Respiration

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Respiration — Key Facts for MDCAT

Definition: The metabolic process by which organic compounds (mainly glucose) are oxidised to release energy for biological activities. Respiration is NOT the same as breathing — it is a cellular process.

Overall Equation: $$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP (energy)}$$ $$\Delta G = -2880 \text{ kJ/mol (approximately)}$$

Two Main Types:

1. Aerobic Respiration: Requires oxygen; occurs in mitochondria; yields 30–32 ATP per glucose
2. Anaerobic Respiration: Does NOT require oxygen; occurs in cytoplasm; yields only 2 ATP per glucose

Anaerobic Respiration (Fermentation):

• In animals and some bacteria: $Glucose \rightarrow 2 lactate + 2ATP$ (lactic acid fermentation)
• In plants and yeast: $Glucose \rightarrow 2 ethanol + 2CO_2 + 2ATP$ (alcoholic fermentation)
• Pyruvate is the key branch point: aerobic → acetyl-CoA enters mitochondria; anaerobic → lactate or ethanol

Key Stages of Aerobic Respiration:

Stage	Location	Input	Output	ATP
Glycolysis	Cytoplasm	Glucose	2 pyruvate, 2 ATP, 2 NADH	Net 2
Link reaction	Mitochondrial matrix	Pyruvate	Acetyl-CoA, CO₂, NADH	0 (directly)
Krebs cycle	Mitochondrial matrix	Acetyl-CoA	CO₂, NADH, FADH₂, GTP	2
ETC + Oxidative phosphorylation	Inner mitochondrial membrane	NADH, FADH₂, O₂	H₂O	~28–30

Respiratory Quotient (RQ): $$RQ = \frac{CO_2 \text{ produced}}{O_2 \text{ consumed}}$$

• Carbohydrates: RQ = 1.0 (e.g., glucose: 6CO₂/6O₂ = 1)
• Fats: RQ ≈ 0.7–0.8 (more O₂ needed for fat oxidation)
• Proteins: RQ ≈ 0.8–0.9

⚡ Exam tip: In glycolysis, the NET gain is 2 ATP and 2 NADH (not 4 ATP — 2 are used early in the preparatory phase). The first investment phase uses 2 ATP; the payoff phase generates 4 ATP. Fermentation in yeast produces ethanol + CO₂ — the CO₂ is what makes bread dough rise. In humans, intense exercise causes lactic acid fermentation leading to oxygen debt.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students who want genuine understanding.

Respiration — Complete Study Guide

Glycolysis (EMP Pathway):

• Occurs in the cytoplasm
• 10-step pathway converting glucose → 2 pyruvate
• Investment phase (steps 1-5): Glucose → glucose-6-phosphate (uses 1 ATP) → fructose-6-phosphate → fructose-1,6-bisphosphate (uses 1 more ATP) → split into 2 × 3-carbon compounds
• Payoff phase (steps 6-10): 2 × 3-phosphoglycerate → 2 × phosphoenolpyruvate → 2 × pyruvate → produces 4 ATP + 2 NADH
• Net yield: 2 ATP, 2 NADH (per glucose)

Link Reaction (Pyruvate Oxidation): $$Pyruvate + NAD^+ + CoA \rightarrow Acetyl-CoA + NADH + CO_2$$ This occurs in the mitochondrial matrix. One pyruvate per glucose (2 total). The acetyl-CoA then enters the Krebs cycle.

Krebs Cycle (Citric Acid Cycle / Tricarboxylic Acid Cycle):

• Acetyl-CoA (2C) + Oxaloacetate (4C) → Citrate (6C)
• Citrate undergoes 8 steps → regenerates oxaloacetate
• Per acetyl-CoA: 3 NADH, 1 FADH₂, 1 GTP (equivalent to ATP), 2 CO₂
• The cycle turns twice per glucose molecule

Electron Transport Chain (ETC):

• Located on the inner mitochondrial membrane
• Complexes I, II, III, IV, and Coenzyme Q and Cytochrome c as carriers
• NADH donates electrons at Complex I → produces ~2.5 ATP
• FADH₂ donates electrons at Complex II → produces ~1.5 ATP
• Oxygen is the final electron acceptor → forms water
• Without oxygen, ETC stops → NADH cannot be reoxidised → glycolysis and Krebs cycle halt

Oxidative Phosphorylation:

• Chemiosmotic theory (Peter Mitchell): ETC pumps protons (H⁺) into intermembrane space
• Creates proton gradient across inner mitochondrial membrane
• Protons flow back through ATP synthase (Complex V) → drives ATP synthesis
• ~3 protons needed per ATP synthesis

ATP Yield per Glucose (Aerobic):

Source	ATP
Glycolysis (substrate-level)	2
Glycolysis NADH (sent to ETC)	3–5 (depends on shuttle)
Link reaction (2 NADH)	5
Krebs cycle (6 NADH)	15
Krebs cycle (2 FADH₂)	3
Krebs cycle (2 GTP)	2
Total	~30–32

⚡ Common mistakes: Confusing cellular respiration with breathing. Thinking fermentation produces no ATP (it produces 2 ATP from glycolysis). Forgetting that 1 NADH from glycolysis produces only 3 ATP (or 2.5) vs 3 from NADH in mitochondria because the mitochondrial membrane has lower efficiency. Anaerobic respiration produces lactate in human muscles — this lactate can be converted back to glucose in the liver (Cori cycle).

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Respiration — Advanced Notes

Anaerobic Respiration vs Fermentation: These are NOT synonymous. Anaerobic respiration uses an external electron acceptor other than O₂ (e.g., sulfate, nitrate). Fermentation does NOT use an external electron acceptor — the pyruvate itself accepts electrons from NADH to form the reduced end product.

Steps of Alcoholic Fermentation (Yeast):

1. Pyruvate → acetaldehyde + CO₂ (pyruvate decarboxylase, requires thiamine pyrophosphate)
2. Acetaldehyde + NADH → ethanol + NAD⁺ (alcohol dehydrogenase)

Lactic Acid Fermentation:

1. Pyruvate + NADH → lactate + NAD⁺ (lactate dehydrogenase)

• Occurs in skeletal muscle during intense exercise when O₂ is insufficient
• Lactate is transported to liver → converted back to glucose (Cori cycle)

Cori Cycle:

• Lactate from muscle → blood → liver
• Liver converts lactate → pyruvate → glucose (via gluconeogenesis)
• Glucose returns to muscle → glycolysis → lactate (again)
• Net: 0 glucose change in the body, but 2 ATP produced anaerobically in muscle

Mitochondrial shuttle systems:

• Malate-aspartate shuttle (most tissues): NADH in cytosol → malate → enters matrix → produces matrix NADH (3 ATP equivalent)
• Glycerol phosphate shuttle (muscle and brain): produces FADH₂ (2 ATP equivalent) — less efficient

Hexose Monophosphate (HMP) Shunt (Pentose Phosphate Pathway):

• Alternative to glycolysis; occurs in cytoplasm
• Produces: NADPH (for biosynthesis), Ribose-5-phosphate (for nucleotide synthesis)
• 6-glucose → 6CO₂ + 12NADPH (oxidative phase) → Ribose-5-P for nucleotide synthesis
• Important in tissues with high biosynthetic demands: liver, adipose tissue, RBCs

Regulation of Respiration:

• Phosphofructokinase (PFK): Key regulatory enzyme of glycolysis; inhibited by ATP and citrate; activated by AMP and fructose-2,6-bisphosphate
• Pyruvate dehydrogenase: Inhibited by its products (NADH, acetyl-CoA); activated by ADP
• Isocitrate dehydrogenase: Inhibited by ATP and NADH; activated by ADP and NAD⁺

Respirometry: Measuring oxygen consumption (RQ respirometer) to determine metabolic rate and RQ value. Useful for diagnosing metabolic disorders.

MDCAT Question Patterns: MDCAT Pakistan respiration questions frequently test: (1) overall equation and RQ values, (2) distinguishing aerobic vs anaerobic respiration, (3) products of glycolysis and Krebs cycle, (4) ETC location and final electron acceptor, (5) ATP yield calculations, (6) fermentation products (lactic acid vs alcohol + CO₂), (7) factors affecting respiration rate. 2–3 questions per paper. Glycolysis steps and location are frequently tested.

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