7: Metabolism and enzymes Higher Biology Unit 2 7: Metabolism and enzymes

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

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

Reversible and irreversible steps Enzyme A Glucose Intermediate 1 IRREVERSIBLE 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.

Reversible and irreversible steps Glycogen (mammals) Enzyme B Intermediate 1 Intermediate 2 REVERSIBLE Starch (plants) 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.

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

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

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

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?

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.

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.

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.

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

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

What is active transport? Does it require energy?

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.

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?

Activation energy an enzyme action

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.

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.

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

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.

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.

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.

Direction of enzyme action 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.