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In respiration, various substances are used as respiratory substrates. The hydrolysis of these respiratory substrates is linked to the production of ATP. ATP production occurs when protons diffuse down an electrochemical gradient through molecules of the enzyme ATP synthase which is embedded in the membranes of cellular organelles. Respiration can be carried out both aerobically (with oxygen) and anaerobically (without oxygen). Both types of respiration produce ATP, but anaerobic respiration produces less. Both processes start with stage one of respiration called glycolysis which takes place in the cytoplasm. Aerobic respiration takes place in the mitochondria. A coenzyme is a molecule that aids the function of enzymes by the transfer of a chemical group from one molecule to another molecule (you saw this in photosynthesis). The coenzymes used in respiration are NAD, coenzyme A and FAD. NAD AND FAD transfer a hydrogen from one molecule to another molecule. This means that they reduce (give hydrogen to) another molecule. Coenzyme A transfers acetate between molecules.
Glycolysis makes pyruvate from glucose. This involves the splitting of one molecule of six carbon glucose into two smaller three carbon molecules of pyruvate. This process takes place in the cytoplasm of cells and is the first stage of both aerobic and anaerobic respiration. Glycolysis doesn’t require oxygen, so it is an anaerobic process.
Glycolysis involves two stages:
Phosphorylation
The glucose molecule is phosphorylated by adding a phosphate from a molecule of ATP which creates one molecule of hexose phosphate and one molecule of ADP. The hexose phosphate is then phosphorylated by ATP to form hexose bisphosphate and another molecule of ADP. The hexose bisphosphate then splits into two molecules of triose phosphate.
Oxidation
Triose phosphate is oxidised (loses hydrogen) forming two molecules of pyruvate. Two molecules of NAD are then reduced (NADH). At the end of glycolysis, four molecules of ATP are produced but two have been used up in stage one so there is a net gain of two ATP.
If respiration is aerobic, pyruvate from glycolysis enters the mitochondrial matrix by active transport. Pyruvate is oxidised to acetate, producing reduced NAD, acetate combines with coenzyme A (CoA) in the link reaction to produce acetyl coenzyme A (acetyl-CoA). In a series of oxidation-reduction reactions, the Krebs cycle generates reduced coenzymes and ATP by substrate-level phosphorylation and CO2 is lost. Acetyl coenzyme A reacts with a four-carbon molecule that enters the Krebs cycle (citric acid cycle). Acetyl-CoA combines with oxaloacetate (a 4-carbon molecule) to form a 6-carbon compound. In the following reactions the 6-carbon compound loses carbon dioxide and hydrogen. We call this process decarboxylation. This produces a 5-carbon compound, and NAD is reduced to NADH (reduced NAD). A single molecule of ATP is also produced as the result of substrate-level phosphorylation. A 4-carbon molecule is produced, and the cycle continues.
Synthesis of ATP by oxidative phosphorylation is associated with the transfer of electrons down the electron transport chain. Hydrogen atoms are released from reduced NAD (NADH) and reduced FAD (FADH2). The hydrogen ion splits into protons (H+) and electrons (e-). The energy is used by electron carriers to pump protons from the mitochondrial matrix into the intermembrane space. Electrons move along the electron transport chain, losing energy at each carrier. The inner mitochondrial membrane is folded into cristae, increasing the surface area, which maximises respiration. The movement drives the synthesis of ATP from ADP and Pi. This process is driven by movement of H+ ions across a membrane. This is called chemiosmosis (chemiosmotic theory). Protons move down the electrochemical gradient, back into the mitochondrial matrix, via ATP synthase. Oxygen is the final electron acceptor.
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RESPIRATION I
