BIOB 160 Principles of Living Systems · Unit 2: Energy and the cell · on Exam 2
2.3 Cellular respiration
Study notes for Cellular respiration in BIOB 160 Principles of Living Systems at Montana State University: the big ideas, key formulas, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in StudyCat.
Big ideas
- Overall: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy (ATP and heat). It is a series of redox reactions: glucose is oxidized and O₂ is reduced. Electrons travel on the carriers NAD⁺ → NADH and FAD → FADH₂.
- Glycolysis (cytosol, no O₂ needed): glucose → 2 pyruvate, with a net gain of 2 ATP and 2 NADH.
- Pyruvate oxidation (mitochondrial matrix): each pyruvate → acetyl CoA + CO₂ + NADH. The citric acid cycle (matrix), per glucose (two turns): 2 ATP, 6 NADH, 2 FADH₂ and 4 CO₂.
- Oxidative phosphorylation (inner mitochondrial membrane): the electron transport chain passes electrons to O₂, the final electron acceptor, forming water, while pumping H⁺ into the intermembrane space. H⁺ flows back through ATP synthase (chemiosmosis), making most of the ATP: about 30–32 ATP per glucose in total.
- Without O₂, fermentation regenerates NAD⁺ so glycolysis can continue: alcohol fermentation (pyruvate → ethanol + CO₂, in yeast) or lactic acid fermentation (in muscle). Fermentation yields only the 2 ATP from glycolysis.
Key formulas
Respiration
$$\text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2 \rightarrow 6\,\text{CO}_2 + 6\,\text{H}_2\text{O} + \text{energy}$$
Worked example
Count, per glucose, the NADH made in glycolysis, pyruvate oxidation and the citric acid cycle. Where is each stage located?
Show the solution
Glycolysis 2 (cytosol), pyruvate oxidation 2 (matrix), citric acid cycle 6 (matrix): 10 NADH in total, plus 2 FADH₂ from the cycle. These feed the electron transport chain on the inner membrane.
Common mistakes
- Saying O₂ is used in the citric acid cycle. It is only used at the end of the electron transport chain.
- Thinking fermentation makes extra ATP. Its job is to regenerate NAD⁺.
- Counting per pyruvate when the question asks per glucose (double everything after glycolysis).
Exam tipReading: Campbell ch. 9 (pp. 164–186). Draw one table: stage, location, inputs, outputs (ATP, NADH, FADH₂, CO₂). Most exam questions are answered from it.
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