dps17bio-blog

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Mass Transport in Plants.

Over large distances, efficient movement of substances to and from surfaces is provided by mass transport. The conducting tissue of the plant is called the vascular tissue and is specialised into xylem and phloem.

XYLEM: transport of water (and mineral salts) from the roots up to the leaves (upwards only)

PHLOEM: transport of organic substances, family sucrose from the leaves to to other parts of the plant (up and down to buds or roots)

Transport of water

evaporation + diffusion => transpiration (pulling force)

Xylem structure

Water is transported in the xylem. The main features of xylem are:

1.Long, hollow tubes made of dead cells so there is unrestricted flow of water along the tube

2.Have extra thickening called lignin in the cell walls. This is a waterproof layer. It also provides the vessel with strength and support. 3.Pits are areas with no lignin. They allow sideways movement of water between xylem vessels. This is important if the xylem vessel is damaged.

Xylem transports water in the stem and leaves

Importance of water to the plant:

Photosynthesis in the palisade cells of the leaf

Keeps cells turgid which provides support for the plant

The movement in this vessel is in one direction only which is upwards.

Cohesion-Tension Theory

Water molecules stick to each other by hydrogen bonding and this is called cohesion. This cohesion maintains a column of water molecules inside the xylem vessel.

Water molecules also stick to the inside of the xylem vessel and this is called adhesion.

As water evaporates from inside the leaf (transpiration) this creates a pulling force, a tension, on the column of water molecules - pulling the entire column of water molecules upwards.

It is the sun that provides the energy (solar energy) to maintain this flow of water, as energy is needed for evaporation (liquid to gas) during transpiration.

Evidence for the water column being under tension.

This has come from measuring tree circumferences/diameters. The minimum values are recorded when the rate of transpiration is at its highest when there is the greatest pulling force on the water column inside the xylem, it is under the greatest tension. As water molecules stick to the inside wall of each vessel, each vessel is pulled inward, so overall there is a reduction in the overall measurement.

From xylem vessel to cells of the leaf: the water molecules continue to move down a water potential gradient. Both cohesion of water molecules and osmosis are involved.

Evaporation of water molecules: the cell walls of the spongy mesophyll cells are moist and water continuously evaporates into the large air spaces inside the leaf. Water diffuses from the airspaces inside the leaf, through the stomata and out of the leaf. This is transpiration.

Water normally exits the leaf as water vapour during transpiration.

Transpiration may occur at different sites:

1. Stomata: evaporation of water from mesophyll cell walls into the air spaces inside the leaf and diffusion of water vapour through stomata.

2. Cuticle: evaporation of water from epidermal cells through the waxy cuticle covering the leaf surface. The cuticle is generally impermeable to water so little is lost here.

Rate of transpiration

This can be measured using a photometer. A photometer actually measures the rate of uptake of water by a cut shoot. It does not take into account that some of the water will be used for photosynthesis in the palisade cells, or some is used to make the cells turgid. The technique involves measuring the time it takes for an air bubble to move.

A photometer is used to measure transpiration rates (but not directly). It contains a leafy shoot (because transpiration happens here), the stem of which was cut will severe a xylem vessel underwater so no air can enter). Assuming that the shoot does not use water for any metabolic processes, for example photosynthesis and keeping cells turgid, the rate at which it takes up water from the photometer is the same as the rate of transpiration. This rate can be measured by following the movement of an air bubble along the graduated (has numbers) scale of the photometer over a measured time interval. The actual volume of water taken up can be found by pir^2 * l when l = distance the bubble has moved and r is the radius of the glass tubing.

The photometer can be used to study factors that affect the rate of transpiration.

Light intensity: the stomata in the leaf open in the light to allow carbon dioxide to enter the leaf for photosynthesis. The stomata close in the dark when photosynthesis cannot occur. Therefore higher light intensity increases the rate of transpiration until the stomata are fully open.

Temperature: increasing the temperature increases the rate of transpiration. At higher temperatures the molecules have more kinetic energy and thus the rate of evaporation also increases.

Humidity: this is the concentration of water vapour in the air. If the air is humid, as on a rainy day, the rate of transpiration decreases because the water potential gradient between leaf air space and the outside air is very low. Very dry air has low humidity; there is a high water potential gradient and the rate of transpiration increases.

Air movement: in still air, a layer of moist air builds up around the surface of the leaf, around the stoma. This decreases the water potential gradient and increases the rate of transpiration. In windy conditions this moist air is blown away and the water potential gradient between air in the leaf air space and outside air is increased, thus increased rate of transpiration.

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Exchange of heat with the environment

Small organisms have a large SA:VOL ratio. They lose a lot of heat to the environment relative to their size; they have a high rate of heat loss. To compensate, they have high rates of respiration; heat is a by-product of respiration

Large organisms have a small SA:VOL ratio and have lower rates of heat loss. Their rates of respiration tend to be lower.

Living in a hot environment

Advantage to be a small organism; large SA:VOL ratio and thus a high rate of diffusion

Advantage to have large ears; increase surface area over which heat is lost

Living in a cold environment

Advantage to be a large organism; low rate of heat loss as small SA:VOL ratio

Advantage to have small ears: to reduce the surface area over which heat is lost

[dps17bio-blog]

Gas exchange in leaf

Waxy cuticle: waxy, to prevent water loss

Mesophyll: palisade - main region for photosynthesis in upper parts

Spongy layer - many large air spaces between the cells (allows gases to diffuse to all the cells)

Stoma - pores on the lower surface for entry and exit of gases

Guard cells - a pair of cells on either side of a stoma - to open/close it - to prevent water loss

During the day: photosynthesis and respiration

At night: respiration only

Gas exchange in leaves occurs between air spaces inside the leaf and mesophyll cells. Therefore the gas exchange surface is the surface of the mesophyll cells

Large surface area: many mesophyll cells in contact with many air spaces

Difference in concentration: gases used up as soon as they enter the cell in either respiration and photosynthesis

Diffusion pathway: short as gases only have to diffuse through the cell wall and the membrane to enter the cell

Pathway of diffusion of an oxygen molecule:

outside air - through stoma - air space - cell wall - cell membrane - into mitochondrion (respiration)

Some plants called xerophytes have special adaptations to reduce water loss at the leaves.

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Gas exchange in fish

Structure:

4 pairs of gills in the pharynx (opercular cavity)

Gills covered by a moveable flap called the operculum

Each gill is composed of large numbers of gill filaments - each with further projections called gill lamellae

Each gill is attached to a bore called the gill arch - this also contains blood vessels

Blood supplies to each gill is in the afferent vessel - deoxygenated blood

Blood removes from each gill in efferent vessel - oxygenated blood

Many capillaries inside each lamella

EXCHANGE SURFACE is the gill lamellae

Oxygen diffuses from water into blood and carbon dioxide from the blood back to the water.

Countercurrent Flow

Blood in the capillary and water flowing over the lamella flow in opposite directions. This ensures that a concentration gradient is maintained along the whole length of the lamella. thus diffusion of gases can occur along the whole length of the lamella.

Ventilation of the gills

One way flow of water over the gills

Water enters through the mouth, passed over and between the gills and is forced out of the side of the fish when the operculum opens

Water is much more dense than air and a two way flow (like air in mammals) would be very inefficient as it would be too difficult to move the water (would use too much energy)

INSPIRATION (WATER TAKEN IN)

1. The mouth opens

2. The floor of the pharynx is pulled down

3. Volume inside pharynx increases

4. Pressure within pharynx decreases

5. Water therefore flows into the mouth and pharynx down the pressure gradient

EXPIRATION (WATER FORCED OUT)

1. The mouth closes

2. The floor of the pharynx goes up

3. Volume inside pharynx decreases

4. Pressure in pharynx increases

5. Water is squeezed out past the gills as the operculum opens

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Gas exchange in insects

Spiracles: openings on either side of the body through which gases can enter and leave the body by diffusion

Valves used for opening and closing the spiracles

Tracheae: a series of large tubes held open by rings of chitin through which air passes

Tracheoles: smaller branched tubes with no chitin that air passes along, ending in the muscles of the insect

Air sacs: used for pumping the air in and out of the tracheal system in very active insects... used for ventilation

Tips of the tracheoles is the exchange surface

Oxygen diffuses through the spiracle down the tracheae and the tracheoles and into the body cells. Likewise carbon dioxide diffuses in the opposite direction.

The tips of the tracheoles contain a small amount of water. Oxygen diffuses in the water and then diffuses across the tip of the tracheoles into the body cells and visa versa for carbon dioxide.

In very small insects this system is insufficient to supply the cells with all the oxygen they need for respiration. However in larger, more active insects, oxygen needs to reach the cells more quickly. To do this the spiracles close and the air sacs are squeezed, using muscles in the abdomen, which pushed the air from the sacs deeper into the tracheoles. This is a ventilation mechanism

When the wing muscles are working very hard, respiration is partly anaerobic and lactate builds up. This lowers the water potential of the muscle cells. As lactate builds up in the muscle cells, water passes from the tracheoles intro the muscle cells by osmosis. As air is drawn deeper into the tracoles this reduces the diffusion distance for oxygen and the oxygen diffuses directly form the air into the muscle cell.

ADAPTATIONS for preventing water loss:

1.Insects have an exoskeleton (outside skeleton) that is covered in a waxy waterproof cuticle

2.Spiracles can be closed using the valves when the insect is not very active

Conflict between gas exchange and conserving water

All terrestrial organisms (land dwelling) must try to conserve water> Any cell exposed directly to the air will lose water by evaporation. SO most of the body must be covered in a waterproof layer; exoskeleton in insects, skins in humans. However this means that diffusion of gases cannot occur through this surface ad therefore a specialised system for gas exchange is needed.

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Gas exchange in a single celled organism

In a single celled organism there is only one cell, so there is a large surface area:volume ratio. Exchange surface is just the cell surface membrane. The cell membrane is only 7nm thick. Oxygen is continually used in respiration so there is a large difference in concentration.

[dps17bio-blog]

Analysing Data concerning Risk Factors and Drug Trials

Correlations: these can be positive or negative. But remember, just because there is a correlation between 2 sets of variables it dos not mean causation. ie that a causal relationship is present, as other factors could be involved. Often you need to look critically at the data for evidence that does not fir with the overall correlation.

Drug testing: often drugs are tested on human volunteers. The following need to be considered:

1.The control group: a group of people that are treated in exactly the same way as the experimental group, but are not given the drug

2,Reason for control group: to see the effect of the drug and to show that it is only the drug that is having the effect that is seen in the experimental group

3.Substitute drug: the control group must be given a placebo (dummy drug) in the same format as the experimental group (tablet form, or inhaler)

4.Volunteers are often divided randomly into groups. This is to avoid bias by the scientists.

5.When selecting volunteers the following are often important: age, gender, ethnicity, existing medical conditions

6. Often the volunteers do not know whether they are in the control group or the experimental group and therefore do not know if they are being given the drug. This is so that the volunteers don’t just ‘feel better’ if they think they are being given the drug. This is called the placebo effect

7.In a double blind trial the scientists do not know which group are getting the drug either... this avoids the scientists being biased in the way they interpret the results in the early stages of the trial

Drug testing using animals: often the following need to be considered:

1.If the drug has been tested on animals, will it have the same effect on humans

2.How many studies have been done... has the trial been repeated

3.Are the long term effects or side effects known

Using surveys to collect data

1.Can be unreliable as volunteers are not truthful in their responses

2. If volunteers are being asked about events in their past, they may have forgotten exact details and hence the data gained could be unreliable

Representing data from different countries:

1.Often results of a study are shown per 100000 of the population. This is to allow for a valid comparison between counties with different sized populations

2.Death rates are sometimes calculated. To calculate the death rate per 100000 you need to know the total number of deaths from the disease, divide it by the size of the population and multiply by 100000

Use of standard deviation: This is a measure of he spread of the data around the mean. A large standard deviation indicated a big spread in the results and therefore vary varied data and visa versa.

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Risk factors, correlations and causal relationships interpreting data

Risk factors

This is a factor that correlates with an increased chance of suffering from a particular condition or disease. This does not mean that the factor causes the disease.

There are specific risk factors associated with cancer and coronary heart disease, and some of these are due to lifestyle choices.

Cancer:

Cigarette smoking increases risk

Diet: high fat, low fibre increases risk

Obesity: being overweight increases risk

Physical activity: lack of exercise increases risk

Sunlight: unprotected exposure to sun increases risk of skin cancer

Coronary Heart Disease

Cigarette smoking increases risk

High blood pressure linked to stress, diet high in saturated fat increases risk

Obesity: increases risk

Diet: high intake of salt and saturated fat increases risk

Physical activity: lack of exercise increases risk

What is a correlation?

A correlation occurs when a change in one variable is reflected by a change in another variable.

However, correlation in data do not prove that one variable causes another variable to change, as other factors may be involved.

A causal relationship shows very clearly that one variable is causing the other variable to change, it is easy to mix the two relationships, but you must be able to distinguish between them and not make assumptions about data.

Scatter diagrams: are used to show the relationship between two variables that are not directly linked.

Consider the 2 diagrams below. It can be concluded that the greater the scatter, the less likely that there is a correlation between the 2 variables.

Scatter diagrams that do not show a correlation between two variables, do not show a causal link. Other factors are involved that cause the correlation (association).

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The Biological Base of Lung Disease

Remember, oxygen diffuses into the blood from the alveolar epithelium and then the capillary wall. A high rate of diffusion is maintained by:

1.A large surface area as there are many alveoli

2.A short diffusion pathway as the alveolar epithelium and capillary endothelium are both a single layer of squamous cells.

3.A large concentration gradient as ventilation maintains a high concentration of oxygen in the alveoli and the continuous circulation of blood maintains a low concentration in the blood.

Any condition which affects these factors will lower the rate of gas exchange by diffusion. If less oxygen diffuses into the blood there will be a lower rate of respiration than normal and less ATP would be produced. Consequently, less energy will be available and patients often feel tired and weak.

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Mechanism of breathing (VENTILATION)

Breathing in: INSPIRATION

1.Muscles of diaphragm contract and the diaphragm flattens

2.External intercostal muscles (between the ribs) contract and thus the rib cage moves up and out

3.Volume of thoracic cavity increases

4.Pressure in the thorax decreases below atmospheric pressure

5.Air is sucked in to the lungs from high to low pressure

Breathing out: EXPIRATION

1.Muscles of diaphragm relax and it returns to it’s domed position

2.External intercostal muscles relax and the rib cage moves back down and in under its own weight. This is quiet breathing (in forced breathing the intercostal muscles actually contract as do abdominal muscles)

3.Volume in thoracic cavity decreases

4.Pressure in the thoracic cavity increases above atmospheric pressure

5.Air is forced out form high to low pressure as the elastic tissue recoils after being stretched during inspiration

Ventilation helps to ensure a steep concentration gradient across the alveolar epithelium, leading to a high rate of diffusion.

The volume of gas exchanged per breath, at rest, is called the tidal volume

Pulmonary ventilation is the volume of gas exchanged per minute.

Pulmonary ventilation - tidal volume * ventilation rate

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The Human Gas Exchange System

Structure of the lungs

Lungs are located in the thorax, protected by your rib cage. There are intercostal muscles (internal and external) between the ribs. The thorax/thoracic cavity is separated from the abdomen by a sheet of muscle called the diaphragm.

Air enters the trachea. It is supported at intervals by C - shaped rings of cartilage in its walls. This stops the trachea collapsing during pressure changed in the lungs.

The trachea divides into a left and right bronchus, each of which is further subdivided into smaller tubes called bronchioles are alveoli.

Each alveolus has many capillaries on it’s surface.

Deoxygenated blood is supplied by the pulmonary artery and oxygenated blood is removed in the pulmonary vein (relating to maintaining a steep concentration gradient for diffusion)

The alveolus wall is a single layer of squamous cells; shortening the diffusion pathway.

The alveolus wall is a single layer of squamous cells called the Alveolar Epithelium, this is the exchange surface. Also the capillary wall is a single layer if squamous cells (capillary endothelium) and this leads to a short diffusion pathway so a higher rate of diffusion can be achieved.

There are many thousands of alveoli in each lung so there’s a large surface are for a faster rate of diffusion.

Ventilation brings fresh air with a high oxygen concentration into the alveoli and removes the stale air with a low oxygen concentration. This helps to maintain the concentration gradient for the diffusion of oxygen.

The continuous flow of blood means there is a low concentration of oxygen in the capillaries at the alveoli helping to maintain the concentration gradient for diffusion.

Alveoli contain elastic tissue allowing them to stretch as they fill the air and spring back during breathing out.

Oxygen diffuses through the alveolar epithelium and capillary endothelium to enter the blood.

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Co transport

This is where 2 substances are moved together using a single protein carrier. Glucose and Na+ (sodium ions) mover into the epithelial cells of the small intestine together.

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Facilitated diffusion

Is also a passive process (no energy from ATP but uses own KE) still goes down a concentration gradient. Needs a protein carrier or a protein channel. Large water soluble molecules and ions use facilitated diffusion. For example, glucose and sodium ions will be repelled by the hydrophilic tails of the phospholipid bilayer without this facilitation.

Protein Carriers

Are highly specific (only a molecule of complementary shape can be transported)

Protein Channels Have a hydrophobic lining and will allow the passage of very small water soluble molecules.

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Simple diffusion

Diffusion is a passive process (no energy from ATP is needed). Molecules move as they have their own kinetic energy.

It is the net movement of particles, molecules or ions from a region of higher concentration to a region of lower concentration... down the concentration gradient.

Some molecules can diffuse through the phospholipid bilayer. These are lipid soluble and are small and uncharged.

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Cell membranes

Each cell has a plasma membrane on it’s surface, separating it form any other cells. Remember, organelles have membranes surrounding them. All these membranes have the same basic structure.

The membrane is composed of:

Phospholipids

Proteins

And sometimes:

Carbohydrate

Cholesterol

Remember, the cell surface membrane is often folded into microvilli. This will increase the available surface area.

Phospholipid: This is a special type of lipid which contains a phosphate group. Three fatty acid tails and 1 glycerol join together by a reaction called condensation.

Some fatty acids are saturated, others unsaturated. Unsaturated have double bonds between carbons

Head is hydrophilic (water loving) (glycerol and phosphate)

Tails are hydrophobic (water hating)

In a cell there is an aquatic environment both inside the cell (cytoplasm) and surrounding the cell (tissue fluid)

The hydrophobic (water loving) heads are next to the cytoplasm inside the cell and tissue fluid outside the cell. The hydrophobic tails are not in contact with either aqueous environment.

Proteins are also present. They can be intrinsic (all the way through the membrane) or extrinsic (half way through).

Carbohydrate is also present on some of the proteins and some of the phospholipids themselves.

Protein + Carbohydrate = Glycoprotein

Phospholipid + Carbohydrate = Glycolipid

These molecules are on the outer side of the membrane only, next to the tissue fluid.

Cholesterol can also be present. Cholesterol is another type of lipid and is found between the phospholipid and protein molecules.

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Methods of studying cells and separating cell components

The techniques of cell fractionation and ultra centrifugation are used to separate cell components. It is separation based on the different densities that organelles.

1.Tissue is homogenised in a homogeniser. This breaks open the cells to release the organelles.

The solution used has the following features:

Ice cold: this reduced enzyme activities but they are not denatured

If the enzymes were active the proteases could start digesting the organelles.

Isotonic: this stops osmotic damage to the organelles as there is no net water movement into or out of the organelles so they do not burst or shrink... no net movement.

Buffer: this keeps the pH constant so enzymes are not denatured.

2.The resulting homogenate is filtered to remove any debris like cells that have not been broken apart.

3.The filtrate is centrifuged. A pellet is formed at the bottom of the tube and the liquid above is called the supernatant. In the first pellet will be the largest most dense organelle (nuclei).

4.The supernatant is poured off and respun for longer at a higher speed. The second pellet will contain the seconds densest organelle (chloroplasts in plant tissues, mitochondria in animal tissues)

5.The process is repeated.

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Units of measurement

It is important to be able to convert one unit into another when working out the actual size of cell organelles from diagrams and electron micrographs (a picture from the electron microscope_

Let the actual size of a structure equal A.

Let the enlarged size equal to E.

Let magnification equal M.

M = e / a Magnification = enlarged/ actual

Always measure images in mm

Calculations

There are two main ways in which information can be presented in questions involving the calculation of the actual size of structures:

1.Magnification e.g X15000

2.Scale bar e.g 30μm

1.Measure the size of the structure in mm enlarged size/ image size. Rearrange the equation M = e / a to make a the subject a = e / m and insert the known values of e and M into it. The answer is in mm at this stage. Convert it into more appropriate units... μm or nm.

2.Measure the length of the scale bar in mm. Work out what 1mm is equivalent to according to the scale bar (scale factor). Now measure the size of the structure, again in mm. As you will have previously calculated what 1mm is equivalent to, simply multiply the size of the specimen in mm by the scale factor.

You need to know the enlarged measurement and an actual measurement.

The SI units for measuring cells are the micrometre and the nanometre

1millimetre = 1000 micrometres (1mm = 1000μm)

1 micrometre = 1000 nanometres (1μm = 100nm)