Key ideas
Respiration releases the energy in food as ATP, and photosynthesis stores light energy in sugar. The two processes are nearly mirror images, and the questions ask where each stage happens and what goes in and out.
- Aerobic respiration: glycolysis in the cytoplasm, the Krebs cycle in the mitochondrial matrix and the electron transport chain on the inner membrane. Oxygen is the final electron acceptor and the yield is about 30 ATP per glucose.
- Without oxygen, cells ferment: lactic acid in animals and ethanol with CO₂ in yeast, giving only 2 ATP per glucose.
- Photosynthesis: the light reactions in the thylakoids make ATP, NADPH and O₂, and the Calvin cycle in the stroma fixes CO₂ into sugar.
- The respiratory quotient RQ = CO₂ produced ÷ O₂ consumed is about 1.0 for carbohydrate, 0.7 for fat and 0.9 for protein.
Key equations
C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ATP6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂RQ = CO₂ produced ÷ O₂ consumedefficiency = energy out ÷ energy in × 100
Worked example
Germinating seeds release 80 mL of CO₂ and use 100 mL of O₂ in the same time, so RQ = 80 ÷ 100 = 0.8, which suggests a mix of fat and carbohydrate. Making 180 g (1 mol) of glucose needs 6 mol of CO₂, which is 6 × 44 = 264 g. If a plant stores 2 kJ out of 100 kJ of light energy, its efficiency is 2 %.
Common mistakes
- Saying photosynthesis happens only in the light and respiration only in the dark. Plants respire all the time.
- Putting the Calvin cycle in the thylakoid membrane. It is in the stroma.
- Confusing the products of the two processes: O₂ is released by photosynthesis and CO₂ by respiration.
Practice
Test yourself with practice questions on this topic: Respiration and photosynthesis