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Three other answers: tracheae, gills and stomata questions
The insect tracheal system from spiracle to tracheole and how tracheal fluid moves during activity, the structure of a bony fish gill and why counter-current flow beats parallel flow, gas exchange in a leaf through stomata and the spongy mesophyll, the opening mechanism of guard cells, and the xerophyte adaptations that limit water loss.
6 original questions · 20 marks · the three other answers: tracheae, gills and stomata notes · Exchange surfaces and gas exchange
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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.
Mark scheme
- 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
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.
Mark scheme
- 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
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.
Mark scheme
- 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
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.
Compare the way an insect delivers oxygen to its flight muscles with the way a bony fish delivers oxygen to its swimming muscles.
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.
Practise three other answers: tracheae, gills and stomata one question at a time
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