A-LevelBiologyEnergy and respiration9700/42 · May/June 2025 · Q110 Marks

Fig. 1.1 is a diagram of part of the inner membrane of a mitochondrion. [Figure 1.1]

(a)(i)The coenzymes NAD and FAD deliver hydrogen atoms to the electron transport chain (ETC). With reference to Fig. 1.1, state which of the four carriers receives hydrogen atoms from reduced FAD.[1]
(a)(ii)Hydrogen atoms that are delivered to the ETC by reduced NAD and reduced FAD split into protons and electrons. Energy is released as electrons pass along the ETC. Describe the events that occur as a result of this release of energy.[2]
(a)(iii)Describe how structure 5 in Fig. 1.1 is involved in oxidative phosphorylation.[3]
(b)Cyanide ions (CN¯) are highly toxic. Cyanide ions bind to carrier 4 in Fig. 1.1 and inactivate the carrier. Suggest and explain how the binding of cyanide ions to carrier 4 can have an effect on respiration.[4]

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Mark scheme

Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.

[1] (a)(i) 2;
[2] (a)(ii) pumps / actively transports, protons / hydrogen ions, into intermembrane space ; ref. to proton gradient;
[3] (a)(iii) any three from: 1 ATP synthase ; 2 protons / hydrogen ions, move (from intermembrane space) to matrix; 3 by facilitated diffusion ; 4 ATP synthesised from ADP + Pi ; A ADP is phosphorylated to ATP 5 ref. to chemiosmosis ;
[4] (b) any four from: 1 electron transport chain / ETC, stops / reduced ; 2 oxygen does not act as (final) electron acceptor ; 3 no / less, protons pumped into intermembrane space or no / less steep, proton gradient ; 4 build-up of, reduced NAD / reduced FAD or no / less, reduced NAD / reduced FAD, oxidised / recycled or no / less, NAD / FAD, regenerate / recycled ; 5 no / less, ATP production or no / fewer, protons pass through ATP synthase ; 6 Krebs cycle / link reaction / oxidative phosphorylation, stops / reduced ; 7 ref. to glycolysis / anaerobic respiration / lactate produced / ethanol produced / substrate level phosphorylation ;

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