Respiration In Plants

• All living organisms need energy for daily life activities, obtained by oxidation of macromolecules called 'food'
• Green plants and cyanobacteria can prepare their own food through photosynthesis, converting light energy into chemical energy stored in carbohydrates like glucose, sucrose, and starch
• Not all cells in green plants photosynthesize; only cells containing chloroplasts carry out photosynthesis, while non-green parts require food for oxidation
• Animals obtain food from plants directly (herbivores) or indirectly (carnivores), while saprophytes like fungi depend on dead and decaying matter
• All food respired for life processes ultimately comes from photosynthesis
• Cellular respiration involves the breakdown of food materials within the cell to release energy and the synthesis of ATP
• Photosynthesis occurs in chloroplasts, while the breakdown of complex molecules to yield energy occurs in the cytoplasm and mitochondria
• Respiration involves the oxidation of respiratory substrates like carbohydrates, proteins, fats, and organic acids to release energy in a series of controlled reactions
• Energy released by oxidation in respiration is used to synthesize ATP, the energy currency of the cell, for various energy-requiring processes
• Plants require O2 for respiration and release CO2, using stomata and lenticels for gas exchange
• Plants do not have specialized organs for gas exchange like animals; each plant part takes care of its own gas-exchange needs
• Glycolysis is a process in respiration where glucose undergoes partial oxidation to form pyruvic acid, occurring in the cytoplasm of all living organisms
• In glycolysis, glucose is derived from sucrose or storage carbohydrates, undergoing a chain of ten reactions to produce pyruvate, with ATP and NADH + H+ utilisation and synthesis at different steps
• Pyruvic acid is the key product of glycolysis
• The metabolic fate of pyruvate depends on the cellular need
• There are three major ways in which different cells handle pyruvic acid produced by glycolysis: lactic acid fermentation, alcoholic fermentation, and aerobic respiration
• Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes
• For the complete oxidation of glucose to CO2 and H2O, organisms adopt Krebs’ cycle, also known as aerobic respiration, which requires O2 supply
• In lactic acid fermentation, pyruvic acid is converted to CO2 and ethanol under anaerobic conditions
• In alcoholic fermentation, pyruvic acid is converted to CO2 and ethanol by yeast
• In animal cells like muscles during exercise, pyruvic acid is reduced to lactic acid by lactate dehydrogenase when oxygen is inadequate for cellular respiration
• In both lactic acid and alcohol fermentation, not much energy is released, and the processes are hazardous as either acid or alcohol is produced
• The net ATPs synthesized when one molecule of glucose is fermented to alcohol or lactic acid can be calculated by deducting the number of ATP utilized during glycolysis
• In aerobic respiration, the final product of glycolysis, pyruvate, is transported from the cytoplasm into the mitochondria
• In aerobic respiration, pyruvate undergoes oxidative decarboxylation by pyruvic dehydrogenase in the mitochondrial matrix
• The acetyl CoA formed from pyruvate enters the tricarboxylic acid cycle (Krebs’ cycle) for further oxidation
• The TCA cycle involves the stepwise oxidation of acetyl CoA to produce CO2, NADH, FADH2, and ATP
• The continued oxidation of acetyl CoA via the TCA cycle requires the replenishment of oxaloacetic acid and regeneration of NAD+ and FAD+ from NADH and FADH2, respectively
• Electrons from NADH produced in the TCA cycle are oxidised through the electron transport system (ETS) in the inner mitochondrial membrane to produce ATP
• The ETS involves the transfer of electrons from one carrier to another, culminating in the production of ATP from ADP and inorganic phosphate
• The presence of oxygen in aerobic respiration is vital as it acts as the final hydrogen acceptor, driving the whole process by removing hydrogen from the system
• In respiration, phosphorylation requires energy, while in respiration, the energy of oxidation-reduction is utilized for the same process, known as oxidative phosphorylation
• The electron transport system is utilized in synthesizing ATP with the help of ATP synthase (complex V), consisting of two major components: F1 and F0
• F1 headpiece is a peripheral membrane protein complex containing the site for ATP synthesis from ADP and inorganic phosphate
• F0 is an integral membrane protein complex forming the channel through which protons cross the inner membrane, coupled to the catalytic site of the F1 component for ATP production
• For each ATP produced, 4H+ passes through F0 from the intermembrane space to the matrix down the electrochemical proton gradient
• During aerobic respiration of one glucose molecule, there can be a net gain of 38 ATP molecules
• Fermentation only partially breaks down glucose, while aerobic respiration completely degrades it to CO2 and H2O
• In fermentation, there is a net gain of only two ATP molecules for each molecule of glucose degraded to pyruvic acid, whereas aerobic respiration generates many more ATP molecules
• NADH is oxidized to NAD+ slowly in fermentation, but vigorously in aerobic respiration