3.1 exchange

Question 1

all organisms exchange substances with their environment in order to absorb useful substances needed for metabolic reactions to occur, and to remove waste products. in multicellular organisms, this exchange of substances takes place via a what?

Answer 1

a specialist exchange surface, known as a mass transport system.

Question 2

what does this mass transport system maintain?

Answer 2

this mass transport system maintains the diffusion gradients that transport substances to and from cell-surface membranes.

Question 3

give the two factors which affect the amount of each material that is exchanged between an organism and its external environment.

Answer 3

• the surface area to volume ratio of the organism.
• the metabolic rate of the organism.

Question 4

explain why organisms with a high metabolic rate require a larger surface area to volume ratio than organisms with a low metabolic rate.

Answer 4

organisms with a high metabolic rate exchange more materials than organisms with a low metabolic rate, so require a larger surface area to volume ratio.

Question 5

state the four main substances that need to be interchanged between an organism and its environment, including an example for each.

Answer 5

• respiratory gases, such as oxygen.
• nutrients, such as glucose.
• excretory products, such as urea.
• heat.

Question 6

except for heat, give the two main ways in which the exchange of substances between an organism and its environment can take place.

Answer 6

• passively (no metabolic energy required), by diffusion and osmosis.
• actively (metabolic energy required), by active transport.

Question 7

exchange takes place at the surface of an organism. give the two main features which organisms have evolved in order to increase the efficiency of exchange with their environment.

Answer 7

• a flattened shape, so that no cell is ever far from the surface of the organism, such as a leaf.
• specialised exchange surfaces, with large surface areas to increase the surface area to volume ratio, such as lungs in mammals, and gills in fish.

Question 8

give three features of specialised exchange surfaces, which allow the efficient exchange of materials across it via diffusion or active transport.

Answer 8

• a large surface area relative to volume ratio, which increases the rate of exchange.
• very thin, so that the diffusion distance is short, allowing materials to move across the exchange surface rapidly.
• a transport system to ensure the movement of an internal medium, such as the blood, in order to maintain a steep diffusion gradient.

Question 9

explain why fish need a specialist exchange surface to exchange respiratory gases.

Answer 9

• fish have a small surface area to volume ratio.
• because of this, their body surface is not accurate to supply and remove their respiratory gases.
• therefore, they have evolved a specialist exchange surface to counteract this.

Question 10

describe what the gills are composed of, and where they are located.

Answer 10

the gills are located within the body of the fish behind the head, made up of gill filaments stacked in a pile.

Question 11

describe the events that occur during the countercurrent exchange principle.

Answer 11

the flow of water over the gill lamellae, and the flow of blood, in opposite directions.

Question 12

give two things that this countercurrent arrangement leads to.

Answer 12

• the diffusion of oxygen from water to the blood.
• a diffusion gradient for oxygen uptake, which is maintained across the entire width of the gill lamellae.

Question 13

explain what would happen to the diffusion gradient if the flow of water and blood occurred in the same direction.

Answer 13

• the diffusion gradient would only be maintained across part of the length of the gill lamellae.
• this means that only 50% of the available oxygen would be absorbed by the blood.

Question 14

explain why specialised exchange surfaces are usually located inside the body of an organism.

Answer 14

• being thin, specialised exchange surfaces are easily damaged and dehydrated.
• therefore, they are usually located inside the body of an organism.

Question 15

give the name of the specialised exchange system insects have evolved in order to carry out gas exchange.

Answer 15

for gas exchange, insects have evolved a specialised internal network of tubes called tracheae, which are further divided into smaller tubes called tracheoles.

Question 16

explain why there is a short diffusion pathway from a tracheole to any body cell within an insect.

Answer 16

• the tracheoles extend throughout all the body tissues of an insect.
• in this way, oxygen is brought directly to the respiring tissues, which results in a short diffusion pathway from a tracheole to any body cell.

Question 17

give the three ways in which respiratory gases move in and out of the tracheal system.

Answer 17

• along a diffusion gradient.
• via mass transport.
• via osmosis.

Question 18

when gas exchange occurs via osmosis, the final diffusion pathway occurs in a gas rather than a liquid phase. explain why diffusion is more rapid in this way.

Answer 18

• during periods of major activity, the muscle cells around the tracheoles respire anaerobically.
• this produces lactate, which lowers the water potential of the muscle cells, drawing water from the tracheoles into the cells via osmosis.
• the volume of water in the tracheoles decreases, which increases the rate at which air moves into the tracheoles.

Question 19

give a limitation of the tracheal system.

Answer 19

• the tracheal system relies mostly on diffusion to exchange gases between the environment and an insect’s cells.
• for diffusion to be effective the diffusion pathway needs to be short, which limits the size that insects can obtain.

Question 20

most gaseous exchange process in plants occur via diffusion in the leaf. explain three ways in which the leaf of a plant is adapted to allow for the rapid diffusion of gases.

Answer 20

• many stomata in the surface of the leaf, which creates a short diffusion pathway between plant cells and stomata.
• interconnecting air-spaces that occur throughout the mesophyll so that gases can readily come into contact with mesophyll cells.
• large surface area of mesophyll cells for rapid diffusion.

Question 21

explain how stomata control the rate of gaseous diffusion, whilst limiting water loss from a plant.

Answer 21

• each stoma is surrounded by guard cells, which open and close the stoma pore.
• guard cells open stomata during gas exchange, and close stomata to limit excessive water loss.
• in this way, stomata control the rate of gaseous diffusion, whilst still limiting water loss from a plant.

Question 22

most insects are terrestrial. this means that water easily evaporates from the surface of their bodies, which can lead to dehydration. give three adaptations that insects have evolved to limit water loss.

Answer 22

• a small surface area to volume ratio, to minimise the area over which water is lost.
• waterproof coverings, which limit the evaporation of water from the surface of the body.
• spiracles, which are openings of the tracheae at the body surface, which can be closed to reduce water loss.

Question 23

xerophytes are plants that are adapted to living in areas where water is in short supply. give three ways in which xerophytes are adapted to limit water loss through transpiration.

Answer 23

• a thick cuticle, which forms a waterproof barrier, preventing the loss of water from the surface of the leaf.
• hairy leaves, which traps still, moist air next to the leaf surface.
• stomata in pits or grooves, which traps still, moist air next to the leaf surface, and reduces the water potential gradient.

Question 24

what is transpiration?

Answer 24

the loss of water through the stoma of a plant leaf.

Question 25

explain how the rolling up of leaves can limit water loss in xerophytic plants.

Answer 25

• most leaves have their stomata largely confined to the lower epidermis on the underside of the leaf.
• the rolling of leaves can trap a region of still, moist air next to the lower epidermis.
• this region becomes saturated with water, resulting in a very high water potential.
• there is no water potential gradient between the inside and outside of the leaf, which limits water loss.

Question 26

to maintain the diffusion of gases across the alveolar epithelium, air is constantly moved into and out of the lungs. give the two muscles responsible for the pressure changes brought about by the movement of air through the lungs.

Answer 26

• the diaphragm - a sheet of muscle that separates the thorax from the abdomen.
• the intercostal muscles - which lie between the ribs, and contract to allow for inspiration and expiration.

Question 27

give the two sets of intercostal muscles, including the processes which they control.

Answer 27

• the internal intercostal muscles, whose contraction leads to expiration.
• the external intercostal muscles, whose contraction leads to inspiration.

Question 28

what does pulmonary ventilation rate measure?

Answer 28

the volume of air taken in and out of the lungs in a given time.

Question 29

give the equation used to calculate pulmonary ventilation rate.

Answer 29

pulmonary ventilation rate = tidal volume x breathing rate.

tidal volume - the volume of air taken in by each breath when the body is at rest.

breathing rate - the number of breaths taken in one minute.

Question 30

give the site of gas exchange in mammals.

Answer 30

the epithelium of the alveoli.

Question 31

give three reasons why the diffusion of gases between the alveoli and the blood is very rapid.

Answer 31

• the walls of both the alveoli, and the capillaries which surround the alveoli are very thin, which results in a short diffusion pathway.
• the alveoli and pulmonary capillaries have a very large total surface area for the diffusion of gases across them.
• blood flow through the pulmonary capillaries maintains a concentration gradient.

Question 32

give four risk factors which could increase the probability of someone suffering from chronic obstructive pulmonary disease (COPD)

Answer 32

• smoking.
• air pollution.
• genetic constitution.
• chest infections.

Question 33

give the piece of apparatus used to measure tidal volume and ventilation rate.

Answer 33

a spirometer.

Question 34

a spirometer can also be used to measure vital capacity. what is vital capacity?

Answer 34

vital capacity - the maximum volume of air that can be breathed into and out of the lungs.

Question 35

what name is given to the term which describes the maximum volume of air than an individual can expire in one second?

Answer 35

forced expiratory volume.

Question 36

explain how the presence of microvilli in the ileum allow it to be adapted to its purpose of absorbing the products of digestion into the bloodstream.

Answer 36

• the inner walls of the ileum are folded into villi, which gives the ileum a large surface area.
• the surface area of the ileum is further increased by microvilli on the epithelial cells of each villus.
• this allows the ileum to be adapted to its purpose of absorbing the products of digestion.

Question 37

describe the two stages involved in the process of digestion.

Answer 37

• the physical breakdown of food into smaller pieces, such as through chewing, and the churning of the muscles in the stomach.
• chemical digestion, which hydrolyses large, insoluble molecules into smaller, soluble ones via the use of digestive enzymes, such as amylase.

Question 38

give the three main types of digestive enzyme, including the molecules that they hydrolyse.

Answer 38

• carbohydrases - hydrolyse carbohydrates to monosaccharides.
• lipases - hydrolyse lipids into glycerol and fatty acids.
• proteases / peptidases - hydrolyse proteins to amino acids.

Question 39

describe the process of starch digestion in the human digestive system, including the enzymes involved and where they are produced.

Answer 39

• amylase, which is produced in the salivary glands and the pancreas, hydrolyses the glycosidic bonds of the starch molecule to break it down into maltose.
• the molecule of maltose is then hydrolysed into the monosaccharide alpha glucose by maltase, which is produced in the lining of the ileum.

Question 40

explain what ‘emulsification’ is and why it occurs.

Answer 40

• emulsification is the splitting of lipids into droplets, called micelles, by bile salts produced by the liver.
• emulsification increases the surface area of the lipid, which in turn increases the action of lipase enzymes in hydrolysing lipids.

Question 41

give the three types of peptidases involved in the digestion of proteins, including the molecules which they hydrolyse.

Answer 41

• endopeptides - hydrolyse the peptide bonds between amino acids in a protein molecule.
• exopeptidases - hydrolyse the peptide bonds between the amino acids of the peptide molecules formed by endopeptidases.
• dipeptidases - hydrolyse the peptide bond between the amino acids of a dipeptide.

Question 42

give three ways in which villi increase the efficiency of the absorption of the products of digestion in the ileum.

Answer 42

• increase the surface area of the ileum for diffusion.
• are very thin walled, so reduce the distance over which diffusion takes place.
• are well supplied with blood vessels, so that blood can carry away absorbed molecules, maintaining a diffusion gradient.

Question 43

explain how triglycerides are absorbed in the ileum.

Answer 43

• emulsification causes the splitting of lipids into droplets, called micelles, by bile salts produced by the liver.
• these micelles come into contact with the epithelial cells lining the villi of the ileum, which causes them to breakdown, releasing monoglycerides and fatty acids.
• these diffuse into the cell and are transported to the endoplasmic reticulum, where they recombine to form triglycerides.
• the triglycerides associate with cholesterol and lipoproteins to form chylomicrons, which move out of the epithelial cells via exocytosis.
• these then enter the lymphatic capillaries found at the centre of each villus.

Question 44

give the role of chylomicrons.

Answer 44

chylomicrons are specialised particles, composed of triglycerides, cholesterol and lipoproteins, which are adapted for the transport of lipids.