Cellular Respiration: Steps & Stages
Aerobic cellular respiration breaks down glucose through four linked stages: glycolysis in the cytoplasm, followed by the link reaction, the Krebs cycle, and oxidative phosphorylation via the electron transport chain inside the mitochondria. Learning where each stage occurs, what enters and leaves, and how reduced NAD and FAD drive ATP synthesis makes every exam question on respiration straightforward.
- Cellular Respiration
- The process by which cells break down glucose and other organic molecules to release energy stored as ATP. In aerobic respiration the overall reaction is: glucose + oxygen → carbon dioxide + water (+ energy).
- Step 1: Glycolysis
- The first stage, occurring in the cytoplasm, where one glucose (6 carbons) is split into two pyruvate molecules (3 carbons each). It produces a net 2 ATP and 2 NADH and does not require oxygen.
- Step 2: Link Reaction
- In the mitochondrial matrix, each pyruvate is converted into acetyl-CoA, releasing one carbon dioxide and producing one NADH. Per glucose that means 2 CO2 and 2 NADH. Some textbooks group this with the Krebs cycle.
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Mitochondria
Function in the Cell:
Site of aerobic respiration, releasing energy (ATP) for the cell. Folded inner membrane increases surface area for enzymes.
Exact GCSE / A-Level Exam Wording:
Mark scheme trap: Never say 'produces energy'! Always write 'site of aerobic respiration, which releases energy'.
Magnification = Image Size ÷ Actual Size (I = A × M). Convert cm → mm (×10) → µm (×1000) before dividing!Key Concepts & Exam Definitions (6)
- Cellular Respiration
- The process by which cells break down glucose and other organic molecules to release energy stored as ATP. In aerobic respiration the overall reaction is: glucose + oxygen → carbon dioxide + water (+ energy).
- Step 1: Glycolysis
- The first stage, occurring in the cytoplasm, where one glucose (6 carbons) is split into two pyruvate molecules (3 carbons each). It produces a net 2 ATP and 2 NADH and does not require oxygen.
- Step 2: Link Reaction
- In the mitochondrial matrix, each pyruvate is converted into acetyl-CoA, releasing one carbon dioxide and producing one NADH. Per glucose that means 2 CO2 and 2 NADH. Some textbooks group this with the Krebs cycle.
- Step 3: Krebs Cycle (Citric Acid Cycle)
- A cycle in the mitochondrial matrix that oxidises acetyl-CoA, releasing carbon dioxide and generating the electron carriers NADH and FADH2 plus a small amount of ATP (as GTP).
- Step 4: Electron Transport Chain & Oxidative Phosphorylation
- A series of protein complexes in the inner mitochondrial membrane where NADH and FADH2 donate electrons; the released energy pumps protons to build a gradient that drives ATP synthase, with oxygen acting as the final electron acceptor to form water.
- ATP (Adenosine Triphosphate)
- The cell's main energy currency. Energy is released when its terminal phosphate bond is broken, converting ATP into ADP + inorganic phosphate, and this energy powers cellular work.
Quick-Fire Flashcards (5)
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Where in the cell does glycolysis take place?
What is the final electron acceptor in aerobic respiration?
What is the approximate net ATP yield per glucose in aerobic respiration?
Write the overall word and symbol equation for aerobic respiration.
What happens during anaerobic respiration (fermentation) in human muscle cells?
Exam-Style Questions & Mark Schemes (3)
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- Q1
Why does aerobic respiration produce far more ATP than anaerobic respiration?
Click “Check Mark Scheme” to compare your answer, Aerobic respiration fully oxidises glucose using oxygen, so the Krebs cycle and electron transport chain can extract energy from NADH and FADH2, giving about 30-32 ATP. Anaerobic respiration relies on glycolysis alone and produces only 2 ATP, because without oxygen the electron transport chain cannot operate.
- Q2
A poison blocks ATP synthase in the inner mitochondrial membrane. What is the immediate effect on ATP production and the proton gradient?
Click “Check Mark Scheme” to compare your answer, Oxidative phosphorylation stops, so almost no ATP is made from the electron transport chain. Protons can no longer flow back through ATP synthase, so the proton gradient builds up (becomes steeper), and this eventually slows electron transport too.
- Q3
In which stage(s) of respiration is carbon dioxide released, and in which is water produced?
Click “Check Mark Scheme” to compare your answer, Carbon dioxide is released during the link reaction (pyruvate → acetyl-CoA) and the Krebs cycle. Water is produced at the end of the electron transport chain, when oxygen accepts electrons and combines with hydrogen ions.
Common Exam Questions (5)
What are the steps of cellular respiration in order?
1) Glycolysis in the cytoplasm splits one 6-carbon glucose molecule into two 3-carbon pyruvate molecules (net 2 ATP and 2 NADH). 2) The link reaction in the mitochondrial matrix converts pyruvate into acetyl-CoA, releasing CO₂ and reducing NAD. 3) The Krebs cycle in the matrix oxidises acetyl-CoA, releasing CO₂ and producing ATP, NADH, and FADH₂. 4) The electron transport chain across the inner mitochondrial membrane uses electrons from NADH and FADH₂ to pump protons and synthesis most of the cell's ATP, with oxygen acting as the final electron acceptor to form water.
Does cellular respiration have 3 or 4 steps?
Most GCSE and introductory courses group respiration into three main stages: glycolysis, the Krebs cycle, and oxidative phosphorylation (the electron transport chain). A-Level specifications treat the link reaction (pyruvate to acetyl-CoA) as a distinct stage between glycolysis and the Krebs cycle, giving 4 stages in total.
What are the three stages of cellular respiration?
The three main stages are glycolysis (in the cytoplasm), the Krebs cycle (in the mitochondrial matrix), and oxidative phosphorylation (across the inner mitochondrial membrane).
How much ATP does aerobic respiration produce?
Aerobic respiration yields approximately 30 to 32 molecules of ATP per molecule of glucose (older textbooks quote a theoretical maximum of 36 to 38 ATP). Only 2 net ATP come from glycolysis and 2 from the Krebs cycle; the rest are generated by oxidative phosphorylation.
What is the difference between aerobic and anaerobic respiration?
Aerobic respiration requires oxygen and completely oxidises glucose to carbon dioxide and water, yielding around 30 to 32 ATP per glucose. Anaerobic respiration occurs without oxygen, relies solely on glycolysis (yielding 2 ATP per glucose), and converts pyruvate into lactate in animals or ethanol and carbon dioxide in yeast and plants.
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