2.1 ATP: the cell's energy currency
Cells can't use the energy in food directly; they first transfer it to ATP (adenosine triphosphate). Removing ATP's last phosphate group gives ADP and phosphate, and the energy released powers muscle contraction, active transport and building reactions. Cells don't store ATP — they keep remaking it: even a resting person makes and uses roughly their own body weight of ATP every day.
The overall equation for cellular respiration is C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy. It resembles burning, but in the cell the energy is released in small, enzyme-controlled steps rather than all at once, and part of it is captured in ATP.
FormulaC₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy (about 30–32 ATP)
2.2 The three stages of respiration
Glycolysis happens in the cytoplasm and needs no oxygen: one glucose is split into two pyruvates, with a net gain of 2 ATP and 2 NADH. If oxygen is present, pyruvate enters the mitochondrion, becomes acetyl-CoA and is broken down completely to CO₂ in the Krebs cycle in the mitochondrial matrix. This makes little ATP but plenty of NADH and FADH₂.
Most ATP is made by the electron transport chain on the inner mitochondrial membrane. As electrons from NADH and FADH₂ pass along a chain of proteins, H⁺ ions are pumped into the intermembrane space. When the H⁺ ions flow back through the enzyme ATP synthase, ATP is made (chemiosmosis). At the end of the chain, oxygen accepts the electrons and water forms. Without oxygen the chain stops and cells switch to fermentation: muscles make lactic acid, yeast makes ethanol and CO₂, and only 2 ATP are gained per glucose.
2.3 Photosynthesis: from light to sugar
Photosynthesis happens in the chloroplast in two stages. The light-dependent reactions take place on the thylakoid membranes: chlorophyll captures light energy, water molecules are split (photolysis) and oxygen is released, while ATP and NADPH are made. The light-independent reactions — the Calvin cycle — take place in the stroma: the enzyme RuBisCO attaches CO₂ to the five-carbon RuBP, and sugar is built using ATP and NADPH.
Making one glucose requires fixing 6 CO₂, which costs 18 ATP and 12 NADPH in the Calvin cycle. The rate of photosynthesis is limited by light intensity, CO₂ concentration and temperature; whichever is scarcest sets the rate (the limiting factor). C4 plants such as maize and CAM plants such as cacti use special pathways that reduce water loss and RuBisCO's wasteful reaction with oxygen in hot, dry places.
Formula6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂
Key terms
- Chemiosmosis
- Making ATP as H⁺ ions flow through ATP synthase.
- Fermentation
- Getting a little ATP from glucose without oxygen.
- Photolysis
- Splitting water using light energy.
- Limiting factor
- The scarcest factor, which sets the rate of a process.
End-of-unit problems
Solve each one yourself first, then open the solution.
Cyanide blocks the last enzyme of the electron transport chain (cytochrome c oxidase). Why does ATP production fall sharply in cyanide poisoning? Which process can still make ATP?
Show the solution
With no way to pass electrons to oxygen, the chain stops, H⁺ is no longer pumped and ATP production by chemiosmosis ends. Because NADH can't be turned back into NAD⁺, the Krebs cycle slows too. Only glycolysis (with fermentation) can keep making 2 ATP per glucose — far too little for the body's needs.
A plant photosynthesises in air containing CO₂ labelled with ¹⁸O. Where do the labelled oxygen atoms appear first: in the released O₂ or in glucose?
Show the solution
The released O₂ comes from water, so it is unlabelled. The oxygen atoms of CO₂ end up in sugar (glucose) and water through the Calvin cycle. The label appears in glucose.
During a short sprint, an athlete breaks down some glucose aerobically (30 ATP per glucose) and some by fermentation (2 ATP per glucose). If 4 of 10 glucose molecules are respired aerobically, how many ATP are made in total?
Show the solution
4 × 30 = 120 ATP; 6 × 2 = 12 ATP. Total 132 ATP. Had all 10 been respired aerobically, the yield would be 300 ATP — which shows why fermentation is "inefficient".
In a greenhouse, raising light intensity doesn't change the rate of photosynthesis, but raising CO₂ concentration does. What is the limiting factor? What would you expect if the temperature were raised to 45 °C?
Show the solution
Since the rate rises only when CO₂ increases, CO₂ concentration is the limiting factor. At 45 °C enzymes such as RuBisCO begin to denature and stomata close to save water, so the rate falls.
🦉 An easier explanation: Professor Owl's lesson: Plants and photosynthesis