Respiration in Plants: Class 11 Biology · Chapter 12

Pull a carrot out of a garden bed. Its orange root has spent its whole life underground in the dark, yet its cells kept dividing, soaking up mineral ions and packing away sugar. Every one of those jobs costs energy. Where does a cell that never sees sunlight get it?

The answer is food, broken down inside the cell. Trace any food back far enough and you reach a photosynthesising cell: a goat eats leaves, a leopard eats the goat, and a mushroom lives on fallen leaves and rotting logs. Inside a plant the story is the same. The chloroplast-bearing cells lie mostly in the outer layers of leaves and young stems, so the carrot's root, like a tree's wood, its flowers and its seeds, lives on sugar delivered from the leaves through the phloem.

Using food for energy is cellular respiration (श्वसन): the cell oxidises energy-rich molecules, breaking their carbon–carbon bonds one at a time and catching part of the released energy as ATP. Whatever a cell oxidises this way is a respiratory substrate. Glucose and other carbohydrates are the everyday fuel; fats, proteins and organic acids serve only in particular plants or situations, such as an oil-rich seed sprouting. Where photosynthesis needs chloroplasts, respiration runs in the cytoplasm and, in eukaryotes, the mitochondria.

Two details matter for the rest of the chapter. The energy comes out in small doses, each step run by its own enzyme, rather than in one burst. None of it powers work directly: it is first banked as ATP, and ATP is spent at the spot where a job needs doing, such as a pump in a root hair or a ribosome building a protein. And the partly dismantled molecules are not thrown away. Their carbon skeletons become starting material for other compounds, an idea that returns in Section 12.6.

12.1 Do Plants Breathe?

In one sense, yes. Plant cells respire day and night, taking in O₂ and giving out CO₂, so gases must keep moving in and out. Yet a plant has no lungs, no gills and no breathing movements. Gases enter through stomata and lenticels and then spread by diffusion alone.

How can a thirty-metre neem tree manage this? Picture a city with no central air-conditioning, where every house simply opens its own windows. That works for plants for three reasons:

1. No commuting. In the city, air never has to be trucked across town. In the same way a leaf, a stem and a root each trade gases with the air beside them; a plant has no blood or pipes to carry O₂ between organs, and needs none.
2. A slow metabolism. Weight for weight, a mango tree's roots or trunk use O₂ far more slowly than a goat's muscles, so diffusion keeps up. Leaves handle big gas volumes only while photosynthesising, and then their own chloroplasts are making O₂ on the spot.
3. Short trips. Diffusion is fast only over tiny distances, and almost every living plant cell is near air. A leaf is a few cells thick. A trunk looks solid, but its living cells form only a thin sleeve under the bark, reached through lenticels; the core is dead wood that just holds the tree up. Everywhere, loosely packed parenchyma leaves gaps that link into a network of air passages.

Why not simply burn the food? Glucose does burn, completely, to CO₂ and water:

In a flame almost all of that energy escapes as heat and light, which is no use for building molecules. A cell instead takes glucose apart in many small steps, like walking down a staircase rather than jumping off the roof. Most steps give off a little heat, but at several of them the energy released matches the cost of making ATP, and the two reactions are linked (Fig. 12.2). This step-by-step release is the whole idea of respiration. Respiration and combustion have the same start and end points, so they release the same total energy; what differs is how much the cell catches.

Burning needs oxygen, and so does the complete breakdown of glucose in a cell. Yet think of the mud at the bottom of a pond, or the inside of a pickle jar: no free oxygen, and microbes thrive there. Some, like yeast, are facultative anaerobes, switching between using O₂ and doing without it. Others, such as Clostridium bacteria in deep mud, are obligate anaerobes, harmed by oxygen itself. Life began this way, since Earth's early atmosphere had almost no free O₂. Every organism alive today, from yeast to banyan, still carries the enzymes for glucose's first, oxygen-free stage: splitting it into pyruvic acid, a pathway called glycolysis.

12.2 Glycolysis

The name joins two Greek words, one for sugar and one for splitting. Glycolysis takes place in the cytoplasm of every living cell and uses no oxygen; in anaerobic organisms it is the only respiratory process. One glucose molecule (six carbons) is partly oxidised to two molecules of pyruvic acid (three carbons each). Because three biochemists pieced the sequence together, it is also called the EMP pathway.

In a plant, the glucose comes from sucrose, the sugar that leaves export, or from stored starch. The enzyme invertase splits sucrose into glucose and fructose. The enzyme hexokinase then uses ATP to attach a phosphate to glucose, giving glucose-6-phosphate, which rearranges (isomerises) into fructose-6-phosphate. Fructose is phosphorylated too and joins the pathway here, so from fructose-6-phosphate onwards both sugars travel the same road. Ten enzyme-catalysed reactions take a glucose all the way to pyruvate. As you scroll, watch for the steps that spend ATP, make ATP or make NADH + H⁺.

12.3 Fermentation

Pyruvic acid is a crossroads. What a cell does with it depends on the organism and on whether oxygen is available (Fig. 12.4). With O₂, pyruvate is oxidised completely in aerobic respiration. Without O₂, many prokaryotes, unicellular eukaryotes and even germinating seeds turn it into either ethanol and CO₂ or lactic acid. Both are kinds of fermentation, in which glucose is only partly oxidised and no oxygen is involved.

In alcoholic fermentation, carried out by yeast, two enzymes act on each pyruvic acid. Pyruvic acid decarboxylase removes a CO₂, leaving acetaldehyde, and alcohol dehydrogenase then reduces acetaldehyde to ethanol, taking hydrogen from NADH + H⁺. In lactic acid fermentation, carried out by some bacteria (the Lactobacillus that sets curd, for example), the enzyme lactate dehydrogenase reduces pyruvic acid straight to lactic acid with NADH + H⁺. Your own muscle cells do the same during a sprint, when O₂ cannot arrive fast enough.

Why bother with that last step at all? Look back at glycolysis: the PGAL → BPGA step needs NAD⁺, and a cell holds only a small stock of it. Without oxygen, NADH cannot hand its hydrogen to the respiratory chain, so the stock would soon be used up and glycolysis would stop. Fermentation unloads the hydrogen onto pyruvic acid (or acetaldehyde) and hands NAD⁺ back to glycolysis. No ATP is made after glycolysis, so fermentation nets just the 2 ATP that glycolysis earns.

As an energy source this is a poor bargain. Under 7 per cent of glucose's energy gets out at all, because most of it stays locked in ethanol or lactic acid, and only part of what does get out is caught as ATP. The products are also harmful: acid or alcohol builds up around the cells that make it.

Getting at the rest of glucose's energy means taking it apart completely, down to CO₂ and water. For that a cell needs O₂ and, if it is eukaryotic, its mitochondria. Plants and animals respire this way almost all the time, and that is the subject of the next section.

12.4 Aerobic Respiration

With oxygen around, pyruvic acid is carried out of the cytoplasm into a mitochondrion. Think of the mitochondrion as a two-room workshop (Fig. 12.5). In the matrix, pyruvic acid is dismantled: its hydrogen atoms are pulled off a few at a time, and its three carbons leave as three CO₂. On the inner membrane, the electrons from those hydrogens are handed along to O₂, and the energy they give up on the way is used to make ATP.

Inside the matrix, each pyruvic acid first undergoes oxidative decarboxylation. A large enzyme complex, pyruvate dehydrogenase, removes one carbon as CO₂ and two hydrogen atoms, which go to NAD⁺. The two carbons that remain are an acetyl group, which is handed on attached to coenzyme A as acetyl CoA. The reaction needs NAD⁺, coenzyme A and Mg²⁺:

One glucose gave two pyruvic acid, so this step yields 2 NADH + H⁺ and 2 CO₂ per glucose. Acetyl CoA now enters a loop of reactions called the tricarboxylic acid cycle, more often called the Krebs' cycle after the biochemist who worked it out.

12.4.1 Tricarboxylic Acid Cycle

The cycle is named after its first product, citric acid, which carries three carboxyl (–COOH) groups. A four-carbon acceptor collects each two-carbon acetyl group, the combined molecule is taken apart again, and the acceptor comes out at the end ready for the next turn. Press Play on Fig. 12.6 and follow the numbered steps below it.

1. Entry. The acetyl group (2C) joins oxaloacetic acid, OAA (4C), with a molecule of water, giving citric acid (6C). The enzyme is citrate synthase, and the CoA is set free to collect another acetyl group.
2. Two CO₂ leave. Citric acid is rearranged (isomerised) to isocitrate. Two oxidative decarboxylations follow in a row, each removing one carbon as CO₂ and passing hydrogen to NAD⁺: isocitrate becomes α-ketoglutaric acid (5C), which becomes succinyl-CoA (4C). As many carbons have left as the acetyl group brought in.
3. A phosphate straight from the substrate. As succinyl-CoA becomes succinic acid, the energy released makes GTP from GDP. Because the phosphate group is transferred without any help from the electron transport system, this is substrate-level phosphorylation. GTP then passes that phosphate on to ADP, so the cycle's energy ends up in ATP.
4. A different hydrogen carrier. Succinic acid is oxidised to fumaric acid, but this time the hydrogen goes to FAD, giving FADH₂. Fumaric acid takes up water to form malic acid.
5. Back to the start. Malic acid is oxidised to OAA, reducing a third NAD⁺. The acceptor has been rebuilt and can pick up the next acetyl group.
6. One turn. Each acetyl group gives 2 CO₂, 3 NADH + H⁺, 1 FADH₂ and 1 ATP. Two things keep the wheel spinning: a fresh OAA at the end of every turn, and a steady return of empty carriers (NAD⁺ and FAD) once NADH and FADH₂ have been emptied. Emptying them is the next stage's job.

Adding the link reaction to one turn of the cycle gives the balance sheet for one pyruvic acid in the matrix:

Double it for one glucose. Every carbon has now left as CO₂, and the matrix holds 8 NADH + H⁺, 2 FADH₂ and just 2 ATP. Notice what is missing: not a single O₂ has been used, and the ATP count is still tiny. Both mysteries are solved by those loaded carriers, NADH and FADH₂.

12.4.2 Electron Transport System (ETS) and Oxidative Phosphorylation

NADH and FADH₂ carry high-energy electrons. Releasing that energy means oxidising them, and the cell does it by passing their electrons along a chain of carriers in the inner mitochondrial membrane until they reach O₂, which takes up protons as well and becomes water. The route the electrons follow is the electron transport system (ETS).

Follow a pair of electrons from NADH made in the matrix. Complex I (NADH dehydrogenase) oxidises the NADH and hands the electrons to ubiquinone (UQ), a small carrier that moves within the membrane. FADH₂, formed when succinate is oxidised in the cycle, feeds its electrons to ubiquinone through complex II (succinate dehydrogenase). Reduced ubiquinone, called ubiquinol, passes the electrons to complex III (the cytochrome bc₁ complex). From there they go to cytochrome c, a small protein loosely bound to the inner membrane on the side facing the intermembrane space, which ferries electrons from complex III to complex IV. Complex IV (cytochrome c oxidase, which contains cytochromes a and a₃ and two copper centres) finally passes them to O₂ (Fig. 12.7).

Complex V, ATP synthase, cashes in the energy released along the chain, joining ADP and inorganic phosphate into ATP. The yield depends on the entry point. An NADH's electron pair runs the whole course from complex I and is worth 3 ATP; an FADH₂'s pair enters at ubiquinone, misses complex I, and is worth only 2 ATP.

Oxygen appears only at the very end, yet nothing works without it. With no final acceptor, the carriers stay loaded with electrons, NADH cannot be oxidised, NAD⁺ and FAD run out, and the Krebs' cycle and the link reaction grind to a halt. Oxygen is the final hydrogen acceptor, the exit at the end of an expressway: block it and traffic backs up all the way to the entry ramps.

The name of the process says where its energy comes from. Chloroplasts make ATP with the same kit (a membrane, a proton gradient, an ATP synthase), but there light powers the gradient, so it is photophosphorylation. Here the power comes from redox reactions, the oxidation of NADH and FADH₂, so it is oxidative phosphorylation.

You met the mechanism, chemiosmosis, in Chapter 11. Here complexes I, III and IV use the energy of electron flow to pump protons out of the matrix into the intermembrane space, piling them up like water behind a dam. ATP synthase is the water mill on that dam (Fig. 12.8). Its F₀ part, an integral protein complex buried in the membrane, is the sluice: the only gate through which protons can get back into the matrix. Its F₁ part, a peripheral headpiece on the matrix side, is the millstone, holding the site where ADP and inorganic phosphate are joined. Protons rushing down their electrochemical gradient through F₀ drive the catalysis in F₁; by NCERT's count, every ATP needs 4 H⁺ to pass.

12.5 The Respiratory Balance Sheet

Can we count how many ATP one glucose is worth? We can, but only for an idealised cell, much as an auditor might value a factory by imagining every machine running flat out with nothing wasted. The count holds only if we assume that:

• the pathway runs as one tidy assembly line, glycolysis then the Krebs' cycle then the ETS, each product feeding the next step;
• the NADH made in the cytoplasm during glycolysis gets into the mitochondria and is oxidised by the ETS like the rest;
• no intermediate is drawn off to build any other compound;
• glucose is the only fuel, with no other substrate joining the pathway part-way.

Real cells break every one of these rules. Glycolysis, the cycle and the ETS all run at once, not in turn. Intermediates are constantly fed in from other pathways or siphoned off to them. ATP never piles up, because it is used the moment it forms, and each enzyme's speed is adjusted by several controls. The audit is still worth doing: like the mileage quoted for a new car, it shows the best the machine can manage. On these assumptions one glucose yields a net gain of 38 ATP (Fig. 12.9).

Now set fermentation beside aerobic respiration:

12.6 Amphibolic Pathway

Picture the respiratory pathway as a highway with glucose at its start. Starch and sucrose join right there, after being converted to glucose, which is why glucose counts as the preferred fuel. Fats and proteins can be respired too, but they use on-ramps further along (Fig. 12.11).

• Fats are first split into glycerol and fatty acids. Glycerol is converted to PGAL and joins glycolysis half-way. Fatty acids are cut into two-carbon pieces that become acetyl CoA.
• Proteins are cut into amino acids by proteases. Each amino acid loses its amino group (deamination), and what remains is a carbon skeleton whose structure decides its on-ramp: pyruvic acid for some, acetyl CoA for others, a Krebs' cycle acid for the rest.

Respiration has traditionally been called a catabolic pathway, since it breaks substrates down. But the doors swing both ways. While a mustard seed is ripening and filling with oil, acetyl CoA is drawn out of the pathway to build fatty acids; when that seed germinates, the fatty acids are broken back down to acetyl CoA and respired. In the same way, pyruvic acid, α-ketoglutaric acid and OAA are drawn off as carbon skeletons for new amino acids. Breaking down is catabolism and building up is anabolism. A pathway that serves both is an amphibolic pathway (amphi, both), a better name for the respiratory pathway than catabolic.

12.7 Respiratory Quotient

Measure the gases around a respiring seed and you can tell what it is burning. The trick is to compare two volumes over the same period: the CO₂ given out and the O₂ taken in. Their ratio is the respiratory quotient (RQ), also called the respiratory ratio:

For a carbohydrate burnt completely, the two volumes match and the RQ is 1. Look back at (12.1): six O₂ go in and six CO₂ come out, and equal numbers of gas molecules occupy equal volumes. Fats are poor in oxygen compared with carbohydrates, so a lot of extra O₂ is needed just to turn their hydrogen into water: their RQ is less than 1, about 0.7. Proteins give about 0.9 (Fig. 12.12).

Gas volumes are measured in a respirometer. A flask that also contains potassium hydroxide (KOH) absorbs every bit of CO₂ the seeds give out, so its gas volume falls by exactly the O₂ used. A twin flask without KOH changes by the difference between the O₂ used and the CO₂ released.

Real tissues rarely run on one fuel. A measured RQ of 0.85, for example, means a blend of carbohydrate and fat is being burnt; no cell lives on pure fat or pure protein alone.