Chapter 12: Respiration in Plants | Summary

Plants do respire — they require O2 and release CO2 — but unlike animals, they have no specialised respiratory organs. Instead, they use stomata and lenticels for gaseous exchange. Each plant part manages its own gas-exchange needs independently, with very little transport of gases between parts. Roots, stems, and leaves respire at rates far lower than animals, so the demand for gas exchange is minimal. In photosynthesising cells, O2 produced internally meets the respiration requirement directly.

The living cells of plants are arranged so that each cell is close to the surface or to air spaces. In woody stems, living cells are located in thin layers under the bark and have lenticels for aeration. The loose packing of parenchyma cells in leaves, stems, and roots creates an interconnected network of air spaces that facilitates diffusion. Dead cells in the interior of stems provide mechanical support but do not respire. This structural arrangement allows plants to survive without a dedicated respiratory system.

Glycolysis, derived from the Greek words glycos (sugar) and lysis (splitting), is the partial oxidation of glucose into two molecules of pyruvic acid. Proposed by Gustav Embden, Otto Meyerhof, and J. Parnas — hence also called the EMP pathway — it occurs in the cytoplasm of all living organisms and is the sole respiratory process in anaerobic organisms. In plants, glucose for glycolysis is derived from sucrose (converted by invertase into glucose and fructose) or from storage carbohydrates. Both glucose and fructose are phosphorylated by hexokinase to glucose-6-phosphate, which isomerises to fructose-6-phosphate before entering the common pathway.

Glycolysis consists of a chain of ten enzyme-controlled reactions. ATP is consumed at two steps: conversion of glucose to glucose-6-phosphate, and conversion of fructose-6-phosphate to fructose-1,6-bisphosphate. Fructose-1,6-bisphosphate is cleaved into dihydroxyacetone phosphate (DHAP) and 3-phosphoglyceraldehyde (PGAL). NADH+H+ is generated when PGAL is oxidised to 1,3-bisphosphoglycerate (BPGA). ATP is synthesised during conversion of BPGA to 3-phosphoglyceric acid (PGA) and again when phosphoenolpyruvate (PEP) is converted to pyruvic acid. The net yield from glycolysis is 2 ATP and 2 NADH+H+ per glucose molecule. Pyruvic acid is the key end product, and its subsequent fate depends on the cellular conditions and organism type.

Fermentation is the incomplete oxidation of glucose under anaerobic conditions. In yeast and many prokaryotes, pyruvic acid is converted to CO2 and ethanol by the enzymes pyruvic acid decarboxylase and alcohol dehydrogenase — this is alcoholic fermentation. In other bacteria and in animal muscle cells under oxygen deficiency, pyruvic acid is reduced to lactic acid by lactate dehydrogenase — this is lactic acid fermentation. In both pathways, NADH+H+ is reoxidised to NAD+, allowing glycolysis to continue.

Fermentation is energetically inefficient: less than 7% of the energy in glucose is released, and not all of it is trapped as ATP. The net gain is only 2 ATP molecules per glucose. The process is also hazardous because it produces either ethanol (toxic to yeast above ~13%) or lactic acid. Compared to aerobic respiration, NADH is reoxidised slowly in fermentation. Fermentation occurs in many prokaryotes, unicellular eukaryotes, and in germinating seeds. It represents an evolutionarily ancient pathway that predates the oxygen-rich atmosphere.

Aerobic respiration is the complete oxidation of organic substrates in the presence of oxygen, releasing CO2, water, and a large amount of energy. It takes place in the mitochondria of eukaryotic cells. Pyruvate, produced in the cytoplasm during glycolysis, is transported into the mitochondrial matrix where it undergoes oxidative decarboxylation catalysed by pyruvic dehydrogenase (requiring NAD+ and Coenzyme A) to yield acetyl CoA, CO2, and NADH+H+. Two molecules of pyruvate from one glucose yield two molecules of acetyl CoA and two molecules of NADH.

Acetyl CoA enters the Tricarboxylic Acid (TCA) or Krebs' cycle in the mitochondrial matrix. The cycle begins with the condensation of acetyl CoA with oxaloacetic acid (OAA) to form citric acid, catalysed by citrate synthase. Through successive decarboxylations and oxidations, the cycle regenerates OAA and produces 3 NADH+H+, 1 FADH2, and 1 GTP per turn. Since two turns occur per glucose molecule, the TCA cycle produces 6 NADH+H+, 2 FADH2, and 2 ATP equivalents in total. The complete oxidation of one glucose via glycolysis and TCA generates 8 NADH+H+, 2 FADH2, and 4 ATP before the electron transport stage.

The Electron Transport System (ETS) is located on the inner mitochondrial membrane and is responsible for the bulk of ATP synthesis. NADH+H+ produced in the mitochondrial matrix is oxidised by NADH dehydrogenase (Complex I), and electrons are passed to ubiquinone. FADH2 (from Complex II, succinate dehydrogenase) also donates electrons to ubiquinone. Reduced ubiquinone (ubiquinol) passes electrons to cytochrome bc1 complex (Complex III), then to cytochrome c (a mobile carrier on the outer surface of the inner membrane), and finally to cytochrome c oxidase (Complex IV), which contains cytochromes a and a3 and two copper centres. Oxygen is the terminal electron acceptor, being reduced to water.

As electrons move through Complexes I to IV, the energy released is used by ATP synthase (Complex V) to synthesise ATP from ADP and inorganic phosphate — a process called oxidative phosphorylation. This is explained by the chemiosmotic hypothesis: proton (H+) gradients generated across the inner membrane drive ATP synthesis. Each NADH oxidised yields 3 ATP; each FADH2 yields 2 ATP. Complex V consists of F1 (peripheral, catalytic) and F0 (integral membrane channel) components; 4H+ pass through F0 per ATP synthesised. The theoretical net gain from complete aerobic respiration of one glucose molecule is 38 ATP.

The respiratory balance sheet attempts to calculate the net gain of ATP per glucose molecule oxidised. This calculation rests on four assumptions: glycolysis, TCA cycle, and ETS operate sequentially; NADH from glycolysis enters mitochondria for oxidative phosphorylation; no intermediates are diverted for other biosynthetic purposes; and only glucose is used as the substrate. Under these assumptions, the theoretical maximum ATP yield is 38 molecules per glucose during aerobic respiration.

However, in living cells these assumptions are not valid. All pathways operate simultaneously; substrates enter and leave the pathway as needed; ATP is used continuously; and enzyme activity is regulated by multiple mechanisms. Therefore, the actual ATP yield is always lower than the theoretical 38. Despite this limitation, the balance sheet is a useful exercise to understand the efficiency and elegance of cellular energy metabolism. It also highlights why aerobic respiration is far more productive than fermentation, which yields only 2 ATP per glucose.

The respiratory pathway has traditionally been viewed as a catabolic pathway because it involves the breakdown of substrates to release energy. However, this view is incomplete. The same intermediates produced during respiration — such as acetyl CoA, pyruvate, and Krebs' cycle intermediates — are also used as starting materials for the biosynthesis of fatty acids, amino acids, and other molecules. Fats are broken down to glycerol (which enters as PGAL) and fatty acids (which enter as acetyl CoA). Proteins are digested by proteases, and amino acids (after deamination) enter as pyruvate, acetyl CoA, or Krebs' cycle intermediates.

Because the respiratory pathway participates in both anabolism (synthesis) and catabolism (breakdown), it is more accurately described as an amphibolic pathway. The same pathway point serves as both an entry and an exit depending on the metabolic needs of the organism. This dual role makes the respiratory pathway central to all aspects of cellular metabolism, not merely energy generation. Recognising this amphibolic nature is essential to understanding the integrated nature of metabolism in living organisms.

The Respiratory Quotient (RQ), also called the respiratory ratio, is the ratio of the volume of CO2 evolved to the volume of O2 consumed during aerobic respiration. It is expressed as: RQ = Volume of CO2 evolved / Volume of O2 consumed. The value of RQ depends on the type of respiratory substrate being oxidised. When carbohydrates are completely oxidised, equal volumes of CO2 and O2 are involved, giving an RQ of 1.0. When fats are the substrate — for example, tripalmitin — more O2 is required relative to CO2 produced, giving an RQ less than 1 (approximately 0.7).

When proteins are used as respiratory substrates, the RQ is approximately 0.9. In living organisms, a single pure substrate is rarely used exclusively; respiratory substrates are usually a mixture of carbohydrates, fats, and proteins. Therefore, the measured RQ in living systems reflects the combined oxidation of multiple substrates simultaneously. The RQ value thus provides useful information about the metabolic status and substrate preference of an organism under given physiological conditions.

Respiration in plants is a highly organised, stepwise process that converts the chemical energy stored in glucose into usable ATP. Beginning with glycolysis in the cytoplasm, through fermentation or the aerobic pathway (Krebs' cycle and ETS) in the mitochondria, the process ensures that energy is released gradually and efficiently rather than all at once. The theoretical net gain of 38 ATP molecules from one glucose molecule in aerobic respiration illustrates the metabolic superiority of the aerobic route over fermentation's 2 ATP. Recognising the respiratory pathway as amphibolic rather than purely catabolic captures its central, integrative role in cellular metabolism.

For CBSE board examinations, Chapter 12 carries significant weightage as it covers foundational concepts in bioenergetics. Key areas frequently tested include the steps and location of glycolysis, the reactions and products of the Krebs' cycle, the structure and function of the ETS and ATP synthase, the comparison of aerobic respiration with fermentation, the concept of the amphibolic pathway, and numerical questions on the Respiratory Quotient. Students should be able to draw and label diagrams of the Krebs' cycle and ETS, write summary equations, and calculate net ATP yields with justification of the assumptions involved.