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) oxaloacetate (4C) S R P (6C) Q Q U ATP ADP T R R 5C compound Q S Fig. 1.1 Identify substances P–U.

Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

1. [1] P - citrate A citric acid
2. [1] Q - NAD/NAD+
3. [1] R - reduced NAD / NADH A NADH2
4. [1] S - carbon dioxide / CO2
5. [1] T - FAD
6. [1] U - reduced FAD / FADH2 ;;;
7. 6 correct = 3 marks
8. 5/4 correct = 2 marks
9. 3/2 correct = 1 mark
10. any two from:
11. [1] transfer of phosphate group to ADP / ADP phosphorylated / ADP + Pi → ATP ;
12. [1] substrate-linked phosphorylation ; A substrate-level phosphorylation R if oxidative phosphorylation
13. [1] enzyme (catalysed reaction) ;
14. any four from:
15. [1] small/water-soluble, so can move around cell ;
16. [1] loss of phosphate / hydrolysis, leads to energy release ;
17. [1] (release energy) immediately / in small packets or ref. 30.5 kJ (mol-1) ;
18. [1] can be, recycled / regenerated or ATP ⇌ ADP + Pi ;
19. [1] link between energy-yielding and energy-requiring reactions / AW ;
20. [1] high turnover / described ;
21. [1] ref to ATPase ;