Respiration in Plants

Respiration is the process by which cells break the C–C bonds of complex organic molecules through oxidation to release energy, which is captured as ATP and used for life's activities. The molecule that is oxidised is the respiratory substrate — usually glucose, but fats, proteins and organic acids can also be respired. The energy is not released all at once but in small, controlled steps so that it can be trapped efficiently rather than lost as heat.

The first stage, common to virtually all organisms, is glycolysis (the EMP pathway, after Embden, Meyerhof and Parnas). A defining NEET fact is that glycolysis occurs in the cytoplasm and does not require oxygen, so it is shared by both aerobic and anaerobic respiration. In it, one molecule of six-carbon glucose is partially oxidised and split into two molecules of the three-carbon pyruvate (pyruvic acid).

Along the way, glucose is first phosphorylated to glucose-6-phosphate and then to fructose-1,6-bisphosphate (an 'investment' that uses 2 ATP), which is split into two 3-carbon sugars; these are then oxidised and converted to pyruvate, generating ATP and NADH. The crucial net yield is 2 ATP and 2 NADH per glucose (four ATP are made but two were used at the start). Knowing this net figure and the location is essential.

The fate of the pyruvate produced depends on whether oxygen is available, and this branch point organises the rest of respiration. In the presence of oxygen (aerobic respiration), pyruvate enters the mitochondria and is completely oxidised through the Krebs cycle and electron transport chain. In the absence of oxygen (anaerobic respiration / fermentation), pyruvate is only partially broken down in the cytoplasm. The two routes differ enormously in how much ATP they ultimately yield, as the next topics show.

When oxygen is absent, cells cannot run the Krebs cycle or electron transport chain, so pyruvate is processed in the cytoplasm by fermentation (anaerobic respiration). Fermentation is an incomplete oxidation of glucose: only a small fraction of its energy is released, and the cell relies entirely on the 2 ATP made earlier in glycolysis. A key purpose of fermentation is to regenerate NAD⁺ from the NADH produced in glycolysis, so that glycolysis can keep running.

There are two main types, and matching each to its organism and product is a guaranteed NEET mark. In alcoholic fermentation, carried out by yeast and some microbes, pyruvate is converted to ethanol (alcohol) and CO₂, using the enzymes pyruvate decarboxylase and alcohol dehydrogenase. This is the basis of brewing and baking.

In lactic acid fermentation, carried out by some bacteria and by our own skeletal muscle during strenuous exercise when oxygen runs short, pyruvate is reduced to lactic acid by the enzyme lactate dehydrogenase. The build-up of lactic acid in muscle is associated with fatigue and cramps.

Both forms of fermentation are energetically poor — they yield only the 2 ATP of glycolysis, far less than aerobic respiration — and both can be hazardous to the cell if the products (alcohol or acid) accumulate. Importantly, in both, the NADH formed in glycolysis is re-oxidised to NAD⁺ as pyruvate (or acetaldehyde) is reduced, which is the chemical 'point' of the process. The contrast to remember is: fermentation = incomplete, cytoplasmic, only 2 ATP, products ethanol+CO₂ (yeast) or lactic acid (muscle).

When oxygen is available, pyruvate is completely oxidised in the mitochondrion, releasing far more energy. First, the pyruvate made in the cytoplasm enters the mitochondrial matrix and undergoes the link reaction (oxidative decarboxylation): catalysed by the pyruvate dehydrogenase complex, each pyruvate loses a CO₂ and joins coenzyme A to form acetyl coenzyme A (acetyl CoA), producing one NADH in the process. Since each glucose gave two pyruvate, this happens twice per glucose.

Acetyl CoA then enters the Krebs cycle — also called the citric acid cycle or TCA cycle — which takes place in the mitochondrial matrix. The 2-carbon acetyl group combines with the 4-carbon oxaloacetate (OAA) to form the 6-carbon citrate, and the cycle then regenerates OAA through a series of oxidations and decarboxylations. Each turn releases 2 molecules of CO₂.

The energy yield of one turn of the Krebs cycle (per acetyl CoA) is a must-memorise figure: 3 NADH, 1 FADH₂ and 1 ATP (the ATP is actually formed as GTP). Because one glucose produces two acetyl CoA, the cycle turns twice per glucose, doubling these numbers. The CO₂ we exhale comes from the link reaction and the Krebs cycle, not from glycolysis.

At this point glucose has been fully dismantled and its carbons released as CO₂, but most of the captured energy is still locked in the reduced coenzymes NADH and FADH₂ rather than in ATP directly. These electron carriers now feed the electron transport chain, the final stage where the bulk of the cell's ATP is actually made. For NEET, the essentials here are: link reaction makes acetyl CoA + CO₂ + NADH; the Krebs cycle is in the matrix; and per acetyl CoA it yields 3 NADH, 1 FADH₂, 1 ATP and 2 CO₂.

The final stage of aerobic respiration is the electron transport chain (ETC) and oxidative phosphorylation, located on the inner mitochondrial membrane. The NADH and FADH₂ made earlier are oxidised here: their electrons pass through a series of carriers (Complexes I–IV, including cytochromes), and at the end oxygen acts as the final electron acceptor, combining with electrons and protons to form water. This is why oxygen is essential — without it the chain backs up and stops.

As electrons move through the chain, protons are pumped across the inner membrane, building a proton gradient. The protons flow back through ATP synthase (Complex V), and this flow drives ATP synthesis — the same chemiosmotic mechanism seen in photosynthesis. The standard NCERT bookkeeping is that each NADH yields about 3 ATP and each FADH₂ yields about 2 ATP when oxidised through the chain.

Putting it all together gives the respiratory balance sheet. From the complete aerobic oxidation of one glucose — glycolysis (2 ATP + 2 NADH), the link reaction (2 NADH), the Krebs cycle (2 turns: 6 NADH + 2 FADH₂ + 2 ATP) — and feeding the 10 NADH and 2 FADH₂ through the ETC, a cell can make a theoretical maximum of about 38 ATP per glucose (often cited as 36–38). This dwarfs the 2 ATP of fermentation and shows why aerobic respiration is so much more efficient.

Two final concepts complete the chapter. The respiratory quotient (RQ) is the ratio of CO₂ released to O₂ consumed: it is 1 for carbohydrates, less than 1 (~0.7) for fats, about 0.9 for proteins, and greater than 1 for organic acids. Lastly, the respiratory pathway is described as amphibolic because it is not purely catabolic (breakdown): its intermediates (such as acetyl CoA and Krebs-cycle acids) are also withdrawn to build (anabolism) fatty acids, amino acids and other molecules. So respiration both releases energy and supplies building blocks — a point NEET likes to test alongside the balance sheet and RQ values.