to form double carbon-carbon bonds. These bonds create bends in the tail, as seen in the image below.
A phospholipid starts as all of these constituent parts, floating around the cytosol. When they come together near the endoplasmic reticulum, special enzymes bind all the parts together, forming a single phospholipid. As many enzymes are functioning together, many phospholipid molecules can be created quickly. As they are created, the phospholipid molecules arrange into a bilayer. This bilayer is eventually large enough to form a small, empty vesicle. These small round balls of phospholipid bilayer are transported to the cell membrane. Here, they can fuse to the cell membrane, growing the cell.
Phospholipid Function
The amphiphilic nature of a phospholipid is extremely important for the functioning of cells. Every phospholipid, because of this dual relationship with water, is self-arranging when grouped together. The hydrophilic tails are drawn together via hydrophobic interactions, such as van der Waals forces. The hydrophilic heads are drawn toward the aqueous solution on either side of the phospholipid bilayer.
This creates one of the most important structures in biology: a semi-permeable membrane. Each phospholipid molecule pushes closely into its neighbor. The hydrophobic core of the membrane helps exclude ions and water. This is extremely important for cells, which must protect themselves from changes in the environment, including excess ions, water, or other substances. Every species of cell has a different arrangement of surface and integral membrane proteins. These channels and enzymes allow the cell to control what goes in and out of the cell, and at what rate.
As part of this important cell membrane, each phospholipid plays an important role. The heads can have various additions and attachments, changing how they interact with water and molecules. The tails have a very important function in establishing the fluidity and strength of the membrane. Some phospholipid molecules have two straight tails, while others have crooked tails. The ratio of straight-to-bent phospholipids determines how closely each phospholipid can get to its neighbor. The closer the phospholipid molecules, the tighter and more rigid the membrane. Other lipid molecules, known as sterols, are also embedded in the membrane and can increase or decrease membrane fluidity.
The composition of cell membranes varies greatly, but in general, animals that live in the cold have more fluid membranes, while those living in hot environments have less fluid membranes. As the fluidity also changes with temperature, this ensures that animals in both environments have adequately fluid membranes. Too stiff, and the cell cannot function properly. Too loose, and the cells will fall apart. That is why organisms adapted to heat cannot survive cold environments and vice versa.
Phospholipid Examples
Cholesterol and Phospholipids
Within human cells, the balance of straight to bend phospholipid molecules, as well as sterol molecules, are important to the fluidity of cells. Cholesterol is especially important, helping to make cell membranes more rigid. Humans naturally produce cholesterol and do not need additional cholesterol in our diets. Our bodies will naturally regulate the cholesterol within our cell membranes, but this process can be overridden by diet.
Only animals produce cholesterol, as plants produce other fats and oils to store energy and support their cell membranes. Cholesterol, in humans and animals, is similar enough that our bodies can easily incorporate it into their cells. A diet high in animal tissue, eggs, and milk will add tons of cholesterol to your bloodstream. Here, it works its way into the cell membranes of your arteries, making them more rigid. Rigid arteries are much more likely to clog and burst, leading to heart attacks, strokes, and aneurysms. Luckily, this process can be combated by eating a plant-based diet and limiting cholesterol intake.
Drug Delivery using Phospholipid Micelles
The phospholipid molecules of the cell membrane are great at keeping substances out, but sometimes doctors want to get substances into a cell, to deliver a medicine or treatment. Many drugs now have phospholipid delivery systems. The drugs are either bound to the phospholipid molecule or enclosed in a micelle. A micelle is a small ball of phospholipids. These can easily fuse with the cell membrane, allowing the medicine to be deposited within the cell as this happens. As the science progresses, scientists are even planning to directly engineer these small capsules. By attaching specific proteins to the surface, they can target receptors on tissue-specific cells, allowing the drug to be delivered specifically to a single organ or place in the body.
Lecithin, A Phospholipid-Based Food Additive
Lecithin is a common food additive, made mostly from phospholipid molecules packed together. These phospholipids are extracted from plant and animal cells, and the rest of the cell is removed. The fatty, but polar nature of these proteins allows them to be used as an emulsifier. These types of culinary additives help dissolve fatty substances in aqueous solutions. Each phospholipid molecule can bind to both the non-polar fatty substances, as well as interact with water molecules and polar substances. This can help dissolve powders into non-polar or fat-laden dishes.
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Biotic Factors
Biotic Factors Definition
The biotic factors of an ecosystem are the living organisms and the molecules they produce. Living things contain DNA, which code for proteins. These proteins carry out important biological chemical reactions, which create wide range of biomolecules, like fats, carbohydrates, and other cellular components. All of these products of DNA, and the organisms themselves are biotic factors.
Biotic Factors and the Ecosystem
Within every ecosystem, there are two types of factors, biotic factors and abiotic factors. The abiotic factors are all of the non-living things within the environment. This can be things like the minerals within the soil, the climate, and natural disasters. The biotic factors react to and influence the abiotic factors of any given environment.
In the following examples, look for interactions between biotic factors and abiotic factors. Sometimes the biotic factors are influenced by things like weather, pH, and the general conditions of their environment. In other cases, the biotic factors change and morph the environment, changing the general conditions as they do so. Biotic factors, unlike abiotic ones, interact in the environment in a way which tends to keep the system in flux.
Examples of Biotic Factors
A Grassy Hillside
Though it might not look like much, a simple grassy hillside is a perfect example of biotic factors altering what changes take place in an environment. If you are lucky enough to live next to a beautiful grassy hillside, look out your window. If not, check out the one below.
The easiest way to imagine the effects of the biotic factors is by taking them away. In this picture, there are only a few easily visible biotic factors. Imagine first that all of the trees were gone. These biotic factors take up nutrients from the ground, but they also provide shade and shelter for other organisms. Their high canopies collect water, directing it to plants below. Without the trees, the biotic factors below them disappear as well. Take away the grass, and you are left with nothing but dirt.
Using your microscope, you could see many more biotic factors in the soil itself. Small insects, bacteria, and a whole underground ecosystem. These biotic factors are harbored and supplied by the roots of the plants. Without the grass, the other organisms would not survive, and only barren minerals would be left. Without the roots, the topsoil would also blow away in the wind, turning the ground into hard rocks and compacted dirt. Any rain would quickly wash away remaining minerals and nutrients, carrying them eventually to the ocean.
As you can see, the abiotic factors of this environment (soil, water, etc.) are quickly changed as the biotic factors disappear. The biotic factors are therefore influencing the distribution, use, and arrangement of the other factors, and changing their environment to better suit life.
Marine Environment
Under the waves of the ocean, things are very similar. The non-living factors most important in the open ocean are sunlight, nutrients, and oxygen. The biotic factors of the open ocean require all three for communities to survive. In this case, it is even easier to see how active the biotic factors are in changing the environment.
In the ocean, the food web is mainly based on algae. These small photosynthesizing autotrophs use energy from the sun to produce sugars and other biologically important molecules. In doing so, they create a small amount of oxygen but use many nutrients. Sometimes, if the nutrients like nitrogen and phosphorous are heavy, these algae will form dense algal blooms. These blooms can be a boon for other species, which come to feed on the massively expanding algae.
However, these blooms can sometimes turn deadly. If all the conditions are just right, the biotic factors can completely change the non-living factors within the environment. When algal blooms are thick, the algae become densely clustered together, in an attempt to capture the sunâs rays. Eventually, the top layer of dense algae completely blocks out the sun. The algae below start to die and drop to the bottom.
Here, a number of small organisms and bacteria rapidly consume the decaying algae. They gather in such numbers and reproduce so fast, they start to deplete the oxygen of water. If the algal bloom is large enough, these biotic factors can create vast zones completely depleted of oxygen. Many organisms, even large sharks and fish, will die if they try to pass through it. This is an extreme case of how the biotic factors of an environment change the abiotic factors.
Biotic Factors throughout Evolutionary History
Life (a.k.a. biotic factors) has been present on this planet for roughly 4,000,000,000 years. The planet before this had no biotic factors. It was essentially a barren rock, with vast oceans, and a slight atmosphere, barely capable of deflecting the sunâs most harmful rays. Then, life happened. The biotic factors started to expand. With them, they brought drastic changes to the environment.
Above is a graph of the oxygen content of the atmosphere, since life first evolved nearly 4 billion years ago. As you can see, for nearly 1.5 billion years, the oxygen content on Earth remained at zero. However, as Earth entered Stage 2 (see graph), something profound happened. Some of the organisms evolved the ability to photosynthesize. During Stages 2 and 3, the oxygen produced by these organisms is rapidly absorbed into the oceans, rock, and atmosphere. An Ozone layer is created, which greatly protects the Earthâs surface from harmful UV radiation.
By the end of stage 3, the oxygen has completely saturated everything it can and begins to fill the atmosphere. This leads to mass extinction, and many of the anaerobic biotic factors are killed off. However, oxygen-based metabolisms are also very efficient. This massive change in the atmosphere and the quality of sunlight leads to an explosion of lifeforms. Around this time, many multicellular lifeforms were taking shape. These biotic factors eventually exploded onto land, gradually transforming the rocky landscape into productive ecosystems with layers of nutrient-rich topsoil.
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Autotroph
Autotroph Definition
An autotroph is an organism capable of synthesizing energy-bound organic molecules, such as sugars, using inorganic molecules and an environmental energy source. This can be directly compared to a heterotroph, which is incapable of synthesizing these molecules and must consume other organisms. An autotroph can use different energy sources, such as the sun or inorganic oxidation, to store the energy it will need for cellular reactions.
An autotroph can range widely in size and distribution. The smallest autotroph is like a cyanobacteria or other unicellular autotroph. The largest autotroph is a tree. In terms of height and girth, this title likely goes the Giant Sequoia Trees, though aspens and other species with multiple stems have been shown to have a greater mass.
These autotroph organisms are much larger than the largest heterotroph and get all of their energy from the sun. An autotroph is the opposite of a heterotroph, which must consume organic materials made by other organisms as food. An autotroph can either use photosynthesis or chemosynthesis to produce food. Each of these types of autotroph is discussed below.
Types of Autotroph
Photosynthesis
An autotroph using photosynthesis to survive is using the sunâs energy to bind carbon dioxide molecules into larger sugar molecules. This provides a source of food for the organism, which then survives by using the energy bound in the sugar molecules to drive other cellular reactions. This process also produces oxygen as a byproduct, which is convenient for us heterotrophic animals which rely on it.
There are several different kinds of photosynthetic autotroph and together they produce a large amount of the biological energy used by the rest of the organisms. The kingdom Plantae contains mainly autotrophic species. These organisms include all terrestrial plants, making up the basis of ecosystems from rainforests to grass plains. Smaller organisms can also be an autotroph, such as algae and cyanobacteria. These organisms are typically single-celled, but they photosynthesize all of the food they need. Other organisms feed on them by filtering them out of the water column. Thus, these autotrophs also form the basis of the marine ecosystem.
Chemosynthesis
Life is not bound to sunlight, and the process of chemosynthesis allows an autotroph to obtain energy from sources other than the sun. There are many types of chemoautotroph, ranging from sulfur-using bacteria on hydrothermal vents deep in the ocean to organisms surviving in the extreme salinity, to organisms buried deep within the earth.
All of these organisms use different methods for obtaining energy, but the basic process is the same. Instead of getting energy from the sunâs rays and using it to create the molecules the cell needs, an autotroph using chemosynthesis will obtain the energy from a naturally occurring chemical reaction. Many natural reactions release energy, and the autotroph often has enzymes to help the process along. Just like the photoautotrophs, these organisms then store the energy in biomolecules. Like every other autotroph, these organisms start a food chain and can sustain entire communities.
Autotrophs in Ecology
An autotroph typically forms the base of any food web. This is because an autotroph is the only organism capable of producing and storing energy. Other organisms can only collect this energy by consuming an autotroph. Below is a typical food web. The autotrophic organisms (called producers in ecology), can be seen as the basis for all other life.
At the producer level, each autotroph takes in energy from the sun. These plants and small organisms (both terrestrial and aquatic) produce excess energy in the form of sugar, fats, and proteins. The tissues of an autotroph are consumed by organisms at the next level. These organisms are herbivores and feed only on the autotroph producers. Larger carnivores feed on the herbivores, and some omnivores feed on both. As you can see here, and in the types of autotroph above, an autotroph is always the basis of more complex ecosystems.
Autotroph Examples
Kelp Forests and Sea Urchins
Kelp is an autotroph and a very advanced form of algae. These underwater plants can tower several meters off the seafloor. Gathering the energy of sunlight into edible materials, a kelp forest can be an incredibly productive ecosystem. The kelp not only provide food but shelter and even nesting grounds for many species.
The above picture shows a diver swimming in a kelp forest. The kelp in this picture is healthy and flourishing. This healthy ecosystem, based on the autotroph, can sustain a variety of life. However, human influences and natural diseases can easily wipe out these productive environments in a very short amount of time. For this to happen, a well-documented series of cascading events must take place.
Above is a sea urchin, one of the only real threats to the kelp. Sea urchins crawl along the sea floor eating algae which grows on the rocks and coral. To them, kelp is a delicious and nutritious prize and they can eat large amounts of it. While other fish eat the leaves of the kelp, these can be regrown and the autotroph will survive and continue to produce. Sea urchins destroy the one thing the autotroph needs to survive, the holdfast.
The holdfast is a small part on the base of the kelp which anchors it to the seafloor. Without the holdfast, the large kelp would wash ashore in the waves, quickly dry out, and die. Luckily for the kelp, the sea urchin has a few natural predators. Both sea otters and starfish prey on kelp, and in a healthy kelp forest they keep sea urchin numbers at bay. But, when these species falter, there can be disastrous consequences for the autotroph, as well as the ecosystem which is built around it.
Off the west coast of the United States, this battle is currently being won by the sea urchins. The fur-trade largely targeted sea otters, which protected the kelp. The starfish numbers increased, for a while. This helped displace the lost otters. However, when a deadly starfish virus struck the waters of the west coast, devastation came. Without the starfish or the otters, the sea urchins won.
Vast areas of the coastline are now called urchin barrens. Here, the only autotroph is the lonely single-celled algae, drifting in the water column. The large kelp forests have been mowed down by a thick army of sea urchins. With no more natural predators, a ravenous diet, and a high tolerance for starvation, the sea urchins practically insure that a large autotroph like the kelp will never become established.
Redwood Trees
The giant coastal redwood trees of California and Oregon are also an interesting autotroph. This autotroph not only provides food, shelter, and oxygen for a number of residents in the surrounding ecosystem, but it also plays a role in the atmosphere of the ecosystem.
Redwood trees are so large, and suck up so much water from the ground that they increase the humidity significantly in the canopy. Though hundreds of feet off the forest floor, the humidity increase also increases the likelihood that the atmosphere will become saturated with water and rain will fall. While a single autotroph adds very little, a vast forest of giants can produce striking results.
Because of the increased rainfall caused by the autotroph, the rest of the plants below thrive. This increases the diversity of autotrophic organisms, which in turn increased the amount of heterotrophs. This cycle of positive reinforcement can lead to healthy and productive ecosystems, which benefit humans greatly. Likewise, cutting a giant autotroph down leads to desertification. The humidity increase is lost, along with the increased rainfall. This supports fewer autotrophs, and in turn leads to a drier, less productive ecosystem.
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