3.2 mass transport

Question 1

haemoglobin is a protein. describe the structure of haemoglobin, including the bonds involved.

Answer 1

• haemoglobin has a quarternary structure, composed of four polypeptide chains made up of sequences of amino acids.
• the quarternary structure of haemoglobin contains peptide bonds, hydrogen bonds, hydrophobic interactions, and ionic bonds.

Question 2

give the role of haemoglobin and describe what makes haemoglobin efficient at this role.

Answer 2

• the role of haemoglobin is to transport oxygen from the lungs to respiring tissues.
• in order to be efficient at transporting oxygen, haemoglobin must readily associate with oxygen at the surface where gas exchange occurs.
• it must also readily dissociate from oxygen at the respiring tissues which require it.

Question 3

which part of the haemoglobin molecule does oxygen bind to?

Answer 3

the Fe²⁺ ions found in the prosthetic haem group.

Question 4

how many molecules of oxygen can bind to a single haemoglobin molecule in humans?

Answer 4

four.

Question 5

describe where the association and dissociation of oxygen via haemoglobin takes place in humans.

Answer 5

• association, the process by which oxygen binds with haemoglobin, takes place in the lungs.
• dissociation, the process by which haemoglobin releases oxygen, takes place in the tissues.

Question 6

give the difference between haemoglobin with a high affinity for oxygen and haemoglobin with a low affinity for oxygen.

Answer 6

• haemoglobin with a high affinity for oxygen binds to oxygen readily, but releases it less easily.
• haemoglobin with a low affinity for oxygen binds to oxygen less easily, but releases it readily.

Question 7

explain why different species have different haemoglobins with different affinities for oxygen.

Answer 7

• different species have different haemoglobins because each species produces a haemoglobin with a slightly different primary structure.
• the haemoglobin of each species therefore has a slightly different quaternary structure, and therefore a different affinity for oxygen.

Question 8

what does a oxygen dissociation curve show?

Answer 8

the relationship between the saturation of haemoglobin with oxygen and the partial pressure of oxygen.

Question 9

explain why little oxygen binds to haemoglobin at low oxygen concentrations, and how this affects the shape of the oxygen dissociation curve.

Answer 9

• the quaternary structure of the haemoglobin molecule means that it is difficult for the first oxygen molecule to bind to one of the sites on the closely packed polypeptide subunits.
• therefore at low oxygen concentrations, little oxygen binds to haemoglobin.
• this causes the gradient of the oxygen dissociation curve to be shallow.

Question 10

describe how ‘positive cooperativity’ causes the gradient of the oxygen dissociation curve to steepen.

Answer 10

• the binding of the first oxygen molecule to one of the peptide subunits on the haemoglobin molecule induces the other subunits to bind to an oxygen molecule.
• it therefore takes a smaller increase in the partial pressure of the oxygen to bind the second oxygen molecule than it did to bind the first molecule.
• this is known as ‘positive cooperativity’, and causes the gradient of the oxygen dissociation curve to steepen.

Question 11

describe the trends in the following oxygen dissociation curves:

• a curve which is further to the left.
• a curve which is further to the right.

Answer 11

• a curve which is further to the left shows a greater affinity of haemoglobin for oxygen - haemoglobin loads oxygen readily, but unloads it less easily.
• a curve which is further to the right shows a lower affinity of haemoglobin for oxygen - haemoglobin loads oxygen less readily but unloads it more easily.

Question 12

explain what is meant by the ‘Bohr effect’.

Answer 12

• haemoglobin has a reduced affinity for oxygen in the presence of carbon dioxide.
• the Bohr effect proposes that the greater the concentration of carbon dioxide, the more readily the haemoglobin dissociates from oxygen.

Question 13

explain why the behaviour of haemoglobin changes in the following regions of the human body:

• the lungs.
• the muscles.

Answer 13

• at gas-exchange surfaces, such as the lungs, the concentration of carbon dioxide is low because it diffuses across the exchange surface and is excreted from the organism.
• the combination of an increased affinity for oxygen, and a high concentration of oxygen in the lungs, means that oxygen is readily loaded by haemoglobin.
• in rapidly respiring tissues, such as the muscles, the concentration of carbon dioxide is high.
• the combination of a reduced affinity for oxygen, and a low concentration of oxygen in the muscles, means that oxygen is readily unloaded from the haemoglobin into the muscle cells.

Question 14

give three common features found in most mammalian mass transport systems.

Answer 14

• a suitable medium through which substances can be carried, e.g. blood.
• a form of mass transport in which the transport medium is moved around in bulk over large distances, and more rapidly than through diffusion.
• a mechanism for moving the transport medium through vessels, so that it can be distributed to all parts of the organism.

Question 15

mammals have a closed, double circulatory system. explain an advantage of having this type of system.

Answer 15

• mammals have a closed, double circulatory system where blood passes twice through the heart for each complete circuit of the body.
• blood pressure is reduced when it passes though the lungs, so would drastically reduce the rate of circulation if it were to pass immediately to the rest of the body.
• blood is therefore returned to the heart to increase its pressure before being circulated to the rest of the body’s tissues.
• as a result, substances are delivered to the rest of the body at a much faster rate.

Question 16

why is it necessary that substances are transported around the body at a high rate?

Answer 16

it is necessary that substances are transported around the body quickly, as mammals have a high body temperature, and hence a high rate of metabolism.

Question 17

give the three types of vessels that make up the circulatory system of a mammal.

Answer 17

• arteries.
• veins.
• capillaries.

Question 18

a transport system is used to exchange substances across larger distances, but these substances eventually enter the cells via diffusion. explain why the final exchange from the blood vessels into the cells takes place rapidly.

Answer 18

the final exchange from blood vessels into the cells is rapid because it takes place over a large surface area, across a short distance, and against a steep diffusion gradient.

Question 19

where in the human body is the heart located?

Answer 19

the heart is located in the thoracic cavity, behind the sternum.

Question 20

describe the differences between the two chambers of the heart.

Answer 20

• atrium - thin-walled and elastic, to stretch as it collects blood.
• ventricle - thick-walled, contracts strongly to pump blood to the lungs or the rest of the body.

Question 21

which side of the heart deals with oxygenated blood, as opposed to deoxygenated blood?

Answer 21

• left side - deals with oxygenated blood.
• right side - deals with deoxygenated blood.

Question 22

explain why the left ventricle has a thicker muscular wall than the right ventricle.

Answer 22

• the right ventricle has a thinner wall than the left ventricle, as it pumps blood to the lungs only.
• in contrast, the left ventricle has a thick muscular wall to enable it to contract and create enough pressure to pump blood to the rest of the body.

Question 23

give the names of the valves that are found between the atrium and the ventricle.

Answer 23

• left atrioventricular (bicuspid) valve.
• right atrioventricular (tricuspid) valve.

Question 24

give the function of the atrioventricular valves.

Answer 24

the function of the atrioventricular valves is to prevent the backflow of blood into the atria when the ventricles contract.

Question 25

give the name and describe the function of the pulmonary vessels, including which chamber of the heart it is connected to.

Answer 25

• aorta - connected to the left ventricle, carries oxygenated blood to all parts of the body, except the lungs.
• vena cava - connected to the right ventricle, carries deoxygenated blood back from the tissues of the body.
• pulmonary artery - connected to the right ventricle, carries deoxygenated blood to the lungs to replenish its oxygen supply.
• pulmonary vein - connected to the left atrium, brings oxygenated blood back from the lungs.

Question 26

give the name of the blood vessels which supply the heart with oxygen.

Answer 26

coronary arteries.

Question 27

explain how blockage of the coronary arteries can lead to myocardial infraction.

Answer 27

• blockage of the coronary arteries can lead to myocardial infraction by depriving an area of the heart from oxygenated blood.
• as a result, the muscle cells in this region are unable to respire aerobically and die.

Question 28

give three factors which could increase the risk of an individual suffering from cardiovascular disease.

Answer 28

• smoking.
• high blood pressure.
• diet high in salt and saturated fat.

Question 29

explain how increased blood pressure within the arteries can lead to the development of cardiovascular disease.

Answer 29

• increased blood pressure within the arteries means that they are more likely to develop an aneurysm and burst, causing haemorrhage.
• as a result of increased pressure within the arteries, the walls tend to thicken and harden, restricting blood flow and increasing the risk of cardiovascular disease.

Question 30

describe the steps that occur during the cardiac cycle.

Answer 30

• blood enters the atria and ventricles via the pulmonary vein and the vena cava.
• the atria are relaxed and fill with blood, resulting in an increase in pressure within the chambers.
• the atria contract and the atrioventricular valves open, forcing blood into the ventricles.
• this causes the pressure within the ventricles to increase, forcing shut the atrioventricular valves to prevent the backflow of blood into the atria.
• increased pressure within the ventricles causes them to contract, forcing blood into the pulmonary artery to be transported to the lungs, and the aorta to be pumped to the rest of the body.

Question 31

define the following terms:
- systole.
- diastole.

Answer 31

• systole - contraction.
• diastole - relaxation.

Question 32

give the name of, and describe the function of the three types of valve found within the cardiovascular and venous systems.

Answer 32

• atrioventricular valves - prevent the backflow of blood into the atria when the ventricles contract.
• semi-lunar valves - prevent the backflow of blood into the ventricles when the pressure in the aorta and pulmonary artery exceeds that of the ventricles.
• pocket valves - occur throughout the venous system and ensure that when the veins are squeezed, blood flows back towards the heart instead of away from it.

Question 33

define the term ‘cardiac output’, and give the two factors upon which it depends.

Answer 33

• cardiac output - the volume of blood pumped by one ventricle of the heart in one minute.

cardiac output depends on:

• heart rate (the rate at which the heart beats)
• stroke volume (the volume of blood pumped out at each beat)

Question 34

give the equation used to calculate cardiac output.

Answer 34

cardiac output = heart rate x stroke volume.

Question 35

describe the functions of the four different types of blood vessel.

Answer 35

• arteries - carry blood away from the heart into the arterioles.
• arterioles - smaller arteries that control blood flow from the arteries into the capillaries.
• capillaries - vessels that link the arterioles to veins.
• veins - carry blood from the capillaries back to the heart.

Question 36

give three features of blood vessels which allow them to be adapted to their function of transporting blood around the body.

Answer 36

• muscle layer - can contract and control the flow of blood.
• elastic layer - helps to maintain blood pressure by stretching and recoiling.
• endothelium - smooth, thin inner wall to reduce friction, and allow for rapid diffusion of substances.

Question 37

describe the function of tissue fluid, including what it is formed from and the substances that it contains.

Answer 37

• tissue fluid is the means by which materials are exchanged between the blood and the body’s cells.
• tissue fluid is formed from blood plasma and contains glucose, amino acids, fatty acids, ions and oxygen.

Question 38

describe how tissue fluid is formed from blood plasma.

Answer 38

• the hydrostatic pressure inside the capillaries at the start of the capillary bed is greater than the hydrostatic pressure in the tissue fluid.
• this difference in hydrostatic pressure results in an overall outward pressure which forces fluid out of the capillaries, forming tissue fluid.
• as fluid leaves, the hydrostatic pressure in the capillaries decreases.

Question 39

explain why, during the formation of tissue fluid, some water re-enters the capillaries from the tissue fluid at the venule end of the capillary bed.

Answer 39

• due to the loss of fluid and increasing concentration of plasma proteins, the water potential at the venule end of the capillary bed is lower than the water potential of the tissue fluid.
• as a result, some water re-enters the capillaries from the tissue fluid at the venule end via osmosis.

Question 40

give the type of pressure created by the pumping of the heart.

Answer 40

hydrostatic pressure.

Question 41

the hydrostatic pressure created by the pumping of the heart causes tissue fluid to move out of the blood plasma. describe two forces which oppose this outward pressure.

Answer 41

• hydrostatic pressure of the tissue fluid outside the capillaries, which resists an outward movement of liquid.
• the lower water potential of the blood, due to plasma proteins, which causes water to move back into the blood within the capillaries.

Question 42

define the term ‘ultrafiltration’, and state what this process is responsible for.

Answer 42

• ultrafiltration - the process through which hydrostatic pressure causes water and dissolved ions to move across a membrane against a concentration gradient.
• ultrafiltration is responsible for the formation of tissue fluid and glomerular filtrate from blood plasma.

Question 43

once tissue fluid has exchanged metabolic materials with the cells it bathes, it is returned to the circulatory system. most tissue fluid returns to the blood plasma via which type of blood vessel?

Answer 43

the capillaries.

Question 44

describe the process by which tissue fluid is returned to the circulatory system.

Answer 44

• the loss of the tissue fluid from the capillaries reduces the hydrostatic pressure inside them.
• as a result, by the time the blood has reached the end of the capillary network, its hydrostatic pressure will be lower than that of the tissue fluid surrounding it.
• this causes tissue fluid to be forced back into the capillaries due to the higher hydrostatic pressure outside.

Question 45

not all tissue fluid surrounding cells returns to the capillaries. describe how the remaining tissue fluid is returned to the circulatory system.

Answer 45

• the remaining tissue fluid is drained into the lymphatic system.
• the lymphatic system transports excess fluid from respiring tissues to the circulatory system by draining the fluid back into the bloodstream, via two ducts that are joined to veins close to the heart.

Question 46

the contents of the lymphatic system are not moved by the pumping of the heart. give two factors which are responsible for the movement of the lymphatic system.

Answer 46

• the hydrostatic pressure of the tissue fluid that has left the capillaries.
• the contraction of body muscles that squeeze the lymph vessels.

Question 47

give the tube through which the vast majority of water is transported in flowering plants.

Answer 47

the xylem.

Question 48

describe the process by which water is pulled through the xylem vessels in the stem of a plant.

Answer 48

the main force that pulls water through the xylem vessels in the stem of a plant is the evaporation of water from the leaves, which occurs via transpiration.

Question 49

explain why transpiration is a passive process.

Answer 49

• the energy required for transpiration to occur is supplied by the sun instead of an energy-storing molecule, such as ATP.
• transpiration is therefore a passive process.

Question 50

explain why there is a water potential gradient from the air spaces through the stomata to the air.

Answer 50

• the humidity of the air is usually less than that of the air spaces next to the stomata.
• as a result, there is a water potential gradient from the air spaces through the stomata to the air.

Question 51

give one way in which plants can control their rate of transpiration.

Answer 51

by changing the size of their stomatal pores.

Question 52

water is lost from plant mesophyll cells by evaporation from their cell walls to the air spaces of the leaf. this is replaced by water reaching the mesophyll cells from the xylem either via cell walls or the cytoplasm. describe how this occurs via the cytoplasmic route.

Answer 52

• mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun.
• these cells now have a lower water potential, which allows water to enter them via osmosis from neighbouring cells.
• these neighbouring cells now have a lower water potential, and the movement of water between the cells continues.
• this establishes a water potential gradient that pulls water from the xylem into the mesophyll cells.

Question 53

describe how water moves through the xylem of plant via the cohesion-tension theory.

Answer 53

• water molecules form hydrogen bonds between one another, which causes them to fuse together.
• this is known as cohesion, and allows water molecules to be pulled through the xylem.
• transpiration occurs - water evaporates from the mesophyll cells in the leaf into the air spaces beneath the stomata.
• a column of water is therefore pulled up the xylem as a result of transpiration - this is known as transpirational pull.
• transpirational pull creates tension in the xylem, further increasing the movement of water molecules through the xylem.

Question 54

give one piece of evidence which supports the cohesion-tension theory.

Answer 54

• if air enters the xylem through a broken vessel, the plant can no longer draw up water.
• this is because the continuous column of water being pulled up the xylem is broken, which prevents the cohesion of water molecules.

Question 55

one way in which the transpiration rate of a plant can be measured is through measuring the amount of water taken up by a plant shoot in a given time. give the piece of apparatus used to measure the rate of water loss in a plant.

Answer 55

a potometer.

Question 56

give the tube through which biological molecules are transported in flowering plants.

Answer 56

the phloem.

Question 57

what is translocation?

Answer 57

the process by which organic molecules and some mineral ions are transported from one part of a plant to another.

Question 58

give two examples of the organic molecules and inorganic ions that are transported via the phloem.

Answer 58

organic molecules:

• sucrose.
• amino acids.

inorganic ions:

• potassium ions.
• magnesium ions.

Question 59

transpiration in a plant occurs in an upward direction only. the translocation of molecules in the phloem can occur in either direction. explain why.

Answer 59

• after producing sugars during photosynthesis, the plant transports them from the sites of production, the sources, to the sink cells for storage.
• these sinks can be located anywhere throughout the plant, either above or below the source.
• as a result, the translocation of molecules in the phloem can occur in either direction.

Question 60

give the name of the mechanism of translocation.

Answer 60

the mass flow theory.

Question 61

what is mass flow?

Answer 61

mass flow refers to the movement of a substance through a channel or area in a specified time.

Question 62

outline the three main stages of the mass flow theory mechanism of translocation.

Answer 62

• the transfer of sucrose into sieve elements from photosynthesising tissue.
• the mass flow of sucrose through sieve tube elements.
• the transfer of sucrose from the sieve tube elements into storage or other sink cells.

Question 63

mass flow is a passive process. explain why environmental factors such as temperature or a lack of oxygen inhibit the process of translocation.

Answer 63

• while mass flow is a passive process, it occurs as a result of the active transport of sugars around a plant.
• the entire process of translocation is therefore active, so is inhibited by environmental factors.

Question 64

give two pieces of evidence which support the mass flow theory, and one piece of evidence which disproves it.

Answer 64

evidence for the mass flow theory:

• the concentration of sucrose is higher in the sources (leaves) than in the sinks (roots)
• environmental factors, such as temperature or a lack of oxygen, inhibit translocation of sucrose in the phloem.

evidence against the mass flow theory:

• not all solutes move at the same speed within the phloem, which they would if movement occurred via mass flow.

Question 65

give the function of companion cells within the phloem.

Answer 65

the main function of the companion cells within the phloem is maintain a pressure gradient within the sieve tube elements, by actively transporting sucrose from the sieve tube elements into the sink cells.