Respiration is the process of breaking down complex organic compounds (C-C bonds) through oxidation within the cells, leading to the release of a considerable amount of energy. This energy is not released in a single step but in a series of slow, enzyme-controlled reactions and is trapped as chemical energy in the form of ATP (Adenosine Triphosphate).
Respiratory Substrates: The compounds that are oxidized during respiration. Glucose is the most favored substrate, but proteins, fats, and organic acids can also be used.
Energy Currency: ATP acts as the energy currency of the cell, utilized in various energy-requiring processes.
Plants require oxygen (O2) for respiration and release carbon dioxide (CO2). Unlike animals, they lack specialized respiratory organs but use stomata and lenticels for gaseous exchange.
Self-Sufficiency: Each plant part takes care of its own gas-exchange needs.
Low Demand: Plants have lower respiratory rates compared to animals.
Diffusion: In large plants, most living cells are located close to the surface, and loose packing of parenchyma cells provides interconnected air spaces.
Glycolysis (from Greek glycos for sugar and lysis for splitting) occurs in the cytoplasm and is the partial oxidation of glucose to form two molecules of pyruvic acid.
Mechanism: A chain of ten reactions discovered by Embden, Meyerhof, and Parnas.
ATP Usage: 2 ATP molecules are used (Glucose to Glucose-6-P and Fructose-6-P to Fructose-1,6-bisphosphate).
Energy Yield: 4 ATP are directly synthesized, and 2 NADH+H+ are formed.
Net Gain: 2 ATP and 2 NADH+H+ per glucose molecule.
Glycolysis Net Gain
Although 4 ATP are produced during the payoff phase, 2 ATP were consumed in the investment phase. Thus, the net gain of energy molecules is exactly 2 ATP and 2 NADH.
While theoretically 38 ATP molecules can be generated from one glucose molecule during aerobic respiration, the actual yield may vary due to metabolic complexities and the utilization of intermediates in other pathways.
Respiration is not purely a catabolic (breaking down) process. Many intermediates are withdrawn from the pathway to synthesize other molecules (e.g., Acetyl CoA for fatty acids). Because it involves both anabolism and catabolism, the respiratory pathway is described as amphibolic.