Mass Transport

Question 1

What Is The Heart?

Answer 1

The heart is a muscular organ that lies in the thoracic cavity behind the sternum (breast bone).

It operates continuously and tirelessly throughout the life of an organism.

Question 2

Vessels Of The Heart?

Answer 2

Each of the four chambers of the heart is connected to large blood vessels that carry blood towards or away from the heart.

The ventricles pump blood away from the heart and into the arteries.

The atria receive blood from the vessels connecting the heart to the lungs are called pulmonary veins vessels.

The vessels connected to the four chamber are therefore as follows:

• Aorta,
• Vena cava,
• Pulmonary artery,
• Pulmonary vein.

Question 3

Two Chambers In The Heart?

Answer 3

Atrium - thin walled and stretches as it collects blood.

Ventricle - has much thicker, muscular wall as it had to contract strongly to pump blood a really long distance, either to lungs or rest of body.

Both chambers hold the same volume of blood.

The right ventricle pumps blood only to the lungs.

The left ventricle however pumps blood to the rest of the body.

Both atria contract together and both ventricles contract together.

Question 4

Why Do We Return The Blood To The Heart After Collecting Oxygen In The Lungs?

Answer 4

Blood must travel through the lungs via capillaries to effectively exchange gases (large surface area allows effectiveness).

The capillaries are tiny and so there is a drop in pressure.

The pressure of blood must be increased again to be pumped from the heart around the body.

Question 5

Valves In The Heart?

Answer 5

They prevent back flow of blood into the atria when the ventricles contract:

Two valves: left atrioventricular valve (bicuspid valve) and right atrioventricular valve (tricuspid valve).

They only open one way. If there’s high pressure behind a valve, it’s forced open.

If there’s high pressure in front of the valve, it’s forced shut.

Question 6

Structure Of The Heart: List?

Answer 6

The left side deals with oxygenated blood from the lungs, while the right side deals with deoxygenated blood from the body.

• Left atrium and right atrium,
• Left ventricle and right ventricle,
• Left atrioventricular valve (bicuspid),
• Right atrioventricular valve (tricuspid),
• Aorta,
• Vena cava,
• Pulmonary artery,
• Pulmonary vein.

Question 7

Aorta?

Answer 7

The aorta is connected to the left ventricle and carries oxygenated blood to all parts of the body except the lungs.

Question 8

Vena Cava?

Answer 8

It’s connected to the right atrium and brings deoxygenated blood back from the tissues of the body (except the lungs).

Question 9

Pulmonary Artery?

Answer 9

Is connected to the right ventricle and carries the oxygenated blood to the lungs, where oxygen is replenished and carbon dioxide is removed.

Unusual for an artery, it carries deoxygenated blood.

Question 10

Pulmonary Vein?

Answer 10

Is connected to the left atrium and brings oxygenated blood back from the lungs. Unusual for a vein, it carries oxygenated blood.

Question 11

The left ventricle?

Answer 11

It’s thicker and has more muscular walls than the right ventricle because it needs to contract powerfully to pump blood all around the body. The right side only pumps blood to the heart.

Question 12

The ventricles?

Answer 12

The ventricles have thicker walls than the atria, because they have to push blood out of the heart. The atria only pushed blood a short distance to the ventricles.

Question 13

The atrioventricular valves?

Answer 13

They link the atria to the ventricles and stop blood flowing back into the atria when the ventricles contract.

Question 14

Supplying The Heart With Oxygen?

Answer 14

Thea heart does not use the blood that passes through it for its own respiratory needs.

The heart muscle is supplied by its own blood vessels, called the coronary arteries, which branch off the aorta shortly after it leaves the heart.

Blockage of these arteries, for example by a blood clot, leads to myocardial infarction (heart attack), because an area of the heart muscle is deprived of blood and, therefore, oxygen also.

The muscle cells in this region are unable to respire (aerobically) and so die.

Question 14

Semi-lunar valves?

Answer 14

Link the ventricles to the pulmonary artery and the aorta, and stop blood flowing back into the heart after the ventricles contract.

Question 14

Cords?

Answer 14

The cords attach the atrioventricular valves to the ventricles to stop them being forced up into the atria when the ventricles contact.

Question 15

Cardiac Output?

Answer 15

Cardiac output is the volume of blood pumped by one ventricle of the heart in one minute.

It is usually measured in dm3min-1.

The cardiac output depends on:

• the heart rate,
• the stroke volume (volume of blood pumped out at each beat).

Measured:
Cardiac output = heart rate x stroke volume.

Question 16

What Is The Cardiac Cycle?

Answer 16

The cardiac cycle is repeated around 70 times each minute when at rest.

There are three phases to the cardiac cycle:

• contraction of the atria (atrial systole),
• contraction of the ventricles (ventricular systole),
• and relaxation (distole).

Contraction occurs separately in the ventricles and the atria and is therefore described in two stages.

Question 17

Atrial Systole?

Answer 17

Contraction step in the cardiac cycle.

1. When the atrioventricular valves are open, both the atria and ventricles can fill with blood (distole stage).
2. Both the atria (left and right) contract and blood passes down into the ventricles due to the atrioventricular valves opening. They open because of the higher blood pressure in the atria than in the ventricles. The ventricles are still relaxed at this stage.
3. 70% of the blood flows passively down to the ventricles so the atria do not have to contract a great amount.

Question 19

Diastole?

Answer 19

Relaxation step of the cardiac cycle.
(I’d use this as the first step).

1. The atria and ventricles are relaxed.
2. Deoxygenated blood returns to heart through the vena cava and oxygenated blood returns from lungs through the pulmonary vein.
3. Both atria fill. This builds pressure in the atria. This pressure soon exceeds the pressure in the relaxed ventricles (the atria at this point are also relaxed).
4. When the pressure exceeds, the atrioventricular valves open and so the blood passes into ventricles.
   This passage is aided by gravity.
5. The relaxation of the ventricle whilst the blood is filling in them allows the ventricles to recoil and reduce pressure. This means the pressure is lower in the ventricles than in the aorta and pulmonary artery.
6. Because of this pressure, the semi-lunar valves close from the previous contraction. This is the second ‘dub’ sound heard from a heart beat.

Question 20

Ventricular Systole?

Answer 20

Second contraction step in the cardiac cycle.

1. The atria relax after blood is passed to the ventricles.
2. The thick, strong ventricle walls contract, forcing the blood out.
3. The pressure of the blood forces the atrio-ventricular valves to shut (producing the heart sound ‘dub’),
4. The pressure of blood opens the semi-lunar valves and so blood passes into the aorta and pulmonary arteries.
5. The pulmonary artery is connected to the right ventricle and pumps deoxygenated blood to the lungs. This is why the walls are only thin.
6. The aorta is connected to the left ventricle and pumps oxygenated blood to the test of the body. This is why the walls of the left ventricle are thick.
7. The whole cycle repeats.

Question 22

Why Does Pressure Change In The Heart?

Answer 22

Mammals have a closed circulatory system.

In other words, the blood is confined to vessels and this allows the pressure within them to be maintained and regulated.

E.g. the pressure in the ventricles is low at first but gradually increases as the ventricles filled with blood as the atria contract. The pressure rises dramatically as the thick muscular walls of the ventricles contract.
Pressure then decreases as the ventricles empty.

E.g. The recoil stage of the ventricles produces a temporary rising pressure at the start of the relaxation phase in the aorta.

Question 23

Haemoglobin?

Answer 23

Red blood rolls contain haemoglobin - large protein with a quaternary structure.

It’s made up of more than one polypeptide chain (four).

Each chain had a haem group, which contains iron ion and gives haemoglobin its red colour.

It has a high affinity for oxygen - each molecular can carry four oxygen molecules.

In the lungs, oxygen joins to haemoglobin in red blood cells to form oxyhemoglobin.

This is a reversible reaction - when oxygen leaves oxyhaemoglobin near the body cells, it turns back to haemoglobin.

Hb + 4O2 —> HbO8 (double arrow).

Question 24

Haemoglobin saturation?

Answer 24

The partial pressure of oxygen (pO2) is a measure of oxygen concentration. The greater the concentration of dissolved oxygen in cells, the higher the partial pressure.

The partial pressure of CO2 (pCO2) is a measure of the concentration of CO2 in a cell.

Haemoglobin’s affinity for oxygen varies depending on the partial pressure of oxygen:

• Oxygen loads onto haemoglobin to form oxyhaemoglobin where there’s a high pO2.
• Oxyhaemoglobin unloads its oxygen where there’s a lower pO2.

Oxygen enters blood capillaries at the alveoli in the lungs. Alveoli have a high pO2 so oxygen loads into haemoglobin to form oxyhemoglobin.

When cells respire, they use up oxygen which lowers the pO2. Red blood cells deliver oxyhaemoglobin to respiring tissues, where it unloads its oxygen.

The haemoglobin then returns to the lungs to pick up more oxygen.

Question 25

Dissociation curves?

Answer 25

A dissociation curve shows how saturated the haemoglobin is with oxygen at any given partial pressure.

100% saturation on a graph means that every haemoglobin molecules is carrying the max 4 molecules of O2.

0% means no haemoglobin is carrying any oxygen.

Where pO2 is high (e.g. in lungs) haemoglobin has a high affinity for oxygen and so the saturation level will be high.

Where pO2 is low (e.g. in respiring tissue), haemoglobin has a low affinity for oxygen and so the saturation level will be low.

The graph is usually s-shaped because when Hb combines with the first of the four O2 molecules, it’s shape alters in a way to make it easier for other molecules to join too.

But as Hb starts to become more saturated, it gets harder for more oxygen molecules to join. When the curve is steep, a small chnage is pO2 caused a big chnage in the amount of oxygen carried by the Hb.

Question 26

How does carbon dioxide affect oxygen unloading?

Answer 26

Haemoglobin gives up its oxygen more readily at higher partial pressures of carbon dioxide (pCO2).

When cells respire, they produce carbon dioxide which raises the PCO2.

This increases the rate of oxygen unloading (the rate at which oxyhaemoglobin dissociates to form haemoglobin and oxygen).

This causes the dissociation curve to shift to the right. The saturation of blood with oxygen is lower for a given pO2, meaning more oxygen is being released.

This is called the Bohr affect.

Question 27

How is haemoglobin different in different organisms?

Answer 27

Different organisms have different types of haemoglobin with different oxygen transporting capacities. This is adaptation which helps the organism survive in a particular environment.

1. Organism that live in environments with a low concentration of oxygen (like in high altitudes) have haemoglobin with a higher affinity for oxygen than human haemoglobin. This dissociation curse is to the LEFT of ours.
2. Organisms that are very active and have a high oxygen demand have haemoglobin with a lower affinity for oxygen than humans. The curve is to the RIGHT of ours.

Question 33

The circulatory system?

Answer 33

The circulatory system is a mass transport system.

Multicellular organisms, like mammals, have a low SA to volume ratio so they need a circulatory system.

It’s made up for the heart and blood vessels. The heat pumps blood through the blood vessels (artistries, arterioles, veins and capillaries) to reach parts of the body.

Blood transports respiratory gases, digestion products, metabolic waste and hormones round the body.

There are two circuits, one that takes blood from the heart to the lungs and then another circuit that takes blood around the rest of the body.

The heart has its own blood supply - the left and right coronary arteries.

Question 34

Arteries adaptations?

Answer 34

Artistries carry blood from the heart to the rest of the body.

Their walls are thick and muscular and have elastic tissue to stretch and recoil as the heart beats, which helps maintain high pressure.

The inner lining (endothelium) is folded, allowing the artery to stretch which also helps to maintain high pressure.

All arteries carry oxygenated blood except for the pulmonary arteries, which take deoxygenated blood to the lungs.

Arteries divide into smaller vessels called arterioles. These form a network throughout the body.

Blood is directed to different areas of demand in the body by muscles inside the arterioles. The muscles contract to restrict blood flow or relax to allow blood flow.

Question 35

Interpreting a cardiac cycle graph?

Answer 35

Will have pressure up the side and time along the bottom.

Might also have volume up the side and time along the bottom.

Use knowledge of pressure increasing and decreasing in the heart and apply to the question.

Remember, if valves are open then the pressure in the atria is higher than in the ventricles.

Question 36

Atheroma formation?

Answer 36

Most cardiovascular diseases begin with the formation of atheroma.

1. The wall of an artery is made up of several layers.
2. The endothelium (inner lining) is usually smooth and unbroken.
3. If damage occurs to the endothelium (e.g. from high blood pressure) white blood cells (mostly macrophages) and lipids (fat) from the blood clump together under the lining to form fatty streaks.
4. Over time more white blood cells, lipids and connective tissues build up and harden to form a fibrous plaque called atheroma.
5. This plaque partially blocks the lumen of the artery and restricts blood flow, which causes blood pressure to increase.
6. Coronary heart disease is a type of cardiovascular diseases which occurs when coronary arteries have lots of atheromas in them, restricting blood flow to heart muscle. It can lead to myocardial infarction.

Question 37

Aneurysm?

Answer 37

Aneurysm is a disease that effects the artery’s.

1. Atheroma plaques damage and weaken arteries. They also narrow arteries, increasing blood pressure.
2. When blood travels through a weakened artery at high pressure, it may push the inner layers of the artery though the outer elastic layer to form a balloon-like swelling. This is called an aneurysm.
3. This aneurysm may burst causing a haemorrhage (bleeding).

Question 38

Thrombosis?

Answer 38

A disease that effects the arteries.

1. An atheroma plaque can rupture (burst through) the endothelium (inner lining) of an artery.
2. This damage the artery wall and leaves a rough surface.
3. Platelets and fibrin (a protein) accumulate at the site of damage and form a blood clot (a thrombus).
4. This blood clot can cause a complete blockage of the artery, or it can become dislodged and block a blood vessel elsewhere in the body.
5. Debris from the rupture can cause another blood clot to form further down the artery.

5.

Question 39

Myocardial infarction?

Answer 39

1. The heart muscle is supplied with blood by the coronary artistries. This blood contains oxygen needed by the heart muscle cells to carry out respiration.
2. If a coronary artistry becomes completely blocked (e.g. by a blood clot) an sea of the heart muscle will be totally cut off from its blood supply, receiving no oxygen.
3. This causes a myocardial interaction (a heart attack).
4. A heart attack can cause damage and death of heart muscle. Symptoms include pain in chest and upper body, shortness of breath and sweating.
5. If large areas of the heart are affected, complete heart failure can occur. This is fatal.

Question 40

Factors that increase cardiovascular disease?

Answer 40

High blood cholesterol and poor diet,
Cigarette smoking,
High blood pressure.

Question 41

How does high blood cholesterol cause cardiovascular disease?

Answer 41

If the blood cholesterol level is high (above 240mg per 100cm3) then the risk of cardiovascular disease is increased.

This is because cholesterol is one of the main constituents of fatty deposits that form atheromas.

Atheroma’s can lead to increased blood pressure and blood clots.

This could block the flow of blood to coronary arteries, which could cause a myocardial infarction.

A high diet in saturated fat is associated with high blood cholesterol levels.

A diet high in salt also increases risk of cardiovascular disease because it increases the risk of getting high blood pressure.

Question 42

How does cigarette smoking cause cardiovascular disease?

Answer 42

Both nicotine and carbon monoxide found in cigarette smoke increase risk of cardiovascular disease.

Nicotine increases the risk of high blood pressure. Carbon monoxide combines with haemoglobin and reduces the emoting of oxygen transported in the blood, and so reduced the amount of oxygen available to tissues. If heart muscle doesn’t receive enough oxygen, it can lead to a heart attack.

Smoking also decreases the amount of antioxidants in the blood - these are important for protecting cells from damage. Fewer antioxidants means cell damage in the coronary artery walls is more likely and this can lead to the formation of atheromas.

Question 43

How does high blood pressure cause cardiovascular disease?

Answer 43

High blood pressure increases the risk of damage to the artery walls.

Damaged walls have an increased risk of atheroma formation, causing a further increase in blood pressure.

Atheromas can also cause blood clots to form. A blood clot could block flow of blood to the heart muscle, resulting in myocardial infraction.

So anything that increases blood pressure also increases the risk of cardiovascular diseases (like being overweight, not exercising and excessive alcohol consumption).

Question 44

Genetic risk factors for cardiovascular disease?

Answer 44

There can be genetic predispositions to coronary heart disease or having high blood pressure as a result of another condition (diabetes).

Question 45

A question asks to evaluate conflicting research - one study says that smoking is not a risk factor for cardiovascular disease and one study says it is. What do you write?

Answer 45

The study design - is it different?

Was one study a small sample size? Bigger sample sizes are more representative of the target population and so the results are more likely to be valid.

Did both studies take into account other variables (risk factors)?

Suggest at the end that more research should be done.

Question 46

Two types of tissue involved in transport in plants?

Answer 46

Xylem tissue - transports water and mineral ions in solution. These substances move up the plant - from roots to leaves.

Phloem tissue - transports organic substances like sugars (also in solution) both up and down the plant.

Xylem and phloem are mass transport systems. They move substances over large distances.

Question 47

Xylem vessels?

Answer 47

Xylem vessels are the part of the xylem tissue that actually transports the water and ions.

They are very long, tube like structures formed from dead cells (vessel elements) joined end to end.

There are no end walls on these cells, making an u interrupted tube that allows water to pass through the middle easily.

Question 48

How does water move up the plant?

Answer 48

Cohesion and tension help water move up plants, from roots to leaves, against gravity.

Water evaporates from the leaves at the ‘top’ of the xylem. This is transpiration.

This creates tension (suction) which pulls more water into the leaf.

Water molecules are cohesive (stick together) so when some are pulled into the leaf, others follow. This means the whole column of water in the xylem, from the leaves down to the roots, moves upwards.

Water enters the stem through roots.

Question 49

Transpiration?

Answer 49

The evaporation of water from a plants surface (especially the leaves).

Water evaporates from the moist cell walls and accumulates in the spaces between cells in the leaf.

When stomata open, it moves out of the leaf down the concentration gradient (more water inside the lead than in the air outside).

Question 50

Four main factors that effect transpiration rate?

Answer 50

Light,

Temperature,

Humidity,

Wind.

Question 51

How does light effect transpiration rate?

Answer 51

The lighter it is, the faster the transpiration rate.

This is because the stomata open when it gets light to let in CO2 for photosynthesis.

When it’s dark, stomata close so there’s less transpiration.

Question 52

How does temperature effect transpiration rate?

Answer 52

The higher the temperature, the faster the transpiration rate.

Warmer water molecules have more energy so they evaporate from the cells inside the leaf faster. This increases the concentration gradient between the inside and the outside of the leaf, making water diffuse out the leaf faster.

Question 53

How does humidity effect transpiration rate?

Answer 53

The lower the humidity, the faster the transpiration rate. Negative correlation.

If the air around the plant is dry, the concentration gradient between the leaf and the air is increased which increases transpiration.

Question 54

How does wind effect transpiration rate?

Answer 54

The windier it is, the faster the transpiration.

Lots of air movement blows away water molecules from around the stomata. This increases the concentration gradient, which increases transpiration.

Question 55

Practical: using a potometer?

Answer 55

A potometer is a piece of apparatus used to estimate transpiration tested by measuring water uptake. It’s assumed that water uptake is directly related to water loss of leaves.

How:
1. Cut a shoot underwater to prevent air from entering the xylem. Cut it on a slant to increase the SA available for water uptake.

2. Assemble the potometer in water and insert the shoot underwater, so no air can enter.
3. Remove the apparatuses from the water but keep the end of the capillary tube submerged in a beaker of water.
4. Check the apparatus is water and air tight.
5. Dry the leaves and allow time for the shoot to acclimatised and then shut the tap.
6. Remove the end of the capillary tube from the beaker of water until one air bubble has formed, then put the end of the tube back in the water.
7. Record the starting position of the air bubble. Start the stopwatch and record the distance moved by the bubble per unit of time (s.g. One hour). The rate of movement of the bubble is the transpiration rate.
8. Only chnage one variable per experiment (s.g. Light or humidity, not both). All other conditions must be constant.

Question 56

Practical: dissecting a plant?

Answer 56

You can look at the xylem and phloem of a plant tissue under a microscope, and then draw them. First, we need to dissect a plant and prepare a cross section.

1. Use a scalpel to cut a cross section of the stem. Cut the sections as thinly as possible - they are better for viewing in a microscope.
2. Use tweezers to gently place the cut sections in water until you come to use them. This stops them from drying out.
3. Transfer each section to a dish containing a stain, like toluidine blue and leave for one minute. TBO stains the lignin in the walls of the xylem vessels blue-green. This will let you see the position of the xylem vessels and examine their structure.
4. Rinse off the sections in water and mount each one onto a slide.

Question 57

Veins adaptations?

Answer 57

Veins take blood back to the heart under low pressure.

They have a wider lumen than equivalent artistries, with very little elastic or muscle tissue.

Veins contain valences to stop the blood flowing backwards.

Blood flow through the veins is helped by contraction of the body muscles surrounding them.

All veins carry deoxygenated blood (because oxygen has been used up by body cells) except for the pulmonary veins, which carry oxygenated blood from the heart to the lungs.

Question 58

How are substances exchanged at capillaries?

Answer 58

Arterioles branch into capillaries, which are the smallest blood vessels.

Substances like glucose and oxygen are exchanged between cells and capillaries, so they’re adapted for efficient diffusion.

1. They’re always found very near cells in exchange tissues (e.g. alveoli in the lungs). This is so there is a short diffusion pathway.
2. Their walls are only one cell thick, which also shortens their diffusion pathway.
3. There are a large number of capillaries, to increase SA for exchange. Networks of capillaries in tissue are called capillary beds.

Question 59

Tissue fluid?

Answer 59

Tissue fluid is the fluid that surrounds cells in tissues. It’s made from small molecules that leave the blood plasma (e.g. oxygen, water and nutrients).

Unlike blood, tissue fluid doesn’t contain red blood cells or big proteins because they’re too large to be pushed out through the capillary walls.

Cells take in oxygen and nutrients from the tissue fluid and release metabolic waste into it.

In a capillary bed, substances move out of the capillaries, into the tissue fluid, by pressure filtration.

Question 60

Pressure filtration?

Answer 60

1. At the start of the capillary bed (network of capillaries in tissue), nearest to the artistries the hydrostatic (liquid) pressure inside the capillaries is greater than the hydrostatic pressure in the tissue fluid.
2. The difference in hydrostatic pressure means an overall outward pressure forces fluid out of the capillaries and into the spaces around the cells. This forms the tissue fluid.
3. As the fluid leaves, the hydrostatic pressure reduces in the capillaries - so the pressure is lower at the venule end of the capillary bed (end nearest to the veins).
4. Due to the fluid loss, and an increasing concentration of plasma proteins (which don’t leave the capillaries), the water potential at the venule end of the capillary bed is lower than the water potential in the tissue fluid.
5. This means that some water re-enters the capillaries from the tissue fluid at the venule end by osmosis.

Any excess tissue fluid is drained into the lymphatic system which transports this excess fluid from the tissue and dumps it back into the circulatory system.

Question 61

Lymphatic system?

Answer 61

A network of tubes that acts like a drain.

Question 62

Phloem tissue?

Answer 62

Solutes are dissolved substances. Phloem transports solutes (mainly sugars like sucrose) round plants.

The phloem is formed from cells arranged in tubes.

Sieve tube elements and companion cells are important cells types in phloem tissue:

1. They are living cells that from the tube transporting solutes. They have no nucleus and few organelles so…
2. There’s a companion cell for each sieve tube element. They carry out living functions for the sieve cells like providing energy for the active transport of solutes.

Question 63

What is translocation?

Answer 63

The movement of solutes (amino acids sucrose and sugars) to where they’re needed in the plant. Solutes are sometimes called assimilates.

It’s an energy requiring process that happens to the phloem.

Translocation moves solutes from sources to sinks. The source of a solute is where it’s made (high conc) and the sink is where it’s used (lower conc). E.g. source of sucrose is leaves and sink is other places on plant, especially storage organs and meristems (areas of growth like stems and roots).

Enzymes maintain a concentration gradient from the source to a sink by changing the solutes at the sink (breaking them down or making them into something else). This makes sure there’s always a lower concentration at the sink.

E.g. sucrose in potatoes is converted to starch in sinks.

Question 64

The mass flow hypothesis?

Answer 64

This attempts to explain phloem transport from source to link by translocation.

1. Active transport is used to actively load the solutes from companion cells into the sieve tubes of the phloem at the source.
2. This lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells.
3. This creates a high pressure inside the sieve tubes at the source end of the phloem.
4. At the sink end, solutes are removed from the phloem to be used. This increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis.
5. The result is a pressure gradient from the source end to the sink end. This gradient pushed solutes along the sieve tubes towards the sink.
6. When they reach the sink, the solutes will be used or stored.

Question 65

Evidence supporting the mass flow hypothesis?

Answer 65

1. If a ring of bark (which includes the phloem, but not the xylem) is removed from a woody stem, a buldge forms above the ring. The fluid from the buldge has a higher conc of sugars than the fluid from below the ring. This is evidence for a downward flow of sugars.
2. A radioactive tracer such as a radioactive carbon (14C) can be used to track the movement of organic substances in the plant.
3. Pressure in the phloem is investigated using aphids (they pierce the phloem, then their bodies are removed leaving the mouthparts behind, which allows the sap to flow out). The sap flows out quicker nearer the leaves than further down the stem - shows a pressure gradient.
4. If a metabolic inhibitor (stops ATP production) is put into the phloem, then translocation stops. This is evidence for active transport.

Question 66

Evidence against the mass flow hypothesis?

Answer 66

1. Sugar travels to many different sinks, not just one with the higher water potential like the model suggests.
2. The sieve plates would create a barrier to mass flow. A lot of pressure would be needed for the solutes to get through at a reasonable rate.

Question 67

How is translocation of solutes demonstrated?

Answer 67

We use radioactive tracers to model translocation of solutes.

1. Supply part of a plant (often a leaf) with organic substances that had a radioactive label. One example is CO2 containing the radioactive isotope 14C. This radioactively-labelled CO2 can be supplied to a single leaf by being pumped into a container which completely surrounds the leaf.
2. The radioactive carbon will then be incorporated into the organic substances produced by the leaf (e.g. sugars) which will be moved around the plant by translocation.
3. The movement of these substances can be tracked using a technique called autoradiography. To reveal where the radioactive tracer had spread to ina plant, the plant is killed and then the whole plant or sections of it is placed on photographic film. The radioactive substance is present where the film turns black.
4. The results demonstrate the translocation of substances from source to sink over time, e.g. autoradiographs of plants killed at different times show an overall movement of solutes from the leaves towards the roots.