explain why the energy yield from respiration in aerobic conditions is much greater than the energy yield from respiration in anaerobic conditions (a detailed account of the total yield of ATP from the aerobic respiration of glucose is not expected)

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology – Respiration

Respiration – Energy Yield in Aerobic vs Anaerobic Conditions

Learning Objective

Explain why the energy yield from respiration in aerobic conditions is much greater than the energy yield from respiration in anaerobic conditions.

Key Concepts

  • Definition of aerobic and anaerobic respiration.

  • Pathways involved: glycolysis, link reaction, Krebs cycle, oxidative phosphorylation.

  • Role of electron carriers (NADH, FADH2).

  • Presence or absence of a terminal electron acceptor.

  • Efficiency of substrate‑level phosphorylation vs oxidative phosphorylation.

Why Aerobic Respiration Yields More Energy

  1. Complete oxidation of glucose. In aerobic respiration glucose is fully oxidised to CO2 and H2O, releasing the maximum amount of chemical energy. In anaerobic pathways the carbon skeleton is only partially oxidised (e.g., to lactate or ethanol).

  2. Generation of more reduced electron carriers. The link reaction and the Krebs cycle produce additional NADH and FADH2 that are not formed in most anaerobic processes.

  3. Use of an electron transport chain (ETC). Aerobic cells use a series of membrane‑bound carriers that pass electrons to molecular oxygen, the final electron acceptor. The energy released pumps protons across the membrane, creating a chemiosmotic gradient.

  4. Oxidative phosphorylation. The proton motive force drives ATP synthase, allowing synthesis of many ATP molecules from each NADH/FADH2. Anaerobic respiration lacks this step; ATP is generated only by substrate‑level phosphorylation.

  5. Higher energetic efficiency. The free‑energy change (\$\Delta G^\circ'\$) for the transfer of electrons to O2 is much larger than for the transfer to organic acceptors (e.g., pyruvate, acetaldehyde). This larger \$\Delta G^\circ'\$ translates into more ATP per electron pair transferred.

Overall Comparison

AspectAerobic RespirationAnaerobic Respiration (e.g., Fermentation)
Final electron acceptorO2 (high‑potential)Organic molecules (e.g., pyruvate, acetaldehyde) – low‑potential
Pathways involvedGlycolysis → Link reaction → Krebs cycle → ETC → Oxidative phosphorylationGlycolysis → Fermentation (no link reaction, no Krebs cycle, no ETC)
Reduced electron carriers produced per glucose\overline{10} NADH + 2 FADH2\overline{2} NADH only (from glycolysis)
ATP generation mechanismsSubstrate‑level phosphorylation + oxidative phosphorylationSubstrate‑level phosphorylation only
Typical net ATP yield (approx.)\overline{30}–32 ATP2 ATP

Chemical Equations (LaTeX)

Overall aerobic 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}\$

Lactic‑acid fermentation (anaerobic):

\$\text{C}6\text{H}{12}\text{O}6 \rightarrow 2\text{CH}3\text{CH(OH)COOH} + 2\text{ATP}\$

Alcoholic fermentation (anaerobic):

\$\text{C}6\text{H}{12}\text{O}6 \rightarrow 2\text{C}2\text{H}5\text{OH} + 2\text{CO}2 + 2\text{ATP}\$

Key Take‑away

The presence of oxygen as a high‑energy terminal electron acceptor enables the electron transport chain to generate a large proton gradient, which drives oxidative phosphorylation. Without oxygen, cells cannot use the ETC and must rely solely on the limited ATP produced directly in glycolysis, resulting in a far lower overall energy yield.

Suggested diagram: Flow chart comparing aerobic and anaerobic pathways, highlighting where NADH/FADH2 are produced and where ATP is generated.