Mass transport

Question 1

Why do large organisms have a transport system ?

Answer 1

• A transport system is needed to take materials from cells to exchange surfaces and from exchange surfaces to cells.
• They also need to be transported between different parts of the organism .

Question 2

What are features of a transport system?

Answer 2

• A suitable medium where substances can be readily dissolved.
• A form of mass transport in which the transport medium is moved around in bulk over large distances
• A closed system of tubular vessels which forms a branching network
• A mechanism for moving the transport medium within vessels.

Question 3

What is the circulatory system like in mammals?

Answer 3

• Closed, double circulatory system
• It has three types of vessels -> arteries, veins and capillaries.

Question 4

Describe the arrangement of the main arteries and veins

Answer 4

• Pulmonary artery ( to lungs)
• Pulmonary vein (from lungs)
• Vena Cava (to heart)
• Aorta (from heart)
• Renal Artery ( to heart)
• Renal vein (from kidneys)
• Hepatic artery ( to liver)
• Hepatic vein ( from liver)
• Hepatic portal vein (stomach to liver)

Question 5

Describe the movement of blood through the heart

Answer 5

• Superior and Inferior Vena Cava bring deoxygenated blood back to the heart
• Blood enters the right atrium, passes through the tricuspid valve and into the right ventricle
• Blood passes out of the semi-lunar valve through the pulmonary artery towards the lungs to pick up oxygen
• Blood returns to the heart, oxygenated, through the pulmonary vein into the left atrium.
• It passes through the bicuspid valve into the right ventricle and is then passes through the semi lunar valve into the aorta to the rest of the body.

Question 6

Describe structures of the heart that are not the chambers, vessels or valves

Answer 6

• Thick muscular wall of left ventricle
• Septum
• Tendinous chords
• Trebeculae Carnae
• Papillary muscles

Question 7

What are the 3 main stages of the heart cycle?

Answer 7

• Atrial and ventricular diastole (chambers are relaxed and filling with blood)
• Atrial systole (atria contracts and remaining blood is pushed into the ventricle)
• Ventricular systole ( ventricles contract and push blood out through the semi-lunar valves)

Question 8

What is the function of the heart?

Answer 8

• To generate pressure differences so that blood flows through the circulatory system

Question 9

What are the rules that help explain the way the heart works?

Answer 9

• Fluids flows from areas of high pressures to areas of low pressures
• Valves will open if the pressure behind them is higher, and valves will close if the pressure in front of them is higher.

Question 10

Describe the cardiac cycle?

Answer 10

• The ventricles relax causing them to recoil and the pressure in the ventricles becomes lower than that of the aorta/ pulmonary artery and the semi-lunar valves close. This is accompanied the ‘lub’ sound of the heart beat.
• During diastole both the atria and the ventricles are relaxed. Blood enters the atria from the vena cava/ pulmonary vein. The atria fill with blood so the pressure is higher in the atria than in the ventricles. The AV valve opens and there is passive filling of the ventricles which is aided by gravity.
• The atria contracts, increasing the pressure further and the remaining blood is forced into the ventricles.
• The ventricles start to contract and the blood pressure in the ventricles increases such that it is higher than in the atria and the AV valves close. This is accompanied by the ‘dub’ sound of the heartbeat. The ventricles continue to contract fully, and with the semi-lunar valves closed, the pressure continues to increase
• When the pressure in the ventricles is higher than in the aorta the semi-lunar valves open.

Question 11

What is the purpose of valves?

Answer 11

• The valves prevent the backflow of blood.
• The valves will open when the pressure behind them is greater than the pressure behind them ad will close when the pressure in front of them is higher than the pressure behind them.

Question 12

What are the tendinous chords?

Answer 12

• They connect the atrioventricular valves to the papillary muscles within the ventricles.
• Multiple chordae tendinae attach to each cusp of the valves
• When the ventricles of the heart contract (ventricular systole), the increased blood pressure in the ventricles push the atrioventricular valves to close, preventing backflow of blood into the atria. The blood pressure in atria is much lower than that in the ventricles and so the tendinous chords prevent the valve from swinging back into the atrial cavity.

Question 13

How does the pressure in the ventricle change during the heart cycle?

Answer 13

• Ventricular pressure is low at first, but gradually increases as the ventricles fill with blood as the atria contract.
• The left atrioventricular valves close and pressure rises dramatically as the thick muscular walls of the ventricle contract.
• As pressure rises above that of the aorta, blood is forced into the aorta past the semi-lunar valves.
• Pressure falls as the ventricles empty and the walls relax.

Question 14

How does the pressure in the aorta change during the heart cycle?

Answer 14

• Aortic pressure rises when ventricles contract as blood is forced into the aorta.
• It then gradually falls, but never below 12KPa, because of the elasticity of its wall, which creates a recoil action-essential if blood is to be constantly delivered to the tissues.
• The recoil produces a temporary rise in pressure at the start of the relaxation phase.

Question 15

How does the pressure in the atrium change during the heart cycle?

Answer 15

• Atrial pressure is always relatively low because the thin walls of the atrium cannot create much force.
• It is highest when they are contracting, but drops when the left atrioventricular valves closes and its walls relax.
• The atria then fill with blood, which leads to a gradual build up of pressure until a slight drop when the left atrioventricular valve opens and some blood moves into the ventricle.

Question 16

What is cardiac output?

Answer 16

• Cardiac output is the volume of blood pumped by one ventricle of the heart in one minute.
• It is meaured in dm^3min^-1 and depends on two factors: Heart rate and stroke volume
• Its equation is therefore: Cardiac Output = heart rate * stroke volume

Question 17

What is tissue fluid?

Answer 17

• Tissue fluid bathes the body cells of the circulatory system.
• Exchange of substances between cells and the blood occurs via the tissue fluid.
• The composition of tissue fluid is the same as plasma although tissue fluid has much fewer proteins as they are too big to fit through gaps in the capillary walls and so remain in the blood.

Question 18

What two forces result in the formation of tissue fluid?

Answer 18

• Hydrostatic pressure
• Water potential gradient

Question 19

What happens in the arterial end of the capillary?

Answer 19

• At the arterial end pressure is high causing water to be forced out the capillaries, into the tissue fluid.
• There are two forces which oppose this outward pressure ->
  Hydrostatic pressure of the tissue fluid outside the capillaries
  The lower water potential of the blood due to the presence of plasma proteins, that causes water to move back into the blood by osmosis.
• However the hydrostatic pressure in the capillaries is greater than these and overall the hydrostatic pressure pushes water out of the capillaries at the arterial end.
• Only small molecules can pass out of the capillary but large molecules such as proteins and cells such as red blood cells cannot pass through the capillaries. This is known as ULTRAFILTRATION.

Question 20

What happens in the venous end of the capillary?

Answer 20

• The loss of water from the capillaries reduces the hydrostatic pressure inside them.
• The hydrostatic pressure inside the capillary becomes lower than that of the tissue fluid outside it.
• Therefore the tissue fluid is forced back into the capillary.
• Additionally, water was lost at the arterial end, yet proteins remain, this decreases the water potential in the capillary at the venous end.
• At venous end of the capillary the water potential of blood plasma is greater than tissue fluid.
• Water moves back into capillary by osmosis. This is known as REABSORBTION.

Question 21

Describe the formation of the lymph

Answer 21

• Not all tissue fluid re-enters the capillaries
• The remaining tissue fluid enter lymph capillaries
• the lymph capillaries are separate from the circulatory system
• They have closed ends and pores that allow large molecules to pass through.
• Once the interstitial fluid has been taken up it is referred to as the lymph.

Question 22

Describe the movement of the lymph

Answer 22

• A network of lymphatic vessels act as drainage for the tissues.
• Enters the lymph capillaries
• the liquid moves along the larger vessels of this system by compression caused by contraction of body muscles.
• Any backflow is prevented by valves.
• This is why people who have been sedentary on planes can experience swollen limbs.

Question 23

What would happen if the blood pressure is too high?

Answer 23

• If blood pressure is high (hypertension) then the pressure at the arteole and is even greater.
• This pushes more fluid out of the capillary and fluid begins to accumulate around the tissues. This is called oedema.

Question 24

Describe the role of all the layers in the vessels and their purpose

Answer 24

• Tough outer layer -> Resistance for pressure changes
• Muscle layer -> Can contract and relax to control blood. It can constrict and dilate the blood vessel.
• Elastic layer -> Can stretch / recoil to maintain blood pressure
• Endothelium -> Smooth layer to prevent friction. Also referred to as the thin inner lining.
• Lumen -> central cavity of the blood vessel through which blood flows

Question 25

What are the different types of blood vessels?

Answer 25

• Arteries carry blood away from the heart and into arterioles
• Arterioles are smaller arteries that control blood flow from arteries to capillaries.
• Capillaries are tiny vessels that link arterioles to veins.
• Veins carry blood capillaries back to the heart.

Question 26

How is the structure of the artery related to its function?

Answer 26

• The muscle later is thick compared to veins -> this means smaller arteries can be constricted and dilated in order to control the volume of blood passing through them.
• The elastic later is relatively thick compared to veins -> because it is important that blood pressure in arteries is kept high if blood is to reach the extremities of the body. The elastic wall is stretched at each beat of heart (systole). It then springs back when the heart relaxes (diastole) . This stretching and recoil action helps to maintain high pressure and smooth pressure surges created by the beating of the heart.
• The overall thickness of the wall is great -> This also resists the vessel bursting under pressure
• There are no valves -> as blood is under constant high pressure due to the heart pumping blood into the arteries.

Question 27

How does the structure of the arteriole relate to its function ?

Answer 27

• The muscle layer is relatively thicker than in arteries -> the contraction of this muscle later allows constriction of the lumen of the arteriole. This restricts the flow of blood and so controls its movement into the capillaries that supply the tissues with blood.
• The elastic layer is relatively thinner than in arteries -> because blood pressure is lower

Question 28

How does the structure of the vein relate to its function?

Answer 28

• The muscle layer is relatively thin -> veins carry blood away from tissues and therefore their constriction and dilation cannot control the flow of blood to the tissues
• The elastic later is relatively thin -> low pressure of blood within the veins will not cause them to burst and pressure is too low to create a recoil action.
• The overall thickness of the wall is small -> no need for a thick wall as the pressure within the veins is too low to create any risk of bursting. It also allows the to be flattened easily, aiding the flow of blood within them.
• There are valves at intervals throughout -> ensuring blood doesn’t flow backwards, which it might otherwise do because the pressure is so low. When body muscles contract, veins are compressed, pressurising the blood within them. The valves ensure that this pressure directs the blood in one direction only.

Question 29

How does the structure of the capillary relate to its function?

Answer 29

• Their walls consist mostly of the lining layer -> making them extremely thin, so the distance over which diffusion takes place is short. This allows for rapid diffusion of materials between the blood and
• They are numerous and highly branched -> providing a large surface for gas exchange.
• They have a narrow diameter -> so permeate tissues, which means that no cell is far from a capillary and there is a short diffusion pathway.
• Their lumen is so narrow -> that red blood cells are squeezed flat against the side of a capillary. This brings them even closer to the cells to which they supply oxygen. This reduces the diffusion distance.
• There are spaces between the lining (endothelial) cells that allow white blood cells to escape in order to deal with infections within tissues.

Question 30

What is the difference between blood and tissue fluid?

Answer 30

IN BLOOD
- Erythrocytes present
- White blood cells present - Large plasma proteins present
- Higher concentration of oxygen
- Lower concentration of carbon dioxide
- Higher concentration of glucose
- Higher concentration of amino acids

IN TISSUE FLUID
- Erythrocytes not present
- White blood cells not present
- Large plasma proteins not present
- Lower concentration of oxygen
- Higher concentration of carbon dioxide
- Lower concentration of glucose
- Lower concentration of amino acids

Question 31

What is haemoglobin?

Answer 31

• Protein which makes up the majority of a red blood cell
• Haemoglobin is made up of 4 polypeptide chains (quaternary structure). These are also called subunits and include 2 identical alpha chains and 2 identical beta chains.
• Each of the polypeptide chains is coiled into a helix
• Each subunit is bound to a prosthetic group called a haem group which contains an Fe 2+

Question 32

Describe how oxygen can bind with haemoglobin

Answer 32

• Each haem group can combine with 1 oxygen molecule.
• Therefore, one molecule can combine with a maximum of 4 oxygen molecules

Question 33

Describe the reaction for the loading and unloading of oxygen on haemoglobin (include equation)

Answer 33

• Hb + 4O2 <=> Hb(O2)4
• Haemoglobin + oxygen <=> oxyhaemoglobin
• It is a reversible reaction

Question 34

When does loading and unloading occur?

Answer 34

• Loading -> when haemoglobin binds with oxygen. In humans this happens in the lungs. Low oxygen concentration to high oxygen concentration.
• Unloads -> When haemoglobin releases oxygen. In humans this takes place in the tissues where there are respiring cells. High oxygen concentration to low oxygen concentration.

Question 35

Describe oxygen dissociation curves

Answer 35

• % saturation of Hb (or affinity of Hb for oxygen) can be plotted against partial pressure of oxygen to produce an oxygen dissociation curve
• At lower partial pressure there is little increase in % saturation of Hb as partial pressure of O2 increases
• At slightly higher pressures, a small change in partial pressure of oxygen can result in a large change in the % saturation of the blood.
• At higher partial pressures there is little increase in % saturation of Hb as partial pressure of O2 increases.

Question 36

Why is the oxygen dissociation curve S shaped?

Answer 36

• The first O2 is harder to load as the subunits of the Hb are closely united. Therefore at low oxygen concentration, little O2 loads and the gradient of the curve is shallow initially.
• When the first O2 loads with Hb, it changes the tertiary structure of Hb, uncovering another binding site so 2nd O2 can load more easity and therefore takes a smaller increase in partial pressure of O2 to load 2nd O2. This then enables the 3rd O2 to load more easily. This is called co-operativity.
• The 4th O2 is harder to load. This is due to probability, as most of the binding sites are already occupied and so it is less likely that a single O2 will find an empty site to associate with.

Question 37

How is the S shaped dissociation curve helpful in loading and unloading oxygen in the blood?

Answer 37

• As oxygenated blood reaches the tissues, a small decrease in partial pressure of O2 causes a large decrease in % saturation. As there are low partial pressures in body cells for example, the oxygen can be unloaded more easily as it has a low affinity.
• As deoxygenated blood approaches the lungs, the steep part means that a small increase in partial pressure of O2 causes a large increase of saturation. As there a high pressure of O2 in the lungs, oxygen can be loaded more easily as it has a higher affinity.

Question 38

Describe the effect of exercise on the unloading of oxygen

Answer 38

• Not all haemoglobin molecules are loaded to their maximum with O2 molecules.
• When haemoglobin reaches a tissue with a low respiratory rate, only one of these molecules will normally be released.
• If tissue is very active then three oxygen molecules will usually be unloaded by each haemoglobin

Question 39

What is meant by affinity?

Answer 39

• Haemoglobin with a high affinity for oxygen loads oxygen more easily, but unloads it less easily.
• Haemoglobin with a low affinity for oxygen loads oxygen less easily, but unloads it more easily.

Question 40

What effect does carbon dioxide have on oxygen loading and unloading?

Answer 40

• Haemoglobin has a reduced affinity for oxygen in the presence of carbon dioxide
• The greater the concentration of CO2, the more easily Hb unloads its oxygen. This is known as the Bohr effect.

Question 41

Why does carbon dioxide lead to the Bohr effect?

Answer 41

• Carbon dioxide dissolves in blood plasma forming a weak acid. This reaction releases H+ ions causing the pH to drop. A lower pH causes a change in shape of haemoglobin which reduces it’s affinity for oxygen, so the association is weaker. The dissociated curve is therefore shifted to the right.
• If curve moves to Right, Hb Releases oxygen more Rapidly.

Question 42

What are the difference in conditions at the gas exchange surface and on the respiring tissues?

Answer 42

GAS EXCHANGE SURFACES
- High oxygen concentration
- Low carbon dioxide concentration
- High affinity of haemoglobin for oxygen
- As a result, oxygen is associated.
RESPIRING TISSUES
- Low oxygen concentration
- High carbon dioxide concentration
- Low affinity of haemoglobin for oxygen
- Hence oxygen is dissociated.

Question 43

What happens at a gas exchange surface in terms of loading and unloading of oxygen?

Answer 43

• There is a higher concentration of oxygen in lungs
• There is a lower concentration of carbon dioxide as it is constantly being removed (exhaled).
• So the pH is slightly raised due to the low concentration of carbon dioxide.
• The higher pH changes the shape of haemoglobin into one that enables it to load oxygen readily.
• The affinity of Hb for oxygen is higher and oxygen is readily loaded.

Question 44

What happens in rapidly respiring tissues in terms of loading and unloading of oxygen?

Answer 44

• Increased rate of respiration results in a higher concentration of carbon dioxide
• Carbon dioxide dissolves in blood plasma forming a weak acid. This reaction releases H+ ions and so the pH drops
• This causes a change in shape of haemoglobin which increases it’s affinity for O2, so its association is weaker.
• The haemoglobin unloads its oxygen more readily.
• The increased CO2 shifted the curve to the right.

Question 45

How do different types of haemoglobin differ?

Answer 45

Different sequences of amino acids and therefore slightly different shapes which gives different affinities for oxygen.

Question 46

How do different types of haemoglobin present on a graph?

Answer 46

• Haemoglobin with higher affinity for oxygen means the curve moves to the left
• Haemoglobin with lower affinity for oxygen means that the curve shifts to the right

Question 47

How does the shape of the graph change for small / active mammals?

Answer 47

• In smaller mammals the curve is shifted to the right
• Haemoglobin has a lower affinity for oxygen and therefore unload oxygen more readily to respiring cells
• This is useful in organisms with a high metabolic rate which need oxygen to be unloaded readily

Question 48

How does the shape of the graph change for organisms that live in an environment with low oxygen conditions?

Answer 48

• If the concentration of oxygen in the blood is lower, organisms will have haemoglobin with a higher affinity for oxygen.
• This means it loads oxygen more readily in the lungs at a lower oxygen concentration.

Question 49

How does the shape of the graph change for foetal haemoglobin?

Answer 49

• Foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin so the oxygen dissociation curve is shifted to the left of the maternal one.
• This means the maternal haemoglobin unloads oxygen in the placenta and the foetal haemoglobin will load with oxygen.
• This helps maximize oxygen uptake from the mother’s blood stream, which has already lost some of its oxygen by the time it reaches the placenta.
• This ensures that oxygen moves from the mother to the foetus.

Question 50

What is myoglobin?

Answer 50

• Iron containing oxygen binding protein found in muscle cells. It has a high affinity for oxygen.
• Myoglobin acts as an oxygen store. It will only unload oxygen when oxygen levels are really low.
• The curve is hyperbolic because myoglobin is composed of single sub unit.
• High concentrations of myoglobin in muscle cells allow organisms to hold their breath for longer period of time.

Question 51

How does water move into the root hair cell?

Answer 51

• Mineral ions like magnesium and nitrates enter the root hair cells from the soil by active transport which requires energy from the hydrolysis of ATP to move these ions against a concentration gradient.
• This lowers the water potential inside the root hair cell and as a result water will move from the higher water potential in the soil to the lower water potential in the root hair cell by osmosis

Question 52

How does water move from the root hair cell move from the root hair cell to the xylem?

Answer 52

• After water has moved into the root hair cell, there is an increase in the water potential of the root hair cell compared to the neighbouring ells further into the root, so water moves into the neighbouring cells by osmosis.
• This increases the water potential of water in those cells compared with the cells further in and so on until the water reaches the xylem in the root. Water enters the xylem via osmosis.

Question 53

What route does water take to reach the xylem?

Answer 53

• Water travels in the apoplast and symplast pathway
• Most water travels via the apoplast pathway, which is the series of spaces running through the cellulose cell walls, dead cells and the hollow tubes of the xylem.
• However, water in the apoplastic pathway is blocked by the casparian, a band of waxy material called suberin that runs around each of the endodermal cells.
• This blockage ensures that the water and dissolved mineral ions have to pass into the cell cytoplasm (the symplast pathway)
• The symplast is the continuous cytoplasm of the living plant cells that is connected through plasmadesmata (channels which traverse the cell walls of plant cells).
• In the symplast pathway, water moves through the cytoplasm and plasmodesmata and vacuole of the cells

Question 54

Describe the different layers of cells from the root hair cell to the xylem

Answer 54

• Epidermis
• Root cortex
• Endodermis
• Pericycle
• Xylem

Question 54

Describe the structure of the leaf from top to bottom

Answer 54

• Waxy cuticle
• Upper epidermis
• Palisade mesophyll
• Spongy mesophyll
• Lower Epidermis
• Guard cells with stoma between them
• Waxy cuticle (again)

Question 55

Describe the force that pulls water up the plant

Answer 55

• The main force that pulls water through the xylem vessels in the stem of plant is the evaporation of water from the leaves is a process called transpiration.
• The energy for this supplied by the sun and the process is therefore passive.

Question 56

Explain the movement of water through the leaf

Answer 56

• Water evaporates from the mesophyll cells in the leaf due to heat from the sun and enters the air spaces
• There’s therefore high water potential in air spaces than in atmosphere.
• Hence water vapour diffuses out of the leaf through open stomata, due to the water potential gradient.
• This process is transpiration
• The mesophyll cells now have a lower water potential and so water enters by osmosis from neighbouring cells.
• The loss of water from these neighbouring cells lowers their water potential and they in turn taken in water from neighbouring cells by osmosis.
• This is repeated in cell sin the leaf all the way back to the xylem and water exits the xylem via osmosis.

Question 57

Describe the structure of xylem vessels and explain how this makes them adapted for their function

Answer 57

• Made from dead cells and gave no cytoplasm or organelles
• The cells are elongated and have no dead cells, forming continuous hollow tubes from roots to leaves .
• This means cohesion-tension is continuous
• The cell wall are lignified which makes them waterproof and very stable and strong
• The lignin forms rings or spirals arounds the vessels.

Question 58

What is the cohesion-tension theory?

Answer 58

• Water evaporates from the mesophyll cells due to heat from the sun and diffuses out of the stomata by transpiration.
• Water is pulled up to replace it due to the cohesive forces between water molecules.
• Water is therefore pulled up the xylem in a continuous column.
• This is called the transpiration pull and it puts the xylem under tension as there is a negative pressure in the xylem.

Question 59

What evidence supports the cohesion-tension theory?

Answer 59

• Change in diameter of tree trunks according to the rate of transpiration
• If a xylem vessel is broke and air enters in, the tree is no longer able to draw up water as the continuous column of water is broken
• When a xylem vessel is broke, water does not leak out, instead air is drawn in which is consistent with it being under tension

Question 60

What are the factors affecting the rate of transpiration?

Answer 60

• Light intensity (increases rate)
• Temperature (increases rate)
• Wind (increases rate)
• Humidity (decreases rate)

Question 61

How does light intensity affect the rate of transpiration?

Answer 61

• The higher the light intensity, the more photosynthesis is taking place in the palisade cells.
• So the stomata are usually open to facilitate gas exchange.
• Open stomata also make it possible for water molecules to diffuse out the air spaces more quickly.
• A higher light intensity increases the rate of transpiration.

Question 62

How does temperature affect the rate of transpiration?

Answer 62

• Increased temperature gives particles more kinetic energy, so they move faster.
• The faster the particles move, the quicker they evaporate from the cell surface into the air spaces and the quicker they diffuse out of the leaf, increasing the rate of transpiration.

Question 63

How does wind affect the rate of transpiration?

Answer 63

• The faster the wind, the faster any water vapour molecules that diffuse out of the leaf are moves away.
• This constant removal of vapour molecules from around the leaf maintains a steeper water potential gradient between the air inside and outside the leaf, so the rate of transpiration is faster.

Question 64

How does humidity affect the rate of transpiration?

Answer 64

• When air is very humid there is a high concentration of water vapour in the air
• The concentration of water vapour molecules inside the air spaces in the leaf is high
• The higher the humidity of the air the lower the water potential gradient between the air outside and the inside of the lead and the slower the rate of transpiration

Question 65

What is the method for a practical to measure the rate of transpiration?

Answer 65

• A leafy shoot is cut under water so that air doesn’t get in and break the continuous column of water
• The potometer is filled completely with water, making sure there are no air bubbles
• Using a rubber tube, the leafy shoot is fitted in the potometer uder water
• The popmeter is removed from under the water and all joins are sealed with waterproof jelly so that water doesn’t get out and air doesn’t get in
• Air bubble is introduced into the capillary tube.
• The distance moved by the air bubble in a given time is measured a number of times and the mean is calculated.
• Using the mean value, the volume of water lost is calculated and the volume of water lost against the time in minutes can he plotted on a graph.
• Once the air bubbles nears the junction of the reservoir tube and the capillary tube, the tap on the reservoir is opened and the syringe is pushed down until the bubble is pushed back to the start of the scale on the capillary tube
• The experiment can be replicated under different conditions

Question 66

What are the disadvantages of the potometer experiment?

Answer 66

• It is impossible to collect the water transpired by the plant and record its volume and instead water uptake is recorded
• Water uptake is different from transpiration as a lot of this water will be used and stored for processes like photosynthesis.

Question 67

What is translocation?

Answer 67

• Translocation is the movement of assimilates, like sucrose, from the site where they are made (source) to the places where they are used or stored (sink).
• This process occurs in tissues called the phloem.

Question 68

Describe the structure of the phloem

Answer 68

• The tissue that transports biological molecules
• Made up of sieve tube elements, ling thin structures arranged end to end.
• Their end walls are perforated to form sieve plates
• Companion cells are associated with the sieve plates
• Plasmodesmata is a hole which allows movement from companion cell to sieve tube element

Question 69

Describe how loading of sucrose takes place

Answer 69

• Hydrogen ion are pumped out of the companion cells by active transport. They contain lots of mitochondria so they can provide the necessary ATP for this process.
• A high concentration of hydrogen ions builds up outside the companion cell. The hydrogen ions move back into the companion cells, down a concentration gradient using a co-transporter protein. Sucrose is co-transported with the hydrogen ions. This results in a build-up in sucrose inside the companion cells.
• Sucrose diffuses into the sieve tube elements via plasmodesmata. This increases the sucrose concentration inside the sieve tube element. The water potential inside the sieve tube now lowers.
• Water moves into the sieve tube elements by osmosis from surrounding cells such as the xylem. The increase in water inside the sieve tube elements increases the hydrostatic pressures inside the sieve tube elements

Question 70

Describe how the unloading of sucrose takes place

Answer 70

• At the sink, sucrose moves into the cells surrounding the sieve tube elements. These cells convert the sucrose into other substances like starch for storage, or glucose and fructose for respiration.
• The reduced sucrose in the sieve tube leads to an increase in water potential. Water moves out of the sieve tube into surrounding cells by osmosis. This decreases the hydrostatic pressure inside the sieve tubes. There is therefore a mass flow of sucrose solution down the hydrostatic gradient in the sieve tubes.

Question 71

What evidence is there supporting the mass flow hypothesis

Answer 71

• There is a pressure within sieve tubes, as shown by sap being released when they are out
• The concentration of sucrose is higher in leaves (source) than in roots (sink)
• Downward flow in the phloem occurs in daylight, but ceases when leaves are shaded or at night
• Increases in sucrose levels in the leaf are followed by similar increases in sucrose levels in the phloem a little later
• metabolic poisons and lack of oxygen inhibit translocation of sucrose in the phloem
• Companion cells possess many mitochondria and readily produce ATP

Question 72

What evidence is their questioning the mass flow hypothesis

Answer 72

• The function of the sieve plates in unclear, as they would seem to hinder mass flow (may have a structural function)
• Not all solutes move at the same speed which they should do if movement is by mass flow
• Sucrose is delivered at more or less the same rate to all regions, rather than going more quickly to ones with lowest sucrose concentration, which the mass flow theory would suggest.

Question 73

What is the ringing experiment?

Answer 73

• A ring of bark is removed from a tree trunk.
• This removes the phloem but not the xylem.
• The stem immediately above the ring swells due to accumulation of sugars of the phloem.
• The flow of sugars to the region below the ring is interrupted and there is death of tissue in this region.
• Therefore it is the phloem rather than the xylem that transports sugars.

Question 74

Explain how radioactive isotopes can be used to trace the movement of substances in the plant

Answer 74

• Plants are given radioactive 14 CO2. They use it for photosynthesis and it will be incorporated into sugars/
• After a short time radiation can be traced in the phloem.
• To monitor the radiation, place thin sections of the plant stem onto X-ray film. Film becomes blackened depending on whether it has been exposed to radiation in sugars.
• These blackened regions correspond to where the phloem tissue is.

Question 75

How can aphids be used to monitor sucrose content in plants?

Answer 75

• Aphids area type of insect that feed on plants.
• They have needle like mouthparts which can penetrate the phloem.
• They can therefore be used to extract the contents of sieve tubes.

Question 76

Describe the processes involved in the transport of sugars in plant stems

Mark scheme answer that is a bit rubbish

Answer 76

• At source sucrose is actively transported into the phloem by companion cells.
• This lowers water potential in phloem and produces a high hydrostatic pressure so that there is mass flow towards sink.
• At the sink sugars are removed