Bioenergetics — section overview
Bioenergetics B4 is about how living organisms obtain and use energy — through photosynthesis (plants trapping light) and respiration (all cells releasing energy from glucose).
Photosynthesis
Word equation: Carbon dioxide + Water → Glucose + Oxygen
Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (using light energy)
Photosynthesis is an endothermic reaction (energy taken in from light).
Chlorophyll in chloroplasts absorbs light energy. Most absorbed in red and blue wavelengths; reflects green (hence green leaves).
Rate of photosynthesis — limiting factors
| Factor | Effect when increased |
|---|---|
| Light intensity | Rate increases (up to a limit) |
| CO₂ concentration | Rate increases (up to a limit) |
| Temperature | Rate increases (up to an optimum) then decreases |
| Chlorophyll concentration | Rate increases |
Inverse square law: light intensity ∝ 1/d², where d = distance from light source.
Uses of glucose in plants
- Respiration (energy)
- Making cellulose (cell walls)
- Making starch (storage)
- Making amino acids and proteins (with nitrates from soil)
- Making lipids/oils (seeds)
Respiration
Respiration releases energy from glucose for use by cells.
Aerobic respiration (with oxygen)
Glucose + Oxygen → Carbon dioxide + Water (+ energy)
6C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Aerobic respiration releases much more energy per glucose molecule than anaerobic.
Anaerobic respiration (without oxygen)
In animals and bacteria: Glucose → Lactic acid (+ small amount of energy)
In yeast (fermentation): Glucose → Ethanol + Carbon dioxide (+ small amount of energy)
Response to exercise
During vigorous exercise:
- Breathing rate and heart rate increase to deliver more O₂ and remove CO₂
- If O₂ cannot be supplied fast enough → anaerobic respiration in muscles
- Lactic acid builds up → muscle fatigue
- After exercise: oxygen debt repaid — lactic acid converted back to glucose in liver
Metabolism
Metabolism = all enzyme-controlled chemical reactions in the body (building up and breaking down molecules). Includes: protein synthesis, photosynthesis, respiration, digestion.
Common exam mistakes in B4
- Photosynthesis produces energy — NO: photosynthesis produces glucose; energy is released by respiration
- Anaerobic respiration produces no energy — FALSE: it produces a small amount
- Yeast fermentation produces CO₂ — yes, plus ethanol (not lactic acid)
- Limiting factor graphs — at a plateau, increasing the current factor won't help; a different factor is limiting
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