Respiration in Plants

12.1 Do Plants Breathe?

Respiration is essential for all living organisms, including plants, as it provides the energy required for various life processes such as absorption, transport, movement, reproduction, and more. Plants do require oxygen (O₂) for respiration and release carbon dioxide (CO₂) as a byproduct. However, unlike animals, plants do not have specialized organs for gaseous exchange. Instead, they rely on structures like stomata on leaves and lenticels on stems and roots to facilitate the diffusion of gases. Each plant part manages its own gas exchange, and there is minimal transport of gases between parts. The rate of respiration in plants is generally lower than in animals, and the distance gases need to diffuse within plant tissues is short due to the proximity of living cells to the surface and the presence of air spaces within loosely packed parenchyma cells. The overall reaction for the complete combustion of glucose in plants is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy This reaction releases a large amount of energy, mostly as heat. To utilize this energy efficiently, plants break down glucose in multiple small steps rather than a single combustion reaction. This stepwise oxidation allows the energy to be trapped in the form of ATP, the energy currency of the cell. Some plant cells may also respire anaerobically when oxygen is scarce, such as in waterlogged soils. This leads to fermentation processes. The initial breakdown of glucose to pyruvic acid (glycolysis) occurs in the cytoplasm and does not require oxygen, making it common to all living organisms. This section sets the stage for understanding how plants respire and manage energy release through cellular respiration.

• Plants require oxygen for respiration and release carbon dioxide.
• Plants lack specialized respiratory organs but use stomata and lenticels for gas exchange.
• Each plant part handles its own gas exchange with minimal gas transport between parts.
• Respiration rates in plants are lower than in animals.
• Gases diffuse over short distances aided by loosely packed parenchyma cells and air spaces.
• Energy from glucose oxidation is released stepwise and trapped as ATP.
• 📌 Respiration: The enzymatic oxidation of organic molecules to release energy.
• 📌 Respiratory substrates: Compounds like carbohydrates, proteins, and fats oxidized during respiration.
• 📌 Stomata: Pores on leaf surfaces facilitating gas exchange.

12.2 Glycolysis

Glycolysis is the initial stage of cellular respiration and involves the enzymatic breakdown of glucose into two molecules of pyruvic acid. The term glycolysis comes from Greek words 'glycos' meaning sugar and 'lysis' meaning splitting. This pathway, also known as the Embden-Meyerhof-Parnas (EMP) pathway, occurs in the cytoplasm of all living cells and does not require oxygen, making it common to both aerobic and anaerobic organisms. In plants, glucose is derived from sucrose, which is either transported from photosynthetic cells or stored carbohydrates. Sucrose is first hydrolyzed by the enzyme invertase into glucose and fructose. Both monosaccharides are phosphorylated by hexokinase to glucose-6-phosphate and fructose-6-phosphate respectively. The pathway then proceeds through a series of ten enzyme-catalyzed reactions, ultimately producing two molecules of pyruvic acid. During glycolysis, ATP is both consumed and produced. Two ATP molecules are used in the early steps: one in converting glucose to glucose-6-phosphate and another in converting fructose-6-phosphate to fructose-1,6-bisphosphate. Later, four ATP molecules are produced by substrate-level phosphorylation, resulting in a net gain of two ATP molecules per glucose molecule. Additionally, the coenzyme NAD⁺ is reduced to NADH + H⁺ during the conversion of 3-phosphoglyceraldehyde (PGAL) to 1,3-bisphosphoglycerate (BPGA). This NADH carries electrons to later stages of respiration for ATP synthesis. The key product of glycolysis, pyruvic acid, can then enter different metabolic pathways depending on oxygen availability and the organism's needs. Under anaerobic conditions, fermentation pathways convert pyruvate into lactic acid or ethanol, while under aerobic conditions, pyruvate enters the mitochondria for further oxidation.

• Glycolysis splits one glucose molecule into two pyruvic acid molecules.
• Occurs in the cytoplasm and is common to all living organisms.
• Sucrose is converted to glucose and fructose before entering glycolysis.
• Two ATP molecules are used; four ATP molecules are produced, net gain is two ATP.
• NAD⁺ is reduced to NADH + H⁺ during glycolysis.
• Pyruvate produced can undergo fermentation or aerobic respiration.
• 📌 Glycolysis: The anaerobic breakdown of glucose into pyruvate with ATP and NADH production.
• 📌 Invertase: Enzyme that hydrolyzes sucrose into glucose and fructose.
• 📌 Hexokinase: Enzyme that phosphorylates glucose to glucose-6-phosphate.