Higher Human Biology

1. Higher Human Biology Unit 1 Human Cells KEY AREA 6: Metabolic Pathways

2. Human Cells Learning Intentions KEY AREA 6 – Metabolic Pathways • Anabolic Pathways • Control of Metabolic Pathways

3. 6a) Metabolic Pathways Metabolism can be defined as all the chemical reactions that occur within the human body Metabolic pathways involve the building up and breaking down of molecules Anabolic pathways require energy and involve biosynthetic (building up) processes e.g. building up of amino acids to form proteins Catabolic pathways release energy and involve the breakdown of molecules e.g. breaking down of glucose in glycolysis Metabolic pathways can have reversible and irreversible steps, and alternative routes

4. 6b) Activation Energy Non-catalysed Reaction The rate of a chemical reaction is indicated by the quantity of chemical change that occurs per unit time The energy needed to break chemical bonds in the reactant chemicals is called the Activation Energy The bonds break when the molecules of the reactant have absorbed enough energy to make them unstable (this is called the Transition State) and the reaction can occur This energy input often takes the form of heat energy and the reaction only proceeds at a high rate if the chemicals are raised to a high temperature Catalysed Reaction

5. 6c) Enzymes & Activation Energy An Enzyme is a Biological Catalyst that:- Lowers the activation energy required by the reactants to form the transition state Speeds up the rate of the reaction is made of Protein Living cells cannot tolerate the high temperatures needed to make chemical reactions proceed at a rapid rate, therefore they make use of enzymes instead

6. 6d) Enzyme Specificity An enzyme contains an active site which is a specific shape (due to the structure and bonding of the amino acids in the polypeptide chain) An enzyme only acts on one substrate because the enzyme is specific to its substrate and the molecules of the substrate are complementary to the enzyme’s active site for which they show an affinity (chemical attraction) After the reaction, the products have a low affinity for the active site and are released, leaving the enzyme free to repeat this process with new molecules of the substrates

7. 6e) Active Site Induced Fit The active site of an enzyme is not a rigid structure. It is flexible and dynamic When a molecule of a substrate enters the active site, the shape of the enzyme molecule and the active site change slightly, making the active site fit very closely round the substrate molecule – this is called induced fit Induced fit ensures that the active site comes into very close contact with the molecules of substrate and increases the chance of the reaction taking place

8. 6f) Orientation of Reactants When the reaction involves two (or more) substrates, the shape of the active site determines the orientation of the reactants. This ensures that they are held together in such a way that the reaction can take place The active site holds the two reactants closely together in an induced fit, then weakens the chemical bonds that must be broken during the reaction. This reduces the activation energy needed to reach the transition state.

9. 6g) Factors affecting Enzyme Action To function efficiently, an enzyme requires a suitable: - Temperature pH Substrate concentration Enzyme concentration Inhibitors may slow down the rate of an enzyme-controlled reaction or bring it to a halt

10. 6h) Enzymes & Substrate Concentration As substrate concentration increases, rate of reaction increases and then the rate of the reaction remains constant as the active sites are all occupied because the enzyme concentration is the limiting factor

11. 6i) Control of Metabolic Pathways by presence/absence of Enzymes Metabolic pathways can be controlled by 1. Presence of an Enzyme 2. Absence of an Enzyme 3. Key enzymes regulating the rate of a reaction within a metabolic pathway Genes for some enzymes are continuously expressed. These enzymes are always present in the cell and their control involves regulation of their rate of reaction Most metabolic reactions are reversible and the presence of a substrate or the removal of a product will drive a sequence of reactions in a particular direction Often an enzyme can catalyse a reaction in both a forward and a reverse direction. The actual direction depends on the concentrations of the reactants & the products.

12. 6n) Control of Enzyme Action – Inhibitors An inhibitor is a substance that decreases the rate of an enzyme controlled reaction A Competitive Inhibitor has a similar molecular structure to the substrate competes with the substrate for the active sites on the enzyme prevents the substrate from binding, so the rate of the reaction decreases Competitive Inhibition can be reversed by increasing the substrate concentration

13. 6o) Control of Enzyme Action – Inhibitors A Non-Competitive Inhibitor does not have a similar molecular structure to the substrate does not combine directly with the active site attaches to a non-active (allosteric) site and changes the shape of the enzyme molecule – indirectly altering the shape of the active site so the substrate cannot bind and rate of the reaction decreases

14. 6p) Control of Enzyme Action - Regulators Enzymes have active sites and non-active (allosteric) sites Enzymes can change their shape if regulatory molecules bind to a non-active (allosteric) site If the regulatory molecule is an activator, the enzyme changes to its active state and enzyme activity is stimulated As activator molecules increase, the enzyme reaction rate increases If the regulatory molecule is a non-competitive inhibitor, the enzyme changes to its inactive state and enzyme activity is inhibited As inhibitor molecules increase, the enzyme reaction rate decreases

15. 6q) Control of Enzyme Action - Feedback Inhibition when the end-product in a metabolic pathway reaches a critical concentration the end product then inhibits an earlier enzyme, blocking the pathway this prevents further synthesis of the end-product This pathway is kept under finely tuned control by negative feedback Feedback inhibition prevents waste of resources

16. Human Cells Questions KEY AREA 6 – Metabolism and Enzymes • Testing Your Knowledge 1 Page 85 Q’s 1-4 2. Testing Your Knowledge 2 Page 96 Q’s 1-4 • Quick Quiz