← Back to Home

AEHBY Task 1 Cheatsheet

15/11/2025

Cellular Respiration Cheatsheet

1. Purpose

  • Cellular respiration = breakdown of glucose (C₆H₁₂O₆) to release energy as ATP.
  • ATP powers muscle contraction, active transport, and biosynthesis.
  • Main product: ATP.
  • Waste products: carbon dioxide (CO₂), water (H₂O), and heat (~60% of energy lost as heat).
  • Importance: without ATP, cells cannot function.

2. ATP Yield

  • Aerobic respiration (with O₂): ~30–32 ATP per glucose.
  • Anaerobic respiration (without O₂): 2 ATP per glucose.
  • Most ATP comes from: Electron Transport Chain (ETC) → 26–34 ATP.
  • Scientific term: ATP yield or bioenergetic yield.

3. Glycolysis

  • Meaning: “sugar splitting.”
  • Location: cytoplasm.
  • Reaction: C₆H₁₂O₆ → 2 × pyruvate (C₃H₄O₃).
  • ATP: 2 used, 4 made → net 2 ATP.
  • Produces 2 NADH.
  • Works with or without oxygen.
    • With O₂: pyruvate → mitochondria.
    • Without O₂: pyruvate → lactic acid (C₃H₆O₃).
  • Importance: first step in all respiration, provides quick ATP.

4. Intermediate Step

  • Location: mitochondrial matrix.
  • Pyruvate → Acetyl‑CoA + CO₂ + NADH.
  • No ATP produced here.
  • Purpose: prepares substrate for Krebs cycle.

5. Krebs Cycle (Citric Acid Cycle)

  • Location: mitochondrial matrix.
  • Input: Acetyl‑CoA.
  • Output (per glucose):
    • 2 ATP
    • 6 NADH
    • 2 FADH₂
    • 6 CO₂ (waste)
  • Purpose: load electron carriers for ETC.
  • Importance: completes oxidation of glucose carbons.

6. Electron Transport Chain (Oxidative Phosphorylation)

  • Location: inner mitochondrial membrane.
  • NADH and FADH₂ donate electrons → pump H⁺ ions → gradient.
  • H⁺ flows back through ATP synthase → ATP.
  • Final electron acceptor: O₂ → forms H₂O.
  • Produces 26–34 ATP.
  • If ETC blocked → ATP production collapses, cell dies.

7. Aerobic vs Anaerobic Respiration

  • Aerobic: needs O₂, ~30 ATP, sustainable, produces CO₂ + H₂O.
  • Anaerobic: no O₂, 2 ATP, fast but short‑term, produces lactic acid.
  • Oxygen debt: extra O₂ needed after exercise to clear lactic acid and restore ATP/PCr.
  • Aerobic threshold: point where aerobic system cannot meet demand → anaerobic begins.

8. Lactic Acid

  • Produced when pyruvate is reduced without O₂.
  • Causes muscle burning, fatigue, lower pH.
  • Cleared by liver (Cori cycle) → converted back to glucose.
  • Importance: allows glycolysis to continue when O₂ is limited.

9. Exercise Energy Systems

  • Sprint (100 m): anaerobic dominates (ATP‑PCr + glycolysis). Quick ATP, lactate buildup.
  • Marathon: aerobic dominates (ETC + Krebs). Long‑term ATP, fuels = glucose + fatty acids.
  • Best for both: aerobic respiration → improves recovery, clears lactate, strengthens heart/lungs.

10. Training Implications

  • Aerobic training: ↑ mitochondria, ↑ O₂ delivery, ↑ endurance.
  • Anaerobic training: ↑ glycolysis enzymes, ↑ creatine phosphate, ↑ lactate tolerance.
  • Sprinter: anaerobic training most important.
  • Marathoner: aerobic training most important.
  • All athletes: need both systems.

11. Higher‑Order Quick Answers

  • Limited O₂: glucose → pyruvate → lactate; 2 ATP.
  • ETC blocked: no proton gradient → ATP drops drastically.
  • Cori cycle: lactate → liver → glucose; clears lactate, repays oxygen debt.
  • Marathoner less fatigue: aerobic = more ATP, less lactate.
  • Short vs long exercise: short = anaerobic (fast, low yield); long = aerobic (slow, high yield, sustainable).

Example Exam Questions

Q1. Where does glycolysis occur and what is its net ATP yield?
A+ Answer: Glycolysis occurs in the cytoplasm. One glucose (C₆H₁₂O₆) is split into two pyruvate molecules (C₃H₄O₃). The process requires 2 ATP to start but produces 4 ATP, giving a net yield of 2 ATP. It also produces 2 NADH, which carry electrons to the electron transport chain. Glycolysis is important because it can occur with or without oxygen, making it the universal first step of respiration.

Q2. What happens to pyruvate before entering the Krebs cycle?
A+ Answer: Pyruvate is transported into the mitochondrial matrix and converted into acetyl‑CoA. This reaction releases one molecule of CO₂ per pyruvate and produces NADH, but no ATP. Acetyl‑CoA then enters the Krebs cycle. This step is essential because it links glycolysis to aerobic respiration and prepares carbon atoms for complete oxidation.

Q3. Why does the electron transport chain produce the most ATP?
A+ Answer: The electron transport chain uses electrons from NADH and FADH₂ to pump hydrogen ions across the inner mitochondrial membrane. This creates a proton gradient. As protons flow back through ATP synthase, their energy is used to phosphorylate ADP into ATP. Because each NADH can yield about 2.5 ATP and each FADH₂ about 1.5 ATP, the ETC produces 26–34 ATP per glucose, far more than glycolysis or the Krebs cycle. Oxygen is vital as the final electron acceptor, forming water.

Q4. Compare aerobic and anaerobic respiration in terms of ATP yield and by‑products.
A+ Answer: Aerobic respiration requires oxygen and produces ~30–32 ATP per glucose. Its by‑products are CO₂ and H₂O, which are easily removed from the body. Anaerobic respiration occurs when oxygen is limited, producing only 2 ATP per glucose. In humans, pyruvate is converted into lactic acid, which accumulates in muscles and causes fatigue. Although anaerobic respiration is less efficient, it provides rapid ATP when oxygen delivery cannot meet demand.

Q5. Explain why a sprinter experiences muscle fatigue faster than a marathon runner.
A+ Answer: A sprinter relies mainly on anaerobic respiration, which produces ATP quickly but in small amounts (2 ATP per glucose). This pathway also generates lactic acid, lowering muscle pH and interfering with enzyme activity, leading to rapid fatigue and burning sensations. In contrast, a marathon runner uses aerobic respiration, which produces much more ATP (~30 per glucose) and avoids large lactate buildup, allowing sustained effort over long periods.

Q6. What is oxygen debt and how is it repaid?
A+ Answer: Oxygen debt is the extra oxygen required after intense exercise to restore the body to resting state. It is used to oxidise lactic acid back into pyruvate or glucose (via the Cori cycle), to replenish ATP and creatine phosphate stores, and to re‑oxygenate myoglobin in muscles. Oxygen debt is repaid during recovery breathing, which is why athletes continue to breathe heavily after exercise.

Memorise These Core Lines

  1. Glycolysis: C₆H₁₂O₆ → 2 pyruvate + 2 ATP (net) + 2 NADH.
  2. Pyruvate → Acetyl‑CoA + CO₂ + NADH (no ATP).
  3. Krebs: 2 ATP + 6 NADH + 2 FADH₂ + CO₂.
  4. ETC: 26–34 ATP, O₂ final acceptor → H₂O.
  5. Aerobic = ~30 ATP, Anaerobic = 2 ATP.
  6. Sprint = anaerobic, Marathon = aerobic, Aerobic helps both.