Respiration in Plants | ISC Class 11 Biology Notes

This note covers energy release from food, gas exchange in plants, respiration with and without oxygen, sugar breakdown, mitochondrial reactions, energy accounting, links between breakdown and synthesis, and the ratio between gases released and consumed during respiration.

What is cellular respiration and why do plants need it?

Cellular respiration is the oxidation of complex organic compounds within cells, breaking carbon-carbon bonds and releasing energy. Oxidation here involves removal of hydrogen or electrons from respiratory substances. The compounds oxidised during respiration are called respiratory substrates.

Plants need energy for activities such as absorption, transport, movement and reproduction. Photosynthesis, the conversion of light energy into chemical energy stored in food, supplies carbohydrates such as glucose, sucrose and starch. Respiration makes energy from food available for cellular work.

How is released energy made usable?

Adenosine triphosphate (ATP) is the cell's energy currency. Energy released during respiration is used to synthesise ATP. ATP is then broken down wherever and whenever energy is required. The energy in a respiratory substrate is not released freely in one single reaction.

Instead, enzymes, biological catalysts that speed reactions, control a series of slow, stepwise reactions. Some steps release enough energy to drive ATP synthesis. This arrangement enables cells to capture chemical energy rather than lose all the liberated energy as heat.

Definition: Respiratory substrates are compounds oxidised during cellular respiration. Usually carbohydrates supply respiratory energy, but proteins, fats and even organic acids can serve as substrates in some plants, under certain conditions.

Glucose is the favoured respiratory substrate. Carbon skeletons, the carbon frameworks left or produced during respiratory reactions, also supply starting materials for the synthesis of other cellular molecules. Thus, respiration contributes both usable energy and materials for building cellular substances.

Why do green plants still require respiration?

Chloroplasts are the cell structures in which photosynthesis occurs. Only cells containing chloroplasts, most often located in the superficial layers, carry out photosynthesis. Non-green cells, tissues and organs also need food for oxidation, so food must be transported to them.

Making food therefore does not remove the need to release its energy. In eukaryotic cells, cells with a membrane-bound nucleus, respiratory breakdown occurs in the cytoplasm and mitochondria. The cytoplasm is the cellular region outside the nucleus; mitochondria are organelles involved in aerobic energy release, meaning energy release through respiration in the presence of oxygen.

How do plants exchange gases without specialised respiratory organs?

Plants use oxygen, written O₂, in aerobic respiration and release carbon dioxide, written CO₂. They lack specialised organs for gaseous exchange. Stomata, pores involved in gas exchange, and lenticels, openings in woody stems, provide routes between internal tissues and the surrounding air.

Why can diffusion meet their needs?

Diffusion is the net movement of particles from a region of higher concentration to one of lower concentration. Several features of plants allow gases to reach living cells without an elaborate respiratory organ system.

1. Each plant part meets its own gas-exchange needs. There is very little transport of gases from one plant part to another.
2. Roots, stems and leaves respire at rates far lower than animals do, so their demands for gas exchange are relatively low.
3. The distance that gases must diffuse is not great, even in large plants. Living cells are located quite close to the plant surface.
4. Loosely packed parenchyma cells, living cells of simple plant tissue, provide an interconnected network of air spaces in leaves, stems and roots.

In woody stems, living cells form thin layers inside and beneath the bark. Interior dead cells provide mechanical support. Consequently, most plant cells have at least part of their surface in contact with air, assisted by the connected spaces between cells.

How does photosynthesis affect gas availability?

Large volumes of gases are exchanged during photosynthesis, and each leaf is adapted to meet its own needs during these periods. Oxygen is released within photosynthesising cells, so its availability is not a problem in those cells while photosynthesis occurs.

Gas exchange and cellular respiration should be distinguished. Gas exchange moves gases between cells and their surroundings. Cellular respiration consists of the chemical reactions that release energy from substrates. The absence of lungs or other specialised respiratory organs does not imply the absence of respiration.

How do aerobic respiration and fermentation differ?

Aerobic respiration completely oxidises organic substances in the presence of oxygen, releasing carbon dioxide, water and a large amount of substrate energy. Water is written H₂O. Anaerobic means occurring without oxygen; fermentation is an anaerobic route involving incomplete glucose oxidation.

Glycolysis is the partial oxidation of glucose to two molecules of pyruvic acid, also called pyruvate. It occurs in the cytoplasm and does not require oxygen. Pyruvate is the three-carbon product whose subsequent fate depends on the organism, oxygen availability and cellular needs.

Fermentation processes pyruvate under anaerobic conditions, producing ethanol and carbon dioxide or lactic acid. Glycolysis therefore precedes both fermentation and the mitochondrial reactions of aerobic respiration. It must not be treated as a pathway restricted to cells without oxygen.

Why is stepwise oxidation different from combustion?

Complete combustion of glucose releases energy, most of which escapes as heat. Respiration also oxidises glucose, but distributes energy release across controlled reactions. This allows part of the energy to be trapped in ATP instead of all being lost as heat.

The overall glucose equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. Here C₆H₁₂O₆ denotes glucose; C, H and O denote carbon, hydrogen and oxygen. Subscripts give the numbers of these atoms in each molecule. The arrow means “yields”, the plus sign joins reactants or products, and coefficients give relative molecular amounts.

How does glycolysis convert glucose into pyruvate?

Glycolysis consists of ten enzyme-controlled reactions in the cytoplasm. It is also called the EMP pathway, after Gustav Embden, Otto Meyerhof and J. Parnas. One six-carbon glucose molecule forms two three-carbon pyruvate molecules through an initial energy-investment stage followed by energy-yielding reactions.

In plants, glucose can come from sucrose or stored carbohydrates. Invertase, the enzyme that splits sucrose, produces glucose and fructose. These simple sugars enter the glycolytic pathway. The sequence below follows glucose and tracks the important energy changes.

Which symbols are used to track energy transfer?

ADP means adenosine diphosphate, the molecule converted to ATP by addition of phosphate. Phosphorylation means adding a phosphate group. Inorganic phosphate, phosphate available separately from an organic molecule, is written Pi.

NAD⁺ is the oxidised form of nicotinamide adenine dinucleotide, an electron-carrying coenzyme. A coenzyme is an organic helper in an enzyme reaction. NADH is its reduced, electron-carrying form; reduction means gaining electrons. H⁺ denotes a hydrogen ion or proton.

What is the reaction sequence?

1. Initial phosphorylation: glucose becomes glucose-6-phosphate using ATP. Hexokinase, the enzyme catalysing this phosphorylation, starts the sequence. Glucose-6-phosphate changes into fructose-6-phosphate, a molecule with a rearranged structure.
2. Second energy investment: fructose-6-phosphate receives another phosphate using ATP and forms fructose-1,6-bisphosphate. Thus, two ATP molecules have been used before the six-carbon compound splits.
3. Formation of three-carbon compounds: fructose-1,6-bisphosphate splits into dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. The latter is also called 3-phosphoglyceraldehyde, abbreviated PGAL. These are triose phosphates, meaning phosphorylated three-carbon sugars, and can interconvert.
4. Oxidation and carrier reduction: PGAL is oxidised and combines with inorganic phosphate to form 1,3-bisphosphoglycerate, abbreviated BPGA. At this step NAD⁺ is reduced to NADH + H⁺. The subsequent reactions occur twice for each original glucose molecule.
5. First ATP-producing step: BPGA becomes 3-phosphoglyceric acid, abbreviated PGA, and ATP is formed from ADP. Because two three-carbon molecules are processed, this step produces two ATP molecules per glucose.
6. Formation of pyruvate: PGA passes through 2-phosphoglycerate and phosphoenolpyruvate, abbreviated PEP. Water is released in PEP formation. PEP then becomes pyruvate, producing another two ATP molecules per original glucose molecule.

A downward sequence begins with glucose and ends with two pyruvic acid molecules. The six-carbon compound branches into two triose phosphates. Side arrows mark ATP use, ATP formation, NADH + H⁺ formation and water release.

See Fig. 12.1 in your NCERT textbook

What is the net result?

Glycolysis directly produces four ATP molecules but consumes two. Subtracting the investment gives 4 − 2 = 2; − means subtraction and = means equality. Its net gain is two ATP per glucose, together with two pyruvate molecules and two NADH molecules. Net gain means production after subtracting the ATP invested in the pathway.

The formation of reduced carriers and the direct formation of ATP are different events. NADH retains energy that can later contribute to ATP synthesis under aerobic conditions. It should not be counted as though it were itself an ATP molecule.

What happens to pyruvate during fermentation?

Fermentation occurs under anaerobic conditions in many organisms. It allows pyruvate from glycolysis to be converted into organic end products while NADH is reoxidised to NAD⁺. Reoxidation restores the electron carrier to the form used during glycolysis.

What are the two major routes?

In alcoholic fermentation, yeast converts pyruvate into ethanol and carbon dioxide. Pyruvic acid decarboxylase and alcohol dehydrogenase are the enzymes catalysing this route. Decarboxylation means removal of carbon dioxide from an organic compound.

In lactic acid fermentation, some bacteria form lactic acid from pyruvate. Animal muscle cells can also reduce pyruvate to lactic acid during exercise when oxygen is inadequate for cellular respiration. Lactate dehydrogenase is the enzyme catalysing this conversion.

Both routes use NADH + H⁺ as the reducing agent, meaning the substance that supplies reducing power. NADH is converted back to NAD⁺. The products differ, but both routes follow glycolysis and leave much of the original glucose energy in organic compounds.

The diagram traces glucose through glycolytic intermediates to pyruvic acid. Arrows then lead towards lactic acid or ethanol plus carbon dioxide. NADH + H⁺ and NAD⁺ labels show changes in the carrier along the pathways.

See Fig. 12.2 in your NCERT textbook

Why is fermentation a limited source of energy?

Less than seven per cent of the energy in glucose is released during alcohol or lactic acid fermentation, and not all of that energy is trapped in ATP. The net ATP gain remains two molecules per glucose, produced during glycolysis.

The products can also be harmful. Yeasts poison themselves to death when alcohol concentration reaches about 13 per cent. This limitation accompanies the low energy yield: incomplete oxidation leaves substantial energy in the products, while their accumulation can damage the organisms producing them.

How do the link reaction and Krebs' cycle oxidise pyruvate?

For aerobic respiration in eukaryotes, pyruvate moves from the cytoplasm into the mitochondria. The mitochondrial matrix is the internal compartment enclosed by the inner mitochondrial membrane. Pyruvate oxidation and the cyclic reactions that follow take place in this matrix.

How does pyruvate enter the cycle?

Pyruvate undergoes oxidative decarboxylation, oxidation accompanied by carbon dioxide removal. The pyruvate dehydrogenase enzyme complex catalyses this conversion. Coenzyme A, abbreviated CoA, carries the resulting two-carbon acetyl group, forming acetyl CoA.

This conversion, often called the link reaction, connects glycolysis to the cycle. Each pyruvate gives one acetyl CoA, one carbon dioxide and one NADH. Therefore, the two pyruvate molecules from one glucose produce two NADH molecules before the cycle itself begins.

The tricarboxylic acid cycle (TCA cycle) is also called the citric acid cycle or Krebs' cycle. Acetyl CoA enters this cyclic pathway. Oxaloacetic acid (OAA), a four-carbon compound that accepts the acetyl group, is regenerated as the cycle proceeds.

What are the principal steps?

1. Citric acid formation: the two-carbon acetyl group combines with four-carbon OAA and water to form six-carbon citric acid. Citrate synthase catalyses the reaction, and CoA is released.
2. Rearrangement: citrate, the form of citric acid in the pathway, is rearranged to isocitrate. The cycle then proceeds through reactions that remove carbon dioxide.
3. Successive decarboxylations: the pathway forms five-carbon α-ketoglutaric acid and then succinyl-CoA. The symbol α, pronounced alpha, is part of the compound's name. Carbon dioxide is released at the two decarboxylation steps.
4. Direct energy capture: conversion of succinyl-CoA to succinic acid forms GTP, guanosine triphosphate. GTP becomes GDP, guanosine diphosphate, in a coupled reaction that forms ATP from ADP.
5. Regeneration: the remaining reactions return the pathway through compounds including malic acid to OAA. Regeneration of this initial acceptor allows another acetyl group to enter the cycle.

Direct ATP or GTP formation linked to a substrate reaction is called substrate-level phosphorylation. This differs from ATP production associated with electron transport. It accounts for only a small part of the energy captured during aerobic respiration.

At three points per turn, NAD⁺ is reduced to NADH + H⁺. At one point, flavin adenine dinucleotide, abbreviated FAD, another electron-carrying coenzyme, is reduced to FADH₂. One turn therefore forms three NADH, one FADH₂ and one ATP through the coupled conversion of GTP.

Pyruvate is shown above the cycle leading to acetyl coenzyme A. The circular pathway labels citric, α-ketoglutaric, succinic, malic and oxaloacetic acids. Side arrows show carbon dioxide release and formation of reduced carriers and GTP.

See Fig. 12.3 in your NCERT textbook

How does the cycle compare with glycolysis?

Two acetyl groups enter for each glucose molecule. Keep the link reaction separate when counting products: its two NADH molecules are additional to the six NADH formed during the two turns of the cycle.

How does the electron transport system support ATP formation?

The electron transport system (ETS) is a series of carriers that transfers electrons to oxygen. It lies in the inner mitochondrial membrane. NADH and FADH₂ donate electrons and return to their oxidised forms, making the carriers available again for earlier respiratory reactions.

What is the flow of electrons?

The numbered complexes are groups of respiratory proteins involved in electron transfer. Ubiquinone is a carrier within the membrane; its reduced form is ubiquinol. Cytochrome c is a small mobile protein carrier that transfers electrons between complexes III and IV.

Draw NADH → complex I → ubiquinone, and FADH₂ → complex II → ubiquinone. Join these routes as ubiquinol → complex III → cytochrome c → complex IV → oxygen. Show water as the product of oxygen reduction.

In the flowchart, I, II, III and IV are Roman numerals identifying complexes one to four. Complex I is NADH dehydrogenase; complex III is the cytochrome bc₁ complex; complex IV is cytochrome c oxidase. The letters and subscripts identify the named cytochrome components.

What is oxidative phosphorylation?

Definition: Oxidative phosphorylation is ATP synthesis from ADP and inorganic phosphate using energy released through oxidation-reduction reactions in respiratory electron transport.

ATP synthase, also called complex V or complex five, is the enzyme complex that synthesises ATP using the energy made available through this system. Oxygen acts at the terminal stage as the final electron acceptor and is reduced to water.

Although oxygen acts at the end, its presence is vital to the whole aerobic process. Removal of electrons through this system enables continued oxidation of reduced carriers. The energy accounting assigns three ATP to oxidation of one NADH and two ATP to oxidation of one FADH₂.

How is the theoretical respiratory ATP balance calculated?

A respiratory balance sheet counts ATP gained during oxidation of one glucose molecule. The calculation includes ATP formed directly and ATP attributed to oxidation of NADH and FADH₂. It is a theoretical exercise whose result depends on specified assumptions.

What contributes to the total?

Use three ATP per NADH and two ATP per FADH₂ in this accounting; × means multiplication. An ATP equivalent includes ATP supplied through coupled conversion of GTP. The table separates the link reaction from the TCA cycle so that the same reduced carriers are not counted twice. Glycolysis contributes its net direct ATP gain.

The direct ATP column includes ATP supplied through the coupled conversion of GTP. Carrier oxidation is accounted for separately, then added to the direct gain. This separation distinguishes ATP already produced during substrate reactions from ATP attributed to the electron transport system.

Which assumptions are needed?

1. The pathways operate in a sequential, orderly manner, with glycolysis followed by the TCA cycle and then the ETS.
2. NADH formed during glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.
3. No intermediate is withdrawn from the pathway to synthesise another compound. An intermediate is a substance formed and then used within a reaction sequence.
4. Glucose is the only substrate respired, with no alternative substrate entering at any intermediate stage.

Note: There can be a net gain of 38 ATP molecules during aerobic respiration of one glucose under these assumptions. This is not an unconditional statement of the yield in a living cell.

In living systems, pathways operate simultaneously. Substances enter and leave according to need, ATP is used when required, and enzyme activity is controlled in several ways. These conditions prevent the theoretical assumptions from accurately describing every actual respiratory event.

The balance sheet is useful for understanding where energy is captured. It also shows why fermentation yields much less ATP: fermentation retains only the net two ATP from glycolysis and does not completely oxidise the organic products.

Why is respiration described as an amphibolic pathway?

Catabolism means breakdown of substances within an organism, while anabolism means synthesis of substances. An amphibolic pathway participates in both. Respiration fits this description because its intermediates connect energy-releasing breakdown with the synthesis of cellular materials.

Where do other substrates enter?

Glucose is the favoured substrate, and carbohydrates are usually first converted into glucose. Other respiratory substrates enter at different points. Their entry depends on the products formed when those substances are broken down.

• Fats: breakdown yields glycerol and fatty acids, the components from which fats are built. Fatty acids are degraded to acetyl CoA before entering the respiratory pathway.
• Glycerol: this component of fats enters after conversion into PGAL, linking it with glycolysis rather than requiring entry as glucose.
• Proteins: proteases, protein-breaking enzymes, release amino acids, the building units of proteins. After deamination, meaning removal of an amino group, they enter as pyruvate, acetyl CoA or a Krebs' cycle intermediate, depending on their structure.

How are the same pathways used for synthesis?

When fatty acids are used as respiratory substrates, they supply acetyl CoA. When the organism needs to synthesise fatty acids, acetyl CoA can instead be withdrawn from the respiratory pathway. The same connection therefore serves opposite metabolic needs.

Respiratory intermediates similarly link protein breakdown and synthesis. Calling respiration purely catabolic overlooks these withdrawals for building new substances. Its amphibolic role also explains one limitation of the ATP balance: an intermediate used for synthesis is unavailable for uninterrupted complete oxidation.

This relationship does not mean every respiratory reaction is itself a synthesis reaction. It means the pathway as a whole is connected to both breakdown and synthesis, with entry and withdrawal of compounds according to the needs of the organism.

What does the respiratory quotient tell us about substrates?

The respiratory quotient (RQ), also called the respiratory ratio, is the volume of carbon dioxide evolved divided by the volume of oxygen consumed during respiration. It depends on the type of respiratory substrate being used.

Definition: RQ = volume of CO₂ evolved ÷ volume of O₂ consumed. The symbol ÷ means division. Use the same volume unit for both gases; the units cancel, leaving a ratio without a unit.

How do the substrate values compare?

For complete glucose oxidation, the equation gives six molecules of carbon dioxide for six molecules of oxygen used. The corresponding gas-volume ratio is 6 ÷ 6 = 1.0. Complete oxidation is part of the condition under which the carbohydrate value applies.

For tripalmitin, the equation is 2(C₅₁H₉₈O₆) + 145O₂ → 102CO₂ + 98H₂O + energy. Here C₅₁H₉₈O₆ is the molecular formula of tripalmitin. Its RQ is given as 102 ÷ 145 = 0.7, using the rounded value.

Why must the values be interpreted carefully?

Respiratory substrates in living organisms are often more than one. Pure proteins or fats are never used as respiratory substrates. Consequently, the characteristic values describe substrate relationships, while respiration within an organism can involve a mixture of substances.

Do not invert the fraction: carbon dioxide belongs in the numerator, the upper quantity, and oxygen in the denominator, the lower quantity. Do not replace “about 0.9” for proteins with an exact claim, or omit complete oxidation when giving the carbohydrate value.

Glossary

• Cellular respiration — Oxidation of organic substances within cells, releasing energy that can be trapped in ATP.
• Respiratory substrate — A compound oxidised during respiration to supply energy for cellular activities.
• ATP — Adenosine triphosphate, the energy currency synthesised using energy released by respiratory reactions.
• Glycolysis — The cytoplasmic sequence that partially oxidises one glucose into two pyruvate molecules.
• Fermentation — Anaerobic processing of pyruvate with incomplete oxidation and regeneration of oxidised electron carriers.
• Oxidative decarboxylation — Oxidation accompanied by carbon dioxide removal, linking pyruvate to acetyl CoA formation.
• Acetyl CoA — A two-carbon acetyl group carried by coenzyme A into the citric acid cycle.
• TCA cycle — A cyclic pathway oxidising acetyl groups while regenerating oxaloacetic acid in the mitochondrial matrix.
• Electron transport system — A series of inner mitochondrial membrane carriers transferring electrons ultimately to oxygen.
• Oxidative phosphorylation — ATP synthesis using energy released through oxidation-reduction reactions during respiratory electron transport.
• Substrate-level phosphorylation — Direct formation of ATP or GTP linked to a substrate reaction in metabolism.
• Catabolism — Breakdown of substances within living organisms, including respiratory breakdown of organic substrates.
• Anabolism — Synthesis of substances within organisms, using materials that can include respiratory intermediates.
• Amphibolic pathway — A pathway involved in both breakdown and synthesis of substances within organisms.
• Respiratory quotient — The ratio of carbon dioxide volume evolved to oxygen volume consumed during respiration.

Common errors and misconceptions

• Misconception: Green plants do not respire because they photosynthesise. Correct: Plants require respiratory energy, and non-green tissues also need food transported to them.
• Misconception: Glycolysis requires oxygen and occurs inside mitochondria. Correct: Glycolysis occurs in the cytoplasm and can proceed without oxygen.
• Misconception: Four ATP formed in glycolysis means four ATP gained. Correct: Two ATP are invested, leaving a net gain of two.
• Misconception: The link reaction and Krebs' cycle are the same step. Correct: Pyruvate first forms acetyl CoA, which then enters the cycle.
• Misconception: Oxygen is directly consumed during glycolysis. Correct: Oxygen accepts electrons at the terminal stage of respiratory electron transport and is reduced to water.
• Misconception: Every living cell obtains exactly 38 ATP per glucose. Correct: This theoretical value depends on assumptions that do not fully describe living systems.
• Misconception: Respiration is solely a breakdown pathway. Correct: Respiratory intermediates also supply synthesis, making the pathway amphibolic.
• Misconception: RQ is oxygen consumed divided by carbon dioxide released. Correct: The ratio is carbon dioxide evolved divided by oxygen consumed; protein RQ is about 0.9.

Exam-style questions with model answers

Q1. Define a respiratory substrate and name the favoured respiratory substrate. [2 marks]

1. A respiratory substrate is a compound oxidised within cells during respiration to release energy.
2. Glucose is the favoured respiratory substrate; carbohydrates are usually used to supply respiratory energy.

Q2. Explain three reasons why plants can meet their gas-exchange needs without specialised respiratory organs. [3 marks]

1. Each plant part meets its own gas-exchange needs, so there is very little transport of gases between different parts.
2. Roots, stems and leaves respire at rates far lower than animals do, producing relatively low demands for gas exchange.
3. Diffusion distances are short, and interconnected air spaces formed by loosely packed cells help bring air close to living cells.

Q3. In glycolysis, one glucose forms two three-carbon intermediates. Two ATP are invested; each intermediate produces one ATP at each of two later steps. Calculate gross ATP production and net ATP gain, and state the number of pyruvate molecules formed. Give four points showing the reasoning. [4 marks]

1. One glucose supplies two three-carbon intermediates, so each of the later ATP-producing steps must be counted twice.
2. There are two ATP-producing steps per intermediate. Gross ATP production is therefore 2 × 2 = 4 ATP molecules.
3. Two ATP molecules were invested earlier, so the net ATP gain is 4 − 2 = 2 ATP molecules.
4. The two three-carbon intermediates finish as two pyruvate molecules, the carbon products of glycolysis for each glucose molecule.

Q4. Compare fermentation and aerobic respiration in five points: oxygen requirement, extent of breakdown, end products, ATP gain and reoxidation of NADH. [5 marks]

1. Fermentation operates under anaerobic conditions. Aerobic respiration requires oxygen, which serves at the terminal stage of the electron transport system.
2. Fermentation involves only partial breakdown of glucose. Aerobic respiration leads to complete oxidation of the organic substrate through its respiratory reactions.
3. Alcoholic fermentation produces ethanol and carbon dioxide, while lactic acid fermentation produces lactic acid. Aerobic respiration produces carbon dioxide and water.
4. Fermentation yields a net two ATP per glucose through glycolysis. Aerobic respiration yields many more ATP molecules by using reduced carriers in oxidative phosphorylation.
5. NADH is reoxidised to NAD⁺ relatively slowly in fermentation. Its oxidation is very vigorous during aerobic respiration through the electron transport system.

Q5. Describe Krebs' cycle in five points, covering acetyl entry, carbon dioxide release, direct energy capture, reduced carriers and regeneration of the acceptor. [5 marks]

1. The two-carbon acetyl group of acetyl CoA combines with four-carbon oxaloacetic acid and water to form six-carbon citric acid. CoA is released.
2. Citrate becomes isocitrate. Two successive decarboxylations lead through α-ketoglutaric acid to succinyl-CoA, releasing two carbon dioxide molecules during each turn.
3. Conversion of succinyl-CoA to succinic acid produces GTP by substrate-level phosphorylation. Its conversion to GDP is coupled to formation of ATP from ADP.
4. Each cycle turn reduces NAD⁺ at three points, forming three NADH, and reduces FAD at one point, forming one FADH₂.
5. The remaining reactions regenerate oxaloacetic acid, allowing the cycle to accept another acetyl group. Continued operation also requires regeneration of the oxidised carriers.

Q6. State four assumptions used to calculate the theoretical ATP balance for complete respiration of one glucose molecule. [4 marks]

1. The pathways function sequentially and in order, with glycolysis followed by the TCA cycle and then electron transport.
2. NADH produced during glycolysis is transferred into the mitochondria, where it undergoes oxidative phosphorylation to contribute to ATP synthesis.
3. None of the intermediate substances formed in the respiratory pathway is withdrawn to synthesise another compound.
4. Glucose is the only respiratory substrate; no alternative substrates enter at intermediate stages and alter the accounting.

Q7. Define RQ and calculate it for complete glucose oxidation, using C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. The gas coefficients give relative gas volumes under the same conditions. Explain the result. [3 marks]

1. Respiratory quotient is the volume of carbon dioxide evolved divided by the volume of oxygen consumed during respiration.
2. The equation gives six relative volumes of carbon dioxide released and six relative volumes of oxygen used, so RQ = 6 ÷ 6 = 1.
3. The value shows equal volumes of the two gases and is the characteristic RQ when carbohydrate is completely oxidised.

Q8. Explain why the respiratory pathway is amphibolic, using fatty acid breakdown and synthesis as the example. [3 marks]

1. An amphibolic pathway participates in both catabolism, the breakdown of substances, and anabolism, the synthesis of substances within organisms.
2. During fatty acid breakdown for respiration, fatty acids are degraded to acetyl CoA, which enters the respiratory pathway for further oxidation.
3. During fatty acid synthesis, acetyl CoA is withdrawn from the respiratory pathway as a building material. The same pathway therefore supports both breakdown and synthesis.

Key takeaways

• Respiration releases energy through controlled oxidation, allowing some energy to be captured in ATP for cellular activities.
• Plants exchange gases through stomata, lenticels and interconnected air spaces, with each part largely meeting its own needs.
• Glycolysis occurs in the cytoplasm, forming two pyruvate molecules and a net two ATP per glucose.
• Fermentation incompletely oxidises glucose, whereas aerobic respiration completes oxidation and generates many more ATP molecules.
• The link reaction forms acetyl CoA; Krebs' cycle regenerates oxaloacetic acid and produces reduced electron carriers.
• The electron transport system transfers electrons ultimately to oxygen, supporting ATP synthesis by oxidative phosphorylation.
• The theoretical 38-ATP balance depends on assumptions; living pathways operate simultaneously and exchange intermediates with other processes.
• Respiration is amphibolic; RQ is one for completely oxidised carbohydrate, below one for fats and about 0.9 for proteins.

Test yourself

What is the difference between gas exchange and cellular respiration?

Gas exchange moves gases between cells and their surroundings. Cellular respiration chemically oxidises substrates to release energy within cells.

Why must food be transported to non-green parts of a plant?

Non-green cells do not photosynthesise but still require food for respiratory oxidation and release of usable energy.

At which two glycolytic steps is ATP invested?

ATP is used when glucose becomes glucose-6-phosphate and when fructose-6-phosphate becomes fructose-1,6-bisphosphate.

What happens to NADH during fermentation?

NADH is reoxidised to NAD⁺ as pyruvate is processed into the organic end products of fermentation.

Where do the TCA cycle and electron transport system occur?

The TCA cycle occurs in the mitochondrial matrix; the electron transport system lies in the inner mitochondrial membrane.

Why is regeneration of oxaloacetic acid necessary?

Oxaloacetic acid accepts the acetyl group at the start of the cycle, so its regeneration permits continued cycle operation.

Why is 38 ATP a conditional value?

It assumes an orderly sequence, mitochondrial use of glycolytic NADH, no withdrawal of intermediates and glucose as the only substrate.

What are the RQ values for completely oxidised carbohydrate, fats and proteins?

Completely oxidised carbohydrate has RQ one; fats have RQ below one; proteins have RQ about 0.9.