Respiration, Photosynthesis and Energy Transfer
What this domain tests
This domain links energy release and energy capture. You must know the stages, locations and products of respiration and photosynthesis and be able to interpret experiments on limiting factors.
Respiration
- Glycolysis occurs in the cytoplasm and yields a small amount of ATP.
- The Krebs cycle occurs in the mitochondrial matrix, releasing CO₂ and producing NADH.
- The electron transport chain on the inner membrane (cristae) uses oxygen as the final electron acceptor, forming water and releasing most of the ATP; about 38 ATP per glucose is the figure used at this level.
- Anaerobic respiration in muscle produces lactic acid; in yeast it produces ethanol and CO₂. Both yield only about 2 ATP.
Photosynthesis
- Light stage in the grana: chlorophyll absorbs light, water is split (photolysis) releasing O₂, and ATP and NADPH are formed.
- Dark stage in the stroma: CO₂ is fixed and reduced using ATP and NADPH to form glucose. It does not require darkness - it requires the products of the light stage.
- Limiting factors: light intensity, CO₂ concentration and temperature. At the compensation point, photosynthesis equals respiration.
Worked example
Elodea in hydrogencarbonate solution produced 5 bubbles per minute at 10 cm from a lamp and 20 per minute at 5 cm. Reasoning: halving the distance raises light intensity, so photosynthesis and oxygen release increase, provided CO₂ and temperature are controlled. If the count stops rising at closer distances, another factor has become limiting.
Traps
Calling the dark stage the 'night-time' reactions; saying oxygen comes from CO₂; describing photosynthesis as the exact reverse of respiration; forgetting to state that temperature and CO₂ must be controlled in a light-intensity experiment.