Re: GAMSAT Biology
Adenosine Triphosphate (ATP)
ATP is the source of energy used in cellular energy transactions. The chemical bonds in ATP are important for the storage and release of energy. An enzyme removes a phosphate group from ATP and energy is released. This energy is used in cellular reactions.
Metabolism is the total of all chemical reactions carried out by an organism. There are two types of reactions:
Catabolism: Reactions that produce energy by breaking down molecules.
Anabolism: Reactions that use energy to build up molecules.
There are 3 basic stages of metabolism. These include:
- Macromolecules are broken down into their constituent parts. E.g. proteins, lipids and polysaccharides are broken down into amino acids, fatty acids and monosaccharides respectively.
- These constituent parts are oxidized to produce acetyl CoA, pyruvate and other metabolites. During this process some ATP is formed, along with NADH and FADH2. This process does not directly use oxygen.
- If oxygen is present and the cell can utilize this oxygen, the metabolites formed (stage 2) can go into the citric acid cycle and oxidative phosphorylation will take place to form large amounts of energy (ATP, NADH, or FADH2).
Stages 1 and 2 both produce energy. These stages are called respiration. If no oxygen is used the respiration is anaerobic. If oxygen is used the respiration is aerobic.
Glycolysis is the first stage of anaerobic and aerobic respiration. Glycolysis involves breaking down a 6-carbon glucose molecule into two 3-carbon pyruvate molecules. Glycolysis will occur in the presence or absence of oxygen. The reactions of glycolysis occur in the cytosol of the cell. Overall 2 ATPs are used and 4 ATPs are produced. Two pyruvate molecules and 2 NADH molecules remain at the end of glycolysis.
Glycolysis is best learnt visually (above video) so students can see the process occurring in the cell.
If oxygen is present in a cell that can undergo aerobic respiration, the products of glycolysis (NADH and pyruvate) will move into the matrix of a mitochondrion. The NADH and pyruvate will diffuse (facilitated diffusion) through the outer membrane. The pyruvate diffuses into the matrix and is converted to acetyl CoA in a reaction that produces CO2 and NADH.
Krebs Cycle (Citric Acid cycle)
Acetyl CoA is a coenzyme, which transfers two carbons from pyruvate to oxaloacetic acid to initiate the Krebs cycle. Each turn of the Krebs cycle produces 1 ATP, 3 NADH and 1 FADH2. During each cycle two carbon atoms are lost as CO2.
Amino acids can be deaminated and converted to pyruvic acid or acetyl CoA. Fatty acids can be converted to acetyl CoA. The acetyl CoA is responsible for the initiation of the Krebs cycle.
The process of producing ATP in the Krebs cycle (citric acid cycle) is termed substrate-level phosphorylation.
Detailed knowledge of each step of the Krebs cycle is not required for the GAMSAT.
Electron Transport Chain
The electron transport chain is a series of proteins in the inner membrane of the mitochondrion. Electrons from NADH and FADH2 are passed through the series of proteins and are accepted by oxygen to form water. As these electrons are passed through the protein series, protons are pumped into the intermembrane space. This creates a proton gradient, which is termed the proton-motive force. This proton-motive force moves protons through ATP synthase to produce ATP.
2-3 ATP molecules are produced from each NADH and 2 ATP molecules are produced from FADH2.
The overall products and reactants for respiration are:
Glucose + O2 à CO2 + H2O
Enzymes are biological catalysts that lower the activation energy of a reaction. A lower activation energy results in the reaction proceeding more quickly than without an enzyme present. The enzyme itself is not changed or consumed in the reaction, so only a small amount of enzyme is needed and it can be recycled.
Enzymes contain active sites that conform to fit the shape of substrates. This allows for the substrate to bind to the active site of the enzyme. Most enzymes are proteins.
Enzyme function can be affected by environmental factors. The rate of an enzyme-catalyzed reaction is affected by the concentration of the reaction substrate and enzymes. Chemical and physical factors can alter the three-dimensional shape of the enzyme, which can affect the ability of the enzyme to catalyse the reaction. These factors include temperature, pH and the presence of regulatory molecules.
The rate of an enzyme catalysed reaction increases with increasing temperature, but only until it reaches a certain temperature called the optimum temperature. At temperatures above the optimum temperature the enzyme will denature and loss functionality.
The optimum temperature of an enzyme usually corresponds to the temperature in which it is found in the body.
Most enzymes have an optimum pH that ranges from a pH of 6-8. Enzymes that are not at their optimal pH will function with decreased efficiency and may even become denatured.
Inhibitors and activators
Different substances can bind to enzymes and alter their shape. An inhibitor is a substance that can bind to an enzyme and decrease its activity. There are two types of enzyme inhibition.
Competitive inhibition: Inhibitors compete with the substrate for the same active site on the enzyme.
Non-competitive inhibition: Inhibitors bind to a location other than the active site. This changes the shape of the enzyme and it will not be able to bind to the substrate.
An allosteric activator binds to allosteric sites in the enzyme. This keeps the enzyme in its active configuration and increases the activity of the enzyme.
Cofactors are chemical components that assist in the functioning of enzymes. Cofactors are usually metals and coenzymes are organic molecules such as vitamins.