Higher Biology Unit 2

1. Higher Biology Unit 2 7: Metabolism and enzymes

2. Cell metabolism • What is a metabolic pathway? • Catabolic pathways – bring about the breakdown of complex molecules to simpler ones, releasing energy. • Give an example of such a reaction • Anabolic pathways – bring about the biosynthesis of complex molecules from simpler building blocks, require energy • Give an example of such a reaction

3. The two types of metabolic pathway Protein Glucose + oxygen anabolic pathway catabolic pathway energy CO2 + water Amino acids

4. Reversible and irreversible steps Enzyme A • Glucose diffuses from a high concentration outside the cell to a low concentration inside the cell, then converted to intermediate 1. • This reaction is irreversible • It is of benefit to the cell as it maintains a low concentration of glucose outside the cell and therefore allows glucose to diffuse constantly into the cell. Glucose Intermediate 1 IRREVERSIBLE

5. Reversible and irreversible steps Glycogen (mammals) Enzyme B • The conversion of Intermediate 1 to Intermediate 2 is reversible. • If more intermediate 2 if formed than the cell needs for the next step hen some can changed back into intermediate 1 and used in al alternative pathway, e.g to build glycogen in animal cells or starch in plant cells. Intermediate 1 Intermediate 2 REVERSIBLE Starch (plants)

6. Reversible and irreversible steps Enzyme C Intermediate 2 Intermediate 3 • The conversion of Intermediate 2 to Intermediate 3 is reversible. • Intermediate 3 will always be converted to pyruvate (through many further steps). IRREVERSIBLE

7. glucose Summary Enzyme A Starch (plants) intermediate 1 Glycogen (mammals) Enzyme B intermediate 2 Enzyme C intermediate 3 Many steps, many enzymes pyruvate

8. Alternative routes glucose Enzyme A intermediate 1 Alternative route • Pathways can be modified and contain alternative routes, so steps can be bypassed. Enzyme B intermediate 2 sorbitol Enzyme C intermediate 3 Several enzyme controlled steps Many steps, many enzymes pyruvate

9. Membranes This mitochondrion has a double membrane – an inner and outer membrane. The inner membrane is folded so that metabolic activity is localised – particular sites are responsible for particular chemical reactions. Here the metabolites are kept close to the enzymes that are required for the reactions to proceed. Which reactions occur n the mitochondrion?

10. Membranes • In this chloroplast the enzymes needed for ATP generation are bound together on flattened sacs containing chlorophyll • The Calvin cycle occurs in the fluid outside the sacs where the required enzymes are also present.

11. Lysosomes • Lysosomes contain strong digestive enzymes. • If these enzymes were free in the cytoplasm they would destroy organelles in the cytoplasm. • The lysosome ensures the enzymes remain compartmentalised until they are needed.

12. Surface area to volume ratio • The smaller the cube the larger the surface relative to its volume. • Membranes, folds and compartments within the cell present a larger surface are upon which metabolic reactions can take place.

13. Membrane transport • Write a definition of diffusion • Does diffusion require energy? • What size of molecules can freely diffuse across the cell membrane?

14. Role of protein pores • Larger molecules depend on protein carrier molecules to allow them to move across the membrane. • These transport proteins contains pores or channels. • These channels only allow specific substances to diffuse across the membrane. pore

15. Active transport • What is active transport? • Does it require energy?

16. Sodium/potassium pump • Certain proteins on the cell membrane act as carrier molecules which transfer specific ion across the membrane. • These proteins are sometimes called pumps. • They can have a dual role, e.g. the sodium/potassium pump actively pumps sodium ions out of the cell and potassium ions into the cell, both against their own concentration gradient.

17. Conditions required by protein pumps • The active transport of molecules across the membrane requires energy from respiration. • Therefore what conditions will affect the rate of active transport?

18. Activation energy an enzyme action

19. Activation energy and enzyme action • A chemical reaction may involve the joining together of simple molecules into more complex ones or the splitting of complex molecules into simpler ones. • Either way, energy is required to initially break the bonds (activation energy) in the reactants to form an unstable compound. The molecules are in the transition state. • When bonds are made to form the product energy is released. • Enzymes serve to lower the activation energy needed for a reaction to proceed. • Thus biochemical reactions are able to proceed rapidly at relatively low temperatures.

20. Enzymes are specific • What are enzymes made of? • Enzymes are specific – describe what this means. • Draw and label an enzyme and substrate fitting together. Label the active site.

22. Induced fit • The active site is not a rigid structure, it is flexible. • When the substrate enters the active site, the enzyme molecule and the active site change slightly making the active site fit very closely round the substrate molecule. • The induced fit ensures that the active site comes into very close contact with the molecules of substrate and increases the chance of a reaction taking place

23. Orientation of reactants • When the reaction involves two (or more) substrates, the shape of the active site helps orientate the reactants in the right position so a reaction can take place. • The active site holds the reactants together in an induced fit. • The chemical bonds in the reactants are weakened, the activation energy is reduced. • The products now have a low affinity for the active site and are released from the active site • The enzyme is free to repeat the process.

24. Factors affecting enzyme action • Describe the effect of temperature on enzyme action. Sketch a graph • Describe the effect of pH on enzyme action. • Enzyme action can also be affected by an inadequate supply of substrate or the presence of an inhibitor.

25. Effect of substrate concentration on enzyme activity • Low concentration – too few substrate molecules present to make use of all the active sites on the enzyme molecules. • Increasing substrate leads to an increase in reaction rate as more active sites are involved. • Further increase in substrate concentration does not increase rate of reaction further (graph levels off) since all the active sites are occupied. • The enzyme concentration has become a limiting factor.

26. Direction of enzyme action Enzyme 2 Enzyme 1 Enzyme 3 Metabolite W Metabolite X Metabolite Y Metabolite Z • The diagram above is metabolic pathway. • Once metabolite W becomes available enzyme 1 is activated and converts W to X. • When metabolite X becomes available enzyme 2 is activated and converts X to Y and so on…… • Most enzyme reactions are reversible, the actual direction of the reaction depends on the relative concentrations of reactants and products. • Thus a balance of metabolites is is always maintained.