Learning objectives
- Write balanced equations for aerobic and anaerobic respiration
- Compare respiration and photosynthesis
- Explain the compensation point and storage losses after harvest
1. Aerobic respiration
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (about 2900 kJ per mole of glucose, captured as roughly 30-32 ATP). It happens in three stages: glycolysis in the cytoplasm, the Krebs cycle in the mitochondrial matrix, and the electron transport chain on the inner mitochondrial membrane.
2. Anaerobic respiration
Without oxygen, plant and yeast cells convert glucose to ethanol and carbon dioxide: C6H12O6 → 2C2H5OH + 2CO2 + a small amount of energy (about 118 kJ). This is alcoholic fermentation, the basis of brewing and bread-making, and it is what happens in waterlogged roots — the ethanol produced is toxic, which is why flooded crops die.
- Aerobic — oxygen used, complete oxidation, large energy yield
- Anaerobic — no oxygen, incomplete breakdown, small energy yield
- Waterlogging forces roots into anaerobic respiration and kills them
3. Compensation point and storage losses
At the compensation point the rate of photosynthesis exactly equals the rate of respiration, so there is no net gas exchange — it occurs around dawn and dusk. Harvested produce keeps respiring, using up its own sugars and releasing heat and moisture. Cool, ventilated, low-humidity storage slows respiration and is the cheapest way to cut post-harvest losses in yam, potato and grain.
Key definitions
- Respiration
- Breakdown of food substances in living cells to release energy in the form of ATP.
- Compensation point
- Light intensity at which the rate of photosynthesis equals the rate of respiration.
- Fermentation
- Anaerobic breakdown of glucose to ethanol and carbon dioxide with a small energy release.
Exam tips
- Never write 'plants respire only at night' — they respire continuously; photosynthesis simply masks it in daylight.
- Show the equation and the energy term; marks are usually allocated separately for the balanced equation.
Practice exam questions & model answers
Q1.Give three differences between respiration and photosynthesis.
Respiration breaks down glucose while photosynthesis builds it up; respiration releases energy while photosynthesis stores light energy as chemical energy; respiration uses oxygen and releases carbon dioxide while photosynthesis uses carbon dioxide and releases oxygen; respiration occurs in all living cells at all times, photosynthesis only in chlorophyll-containing cells in light.
Q2.Explain why waterlogged soil kills crop roots.
Water fills the soil air spaces, so oxygen cannot reach the roots. The roots switch to anaerobic respiration, which yields very little energy and accumulates toxic ethanol, so root cells die, uptake of water and nutrients stops and the plant wilts and dies.
Q3.Why should harvested grain be dried and stored in a cool ventilated place?
Drying and cooling slow the rate of respiration of the grain, reducing loss of stored food, heat and moisture build-up, and discouraging mould and storage insects — so quality and germination are preserved for longer.
Continue revising
Plant Cell Structure and Function
Plant BiologyPhotosynthesis: Process, Equation and Limiting Factors
BotanyLeaf Structure, Stomata and Transpiration
Agricultural SciencePlant Nutrition, NPK and Soil Fertility
Put the theory to work — diagnose a real leaf photo or browse the disease library.