Biology III: Energy and Metabolism
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ATP and Metabolism
ATP (adenosine triphosphate) is the energy currency of the cell: hydrolysis of ATP to ADP + Pᵢ releases energy for work. Metabolism includes catabolism (breakdown, releasing energy) and anabolism (synthesis, using energy). Energy transfers follow the laws of thermodynamics.
Cellular Respiration
The overall equation for aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (about 30 to 32 ATP per glucose in current estimates; older texts give 36 to 38) (Nelson & Cox, 2021).
| Stage | Location | Inputs and outputs |
|---|---|---|
| Glycolysis | Cytoplasm | Glucose (6C) to 2 pyruvate (3C); net 2 ATP, 2 NADH |
| Link reaction | Mitochondrial matrix | Pyruvate to acetyl-CoA; CO₂ released; NADH |
| Krebs cycle (citric acid cycle) | Matrix | Per glucose: 6 NADH, 2 FADH₂, 2 ATP, 4 CO₂ |
| Oxidative phosphorylation | Inner membrane (cristae) | Electron transport chain; proton gradient drives ATP synthase (chemiosmosis); O₂ is the final electron acceptor, forming water |
Anaerobic respiration: lactic acid fermentation in muscle (pyruvate to lactate) and alcoholic fermentation in yeast (ethanol + CO₂). Only 2 ATP per glucose. The respiratory quotient RQ = CO₂ produced ÷ O₂ consumed (about 1.0 for carbohydrate, 0.7 for fat, 0.8 for protein). Link this with the effects of exercise and with diabetes.
Photosynthesis
Overall: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in light, with chlorophyll).
- Light-dependent reactions (thylakoid membranes): light is absorbed by photosystems; water is split (photolysis), releasing O₂; electron transport makes ATP (photophosphorylation) and NADPH.
- Calvin cycle (stroma): CO₂ is fixed onto RuBP by RuBisCO; reduced using ATP and NADPH to G3P, which forms glucose; RuBP is regenerated.
- Limiting factors: light intensity, CO₂ concentration, and temperature. C₃ plants are common; C₄ plants (maize, sugarcane) minimize photorespiration by concentrating CO₂ in bundle-sheath cells; CAM plants (pineapple, cacti) fix CO₂ at night, an adaptation to dry conditions.
- Pigments: chlorophyll a and b, carotenoids; separate by chromatography and measure the absorption and action spectra.
Why This Matters
Respiration explains how we obtain energy from food; photosynthesis is the basis of nearly all food chains and links to agriculture (Chapter 22) and climate (Chapter 13).
Common Mistakes
- Saying photosynthesis is the opposite of respiration without noting the different locations and pathways.
- Forgetting that glycolysis happens in the cytoplasm and needs no oxygen.
- Confusing the products of the light-dependent and light-independent stages.
CHAPTER 6