Bioenergetics
Bioenergetics is the study of energy flow through living systems — how cells capture, store, and release the energy needed for work. The PMDC MDCAT 2026 syllabus focuses on aerobic cellular respiration: glycolysis, the link reaction, the Krebs cycle, the electron transport chain, and the calculation of ATP yield. Expect 3-4 MCQs per paper.
Respiration
Cellular respiration is the controlled oxidation of glucose (or other fuels) to release energy that is captured as ATP. The overall equation is:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + ~36-38 ATP + heat
Aerobic respiration occurs in four sequential phases: glycolysis (cytoplasm), link reaction (mitochondrial matrix), the Krebs / citric acid cycle (matrix), and the electron transport chain & oxidative phosphorylation (inner mitochondrial membrane).
Key definitions
- ATP
- Adenosine triphosphate — the universal energy currency of cells. Hydrolysis of its terminal phosphate releases ~7.3 kcal/mol (30.5 kJ/mol).
- Substrate-level phosphorylation
- Direct transfer of a phosphate group from a high-energy substrate to ADP. Occurs in glycolysis and Krebs cycle.
- Oxidative phosphorylation
- ATP synthesis driven by the proton gradient generated by the electron transport chain. Catalysed by ATP synthase.
- Chemiosmosis
- Movement of H+ ions down their electrochemical gradient through ATP synthase, powering ATP formation. Mitchell's hypothesis (1961).
Splits one glucose (6C) into two pyruvate (3C). It has a preparatory phase (uses 2 ATP) and a pay-off phase (produces 4 ATP and 2 NADH).
Net per glucose: 2 ATP, 2 NADH, 2 pyruvate. Glycolysis is anaerobic — it does not require oxygen and is the only ATP-yielding pathway in obligate anaerobes and red blood cells.
Each pyruvate (3C) enters the mitochondrion and is converted by the pyruvate dehydrogenase complex into acetyl-CoA (2C). One CO2 and one NADH are produced per pyruvate.
Per glucose (2 pyruvates): 2 CO2, 2 NADH, 2 acetyl-CoA. No ATP at this step.
Acetyl-CoA (2C) condenses with oxaloacetate (4C) to form citrate (6C). Through eight enzymatic steps, citrate is oxidised back to oxaloacetate, releasing 2 CO2, 3 NADH, 1 FADH2, and 1 GTP (= 1 ATP) per turn.
Per glucose (2 turns): 4 CO2, 6 NADH, 2 FADH2, 2 GTP/ATP. Discovered by Sir Hans Krebs in 1937 (Nobel Prize 1953).
NADH and FADH2 donate electrons to a chain of carriers (Complexes I → IV) embedded in the inner mitochondrial membrane. Electron flow pumps H+ from matrix to intermembrane space, creating a proton gradient. H+ flows back through ATP synthase (Complex V), driving ATP formation. Oxygen is the final electron acceptor, forming H2O.
Yield: each NADH → ~3 ATP; each FADH2 → ~2 ATP. Per glucose: ~32-34 ATP from the ETC.
| Phase | Location | Inputs | Outputs | ATP / NADH / FADH2 |
|---|---|---|---|---|
| Glycolysis | Cytoplasm (anaerobic) | 1 glucose, 2 ATP, 2 NAD+ | 2 pyruvate | +2 ATP (net), +2 NADH |
| Link reaction | Mitochondrial matrix | 2 pyruvate, 2 NAD+, 2 CoA | 2 acetyl-CoA, 2 CO2 | 0 ATP, +2 NADH |
| Krebs cycle | Mitochondrial matrix | 2 acetyl-CoA | 4 CO2, 6 NADH, 2 FADH2, 2 GTP | +2 ATP (GTP), +6 NADH, +2 FADH2 |
| ETC + oxidative phosphorylation | Inner mitochondrial membrane (cristae) | 10 NADH, 2 FADH2, 6 O2 | 6 H2O, ~34 ATP | +~34 ATP |
| TOTAL | — | 1 glucose + 6 O2 | 6 CO2 + 6 H2O | ~38 ATP (prokaryote), ~36 ATP (eukaryote) |
Aerobic vs Anaerobic respiration
When oxygen is absent, pyruvate is reduced rather than entering the Krebs cycle, regenerating NAD+ so glycolysis can continue.
| Property | Aerobic respiration | Lactic acid fermentation | Alcoholic fermentation |
|---|---|---|---|
| Oxygen required? | Yes | No | No |
| Final electron acceptor | O2 | Pyruvate → lactate | Acetaldehyde → ethanol |
| End products | CO2 + H2O | Lactic acid (3C) | Ethanol + CO2 |
| ATP per glucose | 36–38 | 2 | 2 |
| Site | Cytoplasm + mitochondria | Cytoplasm only | Cytoplasm only |
| Where it happens | Most cells | Muscle cells (oxygen debt), lactobacilli | Yeast, some bacteria |
| Industrial use | — | Yoghurt, cheese, sour-milk | Bread (CO2 rises), beer/wine (ethanol) |
Worked MCQs
Five MCQs that capture the high-yield testing patterns for this chapter. Read the explanation even when you get the answer right — it's where the deeper concept lives.
Q1. The net ATP yield per glucose molecule from glycolysis alone is:
Glycolysis produces 4 ATP gross but consumes 2 ATP in the preparatory phase, giving a net yield of 2 ATP. It also produces 2 NADH and 2 pyruvate per glucose.
Q2. The Krebs cycle takes place in which part of the mitochondrion?
Krebs cycle enzymes are dissolved in the mitochondrial matrix. The electron transport chain & ATP synthase, in contrast, sit on the inner membrane (cristae). Memorising the location is examiner gold.
Q3. In oxidative phosphorylation, the final acceptor of electrons in the electron transport chain is:
Molecular oxygen is reduced at Complex IV (cytochrome c oxidase), accepting electrons and protons to form water. Cyanide and CO block this step, halting the entire chain.
Q4. One molecule of NADH delivered to the ETC produces approximately how many ATP?
Each NADH yields ~3 ATP via oxidative phosphorylation; each FADH2 yields ~2 ATP because it enters the chain at Complex II, bypassing one proton-pumping site.
Q5. During strenuous exercise, muscle cells temporarily switch to which pathway?
When oxygen supply lags behind demand, muscle pyruvate is reduced to lactate to regenerate NAD+ and keep glycolysis running. Build-up of lactic acid contributes to fatigue and the "burning" sensation in muscles.
Quick Recap
- Aerobic respiration = glycolysis + link reaction + Krebs + ETC.
- Glycolysis (cytoplasm): 2 ATP net + 2 NADH + 2 pyruvate.
- Link reaction (matrix): 2 NADH + 2 CO2 per glucose.
- Krebs cycle (matrix): 2 GTP + 6 NADH + 2 FADH2 + 4 CO2 per glucose.
- ETC (inner mitochondrial membrane): O2 is final electron acceptor; chemiosmosis drives ATP synthase.
- NADH → ~3 ATP; FADH2 → ~2 ATP.
- Total: 36 ATP (eukaryote) or 38 ATP (prokaryote) per glucose.
- Anaerobic: lactic acid (muscle) or ethanol + CO2 (yeast); only 2 ATP per glucose.