Cellular Respiration Mark Scheme- Assessment Answers

What This Guide Actually Covers

You're here because you need actual answers for your cellular respiration assessment—not vague explanations that leave you guessing. This is a practical mark scheme breakdown for the questions that keep appearing in exams.

Everything below is based on standard biology curricula. Use it to check your answers, understand what examiners want, and stop losing marks on the same stupid mistakes everyone makes.

The Three Stages You Must Know

Most exam questions test your understanding of these three processes. Know them cold.

• Glycolysis – Happens in the cytoplasm. Splits glucose into two pyruvate molecules. Produces 2 ATP and 2 NADH.
• Krebs Cycle – Happens in the mitochondrial matrix. Produces 2 ATP, 6 NADH, and 2 FADH₂ per glucose molecule.
• Electron Transport Chain – Happens in the inner mitochondrial membrane. Produces up to 34 ATP through oxidative phosphorylation.

The total yield is 38 ATP per glucose in prokaryotes, 36-38 ATP in eukaryotes (some ATP is lost transporting molecules into mitochondria).

Mark Scheme Patterns: What Examiners Actually Look For

Here's the uncomfortable truth: you lose marks not because you don't know the content, but because you write vague garbage. Examiners are looking for specific keywords and processes.

For "Describe the Process" Questions

You need to hit these points to get full marks:

• State where the process occurs (specific location)
• Identify the substrate and products
• Mention key enzymes or electron carriers involved
• Describe what happens to the end products

Wrong: "Glucose is broken down and makes ATP."
Right: "Glycolysis occurs in the cytoplasm, where glucose (C₆H₁₂O₆) is phosphorylated twice and split into two pyruvate molecules, producing a net gain of 2 ATP and 2 NADH."

For "Explain How" Questions

You must show cause and effect. The examiner wants to see that you understand the mechanism, not just the sequence.

• Identify the trigger or initial condition
• Describe the sequence of events
• Explain why each step happens (the biochemistry)
• Connect it to the final outcome

Comparison Table: Aerobic vs Anaerobic Respiration

Practice Questions with Mark Scheme Answers

Question 1: Describe what happens to pyruvate if oxygen is present

Mark Scheme (3 marks):

• Pyruvate moves into the mitochondrial matrix (1 mark)
• It is converted to acetyl CoA, releasing CO₂ and producing NADH (1 mark)
• Acetyl CoA enters the Krebs cycle (1 mark)

Question 2: Why does anaerobic respiration produce far less ATP?

Mark Scheme (4 marks):

• Without oxygen, the electron transport chain cannot function (1 mark)
• NADH cannot be oxidized back to NAD⁺ (1 mark)
• Pyruvate must be reduced to lactate/ethanol to regenerate NAD⁺ (1 mark)
• This allows glycolysis to continue but bypasses Krebs cycle and ETC, yielding only 2 ATP (1 mark)

Question 3: Explain the role of NADH in cellular respiration

Mark Scheme (4 marks):

• NADH is an electron carrier produced during glycolysis and Krebs cycle (1 mark)
• It donates electrons to the electron transport chain (1 mark)
• This creates a proton gradient across the inner mitochondrial membrane (1 mark)
• ATP synthase uses this gradient to produce ATP via chemiosmosis (1 mark)

How to Answer These Questions: A Practical Approach

Stop guessing. Follow this system for every question.

Step 1: Identify the command word

• "State" = one fact, no explanation needed
• "Describe" = list events in order
• "Explain" = give reasons, show understanding
• "Compare" = use a table or identify similarities AND differences

Step 2: Plan your answer in 30 seconds

Write down the key points you need to cover before you start writing. This prevents rambling and missing crucial information.

Step 3: Use the right terminology

Examiners don't guess. They match keywords. If you write "sugar" instead of "glucose," or "energy" instead of "ATP," you lose marks. Learn the exact vocabulary.

Step 4: Answer the question asked

Read the question twice. If it asks about the Krebs cycle, don't spend three sentences explaining glycolysis first. Get straight to the point.

Where Students Lose Marks: The Honest List

• Saying "energy" instead of "ATP" – they're not the same thing
• Forgetting to mention where processes occur
• Confusing substrate-level phosphorylation with oxidative phosphorylation
• Not balancing the word equation for respiration
• Writing about photosynthesis when the question asks about respiration
• Vague answers like "it makes energy" with no biochemical detail

Quick Reference: Essential Equations

Aerobic respiration word equation:
Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP)

Aerobic respiration symbol equation:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP

Anaerobic in animals:
Glucose → Lactate + 2 ATP

Anaerobic in yeast:
Glucose → Ethanol + Carbon dioxide + 2 ATP

The Bottom Line

Cellular respiration questions aren't tricky—they're specific. You either know the biochemistry or you don't. Study the stages, learn the locations, memorize the ATP yields, and practice explaining mechanisms instead of just listing facts.

Use this guide to check your answers. If you're missing marks, it's because you're missing details. Find the gap and fill it.