RESPIRATION IN PLANTS

Cellular respiration is the stepwise oxidation of food within cells to release energy and trap it in ATP. Glucose is the favoured respiratory substrate, but fats, proteins and organic acids can also be respired. ATP is the energy currency used for cellular work.

Respiration breaks C-C bonds by controlled enzyme-catalysed reactions. Energy is not released all at once as heat; it is captured gradually as ATP, while carbon skeletons supply precursors for biosynthesis.

Plants require oxygen for respiration and give out carbon dioxide, but have no specialised respiratory organs. Stomata and lenticels permit diffusion. Each plant part largely meets its own gaseous-exchange needs, and living cells are close to the surface or connected to air through parenchymatous intercellular spaces.

Gas-exchange demand is much lower than in animals. In leaves, photosynthesis itself releases oxygen. In woody stems, the living cells are arranged in thin layers beneath the bark, whereas inner cells are dead and provide mechanical support.

Glycolysis means sugar splitting and is also called the Embden-Meyerhof-Parnas (EMP) pathway. It occurs in the cytoplasm of all living organisms. It does not directly require oxygen and is the only respiratory process in anaerobic organisms.

One glucose molecule (6C) undergoes partial oxidation through ten enzyme-controlled reactions to form two pyruvate molecules (3C each). In plants, sucrose is split by invertase into glucose and fructose; both enter glycolysis after phosphorylation.

The fate of pyruvate depends on oxygen availability and cellular requirement. Under anaerobic conditions, many prokaryotes and unicellular eukaryotes ferment it. Fermentation regenerates NAD⁺ from NADH + H⁺, enabling glycolysis to continue.

Fermentation releases less than 7% of the energy in glucose and yields only 2 net ATP per glucose. Acid or alcohol accumulation is hazardous; yeast dies when alcohol concentration reaches about 13%.

In eukaryotes, pyruvate enters the mitochondrial matrix and undergoes oxidative decarboxylation by pyruvate dehydrogenase. NAD⁺ and CoA participate, forming acetyl CoA, CO₂ and NADH + H⁺. Two pyruvate molecules yield two NADH per glucose at this stage.

The acetyl CoA enters the tricarboxylic acid (TCA) or Krebs cycle. Acetyl CoA condenses with oxaloacetate (OAA) and water to form citrate. Citrate is isomerised to isocitrate; successive decarboxylations yield alpha-ketoglutarate and succinyl CoA. Subsequent reactions regenerate OAA.

The electron transport system (ETS) is located on the inner mitochondrial membrane. It oxidises NADH + H⁺ and FADH₂, transferring electrons ultimately to oxygen, which is reduced to water. Oxygen is therefore vital as the final hydrogen/electron acceptor.

Electrons from NADH enter through NADH dehydrogenase (complex I) and pass to ubiquinone. FADH₂ enters via complex II. Ubiquinone transfers electrons to complex III, then cytochrome c carries them to complex IV (cytochrome c oxidase) before oxygen accepts them.

The theoretical aerobic yield is a net gain of 38 ATP per glucose. This calculation assumes an orderly sequence of glycolysis, TCA cycle and ETS; mitochondrial transfer of glycolytic NADH; no diversion of intermediates for synthesis; and glucose as the sole substrate.

These assumptions are not completely true in living systems: pathways operate simultaneously, intermediates enter and leave as needed, ATP is used continuously and enzyme rates are multiply controlled. The calculation nevertheless illustrates the efficiency of stepwise energy capture.

Respiration is not purely catabolic. Carbohydrates enter as glucose; fats first yield glycerol and fatty acids, with glycerol entering as PGAL and fatty acids as acetyl CoA. Proteins are digested to amino acids, which after deamination can enter as pyruvate, acetyl CoA or at different points of Krebs cycle.

The same respiratory intermediates can be withdrawn for biosynthesis. For example, acetyl CoA is used for fatty-acid synthesis when required. Because the pathway participates in both breakdown (catabolism) and synthesis (anabolism), respiration is an amphibolic pathway.

Respiratory quotient (RQ), or respiratory ratio, is the volume of CO₂ evolved divided by the volume of O₂ consumed during respiration.

1. Why do plants not need specialised respiratory organs?
2. Compare glycolysis, fermentation and Krebs cycle by site, substrate and products.
3. State the three fates of pyruvate.
4. Describe oxidative decarboxylation of pyruvate and the major events of TCA cycle.
5. Trace electron flow through the ETS and explain oxidative phosphorylation.
6. Why is the 38 ATP balance sheet theoretical?
7. Explain why the respiratory pathway is amphibolic.
8. Define RQ and give its value for carbohydrate, fat and protein substrates.