Respiration

• Aerobic respiration is an example of metabolic pathway because it is made of series of metabolic reactions.
• Aerobic respiration releases energy needed for metabolic processes in the body.
• The extra oxygen needed to breakdown lactate is known as Oxygen Debt, which is why we continue to breathe fast and deep after strenuous exercise in order to repay the oxygen debt.
• The overall equation of aerobic respiration is: C6H12O6 + 6O2 ➔ 6CO2 + 6H2O + energy.
• The first three stages of respiration are a series of reactions.
• The products from these reactions are used in the final stage to produce loads of ATP.
• The first stage of respiration, glycolysis, takes place in the cytoplasm of cells and the other three stages take place in the mitochondria.
• Each reaction in respiration is controlled and catalyzed by specific intracellular enzyme.
• The enzyme with the slowest activity is the rate limiting - it determines the overall rate of respiration.
• Coenzymes are used in respiration, for example: NAD and FAD transfer hydrogen from one molecule to another - this means they can reduce (give hydrogen to) or oxidize (take hydrogen from) a molecule.
• Coenzyme A transfers acetate between molecules.
• Glycolysis is the first stage in both aerobic and aerobic respiration and it doesn’t need oxygen to take place.
• There are two steps in glycolysis: Phosphorylation and Oxidation.
• The concentration of protons is higher in the intermembrane space than in the mitochondrial matrix, forming an electrochemical gradient (a concentration gradient of ions).
• Protons move down the electrochemical gradient, back into the mitochondrial matrix, via the enzyme ATP synthase.
• The movement of H+ ions across a membrane, which generates ATP, is called chemiosmosis.
• In the mitochondrial matrix, at the end of the transport chain, the protons, electrons and O2 (from the blood) combine to form water.
• Oxygen is the final electron acceptor.
• Total ATP produced:
• Oxidative phosphorylation makes ATP using energy from the reduced coenzymes, resulting in 3 ATP made from each reduced NAD and 2 ATP made from each reduced FAD.
• The table below shows how much ATP a cell can make from one molecule of glucose in aerobic respiration.
• Inhibition of the electron transport chain can be caused by some metabolic poisons, preventing electrons from moving down the electron transport chain, which stops chemiosmosis.
• Reduced NAD and reduced FAD are no longer oxidized, so NAD and FAD aren’t regenerated for the Krebs cycle, causing it to stop.
• Without oxygen, only glycolysis takes place, producing only 2 ATP.
• The rate of respiration can be measured using a respirometer, which measures the volume of oxygen being taken up in a given time.
• A respirometer measures the volume of oxygen being taken up by some woodlice.
• In the first step of glycolysis, Glucose is phosphorylated by adding 2 phosphates from 2 molecules of ATP, making the sugar more reactive.
• The phosphorylated sugar is then broken down to give two molecules of a 3-carbon sugar called triose phosphate.
• In the second step of glycolysis, two molecules of Triose phosphate are oxidized (lose hydrogen), to form 2 molecules of pyruvate.
• NAD collects the hydrogen atoms, forming 2 reduced NAD and changing pyruvate into acetate.
• Acetate is combined with coenzyme A (CoA) to form Acetyl Coenzyme A (Acetyl CoA).
• The link reaction and the third stage (Krebs cycle) happen twice for every glucose molecule.
• For every glucose molecule, two molecules of acetyl coenzyme A go into the Krebs cycle, two CO2 molecules are released as a waste product of respiration, and two molecules or reduced NAD are formed and are used in the last stage (oxidative phosphorylation).
• The Krebs cycle involves a series of oxidation reduction reactions, each controlled by a specific intracellular enzyme, found in the matrix of the mitochondria.
• In the first step of the Krebs cycle, acetyl CoA (2 carbon) from the link reaction combines with oxaloacetate (4 carbon) to form citrate (6 carbon), with coenzyme A going back to the link reaction to be used again.
• In the second step of the Krebs cycle, CO2 is removed from citrate (6 carbon), a process called decarboxylation, and hydrogen is also removed, a process called dehydrogenation, producing one molecule of reduced NAD and two molecules of reduced FAD.
• The 5C molecule is then converted to oxaloacetate (4C molecule) in the Krebs cycle, with decarboxylation and dehydrogenation occurring, producing one molecule of reduced FAD and two molecules of reduced NAD.
• ATP is produced by the direct transfer of phosphate group from an intermediate compound to ADP, a process called substrate-level phosphorylation.
• The cycle happens once for every pyruvate molecule, so it goes twice every glucose molecule.
• Some products of Krebs cycle are reused, some are released and others are used for the next stage of respiration.