Aerobic Respiration

Glycolysis
Glucose cannot be used as directly by cell as a source of energy so cells use ATP as their immediate source of energy. * This conversion of glucose into ATP takes place during cellular respiration and there are 2 different forms of cellular respiration depending upon whether oxygen is available or not * Aerobic respiration requires oxygen and produces CO2, water and lots of ATP. * Aerobic has 4 stages: 1) Glycolysis – the splitting of the 6-carbon glucose molecule into 2 3-carbon molecule pyruvate molecules. 2) Link reaction – the conversion of the 3-carbon pyruvate molecule into CO2 and a 2-carbon molecule called acetylcoenzyme A. 3) Krebs cycle – the introduction of acetylcoenzyme A into a cycle of oxidation-reduction reactions that yield some ATP and a large number of electrons. 4) Electron Transport Chain – the use of the electrons produced in Krebs to synthesis 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, usually glucose, is split into two molecules of pyruvate * Although there a number of smaller enzyme-controlled reactions in glycolysis, these can be grouped into 4 stages: 1) Activation of glucose by phosphorylation. Before it can be split into two, glucose must first be made more reactive by the addition of 2 phosphate molecules (phosphorylation). The phosphate molecules come from the hydrolysis of 2 ATP molecules to ADP. This provides the energy to activate glucose (lowers the activation energy) 2) Splitting of the phosphorylated glucose. Each glucose molecule is split into 2 3-carbon molecules known as triose phosphate 3) Oxidation of triose phosphate. Hydrogen is removed from each of the 2 triose phosphate molecules and transferred to a hydrogen-carrier molecule known as NAD to form reduced NAD 4) The production of ATP. Enzyme-controlled reactions convert each triose phosphate into another 3-carbon molecule called pyruvate. In the process, 2 molecules of ATP are regenerated from ATP
Energy Yields from Glycolysis * The overall yield from one glucose molecule undergoing glycolysis is therefore * 2 molecules of ATP (4 are produced but 2 were used in the initial phosphorylation of glucose so the net increase is 2) * 2 molecules of NADH (these go on into ETC) * 2 molecule of pyruvate * Glycolysis is a universal feature of every living organism, the enzymes are found in the cytoplasm of cells and so glycolysis does not need any organelle for it to take place. * It doesn’t need oxygen and so can take place when it is present or not, in the absence of oxygen the pyruvate can be converted into either lactate or ethanol and CO2.
Link Reaction and Krebs cycle
The Link Reaction * The pyruvate molecules produced in the cytoplasm during glycolysis are actively transported into the matrix of the mitochondria. * Here pyruvate undergoes a series of reactions during which the following changes take place: * The pyruvate is oxidised by removing hydrogen. This hydrogen is accepted by NAD producing NADH, which is later used to produce ATP. * The 2-carbon molecule, called an acetyl group, that is thereby formed combines with a molecule called coenzyme A (CoA) to produce a compound called acetylcoenzyme A * A CO2 molecule is formed from each pyruvate * The overall equation is:
Pyruvate + NAD + CoA acetyl CoA + NADH + CO2
The Krebs cycle * Reactions that take place in the matrix of the mitochondria. * It summarised as: * The 2-carbon acetylcoenzyme A from the link reaction combines with a 4-carbon molecule to form a 6-carbon molecule * This 6-carbon molecule loses CO2 and hydrogen’s to give a 4-carbon molecule and a single molecule of ATP produced as a result of substrate-level phosphorylation * The 4-carbon molecule can now combine with a new molecule of acetylcoenzyme A to begin the cycle again * For each molecule of pyruvate, the link reaction and the Krebs cycle therefore produce: * Reduced coenzymes such as NAD and FAD. These have the potential to produce ATP molecules and are therefore the important products of Krebs cycle * 1 molecule of ATP * 3 molecules of CO2 * As two pyruvate molecules are produced for each original glucose molecule, the yield from a single glucose molecule is double the above * 2 ATP * 6 CO2
Electron Transport Chain * Hydrogen atoms from Krebs are taken into the ETC – where the energy of the electrons within the hydrogen atoms is converted into ATP.
The Electron Transport Chain and Mitochondria * Mitochondria – rod shaped organelles with a double membrane (forming cristae with the inner membrane) and the matrix (made up of semi-rigid materials of protein, lipids and traces of DNA, found in eukaryotic cells). They are the site of the ETC and attached to the cristae are the enzymes and other proteins involved in the ETC and hence ATP synthesis.
The Electron Transport Chain and the Synthesis of ATP * ATP is synthesised using the ETC as follows: * The hydrogen atoms produced during glycolysis and Krebs combine with the coenzymes NAD and FAD that are attached to the cristae. NADH and FADH donate electrons of the hydrogen atom they are carrying to the 1st molecule in the ETC. * This releases protons from the hydrogen atoms and these are actively transported across the inner mitochondrial membrane. The electrons, meanwhile, pass along a chain of electron transport carrier molecules in a series of oxidation-reduction reactions. The electrons lose energy as they pass down the chain and some is used to create a minimal amount of ATP. * The protons accumulate in the space between the 2 mitochondrial membranes before the concentration becomes too high and so diffuse back into the matrix through a protein channel which have ATP synthase attached. This allows ATP synthase to produce ATP. At the end of the chain the electrons combine with these protons and oxygen to form water. * The importance of oxygen in respiration is to act as the final acceptor of the hydrogen atoms produced in glycolysis and Krebs.