Topic 12 - Respiration

• Aerobic respiration requires oxygen and produces carbon dioxide, water and lots of ATP.
• Anaerobic respiration occurs in the absence of oxygen and produces lactate (in animals) or ethanol and carbon dioxide (in plants and fungi), but little ATP.
• Aerobic respiration can be split into four stages : glycolysis, the Link Reaction, Krebs Cycle and oxidative phosphorylation.
• Glycolysis involves the splitting of the 6-carbon glucose molecule into two 3-carbon pyruvate molecules.
• The Link Reaction is when the 3-carbon pyruvate molecules enter into a series of reactions which lead to the formation of acetylcoenzyme A, a 2-carbon molecule.
• The Krebs Cycle involves the introduction of acetylcoenzyme A into a cycle of oxidation-reduction reactions that yield some ATP and a large quantity of reduced NAD and FAD.
• Oxidative phosphorylation involves the use of electrons, associated with NAD and FAD, released from the Krebs Cycle to synthesise ATP with water produced as a by-product.
• Glycolysis is the initial stage of both aerobic and anaerobic respiration. It occurs in the cytoplasm of all living cells, and is the process by which a hexose sugar is split into two molecules of 3-carbon pyruvate.
• The first stage of glycolysis is phosphorylation of glucose to hexose (glucose) phosphate and then to hexose bisphosphate. Two phosphate molecules are added to glucose prior to splitting, which come from the hydrolysis of two molecules of ATP to ADP.
• The hydrolysis of ATP to ADP in the first stage of glycolysis provides energy to activate the glucose and lowers the activation energy for the enzyme-controlled reactions that follow.
• The second stage of glycolysis involves the splitting of the phosphorylated glucose. Each glucose molecule is split into two 3-carbon molecules known as triose phosphate.
• The third stage of glycolysis is the oxidation of triose phosphate, where is hydrogen is removed from each molecule of triose phosphate. This hydrogen is transferred to NAD to form reduced NAD.
• The fourth stage of glycolysis involves the production of ATP. This is where enzyme-controlled reactions convert each triose phosphate into another 3-carbon molecule called pyruvate. During this process, two molecules of ATP are regenerated from ADP.
• The overall yield for one glucose molecule undergoing glycolysis is two ATP molecules, two reduced NAD molecules and two molecules of pyruvate.
• Although the net yield of ATP is 2 for the glycolysis of one glucose molecule, overall four molecules are produced across the process. However, two are used up in the initial phosphorylation of glucose, giving a net yield of two.