cellular respiration

• respiration is a series of enzyme controlled reactions that generates ATP by the release of energy from food. ATP is the link between anabolic and catabolic reactions ; it is used to transfer energy to cellular processes which requires energy.
• Adenosine triphosphate (ATP) is an adenosine molecule with three inorganic phosphate (Pi) groups attached. Energy is released when the terminal (last) phosphate is removed.
• examples of cellular activities include muscle contraction, protein synthesis, cell division, and nerve impulse transmission.
• there are 3 stages of aerobic respiration : glycolysis, citric acid cycle, and electron transport chain
• glycolysis occurs in the cytoplasm of the cell. glucose is broken down into a series of intermediate molecules and finally to pyruvate. part 1 of glycolysis is called energy investment, as 2 ATP molecules are needed for the phosphorylation of the intermediates. part 2 is called energy gain because 4 ATP are generated (net gain of 2 ATP). Dehydrogenase enzymes remove hydrogen ions and electrons from intermediates. Hydrogen ions and electrons are picked up by NAD (hydrogen carrier/accepter) to form NADH and carried to the electron transport chain.
• the citric acid cycle takes place in the mitochondrial matrix, and only occurs is oxygen is present. pyruvate is broken down into an acetyl group, which binds with coenzyme A to form acetyl coenzyme A. Acetyl from acetyl CoA combines with oxaloacetate to form citrate. hydrogen and electrons combine with NAD to form NADH, which goes to the electron transport chain. one ATP is formed and CO2 is released. oxaloacetate is regenerated for the continuation of the cycle.
• The electron transport chain is a chain of protein molecules attached to the inner membrane of the mitochondria.
• NADH from glycolysis and the citric acid cycle passes electrons to the electron transport system.
• Electrons flow along the electron transport system and release energy.
• The energy from electron flow is used to pump hydrogen ions across the mitochondrial membrane to the inter-membrane space.
• The returning flow of hydrogen goes back into the matrix through ATP synthase, generating ATP from ADP + Pi.
• At the end of the chain, electrons and hydrogen combine with oxygen to form water.
• Oxygen is the final hydrogen ion and electron acceptor.
• in the absence of oxygen, fermentation takes place in the cytoplasm as it cannot progress to the citric acid cycle. it results in much less ATP being produced than in aerobic respiration.
• in animal cells, fermentation takes place during strenuous exercise. pyruvate is converted to lactate, but the process is reversible when oxygen becomes available.
• in plants and yeast, fermentation involves the conversion of pyruvate to ethanol and CO2. this process is irreversible because cells are damaged by the high concentrations of alcohol and the loss of carbon dioxide from the cells.