Respiration in Plants

Every living cell needs energy to grow, divide, transport substances across membranes and make its own molecules. That energy comes from the oxidation of macromolecules in the cell, and the process is called cellular respiration. The compounds that are oxidised are the respiratory substrates: carbohydrates, proteins, fats and organic acids. Glucose is the most common respiratory substrate, and the exam expects you to say so in those exact words. In plants, sucrose made in the leaf is first split by the enzyme invertase into glucose and fructose, and both enter the pathway.

Write the overall equation once and remember its shape:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Two things about this equation matter for marks. First, it is a summary, not a single step. Glucose is broken down in a long series of small enzyme-catalysed steps, each releasing a little energy. NCERT stresses that the energy is not released free into the cell in a single step; it is released in a series of slow, enzyme-controlled steps and trapped as ATP. Combustion releases it all at once as heat and light, respiration does not. The step-wise release is what allows some of the energy to be trapped rather than lost as heat. Second, the energy that is released is not used directly. It is used to make ATP (adenosine triphosphate) from ADP and inorganic phosphate, and ATP is broken down wherever and whenever the cell needs energy. That is why ATP is called the energy currency of the cell.

The chapter therefore has one story: how does glucose become CO₂ and water, and how many ATP does the cell get on the way? Keep the sites in mind from the start. Glycolysis happens in the cytoplasm of every cell, whether it has oxygen or not; everything after pyruvic acid that needs oxygen happens inside the mitochondrion, the link reaction and the citric acid cycle in the matrix and the electron transport system on the inner membrane. Almost every 1-mark question in this chapter is a question about a site or an enzyme, so learn the place of each step as you learn the step.

Animals breathe: they have lungs or gills that move gases in and out. Plants do not. Yet plants need O₂ for respiration and give out CO₂, so gas exchange must happen somehow. The NCERT answer is that plants have no specialised organs for gas exchange, and use stomata on leaves and young stems and lenticels on woody stems and roots. Each plant part takes care of its own gas exchange needs; there is no transport of gases from one organ to another, and only a very small quantity of gas is carried by the transport system.

Why can plants get away without lungs? The chapter gives reasons you should be able to list as a 2-mark or 3-mark answer:

• Each part of the plant takes care of its own gas-exchange needs, so there is no need to move gases around the body.
• Plants do not present great demands for gas exchange. Roots, stems and leaves respire at rates far lower than animals do.
• During photosynthesis, O₂ is released within the cell, so the leaf cells are supplied with oxygen while they are photosynthesising.
• Cells that are living are located close to the surface of the plant. In stems the living cells are arranged in thin layers under the bark, and they have openings (lenticels); in leaves most cells are near the surface anyway.
• A large part of the bulky plant body is made of dead cells (wood) or loosely packed parenchyma with large intercellular spaces, so gases diffuse easily through the plant body. The distance a gas has to diffuse is never very large.

The examiner also likes the distinction between breathing and respiration. Breathing is the physical process of taking in O₂ and giving out CO₂, whatever organ does it. Respiration is the chemical process inside the cell where the substrate is oxidised and energy is trapped as ATP. Plants respire in every living cell, but they do not breathe in the animal sense; they simply let gases diffuse in and out through stomata and lenticels.

One more fact that gets asked: because glycolysis happens in the cytoplasm and needs no oxygen, all living cells, plant or animal, aerobic or anaerobic, carry out glycolysis. Only the fate of pyruvic acid depends on whether oxygen is available.

The name means splitting of sugar (glycos, sugar; lysis, splitting). The scheme was worked out by Gustav Embden, Otto Meyerhof and J. Parnas, so it is also called the EMP pathway. It is the partial oxidation of one molecule of glucose (6C) to two molecules of pyruvic acid (3C). It takes place in the cytoplasm, needs no oxygen and occurs in all living organisms. In anaerobic organisms it is the only process in respiration.

NCERT describes ten steps, each with an enzyme, and the marking scheme rewards the schematic in Fig. 12.1 drawn in order. Learn it as a story in three parts.

Part 1, spend two ATP to activate the sugar. Glucose is phosphorylated by hexokinase to glucose-6-phosphate (one ATP used). This isomerises to fructose-6-phosphate. Phosphofructokinase then adds a second phosphate using a second ATP to give fructose 1,6-bisphosphate. These are the only two ATP-consuming steps; NCERT names them as the conversion of glucose to glucose 6-phosphate and of fructose 6-phosphate to fructose 1,6-bisphosphate.

Part 2, split the sugar. Aldolase splits fructose 1,6-bisphosphate into two 3-carbon molecules, dihydroxyacetone phosphate (DHAP) and 3-phosphoglyceraldehyde (PGAL). DHAP is isomerised to PGAL, so from here on everything happens twice per glucose.

Part 3, oxidise and harvest. PGAL is oxidised and phosphorylated to 1,3-bisphosphoglycerate (BPGA); the two hydrogen atoms (redox equivalents) removed from PGAL are transferred to NAD⁺, forming NADH + H⁺. BPGA gives up one phosphate to ADP and becomes 3-phosphoglyceric acid (3-PGA); this is an ATP-forming step. 3-PGA rearranges to 2-phosphoglycerate, which loses a water molecule (enolase) to become phosphoenolpyruvate (PEP). PEP transfers its phosphate to ADP (pyruvate kinase) to give pyruvic acid and the second ATP. Both ATP-forming steps are examples of substrate-level phosphorylation: the phosphate comes straight from a substrate molecule, not from an electron transport chain.

Now count. Two ATP are used in part 1. Four ATP are made in part 3 (two steps, each occurring twice). Net gain: 2 ATP and 2 NADH + H⁺ per glucose, plus two pyruvic acid. The exam asks this count in every form.

Pyruvic acid is the key product of glycolysis, and its fate depends on the cell and on oxygen. NCERT names three major ways: lactic acid fermentation, alcoholic fermentation and aerobic respiration. Fermentation happens under anaerobic conditions in many prokaryotes and unicellular eukaryotes; aerobic respiration needs oxygen and, in eukaryotes, mitochondria.

When there is no oxygen, pyruvic acid cannot enter the mitochondrion for complete oxidation. The cell then does the minimum needed to keep glycolysis running: it reoxidises the NADH + H⁺ made in glycolysis back to NAD⁺, because without NAD⁺ the PGAL step would stop and no more ATP could be made. This incomplete oxidation of glucose is called fermentation, and NCERT describes two kinds.

Alcoholic fermentation in yeast. Pyruvic acid is first decarboxylated to acetaldehyde by pyruvic acid decarboxylase, releasing CO₂. Then acetaldehyde is reduced by alcohol dehydrogenase to ethanol, and NADH + H⁺ is oxidised to NAD⁺ in the process. Overall, one glucose gives two ethanol and two CO₂. This is the reaction that makes bread rise and makes beer and wine.

Lactic acid fermentation in some bacteria and in animal muscle during heavy exercise, when oxygen is inadequate. Pyruvic acid is reduced directly to lactic acid by lactate dehydrogenase, again oxidising NADH + H⁺ to NAD⁺. No CO₂ is released. In both cases the reducing agent is NADH + H⁺, which is reoxidised to NAD⁺; this recycling is the point of fermentation.

Three exam facts about the yield:

• In both fermentations very little energy is released: less than seven per cent of the energy in glucose is released, and not all of it is trapped as high-energy bonds of ATP. The net gain is only the 2 ATP of glycolysis.
• Fermentation is a partial breakdown of glucose; aerobic respiration degrades glucose completely to CO₂ and H₂O.
• The products are toxic. Yeasts poison themselves to death when the concentration of alcohol reaches about 13 per cent, which is why naturally fermented beverages are not stronger than that; spirits are made by distillation.

NCERT's Fig. 12.2 summarises the major pathways of anaerobic respiration: glucose → pyruvic acid, and then three arrows, to lactic acid (lactate dehydrogenase), to ethanol + CO₂ (pyruvic acid decarboxylase then alcohol dehydrogenase) and to acetyl CoA + CO₂ for aerobic respiration. Draw it for any question on the fate of pyruvic acid.

NCERT treats fermentation and anaerobic respiration together, and the comparison the examiner cares about is with aerobic respiration: no oxygen needed, no mitochondrion, no complete oxidation, no ETS, NADH oxidised by the organic acceptor (pyruvate or acetaldehyde) rather than by oxygen, and only 2 ATP.

Aerobic respiration is the process that leads to the complete oxidation of organic substances in the presence of oxygen, releasing CO₂, water and a large amount of energy. In eukaryotes it happens in the mitochondria, so the pyruvic acid made in the cytoplasm must first be transported into the mitochondrial matrix. The chapter describes two crucial events that follow: the complete oxidation of pyruvate by step-wise removal of all the hydrogen atoms, leaving three molecules of CO₂; and the passing of those electrons to molecular O₂ with simultaneous synthesis of ATP. The first happens in the matrix, the second on the inner membrane.

The link reaction (oxidative decarboxylation of pyruvate). Inside the matrix, pyruvic acid (3C) undergoes oxidative decarboxylation by a complex set of reactions catalysed by pyruvic dehydrogenase. The reaction needs the participation of several coenzymes, including NAD⁺ and Coenzyme A. One carbon leaves as CO₂, the remaining two-carbon acetyl group is attached to CoA to give acetyl CoA, and NAD⁺ is reduced to NADH + H⁺.

Pyruvic acid + CoA + NAD⁺ → Acetyl CoA + CO₂ + NADH + H⁺ (pyruvic dehydrogenase, Mg²⁺)

Since one glucose gives two pyruvic acid, this step produces 2 NADH + H⁺ and 2 CO₂ per glucose. Call it oxidative decarboxylation in the exam: oxidative because NAD⁺ is reduced (pyruvate is oxidised), decarboxylation because CO₂ is removed. Acetyl CoA then enters the cyclic pathway, the tricarboxylic acid cycle, described next.

Why is acetyl CoA important beyond this chapter? It is the molecule where the breakdown of fats, carbohydrates and proteins meet: fatty acids are broken down to acetyl CoA, several amino acids are converted to it, and glucose reaches it through pyruvate. NEET has asked exactly this twice: the biomolecule common to respiration-mediated breakdown of fats, carbohydrates and proteins is acetyl CoA, not pyruvic acid.

Keep the sites straight for the whole of aerobic respiration: glycolysis in the cytoplasm; link reaction and citric acid cycle in the mitochondrial matrix; ETS and oxidative phosphorylation on the inner mitochondrial membrane. Draw a mitochondrion once with these three labels and the 1-mark site questions stop being a risk.

The cycle was first elucidated by Hans Krebs in 1937, hence Krebs cycle. Because the first product, citric acid, has three carboxyl groups it is also the tricarboxylic acid (TCA) cycle or citric acid cycle. It runs in the mitochondrial matrix.

Entry. Acetyl CoA (2C) condenses with oxaloacetic acid (OAA, 4C) and water to give citric acid (6C), catalysed by citrate synthase; a molecule of CoA is released and goes back to pick up another acetyl group. Citrate is then isomerised to isocitrate.

Two decarboxylations. Isocitrate loses one CO₂ and is oxidised (NAD⁺ → NADH + H⁺) to α-ketoglutaric acid (5C). α-Ketoglutaric acid loses a second CO₂, is oxidised again (a second NADH + H⁺) and joins CoA to become succinyl-CoA (4C). These are the two CO₂ molecules of one turn; together with the CO₂ of the link reaction they account for the three carbons of one pyruvate.

The GTP step. Succinyl-CoA is converted to succinic acid and CoA is released; the energy of the thioester bond is used to make a molecule of GTP from GDP and Pi. This is a substrate-level phosphorylation, the only one in the cycle. In a coupled reaction GTP is converted to GDP with simultaneous synthesis of ATP from ADP, so it is counted as one ATP.

Regeneration of OAA. Succinic acid is oxidised to fumaric acid by succinate dehydrogenase; here the acceptor is FAD, which becomes FADH₂ (the only FADH₂ of the cycle). Fumaric acid takes up water to give malic acid, and malic acid is oxidised (third NADH + H⁺) to oxaloacetic acid, which is ready to accept the next acetyl CoA. The continued oxidation of acetyl CoA therefore needs the continued replenishment of OAA and the regeneration of NAD⁺ and FAD.

Tally per turn (per acetyl CoA): 2 CO₂, 3 NADH + H⁺, 1 FADH₂, 1 GTP (= 1 ATP). Per glucose the cycle turns twice, so 4 CO₂, 6 NADH + H⁺, 2 FADH₂, 2 ATP. NCERT gives the summary equation for one pyruvic acid, which includes the link reaction:

Pyruvic acid + 4NAD⁺ + FAD + 2H₂O + ADP + Pi → 3CO₂ + 4NADH + 4H⁺ + FADH₂ + ATP

Note what the cycle does not do: no oxygen is used directly, and only one ATP is made by substrate-level phosphorylation. Its real output is reduced coenzymes, NADH + H⁺ and FADH₂, whose oxidation in the ETS produces most of the cell's ATP. Also note that the cycle needs oxygen indirectly, because the NAD⁺ and FAD it requires are regenerated only when the ETS passes their electrons to oxygen.

So far the energy of glucose has been parked in NADH + H⁺ and FADH₂. The electron transport system on the inner mitochondrial membrane releases and uses that energy: electrons are passed from one carrier to another and finally to oxygen, and the energy released on the way is used to make ATP. This is the metabolic pathway through which the electron passes from one carrier to another.

The carriers, in order (NCERT Fig. 12.4).

• Complex I, NADH dehydrogenase. Electrons from NADH produced in the mitochondrial matrix during the citric acid cycle are oxidised here and passed to ubiquinone (UQ), a mobile carrier within the inner membrane.
• Complex II, succinate dehydrogenase. Ubiquinone also receives reducing equivalents from FADH₂, which is generated during the oxidation of succinate in the citric acid cycle. This is why the FADH₂ electrons skip complex I and yield fewer ATP.
• Complex III, cytochrome bc₁ complex. Reduced ubiquinone (ubiquinol) is oxidised and the electrons are transferred to cytochrome c.
• Cytochrome c is a small protein attached to the outer surface of the inner membrane; it is a mobile carrier of electrons between complex III and complex IV.
• Complex IV, cytochrome c oxidase. Contains cytochromes a and a₃ and two copper centres. It hands the electrons to molecular oxygen, the final acceptor, which is reduced to water.

Where the ATP comes from. When the electrons pass through the chain, they are coupled to ATP synthase (complex V) for the production of ATP from ADP and Pi. The number of ATP depends on the electron donor: oxidation of one NADH gives 3 ATP, of one FADH₂ gives 2 ATP.

Chemiosmosis and ATP synthase. As electrons move through the complexes, protons are pumped from the matrix into the intermembrane space, creating a proton gradient across the inner membrane. ATP synthase has two parts. F₁ is the headpiece, a peripheral membrane protein complex that contains the site for synthesis of ATP from ADP and Pi. F₀ is an integral membrane protein complex that forms the channel through which protons cross the inner membrane. The passage of protons through the channel is coupled to the catalytic site of F₁. NCERT's number: for each ATP produced, 2H⁺ pass through F₀ from the intermembrane space to the matrix down the electrochemical proton gradient. Because the ATP is made by oxidation of the coenzymes with oxygen as the acceptor, the process is called oxidative phosphorylation, as opposed to photophosphorylation in the chloroplast where the energy comes from light.

Role of oxygen. Oxygen is used only at the end of the pathway, yet it is essential for the whole of aerobic respiration. NCERT puts it simply: the role of oxygen is limited to the terminal stage, but its presence is vital because it drives the whole process by removing hydrogen from the system. Oxygen acts as the final hydrogen (electron) acceptor. Without it the carriers stay reduced, NAD⁺ and FAD are not regenerated, and the citric acid cycle stops.

The balance sheet is the exam's favourite arithmetic. NCERT is honest that the calculation is theoretical and rests on four assumptions that are not really valid in a living system; the assumptions themselves are a 2-mark or 3-mark question (NCERT exercise 12.8).

1. There is a sequential, orderly pathway functioning, with one substrate forming the next and with glycolysis, TCA cycle and ETS following one after another.
2. The NADH synthesised in glycolysis is transferred into the mitochondria and undergoes oxidative phosphorylation.
3. None of the intermediates in the pathway are utilised to synthesise any other compound.
4. Only glucose is being respired; no other alternative substrates enter the pathway at any of the intermediary stages.

In reality all pathways work simultaneously, substrates enter and leave at many points, ATP is used as and when needed, and enzyme rates change. Still, the calculation gives a useful figure.

The tally per glucose.

• Glycolysis (cytoplasm): 2 ATP (substrate level) + 2 NADH + H⁺.
• Link reaction (matrix): 2 NADH + H⁺.
• Citric acid cycle, two turns (matrix): 2 ATP via GTP + 6 NADH + H⁺ + 2 FADH₂.
• Totals of reduced coenzymes: 10 NADH + H⁺ and 2 FADH₂.
• ETS (inner membrane): 10 × 3 = 30 ATP from NADH, 2 × 2 = 4 ATP from FADH₂.

Adding: 2 + 2 + 30 + 4 = 38 ATP if every NADH gives three ATP. NCERT states that there can be a net gain of 36 ATP molecules during aerobic respiration of one molecule of glucose; the difference is that the 2 NADH made in the cytoplasm during glycolysis have to be moved into the mitochondrion, and the transport costs the equivalent of 2 ATP. Give 36 as the NCERT answer and explain the 38 when asked to show working.

Fermentation versus aerobic respiration, the comparison NCERT draws at the end of this section, is asked every year:

• Fermentation is a partial breakdown of glucose; in aerobic respiration glucose is completely degraded to CO₂ and H₂O.
• Fermentation gives a net gain of only 2 ATP per glucose; aerobic respiration gives many more, 36.
• In fermentation NADH is oxidised to NAD⁺ rather slowly, by pyruvate or acetaldehyde; in aerobic respiration it is oxidised vigorously by the ETS with oxygen as the acceptor.

Glucose is the favoured substrate, but cells respire other things too, and they enter the same pathway at different points. NCERT's Fig. 12.5 (interrelationship among metabolic pathways) is the picture to remember.

• Fats are first broken down into glycerol and fatty acids. Fatty acids are degraded to acetyl CoA, which enters the citric acid cycle. Glycerol enters glycolysis as PGAL (3-phosphoglyceraldehyde).
• Proteins are degraded by proteases to amino acids, and the amino acids, after deamination, enter at different points: some as pyruvate, some as acetyl CoA, some as intermediates of the citric acid cycle such as α-ketoglutaric acid or oxaloacetic acid.
• Carbohydrates other than glucose (starch, sucrose, fructose) are converted to glucose or its phosphates and enter glycolysis.

So far this is catabolism, the breakdown of molecules. The important point is that the same pathway also runs the other way. When the organism needs to synthesise fatty acids, acetyl CoA is withdrawn from the respiratory pathway. Many amino acids are made from intermediates of the citric acid cycle. Glycerol for fat synthesis comes from PGAL. That is anabolism, biosynthesis, using respiratory intermediates as starting material.

Since the respiratory pathway is involved in both breakdown and synthesis, it is better to call it an amphibolic pathway rather than a purely catabolic one. That single sentence, with two examples on each side, is the model 2-mark answer for NCERT exercise 12.9. For a 3-mark version, add that this is why a cell cannot stop respiring even when it has plenty of ATP: the intermediates are needed as building blocks, which is also assumption 3 of the balance sheet being violated in real life.

Two definitions to keep separate. Catabolic pathways break large molecules down and release energy; anabolic pathways build molecules and use energy; amphibolic pathways do both. Respiration is often described as catabolic in short answers (and 'catabolic' is the accepted answer to 'is respiration catabolic or anabolic'), but when the question uses the word amphibolic, the examiner wants the dual role explained.

During aerobic respiration O₂ is consumed and CO₂ is released. The ratio of the volume of CO₂ evolved to the volume of O₂ consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio.

RQ = volume of CO₂ evolved / volume of O₂ consumed

The value depends on the respiratory substrate, and the exam asks the three standard cases.

Carbohydrates, RQ = 1. From the overall equation, 6 O₂ consumed and 6 CO₂ evolved, so RQ = 6CO₂/6O₂ = 1.0.

Fats, RQ less than 1. Fats contain very little oxygen relative to carbon and hydrogen, so they need extra O₂ to oxidise their hydrogen, and less CO₂ comes out per O₂ used. NCERT's example is tripalmitin:

2(C₅₁H₉₈O₆) + 145O₂ → 102CO₂ + 98H₂O + energy

RQ = 102CO₂/145O₂ = 0.7. Check the arithmetic once so you can reproduce it: 102 ÷ 145 = 0.703, written as 0.7.

Proteins, RQ about 0.9. NCERT states that when proteins are the respiratory substrate the ratio would be about 0.9.

Two qualifications the examiner may probe. First, in living organisms pure substrates are rarely used alone; the RQ of a tissue reflects a mixture. Second, RQ is defined for aerobic respiration; in anaerobic conditions (fermentation) CO₂ is released without O₂ being consumed, so the ratio is not meaningful (it would tend to infinity). Case-based questions build on this with a germinating-seed experiment: starchy seeds such as wheat give RQ close to 1, oily seeds such as castor give a value below 1, and a seed sealed without oxygen releases CO₂ without consuming O₂.

Why does RQ matter beyond the number? Measuring it tells a physiologist which substrate a tissue is burning, which is useful in studying germinating seeds, ripening fruit or a stressed plant. But for the exam the definition, the formula, the three values and the tripalmitin calculation are the four things to have ready.