Bioenergetics
Bioenergetics is the study of energy flow through living systems — how cells capture, store, and release the energy needed for work. The AAMC MCAT content outline focuses on aerobic cellular respiration: glycolysis, the link reaction, the Krebs cycle, the electron transport chain, and the calculation of ATP yield. This is a high-yield area for the Bio/Biochem section.
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 + ~30-32 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. Catalyzed by ATP synthase.
- Chemiosmosis
- Movement of H+ ions down their electrochemical gradient through ATP synthase, powering ATP formation. Mitchell's hypothesis (1961).
ATP is a nucleotide: adenine + ribose + three phosphate groups. The two terminal phosphoanhydride bonds are "high-energy" because hydrolysis relieves charge repulsion and yields resonance-stabilized products. Hydrolysis of ATP → ADP + Pi releases ~7.3 kcal/mol (ΔG°′ ≈ −30.5 kJ/mol). Cells couple this favorable reaction to endergonic reactions — often by transferring the phosphate to a substrate (phosphorylation) — so that the summed ΔG is negative and the unfavorable step can proceed.
Respiration is a series of redox reactions: glucose is oxidized (loses electrons/H) while O2 is ultimately reduced (OIL RIG — Oxidation Is Loss, Reduction Is Gain). The electrons are ferried by coenzymes: NAD+ + 2e− + H+ → NADH and FAD + 2e− + 2H+ → FADH2. These reduced carriers deliver their electrons to the electron transport chain, where the energy is used to make ATP.
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 oxidized 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.
Modern yield: each NADH → ~2.5 ATP; each FADH2 → ~1.5 ATP (FADH2 enters at Complex II and bypasses one proton-pumping site). Per glucose the ETC contributes ~26-28 ATP.
| 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, ~26-28 ATP | +~26-28 ATP |
| TOTAL | — | 1 glucose + 6 O2 | 6 CO2 + 6 H2O | ~30-32 ATP (modern estimate) |
Older textbooks quote 36-38 ATP, but current estimates give ~30-32 ATP per glucose. The lower number reflects two facts: the measured yields per NADH (~2.5) and FADH2 (~1.5) are non-integer, and the 2 NADH made in the cytosol during glycolysis must be shuttled into the mitochondrion — the glycerol-3-phosphate shuttle converts them to FADH2-equivalents (costing ATP), while the malate-aspartate shuttle preserves them as NADH.
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 | ~30–32 | 2 | 2 |
| Site | Cytoplasm + mitochondria | Cytoplasm only | Cytoplasm only |
| Where it happens | Most cells | Muscle cells (oxygen debt), lactobacilli | Yeast, some bacteria |
| Industrial use | — | Yogurt, 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. Using modern yield estimates, the complete aerobic oxidation of one glucose molecule produces approximately how many ATP?
Current biochemistry gives ~30-32 ATP per glucose: 2 (glycolysis) + 2 (Krebs GTP) from substrate-level phosphorylation, plus ~26-28 from oxidative phosphorylation (each NADH ≈ 2.5 ATP, each FADH2 ≈ 1.5 ATP). The older "36-38" figure used rounded integer yields (3 and 2) and ignored the cost of shuttling cytosolic NADH into the mitochondrion.
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 → ~2.5 ATP; FADH2 → ~1.5 ATP.
- Total: ~30-32 ATP per glucose (modern estimate; the old "36-38" is outdated).
- Anaerobic: lactic acid (muscle) or ethanol + CO2 (yeast); only 2 ATP per glucose.