Unit 2

• Metabolism - all reactions which take place in a living cell.
• Anabolic - small molecules built up into larger molecules. Energy is required for this to happen.
• Catabolic - large molecules are broken down into smaller molecules. Energy is released.
• Metabolic pathways are integrated and controlled pathways of enzyme-catalysed reactions within a cell.
• Metabolic pathways can have reversible steps ( ofthen the same enzyme can catalyse the reaction in both directions ), irreversible steps and alternative routes.
• The plasma membrane of the cell controls which substances enter and leave the cell.
• Plasma Membrane is composed of a phospholip bilayer that has various proteins embed in it.
• Membranes can be around organelles within eukaryotic cells.
• Molecules pass the plasma membrane through passive ( diffusion or osmosis ) or active transport.
• Pore protein - forms a channel through the phospholipid bilayer.

  Allows larger molecules to cross the membrane by diffusion.
• Pump protein - Carrier molecule which spans the membrane.

  Moves substances across the membrane against their concentration gradien.
• Enzyme - proteins found embedded on the outside or inside of the membrane.

  Catalyse chemical reactions.
• Metabolic pathways are controlled either by the presence or absence of particular enzymes.
• Enzymes speed up the rate of reaction by lowering activation energy of a reaction.
• Induced Fit

  * Substrate has high affinity for active site = high tendency to bind with enzyme.

  * Products have low affinity for active site = low tendency to bind with enzyme.
• Substrate Concentration

  * Enzyme activity increases with increasing substrate concentration.

  > Active sites are occupied with substrate at all times, enzyme concentration is acting as a limiting factor.

  >> increase enzyme concentration to increase number of active sites.
• Inhibitors are substances which interfere with enzyme action and decrease the rate of reaction.
• Competitive inhibitors are molecules with a similar shape to the substrate. They bind at the active site preventing the substrate from doing so.

  Can be reversed by increasing substrate concentration.
• Non-Competitive inhibitors are molecules which bind away from active site. They change the shape of the active site preventing substrate from doing so.

  Cannot be reversed by increasing substrate concentration.
• Competitive inhibitor

  * low substrate concentration = lower rate of reaction.

  * high substrate concentration = normal rate of reaction.

  > inhibitor less likely to enter active site.
• Non Competitive inhibitor

  * Distort active site of some enzyme molecules and enzymes will not function at all.

  > low reaction rate
• Feedback inhibition occurs when end-product in the metabolic pathway reaches critical condition.

  The end-product then inhibits an earlier enzyme, preventing further synthesis of the end-product.

  When the concentration of the end product decreases, the inhibition is lifted.
• Respiration is a series of enzyme controlled reactions that releases energy from glucose.
• Glycolysis is the first stage of aerobic respiration. Takes place in the cytoplasm and does not require oxygen.
• During glycolysis, 2 ATP are used in the investment stage to produce 4 ATP in the energy pay off stage - results in a net gain of 2 ATP.
• Dehydrogenases remove hydrogen and electrons from intermediates during glycolysis and the citric acid cycle and pass them to Coenzyme A to be transferred to ETC
• The citric acid cycle takes place in the matrix of the mitochondria. This stage requires oxygen.
• Pyruvate is broken down to form an acetyl group which combines with Coenzyme A to form Acetyl Coenzyme A.
• The acetyl group combines with oxaloacetate to form citrate.
• During the citric acid cycle, 2 molecules of carbon dioxide are released, hydrogen and electrons are removed by dehydrogenases and combine with NAD to form NADH and enough energy is produced to form a molecule of ATP.
• NADH transfers the hydrogens and electrons that are combined with it to the electron transport chain.
• The electron transport chain takes place in the cristae of the mitochondria. Also requires oxygen to take place.
• An electron transport chain is a series of carrier proteins attached to the inner membrane of the mitochondria.
• The electrons brought by NADH flow along the electron transport chain releasing energy.
• Energy is used to actively transport hydrogen ions across the membrane. As the hydrogen ions flow back through the membrane protein ATP Synthase, they rotate and synthesise ATP.
• The hydrogen ions and electrons combine with oxygen to form water.
• If oxygen is not available, fermentation takes place.

  • Animals - pyruvate -> lactate
  • Plants - pyruvate -> ethanol and CO2
• ATP is used to transfer energy from respiration to the cellular processes.
• Metabolic rate is the rate of consumption of energy of an organism. Can be measured by Heat Production, Oxygen Consumption and Carbon Dioxide Production.
• To measure metabolic rate accurately, use respirometer or calorimeter