Respiration in Plants notes

Respiration in plants is the stepwise enzymatic breakdown of complex organic molecules, mainly glucose, to release energy as ATP. Unlike photosynthesis, respiration occurs in all living plant cells throughout day and night. The chapter begins with glycolysis in the cytoplasm, where glucose becomes pyruvate with a small ATP gain. In absence of oxygen, pyruvate enters fermentation; in presence of oxygen, it enters mitochondria for pyruvate oxidation, Krebs cycle and electron transport chain. The chapter also explains respiratory quotient, which identifies the respiratory substrate, and the amphibolic nature of respiration, because respiratory intermediates are used both for breakdown and biosynthesis. For NEET, ATP accounting, pathway locations, end products and NCERT statements are highly important.

Glycolysis is the first stage of respiration and occurs in the cytoplasm of plant cells. It is also called the EMP pathway after Embden, Meyerhof and Parnas. One glucose molecule is converted into two molecules of pyruvic acid through ten enzyme-controlled steps. The pathway has an energy investment phase, where 2 ATP are consumed, and an energy payoff phase, where 4 ATP and 2 NADH are produced. Thus, the net gain is 2 ATP and 2 NADH per glucose. Glycolysis does not directly require oxygen, so it operates in both aerobic and anaerobic conditions. The fate of pyruvate depends on oxygen availability: fermentation in anaerobic conditions or mitochondrial oxidation in aerobic conditions.

Fermentation is the anaerobic, incomplete breakdown of glucose in which pyruvate is converted into organic end products such as ethanol or lactic acid. It occurs in the cytoplasm and does not use oxygen as the final electron acceptor. Its main purpose is to regenerate NAD+ from NADH so that glycolysis can continue. Alcoholic fermentation occurs in yeast and some plant tissues, producing ethanol and carbon dioxide. Lactic acid fermentation occurs in some bacteria and animal muscle cells, producing lactic acid without carbon dioxide release. Because fermentation stops after glycolysis, only 2 ATP are obtained per glucose. NEET frequently tests end products, organisms, CO2 release and ATP yield.

Aerobic respiration is the oxygen-dependent complete oxidation of glucose into carbon dioxide and water, producing a large amount of ATP. After glycolysis, pyruvate enters the mitochondrion and undergoes oxidative decarboxylation to form acetyl CoA, releasing CO2 and NADH. Acetyl CoA enters the Krebs cycle, also called TCA or citric acid cycle, in the mitochondrial matrix. Each turn of Krebs cycle oxidizes one acetyl CoA and produces CO2, NADH, FADH2 and ATP/GTP. Since one glucose gives two pyruvates, the link reaction and Krebs cycle occur twice per glucose. The reduced coenzymes NADH and FADH2 then donate electrons to ETC, where most ATP is produced through oxidative phosphorylation.

Electron transport is the final stage of aerobic respiration and occurs on the inner mitochondrial membrane. Reduced coenzymes NADH and FADH2 donate high-energy electrons to a series of electron carriers. As electrons move through complexes, their energy pumps protons from the matrix into the intermembrane space. This creates a proton gradient and electrochemical potential. According to chemiosmotic theory, protons flow back into the matrix through ATP synthase, driving ATP formation from ADP and inorganic phosphate. Oxygen acts as the final electron acceptor and combines with electrons and protons to form water. This process is called oxidative phosphorylation because oxidation of NADH/FADH2 is coupled with ATP synthesis.

Respiratory quotient, or RQ, is the ratio of the volume of carbon dioxide released to the volume of oxygen consumed during respiration. It is useful because different substrates have different chemical compositions and therefore require different amounts of oxygen for complete oxidation. Carbohydrates such as glucose have RQ equal to 1 because the volume of CO2 produced equals the volume of O2 used. Fats have RQ less than 1 because they are oxygen-poor and require more oxygen for oxidation. Organic acids may show RQ greater than 1. Proteins usually have an RQ around 0.8. In NEET, RQ is tested through formulas, numerical values, substrate identification and special cases such as germinating fatty seeds.

Respiration is traditionally described as a catabolic pathway because it breaks down glucose and other substrates to release energy. However, NCERT emphasizes that the respiratory pathway is amphibolic, meaning it participates in both catabolism and anabolism. Many intermediates of glycolysis and Krebs cycle are withdrawn for biosynthesis of amino acids, fatty acids, nucleotides and other cell materials. Conversely, fats and proteins can be converted into respiratory intermediates and enter the pathway for energy production. Thus, respiration acts as a metabolic hub integrating breakdown, biosynthesis and energy flow. The respiratory balance sheet is a simplified ATP accounting model, but real cells adjust pathways according to energy demand, substrate availability and biosynthetic needs.