Energy and respiration

• Carbon leaves as CO2; hydrogen goes to NAD and FAD; protons are pumped matrix → intermembrane space and come back through ATP synthase; oxygen takes the electrons at the end. No oxygen, no chain — and only two ATP per glucose.

• 12.1.1 I can outline — outline the need for energy in living organisms, as illustrated by active transport, movement and anabolic reactions, such as those occurring in DNA replication and protein synthesis
• 12.1.2 I can describe — describe the features of ATP that make it suitable as the universal energy currency
• 12.1.3 I can state — state that ATP is synthesised by: • transfer of phosphate in substrate-linked reactions • chemiosmosis in membranes of mitochondria and chloroplasts
• 12.1.4 I can explain — explain the relative energy values of carbohydrates, lipids and proteins as respiratory substrates
• 12.1.5 I can state — state that the respiratory quotient (RQ) is the ratio of the number of molecules of carbon dioxide produced to the number of molecules of oxygen taken in, as a result of respiration
• 12.1.6 I can calculate — calculate RQ values of different respiratory substrates from equations for respiration
• 12.1.7 I can describe — describe and carry out investigations, using simple respirometers, to determine the RQ of germinating seeds or small invertebrates (e.g. blowfly larvae)
• 12.2.1 I can state — state where each of the four stages in aerobic respiration occurs in eukaryotic cells: • glycolysis in the cytoplasm • link reaction in the mitochondrial matrix • Krebs cycle in the mitochondrial matrix • oxidative phosphorylation on the inner membrane of mitochondria
• 12.2.2 I can outline — outline glycolysis as phosphorylation of glucose and the subsequent splitting of fructose 1,6-bisphosphate (6C) into two triose phosphate molecules (3C), which are then further oxidised to pyruvate (3C), with the production of ATP and reduced NAD
• 12.2.3 I can explain — explain that, when oxygen is available, pyruvate enters mitochondria to take part in the link reaction
• 12.2.4 I can describe — describe the link reaction, including the role of coenzyme A in the transfer of acetyl (2C) groups
• 12.2.5 I can outline — outline the Krebs cycle, explaining that oxaloacetate (4C) acts as an acceptor of the 2C fragment from acetyl coenzyme A to form citrate (6C), which is converted back to oxaloacetate in a series of small steps
• 12.2.6 I can explain — explain that reactions in the Krebs cycle involve decarboxylation and dehydrogenation and the reduction of the coenzymes NAD and FAD
• 12.2.7 I can describe — describe the role of NAD and FAD in transferring hydrogen to carriers in the inner mitochondrial membrane
• 12.2.8 I can explain — explain that during oxidative phosphorylation: • hydrogen atoms split into protons and energetic electrons • energetic electrons release energy as they pass through the electron transport chain (details of carriers are not expected) • the released energy is used to transfer protons across the inner mitochondrial membrane • protons return to the mitochondrial matrix by facilitated diffusion through ATP synthase, providing energy for ATP synthesis (details of ATP synthase are not expected) • oxygen acts as the final electron acceptor to form water
• 12.2.9 I can describe — describe the relationship between the structure and function of mitochondria using diagrams and electron micrographs
• 12.2.10 I can outline — outline respiration in anaerobic conditions in mammals (lactate fermentation) and in yeast cells (ethanol fermentation)
• 12.2.11 I can explain — explain why the energy yield from respiration in aerobic conditions is much greater than the energy yield from respiration in anaerobic conditions (a detailed account of the total yield of ATP from the aerobic respiration of glucose is not expected)
• 12.2.12 I can explain — explain how rice is adapted to grow with its roots submerged in water, limited to the development of aerenchyma in roots, ethanol fermentation in roots and faster growth of stems
• 12.2.13 I can describe — describe and carry out investigations using redox indicators, including DCPIP and methylene blue, to determine the effects of temperature and substrate concentration on the rate of respiration of yeast
• 12.2.14 I can describe — describe and carry out investigations using simple respirometers to determine the effect of temperature on the rate of respiration

Say where the energy comes from correctly. Energy is released when ATP is hydrolysed; the answer that credits a special bond with holding energy is not a description of hydrolysis, and ATP itself is a molecule, not energy. Write “hydrolysis of ATP releases energy”.

• “Protons are pumped into the matrix.” Correct Protons are pumped from the matrix into the intermembrane space, using energy released by electrons in the electron transport chain. They return into the matrix by facilitated diffusion through ATP synthase, and that return provides the energy for ATP synthesis.
• “Respiration makes energy” / “ATP is energy.” Correct Energy cannot be made. Respiration releases energy from a substrate and transfers it to ATP; energy is released when ATP is hydrolysed to ADP and Pi.
• “Glycolysis happens in the mitochondria.” Correct Glycolysis is in the cytoplasm. The link reaction and Krebs cycle are in the matrix; oxidative phosphorylation is on the inner mitochondrial membrane.
• “Glycolysis produces 4 ATP.” Correct It makes 4 but uses 2 in phosphorylating glucose: a net gain of 2 ATP per glucose, with 2 reduced NAD and 2 pyruvate.
• “Oxygen is used to make energy.” Correct Oxygen is the final electron acceptor: it combines with electrons and protons to form water, which keeps electrons flowing along the chain.
• “Lipids release more energy because they are bigger molecules.” Correct Energy value is per gram, so size is irrelevant. Lipids carry more hydrogen atoms per gram, so more reduced NAD and FAD, more protons pumped and more ATP.
• “RQ = O2 taken in ÷ CO2 produced.” Correct Upside down. RQ = CO2 produced ÷ O2 taken in, with no unit.
• “Respiration produces energy.” Repair Respiration releases energy from a substrate and transfers it to ATP.
• “ATP is an energy store.” Repair ATP is an immediate source, recycled within seconds from a small pool. Glycogen, starch and lipid are stores.
• “Glycolysis happens in the mitochondria.” Repair In the cytoplasm. Link reaction and Krebs cycle: matrix. Oxidative phosphorylation: inner membrane.
• “Glycolysis gains 4 ATP.” Repair 4 made, 2 used: a net gain of 2.
• “The Krebs cycle makes most of the ATP.” Repair One ATP per turn. Its reduced NAD and reduced FAD drive oxidative phosphorylation, which makes most.
• “Oxaloacetate combines with pyruvate to make citrate.” Repair With the acetyl (2C) group of acetyl coenzyme A: 4C + 2C = 6C. Pyruvate is 3C and never enters the cycle.
• “Coenzyme A is used up in the link reaction.” Repair CoA carries the acetyl group to oxaloacetate and is released to be reused.
• “The electrons are pumped into the intermembrane space.” Repair Protons are pumped, from the matrix into the intermembrane space; electrons pass along the chain in the membrane to oxygen.
• “ATP synthase pumps protons to make ATP.” Repair Protons diffuse back through ATP synthase, down their gradient, by facilitated diffusion; that flow provides the energy for ATP synthesis. The pumping is done with energy from the electron transport chain.
• “Oxygen is needed to make energy.” Repair Oxygen is the final electron acceptor, forming water and keeping the chain running.
• “Lactate fermentation makes ATP.” Repair It regenerates NAD so that glycolysis (net 2 ATP) can continue. The conversion of pyruvate to lactate makes no ATP.
• “RQ = O2 taken in ÷ CO2 produced.” Repair CO2 produced ÷ O2 taken in, no unit.
• “With soda-lime, the fluid moves because CO2 is made.” Repair Soda-lime absorbs CO2, so the movement measures O2 taken in.
• “Shake the methylene blue tube so it mixes.” Repair Shaking lets oxygen re-oxidise the dye to blue. Invert once, then leave it still.
• “Rice roots have aerenchyma so they can photosynthesise under water.” Repair Aerenchyma carries oxygen from the shoot to the roots by diffusion, so root cells can respire aerobically. Roots do not photosynthesise.

• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. In the syllabus’s own meanings: state is to express in clear terms; give is to produce an answer from a given source or from memory; identify is to name, select or recognise. Define is to give a precise meaning. Outline is to set out the main points; describe is to state the points of a topic, or give its characteristics and main features. Explain is to set out purposes or reasons, make the relationships between things clear, and say why and/or how, supported with relevant evidence. Compare is to identify or comment on similarities and/or differences; contrast is to identify or comment on differences. Discuss is to write about issues or topics in depth in a structured way; assess is to make an informed judgement. Suggest is to apply knowledge and understanding to situations where there is a range of valid responses, making proposals or putting forward considerations.
• Interleave with the chapters that use this one. Topic 13 (photosynthesis) reuses chemiosmosis in the thylakoid membranes: when you reach it, re-answer “where are protons pumped, and where do they return?” for both organelles side by side. Topic 15 (control and coordination) spends ATP in the sodium–potassium pump and in muscle contraction: re-answer 12.1.1. Topic 14 (homeostasis) breaks down glycogen to glucose that feeds glycolysis: re-answer 12.2.2. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Biology 9700 has five components. Topic 12 is A Level content, so it is examined in Papers 4 and 5. A Level content: examined in Paper 4 (A Level structured, which also requires the AS content) and, as practical context, Paper 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E. There is no data booklet in Biology. At A Level, the statistical formulae (Hardy–Weinberg, the Lincoln index, Simpson’s index, standard deviation, standard error, 95% confidence intervals, the χ² test, the t-test, and Pearson’s and Spearman’s correlation) are printed in a question when it needs them, and so are the tables of critical values; degrees of freedom you must work out yourself. Everything else — magnification, surface area to volume, RQ, Rf, rates — you must recall, and this chapter says which is which.
• There is no multiple-choice paper at A Level, so every Paper 4 mark here is written. A structured question may ask you to state the site of a stage, outline glycolysis or the Krebs cycle with the carbon numbers, describe the link reaction or the role of NAD and FAD, explain oxidative phosphorylation in its five points or why aerobic respiration yields far more ATP, and calculate an RQ from an equation.
• An electron micrograph of a mitochondrion to label and relate to function; a pathway diagram with compounds or products to identify; a balanced equation to turn into an RQ; respirometer distances to turn into volumes (V = πr2l, recalled), rates and an RQ; energy values of substrates, supplied, to explain.
• Plan a respirometer investigation (temperature as the independent variable, O2 uptake as the dependent, a glass-bead control, soda-lime as a medium risk) or a yeast investigation with DCPIP or methylene blue (time to decolourise, rate = 1/time, a boiled-yeast control, no shaking). Then analyse: a t-test on two sets of rates, with the formula and critical values printed in the question.
• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. In the syllabus’s own meanings: state is to express in clear terms; give is to produce an answer from a given source or from memory; identify is to name, select or recognise. Define is to give a precise meaning. Outline is to set out the main points; describe is to state the points of a topic, or give its characteristics and main features. Explain is to set out purposes or reasons, make the relationships between things clear, and say why and/or how, supported with relevant evidence. Compare is to identify or comment on similarities and/or differences; contrast is to identify or comment on differences. Discuss is to write about issues or topics in depth in a structured way; assess is to make an informed judgement. Suggest is to apply knowledge and understanding to situations where there is a range of valid responses, making proposals or putting forward considerations.