A Level Biology - How to Solve: Respiration (Glycolysis, Link Reaction, Krebs, ETC, ATP Yield Calculations)

How to Solve: Respiration (Glycolysis, Link Reaction, Krebs, ETC, ATP Yield Calculations)

Complete Guide for GCSE/A-Level Biology

Introduction

"Mastering respiration unlocks 10–15% of your GCSE/A-Level Biology exam—including the infamous 6-mark ATP yield question. One wrong step here costs you easy marks. Today, you’ll learn the exact method to calculate ATP, track carbon atoms, and explain every stage like a pro."

WHAT YOU NEED TO KNOW FIRST

1. Basic cell structure: Know mitochondria (matrix, inner membrane, cristae).
2. Redox reactions: Understand oxidation (loss of electrons/H) and reduction (gain of electrons/H).
3. ATP structure: Adenosine triphosphate = adenine + ribose + 3 phosphates. Hydrolysis → ADP + Pi + energy.

KEY TERMS & FORMULAS

Key Terms

Formulas

1. ATP Yield per Glucose
2. Glycolysis: 2 ATP (net) + 2 NADH
3. Link Reaction (x2): 2 NADH
4. Krebs Cycle (x2): 2 ATP + 6 NADH + 2 FADH₂
5. ETC: 1 NADH → 2.5 ATP (eukaryotes) / 3 ATP (prokaryotes)
   1 FADH₂ → 1.5 ATP
6. Total ATP (eukaryotes): 30–32 ATP (depends on shuttle system for NADH into mitochondria).

MEMORISE THIS:
- Glycolysis: 2 ATP (net)
- Krebs: 2 ATP
- ETC: 26–28 ATP (from NADH/FADH₂)

1. Carbon Tracking
2. Glucose (6C) → 2 Pyruvate (3C each) → 2 Acetyl-CoA (2C each) + 2 CO₂ → 4 CO₂ (Krebs).
3. MEMORISE: 6C in → 6CO₂ out.

STEP-BY-STEP METHOD

Step 1: Draw the Respiration Overview

Sketch this every time:

Glucose (6C)

   ↓ Glycolysis (Cytoplasm)
2 Pyruvate (3C) + 2 ATP + 2 NADH

   ↓ Link Reaction (Mitochondrial Matrix)
2 Acetyl-CoA (2C) + 2 CO₂ + 2 NADH

   ↓ Krebs Cycle (Matrix)
4 CO₂ + 2 ATP + 6 NADH + 2 FADH₂

   ↓ ETC (Inner Membrane)
~26–28 ATP + H₂O

Step 2: Track Carbon Atoms

• Glycolysis: 6C → 2x 3C (pyruvate). No carbon lost.
• Link Reaction: 3C → 2C (Acetyl-CoA) + 1 CO₂. 2 CO₂ lost per glucose.
• Krebs Cycle: 2C (Acetyl-CoA) + 4C (oxaloacetate) → 6C (citrate) → 4C (oxaloacetate) + 2 CO₂. 4 CO₂ lost per glucose.

Check: 6C in → 6 CO₂ out.

Step 3: Count ATP, NADH, FADH₂ per Stage

Step 4: Calculate ATP from ETC

• NADH: 10 × 2.5 ATP = 25 ATP
• FADH₂: 2 × 1.5 ATP = 3 ATP
• Total ETC ATP: 25 + 3 = 28 ATP
• Add substrate-level ATP: 4 ATP (from glycolysis/Krebs)
• Total ATP per glucose: 30–32 ATP (shuttle system varies).

MEMORISE: 30 ATP (eukaryotes), 32 ATP (prokaryotes).

Step 5: Answer Exam Questions

• Describe: Use the overview diagram.
• Calculate ATP: Follow Steps 3–4.
• Explain ETC: "NADH/FADH₂ donate electrons → H⁺ pumped → ATP synthase makes ATP via chemiosmosis."

WORKED EXAMPLES

Example 1 – Basic: ATP Yield from 1 Glucose

Question: Calculate the total ATP yield from 1 glucose molecule in a eukaryotic cell.

Solution: 1. Glycolysis: 2 ATP + 2 NADH 2. Link Reaction: 2 NADH 3. Krebs Cycle: 2 ATP + 6 NADH + 2 FADH₂ 4. ETC:
- 10 NADH × 2.5 ATP = 25 ATP
- 2 FADH₂ × 1.5 ATP = 3 ATP 5. Total: 25 + 3 + 4 (substrate-level) = 30 ATP

What we did and why: Added ATP from all stages, converted NADH/FADH₂ to ATP using standard values.

Example 2 – Medium: Carbon Tracking

Question: A student says, "In the Krebs cycle, 6 CO₂ are released per glucose." Explain why this is incorrect.

Solution: 1. Krebs cycle runs twice per glucose (once per Acetyl-CoA). 2. Each cycle releases 2 CO₂. 3. Total CO₂ from Krebs: 2 × 2 = 4 CO₂. 4. Link reaction releases 2 CO₂. 5. Total CO₂ per glucose: 4 + 2 = 6 CO₂ (but Krebs alone = 4 CO₂).

What we did and why: Separated CO₂ from link reaction vs. Krebs to show the student’s error.

Example 3 – Exam-Style: ATP from FADH₂ Only

Question: A mutant yeast strain produces only FADH₂ (no NADH). How many ATP can it make from 1 glucose? Assume 1 FADH₂ = 1.5 ATP.

Solution: 1. Glycolysis: 0 NADH, 0 FADH₂ (but 2 ATP net). 2. Link Reaction: 0 NADH, 0 FADH₂. 3. Krebs Cycle: 0 NADH, 2 FADH₂. 4. ETC: 2 FADH₂ × 1.5 ATP = 3 ATP. 5. Add substrate-level ATP: 2 (glycolysis) + 2 (Krebs) = 4 ATP. 6. Total: 3 + 4 = 7 ATP.

What we did and why: Ignored NADH, focused on FADH₂ and substrate-level ATP.

COMMON MISTAKES

1. Mistake: Counting 3 ATP per NADH (prokaryote value) in eukaryotes.
   Why it happens: Confusing prokaryote/eukaryote ATP yields.
   Correct approach: Use 2.5 ATP per NADH for eukaryotes.

2. Mistake: Forgetting substrate-level ATP (only counting ETC).
   Why it happens: Overlooking ATP made in glycolysis/Krebs.
   Correct approach: Add 4 ATP (2 glycolysis + 2 Krebs).

3. Mistake: Saying "6 CO₂ from Krebs."
   Why it happens: Not accounting for link reaction CO₂.
   Correct approach: Krebs = 4 CO₂; link reaction = 2 CO₂.

4. Mistake: Misplacing stages (e.g., ETC in cytoplasm).
   Why it happens: Not memorising locations.
   Correct approach: Glycolysis = cytoplasm; rest = mitochondria.

5. Mistake: Ignoring the "net" ATP in glycolysis.
   Why it happens: Counting 4 ATP (gross) instead of 2 ATP (net).
   Correct approach: Glycolysis uses 2 ATP, makes 4 → net 2 ATP.

EXAM TRAPS

1. Trap: "How many ATP from 1 pyruvate?"
   How to spot it: Question specifies 1 pyruvate, not 1 glucose.
   How to avoid it:
2. Link reaction: 1 NADH
3. Krebs: 1 ATP + 3 NADH + 1 FADH₂
4. ETC: 4 NADH × 2.5 + 1 FADH₂ × 1.5 = 11.5 ATP

5. Total: 11.5 + 1 (substrate-level) = 12.5 ATP per pyruvate.

6. Trap: "Explain why ATP yield varies."
   How to spot it: Question asks for reasons, not a number.
   How to avoid it:

7. Shuttle system (NADH from glycolysis enters mitochondria differently).
8. Prokaryotes vs. eukaryotes (3 ATP vs. 2.5 ATP per NADH).

9. Leaky membranes (H⁺ gradient not 100% efficient).

10. Trap: "Describe the role of oxygen."
    How to spot it: Vague question about ETC.
    How to avoid it:

11. Oxygen is the final electron acceptor in ETC.
12. Combines with H⁺ to form H₂O.
13. Without oxygen, ETC stops → no ATP from oxidative phosphorylation.

1-MINUTE RECAP

"Here’s the night-before cheat sheet: 1. Glycolysis: 2 ATP net, 2 NADH, 0 CO₂. Happens in cytoplasm. 2. Link Reaction: 2 NADH, 2 CO₂. Pyruvate → Acetyl-CoA. 3. Krebs Cycle: 2 ATP, 6 NADH, 2 FADH₂, 4 CO₂. Runs twice per glucose. 4. ETC: 10 NADH × 2.5 = 25 ATP; 2 FADH₂ × 1.5 = 3 ATP. Total 30 ATP. 5. Carbon: 6C in → 6 CO₂ out. Track it! 6. Exam traps: Watch for ‘per pyruvate,’ ‘why ATP varies,’ and oxygen’s role. Draw the diagram, count the ATP, and you’ve got this!"