Cellular Respiration IGCSE Biology: Comprehensive Guide with Memory Techniques

⚡ Understanding Cellular Respiration in IGCSE Biology

Cellular respiration is the process by which cells release energy from glucose to produce ATP. This essential IGCSE Biology topic requires understanding of complex biochemical pathways. Our comprehensive guide will help you master this crucial concept with proven memory techniques.

📚 Key Theory: Cellular Respiration Process

Overall Respiration Equation

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP (38 molecules)

Word Equation: Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP)

Two Types of Cellular Respiration

1. Aerobic Respiration (With Oxygen)

Location: Mitochondria ATP Yield: 38 molecules per glucose Efficiency: High (complete glucose breakdown)

2. Anaerobic Respiration (Without Oxygen)

Location: Cytoplasm ATP Yield: 2 molecules per glucose Efficiency: Low (incomplete glucose breakdown)

🧠 Memory Tips and Techniques

1. Aerobic Respiration Stages: “Go Link Krebs Electron”

• Glycolysis (cytoplasm)
• Link reaction (mitochondrial matrix)
• Krebs cycle (mitochondrial matrix)
• Electron transport chain (inner mitochondrial membrane)

2. ATP Production Memory: “2-2-32-2”

• Glycolysis: 2 ATP
• Link Reaction: 2 ATP (via NADH)
• Krebs Cycle: 32 ATP (via NADH and FADH₂)
• Direct: 2 ATP from Krebs cycle
• Total: 38 ATP

3. Anaerobic Memory: “No Oxygen, No Problem, Less Power”

• No Oxygen: Anaerobic conditions
• No Problem: Still produces energy
• Less Power: Only 2 ATP vs 38 ATP

4. Fermentation Types: “Animals Lactic, Yeast Alcoholic”

• Animals: Lactic acid fermentation
• Yeast: Alcoholic fermentation

📖 Detailed Process Breakdown

Stage 1: Glycolysis

Location: Cytoplasm Oxygen Required: No

Key Steps:

1. Glucose activation: Phosphorylation using 2 ATP
2. Sugar splitting: 6-carbon → two 3-carbon molecules
3. Oxidation: NAD⁺ reduced to NADH
4. ATP formation: 4 ATP produced (net gain: 2 ATP)

Products: 2 Pyruvate + 2 ATP + 2 NADH

Stage 2: Link Reaction (Pyruvate Oxidation)

Location: Mitochondrial matrix Oxygen Required: Yes

Process:

• Pyruvate → Acetyl-CoA
• CO₂ released (decarboxylation)
• NAD⁺ → NADH (per pyruvate)
• Coenzyme A attached

Products: 2 Acetyl-CoA + 2 CO₂ + 2 NADH

Stage 3: Krebs Cycle (Citric Acid Cycle)

Location: Mitochondrial matrix Oxygen Required: Yes

Key Steps:

1. Acetyl-CoA + Oxaloacetate → Citrate (6C)
2. Decarboxylation: 2 CO₂ released
3. Substrate-level phosphorylation: 1 ATP
4. Reduction: 3 NADH + 1 FADH₂
5. Regeneration: Oxaloacetate reformed

Products per cycle: 1 ATP + 3 NADH + 1 FADH₂ + 2 CO₂

Stage 4: Electron Transport Chain

Location: Inner mitochondrial membrane Oxygen Required: Yes (final electron acceptor)

Process:

• NADH and FADH₂ donate electrons
• Electron carriers (cytochromes) transfer electrons
• Proton pumping creates gradient
• ATP synthase produces ATP
• Oxygen accepts final electrons → H₂O

ATP Yield:

• Each NADH → 3 ATP
• Each FADH₂ → 2 ATP

🔄 Anaerobic Respiration Details

Lactic Acid Fermentation (Animals)

Equation: Glucose → 2 Lactic acid + 2 ATP

Process:

1. Glycolysis occurs (2 ATP produced)
2. Pyruvate reduced to lactic acid
3. NAD⁺ regenerated for continued glycolysis
4. No oxygen required

Examples: Muscle fatigue, yogurt production

Alcoholic Fermentation (Yeast)

Equation: Glucose → 2 Ethanol + 2 CO₂ + 2 ATP

Process:

1. Glycolysis occurs (2 ATP produced)
2. Pyruvate → Acetaldehyde + CO₂
3. Acetaldehyde → Ethanol
4. NAD⁺ regenerated

Examples: Bread making, alcohol production

🎯 IGCSE Exam Focus Areas

Common Exam Questions

1. Compare aerobic and anaerobic respiration (6 marks)
2. Describe the role of mitochondria (4 marks)
3. Explain oxygen debt in muscles (5 marks)
4. Calculate ATP yield from glucose (3 marks)

Key Experimental Knowledge

• Respirometer experiments
• Yeast fermentation investigations
• Oxygen consumption measurements
• CO₂ production detection

📊 Respiration Comparison Table

Aspect	Aerobic	Anaerobic
Oxygen needed	Yes	No
Location	Mitochondria	Cytoplasm
ATP yield	38 per glucose	2 per glucose
Products	CO₂ + H₂O	Lactic acid/Ethanol
Efficiency	High	Low
Duration	Continuous	Short-term

🏃‍♂️ Respiration in Exercise

Light Exercise

• Aerobic respiration meets energy demands
• Oxygen supply adequate
• Sustainable for long periods

Intense Exercise

• Oxygen demand exceeds supply
• Anaerobic respiration begins
• Lactic acid accumulates
• Muscle fatigue occurs

Recovery Period

• Oxygen debt repaid
• Lactic acid converted back to pyruvate
• ATP and phosphocreatine restored
• Normal respiration resumes

🔬 Factors Affecting Respiration Rate

1. Temperature

• Higher temperature: Increased enzyme activity
• Optimum range: 35-40°C for human cells
• Too high: Enzyme denaturation

2. Oxygen Availability

• High O₂: Aerobic respiration favored
• Low O₂: Switch to anaerobic
• No O₂: Only anaerobic possible

3. Glucose Concentration

• More glucose: Higher respiration rate
• Limited glucose: Rate decreases
• Alternative substrates: Fats, proteins

4. pH Levels

• Optimal pH: 7.4 for human cells
• Acidic conditions: Enzyme inhibition
• Basic conditions: Enzyme denaturation

🧬 Mitochondrial Structure and Function

Key Adaptations for Respiration

• Double membrane: Compartmentalization
• Cristae: Increased surface area for ETC
• Matrix: Contains Krebs cycle enzymes
• Inner membrane: Impermeable to protons
• ATP synthase: Harnesses proton gradient

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