Respiration
Cambridge A-Level Biology (9700) · Unit 12: Energy and respiration · 16 flashcards
Respiration is topic 12.2 in the Cambridge A-Level Biology (9700) syllabus , positioned in Unit 12 — Energy and respiration , alongside Energy. In one line: Glycolysis occurs in the cytoplasm of eukaryotic cells. It's the first stage of both aerobic and anaerobic respiration, breaking down glucose into pyruvate.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 16 flashcards — 5 definitions and 11 key concepts — covering the precise wording mark schemes reward. Use the 5 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Where does glycolysis occur in eukaryotic cells
Glycolysis occurs in the cytoplasm of eukaryotic cells. It's the first stage of both aerobic and anaerobic respiration, breaking down glucose into pyruvate.
What the Cambridge 9700 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- State where in a eukaryotic cell each of the four stages of aerobic respiration happens: glycolysis takes place in the cytoplasm; the link reaction and the Krebs cycle both take place in the matrix of the mitochondrion; oxidative phosphorylation takes place on the inner mitochondrial membrane.
- Outline glycolysis: glucose is phosphorylated, the resulting fructose 1,6-bisphosphate (6C) splits into two molecules of triose phosphate (3C), and each of these is oxidised further to pyruvate (3C), producing ATP and reduced NAD.
- Explain that, if oxygen is present, pyruvate moves into the mitochondria and is used in the link reaction.
- Describe what happens in the link reaction, including how coenzyme A transfers acetyl (2C) groups.
- Outline the Krebs cycle. Explain that oxaloacetate (4C) accepts the 2C acetyl fragment carried by acetyl coenzyme A, forming citrate (6C), and that citrate is turned back into oxaloacetate through a sequence of small steps.
- Explain that the reactions of the Krebs cycle include decarboxylation and dehydrogenation, and that the coenzymes FAD and NAD are reduced.
- Describe how NAD and FAD carry hydrogen to the carriers located in the inner membrane of the mitochondrion.
- Explain these events of oxidative phosphorylation: hydrogen atoms separate into protons and high-energy electrons; as these electrons move along the electron transport chain they release energy (the individual carriers are not required); this energy is used to pump protons across the inner membrane of the mitochondrion; the protons diffuse back into the matrix through ATP synthase (facilitated diffusion), and this supplies the energy to make ATP (the details of ATP synthase are not required); oxygen is the final electron acceptor, forming water.
- Use diagrams and electron micrographs to describe how the structure of mitochondria relates to their function.
- Outline what happens when respiration is anaerobic, both in yeast (ethanol fermentation) and in mammals (lactate fermentation).
- Explain why respiration yields far more energy in aerobic conditions than in anaerobic conditions (you do not need a detailed count of the total ATP yield when glucose is respired aerobically).
- Explain the adaptations that let rice grow with submerged roots, limited to: aerenchyma developing in the roots, ethanol fermentation in the roots, and faster stem growth.
- Carry out and describe investigations using redox indicators (including methylene blue and DCPIP) to find how temperature and substrate concentration affect the rate at which yeast respires.
- Carry out and describe investigations with simple respirometers to find out how temperature affects the rate of respiration.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9700 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Respiration
- › State that energy is 'released' as electrons move along the chain and that reduced NAD/FAD release hydrogen atoms which then split into protons and electrons.
Where does glycolysis occur in eukaryotic cells?
Glycolysis occurs in the cytoplasm of eukaryotic cells. It's the first stage of both aerobic and anaerobic respiration, breaking down glucose into pyruvate.
Outline the process of glycolysis.
Glycolysis involves the phosphorylation of glucose, followed by the splitting of fructose 1,6-bisphosphate (6C) into two triose phosphate molecules (3C). These are then oxidized to pyruvate (3C), producing ATP and reduced NAD.
What is the role of coenzyme A in the link reaction?
Coenzyme A combines with the 2-carbon acetyl group formed from pyruvate, creating acetyl coenzyme A. This transports the acetyl group to the Krebs cycle.
Describe the key events of the Krebs cycle, including decarboxylation and dehydrogenation.
The Krebs cycle involves oxaloacetate (4C) accepting a 2C fragment from acetyl coenzyme A to form citrate (6C). Through a series of steps, citrate is converted back to oxaloacetate. Decarboxylation removes carbon dioxide, and dehydrogenation removes hydrogen (reducing NAD and FAD).
What is the role of NAD and FAD in respiration?
NAD and FAD are coenzymes that accept hydrogen atoms (and electrons) during glycolysis, the link reaction, and the Krebs cycle. They then transport these hydrogen atoms to the electron transport chain on the inner mitochondrial membrane.
Outline the process of oxidative phosphorylation.
Hydrogen atoms split into protons and electrons. Electrons release energy as they pass through the electron transport chain. This energy pumps protons across the inner mitochondrial membrane, creating a gradient. Protons then diffuse back through ATP synthase, driving ATP synthesis. Oxygen is the final electron acceptor, forming water.
Explain how the structure of the mitochondria relates to its function in respiration.
The highly folded inner membrane (cristae) increases surface area for electron transport and ATP synthesis. The matrix contains enzymes for the Krebs cycle and link reaction. The intermembrane space accumulates protons for chemiosmosis.
Describe ethanol fermentation in yeast cells under anaerobic conditions.
In ethanol fermentation, pyruvate is decarboxylated to acetaldehyde, which is then reduced by reduced NAD to ethanol, regenerating NAD+ so glycolysis can continue. This process allows yeast to produce ATP in the absence of oxygen.
Describe lactate fermentation in mammalian cells under anaerobic conditions.
In lactate fermentation, pyruvate is reduced by reduced NAD to lactate, regenerating NAD+ so glycolysis can continue. This process allows muscle cells to produce ATP in the absence of sufficient oxygen, but lactate accumulation causes muscle fatigue.
Explain why aerobic respiration yields significantly more energy than anaerobic respiration.
With oxygen as the final electron acceptor, pyruvate enters the link reaction and Krebs cycle, and the reduced NAD and FAD are reoxidised by the electron transport chain, so oxidative phosphorylation makes many ATP. Without oxygen these stages stop, glucose is only partly broken down (energy stays in ethanol or lactate), and only the 2 ATP from glycolysis are made.
Describe two adaptations of rice plants that allow them to survive in submerged conditions.
Rice plants develop aerenchyma in their roots, creating air spaces for oxygen diffusion to submerged tissues. They also undergo ethanol fermentation in roots to produce ATP in the absence of oxygen, and exhibit faster stem growth to reach above the water surface for gas exchange.
What is the role of DCPIP in investigations of respiration?
DCPIP is a redox indicator that acts as an electron acceptor. As it accepts electrons during respiration, it becomes reduced and changes color (
How does increasing temperature generally affect the rate of respiration in yeast, up to a certain point?
Increasing temperature generally increases the rate of respiration in yeast, as enzyme activity increases with temperature, leading to faster metabolic reactions. However, beyond an optimal temperature, enzymes denature and the rate decreases.
Describe how a simple respirometer can be used to measure the rate of respiration.
A respirometer measures the rate of oxygen consumption. The organism is placed in a sealed chamber, and the decrease in oxygen volume (often indicated by a change in fluid level in a connected manometer) is measured over time. This change reflects the rate of respiration.
Explain the purpose of using a control in an experiment investigating the effect of temperature on yeast respiration.
A control is set up at each temperature, identical except that the yeast is boiled (killed) or replaced with water. If the indicator does not change colour in the control, the change in the experimental tubes must be due to respiration by living yeast and not to temperature acting on the other reagents.
What is the role of methylene blue in investigating respiration?
Methylene blue functions as a redox indicator, accepting electrons during respiration and becoming colorless when reduced. The time taken for methylene blue to decolorize is inversely proportional to the rate of respiration.
Review the material
Read full revision notes on Respiration — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 12 — Energy and respiration
Respiration sits alongside these A-Level Biology decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Respiration deck
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