Chapter 12: Respiration in Plants

In the figure : Respiration releases energy from glucose in small steps and stores it as ATP.

Rule 1: Why plants do not need a breathing system — Plants need less gas exchange than animals because each part takes in oxygen and gives out CO2 on its own. Cells are close to the surface, only a small part of the plant is living and active, and leaves also produce oxygen during photosynthesis. Gases move by simple diffusion through stomata in leaves, lenticels in woody stems, and the root surface.

• Easy : A potato tuber exchanges gas through small pores called lenticels.
• Medium : In a leaf, the oxygen made in photosynthesis is available for respiration in the day.
• Exam-type : Q: Name two plant structures that allow gas exchange. A: Stomata and lenticels.

Rule 2: Respiration and ATP — Energy in glucose is released in many small steps by enzymes so that it can be trapped in ATP, rather than lost as heat all at once. The cell uses ATP wherever energy is needed, which is why ATP is called the energy currency. The substrate is usually carbohydrate, but fats and proteins can also be used.

• Easy : Burning glucose in a flame gives all energy as heat; the cell releases it gradually.
• Medium : ATP made in respiration is used for active transport, growth and synthesis.
• Exam-type : Q: Why is ATP called the energy currency of the cell? A: Energy is stored in it and spent wherever needed, like money.

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In the figure : Glycolysis: glucose is phosphorylated, split into two 3-carbon sugars and changed to pyruvic acid.

Rule 1: What happens — Glucose is first phosphorylated using 2 ATP, then split into two 3-carbon sugars (the energy-investing phase). Each 3-carbon sugar is then oxidised: NAD+ is reduced to NADH + H+ and ATP is made directly by substrate-level phosphorylation. Each glucose gives 4 ATP in these steps.

• Easy : Two ATP are spent at the start and four are made later.
• Medium : Two 3-carbon sugars give 2 NADH from 2 NAD+.
• Exam-type : Q: Where does glycolysis occur? A: In the cytoplasm.

Rule 2: Net result — Net gain of 2 ATP (4 made minus 2 used), 2 NADH + H+ and 2 pyruvic acid per glucose. Pyruvic acid is the key product: its fate depends on the cell. It enters fermentation if oxygen is absent, or enters the mitochondrion for aerobic respiration if oxygen is present.

• Easy : Net ATP = 4 - 2 = 2.
• Medium : In low-oxygen soil, root cells send pyruvic acid to fermentation; with oxygen, to the mitochondrion.
• Exam-type : Q: What are the products of glycolysis per glucose? A: 2 pyruvic acid, 2 NADH and a net 2 ATP.

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In the figure : Pyruvic acid can be changed to ethanol and CO2 or to lactic acid when oxygen is absent.

Rule 1: Alcoholic fermentation — In yeast, pyruvic acid is first changed to acetaldehyde and CO2 by pyruvate decarboxylase; then acetaldehyde is reduced to ethanol by alcohol dehydrogenase, using NADH + H+ which becomes NAD+. The NAD+ allows glycolysis to continue. Yeast stops growing when alcohol reaches about 13 per cent.

• Easy : CO2 from yeast fermentation makes bread dough rise.
• Medium : NADH is changed back to NAD+, which glycolysis needs.
• Exam-type : Q: Which enzyme turns acetaldehyde to ethanol? A: Alcohol dehydrogenase.

Rule 2: Lactic acid fermentation — In some bacteria and in animal muscle cells during heavy exercise when oxygen runs short, pyruvic acid is reduced to lactic acid by lactate dehydrogenase. NADH is used and NAD+ is formed again. No CO2 is released.

• Easy : Curd formation is due to lactic acid bacteria.
• Medium : During a sprint, muscles produce lactic acid, which causes fatigue.
• Exam-type : Q: Is CO2 released in lactic acid fermentation? A: No.

Rule 3: Why it is less efficient — Fermentation gives only 2 ATP per glucose (from glycolysis), and only less than 7 per cent of the energy in glucose is released. The end products are still energy-rich and can be harmful in large amounts.

• Easy : Fermentation makes only 2 ATP per glucose, whereas aerobic respiration makes many more.
• Medium : Alcohol accumulation can kill yeast.
• Exam-type : Q: Why is fermentation inefficient? A: Glucose is only partly broken down.

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In the figure : Pyruvic acid is changed to acetyl CoA and then oxidised in the TCA cycle.

Rule 1: Link step — Pyruvic acid is oxidatively decarboxylated to acetyl CoA, with CO2 released and NAD+ reduced to NADH. This happens twice per glucose, producing 2 acetyl CoA, 2 CO2 and 2 NADH. It needs several coenzymes and Mg2+.

• Easy : One pyruvic acid gives one acetyl CoA, one CO2 and one NADH.
• Medium : Per glucose, 2 pyruvic acid give 2 NADH in the link step.
• Exam-type : Q: What is released when pyruvic acid forms acetyl CoA? A: CO2 and NADH.

Rule 2: TCA cycle — Acetyl CoA (2 C) joins oxaloacetic acid (4 C) to form citric acid (6 C). Through a series of steps citric acid loses two CO2 and is changed back to oxaloacetic acid, so the cycle can continue. Per turn: 3 NADH, 1 FADH2, 1 ATP (or GTP) and 2 CO2. Per glucose (two turns): 6 NADH, 2 FADH2, 2 ATP and 4 CO2.

• Easy : Two turns of the cycle are needed per glucose.
• Medium : 6 carbons enter as 2 acetyl groups (4 C) and complete oxidation of glucose releases 6 CO2 in all (2 in link step + 4 in TCA).
• Exam-type : Q: Where does the TCA cycle occur? A: In the mitochondrial matrix.

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In the figure : Electrons pass through carriers to oxygen, giving water and ATP.

Rule 1: How the ETS works — NADH is oxidised by complex I and FADH2 by complex II. Electrons pass through ubiquinone, complex III, cytochrome c and complex IV (cytochrome c oxidase) and finally reduce oxygen to water. Protons are pumped into the intermembrane space; they flow back through ATP synthase (complex V) to make ATP. One NADH gives 3 ATP and one FADH2 gives 2 ATP in this model. Oxygen is the final electron and hydrogen acceptor.

• Easy : Without oxygen, the chain stops because electrons have nowhere to go.
• Medium : NADH gives more ATP than FADH2 because it enters at complex I and so pumps more protons.
• Exam-type : Q: What is the final acceptor of electrons? A: Oxygen.

Rule 2: Respiratory balance sheet — Counting one glucose with the 3-and-2 model: glycolysis 2 ATP + 2 NADH (6); link step 2 NADH (6); TCA cycle 2 ATP + 6 NADH (18) + 2 FADH2 (4). Total = 2 + 6 + 6 + 2 + 18 + 4 = 38 ATP. This is a theoretical value; in a real cell intermediates are withdrawn and pathways overlap.

• Easy : Add up: substrate-level ATP = 2 + 2 = 4; from NADH and FADH2 = 34; total 38.
• Medium : Ten NADH x 3 = 30, two FADH2 x 2 = 4, plus 4 direct ATP = 38.
• Exam-type : Q: What is the theoretical net ATP yield from one glucose in aerobic respiration? A: 38.

Spot the difference

In the figure : Fats, proteins and carbohydrates can all enter the respiratory pathway.

Rule 1: Amphibolic nature — Fats are broken down into glycerol and fatty acids; fatty acids form acetyl CoA, and glycerol enters glycolysis. Proteins are broken down to amino acids that enter as pyruvic acid, acetyl CoA or TCA cycle intermediates. In the opposite direction, intermediates are used to make fatty acids, amino acids and other molecules when food is plentiful. So the pathway is both breakdown and synthesis.

• Easy : Fatty acids are changed to acetyl CoA before respiration.
• Medium : Excess carbohydrate can be changed to fat through acetyl CoA.
• Exam-type : Q: Why is the respiratory pathway called amphibolic? A: It is used for both breakdown and synthesis.

Rule 2: Respiratory quotient — RQ = volume of CO2 released / volume of O2 consumed. For carbohydrate, RQ = 1.0 (6 CO2 / 6 O2). For fats, RQ is about 0.7 (e.g. tripalmitin: 102 CO2 / 145 O2). For proteins, RQ is about 0.9. When respiration is anaerobic, RQ is more than 1 because CO2 is released but little or no O2 is used.

• Easy : RQ of glucose: 6 CO2 / 6 O2 = 1.
• Medium : If a seed shows RQ 0.7, it is probably respiring fat.
• Exam-type : Q: What does an RQ below 1 suggest? A: Fats (or proteins) are being respired.

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