Questions on gas exchange in insects, fish and plants

Question 1

A bony fish removes about 80 per cent of the dissolved oxygen from the water passing over its gills, while parallel flow could not exceed 50 per cent. Explain this difference.

Accepted Answer

B1 in counter-current flow the water and the blood travel in opposite directions along the lamella B1 the water alongside any point on the lamella therefore always holds more oxygen than the blood beneath it B1 a concentration gradient is maintained along the whole length of the lamella, so oxygen diffuses into the blood across the entire exchange surface A1 in parallel flow the two fluids converge part-way along, and once they reach the same concentration diffusion stops and the rest of the surface is wasted, so at best both leave at about half the starting value

Question 2

Describe what happens to the fluid at the ends of an insect's tracheoles during a period of sustained activity, and describe the effect this has on the delivery of oxygen.

Accepted Answer

B1 the muscle cells respire anaerobically in part, so lactate accumulates in their cytoplasm B1 the water potential of the muscle cells falls below that of the tracheal fluid B1 water therefore moves out of the end of the tracheole into the muscle cell by osmosis, and air is drawn further along the tracheole to fill the space A1 more of the final stretch of the pathway is now gas rather than liquid, and gases diffuse far faster through air than through water, so oxygen reaches the cell sooner

Question 3

A leaf has no ventilation and no transport system for gases, yet it supplies every mesophyll cell with carbon dioxide. Explain how it manages this, and explain why the same arrangement costs the plant water.

Accepted Answer

B1 gas enters through the stomata into the large air spaces of the spongy mesophyll, so it diffuses freely through the interior of the leaf B1 no cell is more than a fraction of a millimetre from an air space, so the diffusion distance is short enough for diffusion alone to be fast enough B1 the moist walls of the mesophyll cells act as the exchange surface, and together they come to several times the area of the leaf itself A1 the leaf interior is saturated with water vapour while the air outside rarely is, so a pore open enough to admit carbon dioxide also lets water vapour diffuse out

Question 4

Explain how changes in the volume of the buccal cavity and the opercular cavity keep water flowing over a bony fish's gills in one direction and almost without a break.

Accepted Answer

B1 the fish first lowers the floor of its buccal cavity with its mouth open, increasing the volume of the cavity, which drops the pressure inside it and draws water in through the mouth B1 the fish then closes its mouth and raises the floor of the buccal cavity, decreasing the volume and raising the pressure, which forces water backwards over the gills B1 the operculum acts as a valve, opening to let water leave as the pressure rises and closing to prevent it being drawn back in during the first stage B1 while the buccal cavity fills, the opercular cavity expands with the operculum shut and draws water across the gills, so flow over them continues through both stages and stays in one direction, rather than the same, partly deoxygenated water being moved back and forth across the exchange surface

Question 5

Explain, in terms of ATP, hydrogen ions and potassium ions, how a guard cell opens a stoma.

Accepted Answer

B1 a proton pump in the guard cell membrane uses ATP to actively move hydrogen ions out of the cell B1 the resulting electrochemical gradient allows potassium ions to enter the guard cell through channel proteins B1 the water potential of the guard cell falls as potassium ion concentration rises inside it, so water enters the cell by osmosis and the cell becomes turgid B1 because the guard cell wall facing the pore is thicker than the outer wall, the cell lengthens by curving rather than swelling evenly, and the two guard cells bow apart, opening the pore

Question 6

Compare the tracheal system of an insect with the stomata and air spaces of a leaf as routes for gas exchange, referring to what drives movement through each system and to what limits the size each system can serve.

Accepted Answer

B1 gas moves through most of an insect's tracheal system by diffusion, supplemented by mass transport when abdominal pumping squeezes the tracheae in larger, more active insects, whereas gas moves through a leaf's internal air spaces by diffusion alone, since a leaf has no equivalent muscular pumping mechanism B1 both systems deliver gas directly to or from individual cells rather than relying on a circulatory fluid, the tracheoles ending at or in insect cells and the air spaces reaching the moist walls of every mesophyll cell B1 the insect's tracheal system limits the size an insect can reach, because diffusion down a long tracheal tube becomes too slow over a large body, whereas a leaf's flat, thin shape keeps every mesophyll cell close to an air space regardless of how large the whole leaf's surface area becomes B1 both are therefore diffusion-based systems whose reach is set by distance rather than by a lack of surface area, but the tracheal system is constrained by overall body size while the leaf's internal geometry avoids that constraint by staying thin

Question 7

A fish stranded on a riverbank dies of oxygen shortage in air containing about thirty times more oxygen than the water it came from. Suggest why.

Accepted Answer

B1 in water the lamellae are supported and held apart, but in air they stick together and collapse into a wet mass B1 the surface area available for gas exchange therefore falls sharply A1 rate of diffusion is proportional to surface area, so too little oxygen crosses into the blood however much is present in the air around the fish

Question 8

Compare the way an insect delivers oxygen to its flight muscles with the way a bony fish delivers oxygen to its swimming muscles.

Accepted Answer

B1 the insect pipes oxygen as a gas along tracheae and tracheoles directly to the cells, whereas the fish takes oxygen up at the gill and carries it in the blood B1 the insect's blood plays little or no part in oxygen transport, whereas the fish depends on haemoglobin to raise the amount of oxygen each volume of blood can carry A1 the insect's diffusion distance from air to cytoplasm is under a micrometre, whereas the fish's gill may be many centimetres from a working muscle, so a mass transport system is unavoidable

Question 9

Explain why a plant closes its stomata in darkness, and explain why it also closes them in drought even though carbon dioxide is still needed for photosynthesis in daylight.

Accepted Answer

B1 in darkness photosynthesis has stopped, so carbon dioxide is not needed and closing the stomata avoids the unnecessary loss of water vapour that keeping them open would cost B1 in drought, the hormone abscisic acid, produced by roots in dry soil and by the leaf's own tissue as it loses water, triggers closure of the stomata B1 closing the stomata in drought reduces water loss and protects the plant from dehydration, even at the cost of reducing photosynthesis by restricting the carbon dioxide supply

Question 10

Describe how rolling its leaves and growing hairs on its epidermis each reduce water loss in a plant such as marram grass, referring in each case to humid air.

Accepted Answer

B1 rolling the leaf encloses the stomata, which are on the inner, rolled-in surface, within a humid chamber, cutting the leaf surface actually exposed to moving air B1 hairs on the epidermis hold a layer of still, humid air against the surface, so the water potential gradient between the leaf interior and the air immediately outside the stomata is reduced B1 in both cases the mechanism is the same: trapping humid air near the stomata lowers the difference in water potential that drives water loss, rather than physically blocking the stomata themselves

Question 11

A succulent plant opens its stomata only at night, storing carbon dioxide as an organic acid until it is needed for photosynthesis the following day. Suggest the advantage of this pattern in a hot, dry habitat, and suggest two disadvantages of storing carbon dioxide this way rather than taking it in directly during the day.

Accepted Answer

B1 the advantage is that opening the stomata at night, when the air is cooler and more humid, restricts most water loss to the part of the day when the water potential gradient between the leaf and the air is smallest B1 a disadvantage is that storing carbon dioxide as an organic acid costs the plant energy, beyond the energy cost of ordinary photosynthesis B1 a second disadvantage is that the amount of carbon dioxide available for photosynthesis the next day is limited by how much acid the plant can store overnight, capping the daily supply regardless of demand

Question 12

Fossil dragonflies from the Carboniferous period had wingspans far larger than any insect flying today, at a time when the atmosphere contained a notably higher percentage of oxygen than it does now. Suggest how a higher atmospheric oxygen concentration could have allowed insects of that size to be sustained by a tracheal system.

Accepted Answer

B1 gas exchange through a tracheal system relies on diffusion down tracheae and tracheoles, and the rate of diffusion depends on the concentration difference driving it B1 a higher atmospheric oxygen concentration would increase the difference in oxygen concentration between the air in the tracheal system and the tissue at the end of it, increasing the rate of diffusion for a tube of any given length B1 a faster rate of diffusion would let oxygen reach cells at the end of longer tracheae fast enough to meet their demand, allowing a larger tracheal system, and so a larger insect, than today's lower atmospheric oxygen concentration can sustain

Question 13

A student dissects a locust under water and sees silvery tubes running inwards from small pores along the sides of the abdomen. A short length of one tube, mounted on a slide, shows a spiral thickening in its wall. Explain these observations.

Accepted Answer

B1 the tubes are tracheae, and they look silvery because they are filled with air, which reflects light at its boundary with the water B1 the pores are spiracles, and each trachea runs inwards from a spiracle B1 the spiral thickening is chitin, which holds the trachea open so that air can move along it

Question 14

A gill arch cut from a fish head is examined in a dish of water, then lifted out into the air. Explain why the filaments spread apart in water but stick together in air, and why this means a fish cannot survive in air even though air contains more oxygen than water.

Accepted Answer

B1 in water the filaments and lamellae are supported by the water and held apart B1 in air nothing supports them, and the film of water between them draws them together so they collapse into a mass B1 the surface area available for gas exchange falls greatly, so too little oxygen diffuses into the blood to meet the fish's demand

Question 15

State two features of a gill lamella, other than the direction in which water flows over it, that increase the rate at which oxygen enters the blood.

Accepted Answer

B1 each lamella is covered by a single layer of cells, so the diffusion pathway is very short B1 the lamellae are stacked in large numbers along each filament, giving a very large surface area; accept a capillary network inside each lamella that carries oxygen away and maintains the gradient

Question 16

Name the pores through which air enters an insect's tracheal system, and name the tubes into which they lead.

Accepted Answer

A1 the pores are spiracles A1 tracheae, which lead inward from each spiracle and are held open by rings of chitin

Question 17

Describe two ways in which an insect's spiracles reduce water loss when the insect is not actively gas exchanging.

Accepted Answer

B1 each spiracle is guarded by a valve, which is closed when the insect is inactive and opened only as demand for gas exchange requires B1 spiracles are often fringed with hairs, which hold a layer of humid air close to the opening, reducing the concentration gradient down which water vapour is lost

Question 18

After dissecting a locust, a student concludes that the tracheal system delivers oxygen directly to the muscle cells. Suggest two reasons why the dissection itself could not show this.

Accepted Answer

B1 the tracheoles that reach the cells are under a micrometre across and end among the muscle fibres, too fine to trace with a hand lens, so only the larger tracheae are seen B1 the locust is dead, so no movement of air, tracheal fluid or oxygen into the cells can be observed