Energy and respiration — AS & A Level Biology (9700) questions by topic
Describe, explain and identify are the command words to prepare for, applied to glycolysis, the Krebs cycle and electron transport chain reasoning: naming the first Krebs cycle intermediate, describing substrate-linked phosphorylation, and explaining why reduced NAD must be re-oxidised for glycolysis to continue. Describe wants the correct named process, such as substrate-linked phosphorylation, not oxidative phosphorylation used interchangeably.
With 18 real questions from 2023 to 2025 papers, marks run from 5 to 12, so most answers require several linked points rather than a single fact. Submitting respiration-pathway explanations for instant marking is useful here, since confusing reduced NAD with NADH, or ATP with ADP, is flagged immediately rather than repeated across a whole answer.
What examiners look for
- A frequent error misidentifies the first Krebs cycle intermediate, citrate, along with confusion naming coenzymes Q, R, T and U.
- Substrate-linked phosphorylation is sometimes named correctly but then contradicted by adding that it occurs during oxidative phosphorylation.
- ATP's small, water-soluble nature is often stated without explaining that this property allows it to move around the cell.
- Reduced NAD is frequently confused with NADH, or muddled with ATP and ADP, when describing respiration substances.
- Strong answers describe the transfer of a phosphate group to ADP with a correct equation, and explain that reduced pyruvate or ethanal regenerates NAD so glycolysis can continue.
May–June 2025, Paper 41
Name two energy-rich molecules that enter a respiration pathway to synthesise ATP.
Answer this question and get it marked →Fig. 2.2 shows a respirometer set up to measure the rate of respiration of woodlice. The apparatus includes NaOH pellets, a boiling tube, and a graduated capillary tube with a…
Answer this question and get it marked →May–June 2025, Paper 42
Fig. 1.1 shows a diagram of part of the inner membrane of a mitochondrion with five numbered structures. Structures 1, 2, 3 and 4 are electron carriers (ETC components). Structure…
Answer this question and get it marked →The grey seal, Halichoerus grypus, is an aquatic mammal that lives in the North Atlantic Ocean. It feeds on fish, which it hunts at depths of up to 70 metres. Diving to hunt for…
Answer this question and get it marked →May–June 2025, Paper 43
Fig. 1.1 shows how a liver cell obtains glucose and how pyruvate produced by glycolysis is used. Glucose enters via transporter A, glycolysis (labelled B) takes place in the…
Answer this question and get it marked →May–June 2025, Paper 44
A hummingbird feeds on nectar but also eats insects. Scientists measured the respiratory quotient (RQ) of a hummingbird every 10 minutes over 120 minutes. Define the term…
Answer this question and get it marked →May–June 2024, Paper 41
Oxygen is needed for aerobic respiration in cells. In eukaryotic cells, the mitochondrion is the organelle used for aerobic respiration. Fig. 1.1 shows a transmission electron…
Answer this question and get it marked →May–June 2024, Paper 42
Fig. 6.1 is a diagram of part of a mitochondrion, showing the outer mitochondrial membrane with proteins, the intermembrane space, and the inner mitochondrial membrane with…
Answer this question and get it marked →October–November 2024, Paper 42
DCPIP is an indicator and can be used to determine the rate of respiration of organisms such as yeast. State the category of indicators to which DCPIP belongs.
Answer this question and get it marked →October–November 2024, Paper 43
During aerobic respiration, cells respire substrates such as glucose to produce ATP. Some events that occur during aerobic respiration are: the respiratory substrate breaks down…
Answer this question and get it marked →February–March 2023, Paper 42
Fig. 6.1 is a diagram of a section through a mitochondrion. Four arrows A, B, C and D show the movement of molecules and ions. Use the letters to identify all the arrows (one or…
Answer this question and get it marked →May–June 2023, Paper 41
Farmers flood fields of rice because this encourages faster growth and higher yields. An adaptation of rice plants that allows them to grow in water is the development of…
Answer this question and get it marked →Fig. 7.1 shows the structures of five amino acids with varying numbers and arrangements of carbon atoms: asparagine (4 carbon atoms), leucine (6 carbon atoms), methionine (5…
Answer this question and get it marked →May–June 2023, Paper 42
Explain why glucose is phosphorylated at the beginning of glycolysis.
Answer this question and get it marked →May–June 2023, Paper 43
The Krebs cycle was named after the biochemist Sir Hans Krebs, who worked out the sequence in 1937. Fig. 1.1 is an outline of the Krebs cycle: acetyl coenzyme A (2C) combines with…
Answer this question and get it marked →October–November 2023, Paper 41
The respiratory quotient (RQ) values for different respiratory substrates can be calculated. Table 5.1 shows the formulae of four respiratory substrates and the number of oxygen…
Answer this question and get it marked →October–November 2023, Paper 42
Some organisms carry out respiration in anaerobic conditions when oxygen is not available or in low concentration. In yeast and some plants this is called ethanol fermentation; in…
Answer this question and get it marked →October–November 2023, Paper 43
The respiratory quotient (RQ) values for different respiratory substrates can be calculated. Table 5.1 shows the formulae of four respiratory substrates and the number of oxygen…
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