Form 4 · Revision notes

Metabolism and Enzymes, revision notes

Complete revision notes for Metabolism and Enzymes: every content standard, the key definitions, comparison tables and a recall checklist for SPM Biology Form 4.

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Overview

Metabolism is the sum of all chemical reactions in a cell, building up (anabolism) and breaking down (catabolism). Almost all of these reactions are controlled by enzymes.

This chapter covers what enzymes are, how they work, the factors that change their activity, and their uses in daily life.

Two other factors also change the rate of an enzyme-catalysed reaction: substrate concentration and enzyme concentration. Increasing either one raises the rate because more collisions between enzyme and substrate can occur, but only up to the point where all the available active sites are already occupied at any instant, after which the rate levels off.

Enzymes are named mostly by adding the suffix '-ase' to the name of their substrate or the reaction they catalyse, such as amylase acting on starch or protease acting on protein. Recognising this naming pattern helps you work out what an unfamiliar enzyme does directly from its name in an exam question.

Anabolic reactions include photosynthesis and protein synthesis, both of which build larger molecules and require an input of energy; catabolic reactions include respiration and digestion, both of which break large molecules into smaller ones and release energy that the cell can use.

Digestive enzymes illustrate how optimum pH varies between locations in the body: pepsin in the stomach works best in strongly acidic conditions, while trypsin in the small intestine works best in mildly alkaline conditions, showing that 'optimum pH' is a property of each individual enzyme rather than a single fixed value for all enzymes.

A rate of reaction is calculated from a graph as the gradient of the curve, or more simply as the amount of product formed, or substrate used up, divided by the time taken; SPM questions often ask for this rate to be calculated directly from a results table rather than only described in words.

Metabolism: anabolism and catabolism (CS 5.1)

Content Standard 5.1 defines metabolism as the sum of every chemical reaction that takes place inside a living cell. These reactions fall into two opposite groups.

Anabolism builds large, complex molecules from smaller ones and needs an input of energy, as when amino acids are joined into a protein during protein synthesis, or when carbon dioxide and water are built into glucose during photosynthesis. Catabolism breaks large molecules down into smaller ones and releases energy the cell can use, as when glucose is broken down during respiration or food is broken down during digestion.

A frequent exam trap is treating 'metabolism' and 'digestion' as the same word. Digestion is only one catabolic process, the breakdown of food; metabolism is the far wider term that covers every reaction in the cell, anabolic and catabolic together.

Almost all of these reactions are controlled by enzymes, which is why this chapter moves straight from metabolism into the study of enzymes.

The energy released by catabolism does not disappear: it is transferred to drive the anabolic reactions that build the cell and to power other cell activities such as active transport and movement. Seeing metabolism as this constant two-way traffic, building up and breaking down at the same time, is what the definition is really testing.

Enzymes as biological catalysts and the lock-and-key model (CS 5.2)

An enzyme is a protein that acts as a biological catalyst: it speeds up a specific chemical reaction by lowering the activation energy needed to start it, yet is not used up or permanently changed by that reaction, so a single enzyme molecule can be used again and again. Because an enzyme stays unchanged, a cell needs only a small quantity of it.

Each enzyme is specific; it catalyses only one reaction or one type of substrate. The lock-and-key model explains this specificity.

A small region of the enzyme, the active site, has a three-dimensional shape that is complementary to one particular substrate, just as one key fits one lock. The substrate binds to the active site to form an enzyme-substrate complex; the reaction then takes place and the products leave, freeing the active site to bind another substrate molecule.

The active site owes its precise shape to the way the protein is folded. This is why anything that changes the folding, a large rise in temperature or an extreme pH, destroys the fit between enzyme and substrate, a point the next sections develop.

Factors affecting enzyme activity at a glance

Four factors change the rate of an enzyme-catalysed reaction, and each is examined through a graph. Setting them out together shows what happens as each factor increases and why, which is the explanation a marker looks for rather than the graph shape alone.

Factor	Effect as the factor increases	Reason	Shape of the graph
Temperature	Rate rises to an optimum, then falls sharply	Warmth gives molecules more kinetic energy and more collisions up to the optimum; beyond it the enzyme denatures	Rises to a peak, then drops steeply to zero
pH	Rate is highest at the optimum pH and lower on either side	Away from the optimum the active site changes shape; an extreme pH denatures the enzyme	A bell-shaped curve peaking at the optimum pH
Substrate concentration	Rate rises, then levels off	More substrate means more collisions until every active site is occupied and the enzyme becomes the limiting factor	Rises steeply, then flattens into a plateau
Enzyme concentration	Rate rises as long as substrate is in excess	More enzyme means more active sites available; the rate levels off once substrate becomes limiting	Rises steadily, then plateaus when substrate runs low

Temperature and pH: optimum and denaturation (CS 5.2)

As temperature rises from low values, the rate of an enzyme reaction increases because the enzyme and substrate molecules move faster and collide more often, so more enzyme-substrate complexes form each second. The rate is highest at the optimum temperature, which for human enzymes is around body temperature.

Above the optimum the rate falls steeply: the heat distorts the precise folding of the enzyme, the active site changes shape, and the substrate no longer fits. The enzyme is then denatured, and because the folding cannot reform, this change is permanent.

A low temperature, by contrast, does not denature an enzyme; it only slows it down, and activity returns when the enzyme is warmed again.

pH affects enzymes in a similar way. Each enzyme has an optimum pH at which its active site has the correct shape and the rate is highest; moving away from the optimum lowers the rate, and an extreme pH denatures the enzyme.

Different enzymes have different optimum pH values suited to where they work. Pepsin works in the strongly acidic stomach and has a low optimum pH, while trypsin works in the mildly alkaline small intestine and has a higher optimum pH, proof that 'optimum pH' is a property of each individual enzyme, not a single fixed value for all.

Substrate concentration and enzyme concentration (CS 5.2)

At a low substrate concentration, raising it increases the rate of reaction because more substrate molecules are available to collide with the enzymes' active sites, so more complexes form each second. Once the substrate concentration is high enough that every active site is occupied at any given moment, adding still more substrate has no further effect: the enzyme has become the limiting factor and the rate levels off into a plateau.

The graph therefore rises steeply and then flattens.

Enzyme concentration behaves in a matching way when substrate is plentiful. Increasing the amount of enzyme provides more active sites, so the rate rises.

This continues only while substrate is in excess; as soon as substrate becomes scarce it becomes the limiting factor, and adding more enzyme no longer raises the rate. Rate itself is measured as the amount of product formed, or substrate used up, divided by the time taken, and on a graph of amount against time it is the gradient, steepest and so highest near the start before the substrate is used up.

Applications of enzymes in daily life (CS 5.3)

Content Standard 5.3 looks at how enzymes are put to use outside the body. Biological detergents contain protease and lipase, which break down protein-based stains such as blood or egg and fat-based stains into smaller, soluble molecules that rinse away easily.

Because these enzymes work well around body temperature, the detergent cleans effectively at a lower washing temperature, which also saves the energy a hotter wash would need.

The food industry uses enzymes widely: pectinase breaks down pectin to clarify cloudy fruit juice and raise the volume extracted, amylase converts starch to sugar in the production of syrups, and rennin (from rennet) clots milk protein in cheese-making. Immobilised enzymes, enzymes fixed onto or trapped within an inert support such as alginate beads, are especially valuable, because the enzyme can be recovered and reused repeatedly and is easily separated from the product, lowering cost and keeping the product free of enzyme.

Immobilised lactase, for example, is used to make lactose-free milk for people who cannot digest lactose. Enzymes are also used in medicine, such as in diagnostic test strips, and in brewing, where they break down starch during fermentation.

Key concepts to master

Metabolism, Anabolism builds large molecules from small ones (uses energy); catabolism breaks large molecules down (releases energy).
Enzymes as biological catalysts, Enzymes are proteins that speed up reactions without being used up, and are specific to one substrate.
Lock-and-key model, A substrate fits the enzyme's active site like a key in a lock, forming an enzyme-substrate complex.
Effect of temperature, Rate rises to an optimum (around body temperature) then falls sharply as the enzyme denatures at high temperature.
Effect of pH, Each enzyme has an optimum pH; away from it the rate drops, and extreme pH denatures the enzyme.
Applications, Enzymes are used in detergents, food processing, brewing and medicine.
Effect of substrate concentration, As substrate concentration increases, the rate of reaction increases because more substrate molecules collide with the available active sites; beyond a certain point the rate plateaus because every active site is already occupied at any moment.
Effect of enzyme concentration, As enzyme concentration increases, with excess substrate present, the rate of reaction increases because there are more active sites available; the rate eventually levels off once substrate becomes the limiting factor instead.
Naming enzymes, Most enzyme names end in '-ase' and are built from the substrate they act on or the reaction they catalyse, such as amylase for starch, protease for protein and lipase for fat.
Enzymes in daily life, Biological detergents contain protease and lipase to remove protein and fat stains at lower washing temperatures; the food industry uses enzymes such as pectinase to clarify fruit juice and rennin in cheese-making.
Active site and tertiary structure, An enzyme is a globular protein folded into a precise three-dimensional shape; the active site is a small region of this shape whose contours and chemical groups are complementary to one specific substrate.
Immobilised enzymes, Enzymes can be fixed onto or within an inert support, such as alginate beads, so they can be reused repeatedly and easily separated from the product, which is common in industrial applications like producing lactose-free milk.
Calculating rate from data, Rate of reaction can be found by dividing the change in the amount of product or substrate by the time taken, or by finding the gradient of the steepest part of a graph of amount against time.

Quick recall checklist

Can you define and explain Metabolism?
Can you define and explain Enzymes as biological catalysts?
Can you define and explain Lock-and-key model?
Can you define and explain Effect of temperature?
Can you define and explain Effect of pH?
Can you define and explain Applications?
Can you define and explain Effect of substrate concentration?
Can you define and explain Effect of enzyme concentration?
Can you define and explain Naming enzymes?
Can you define and explain Enzymes in daily life?
Can you define and explain Active site and tertiary structure?
Can you define and explain Immobilised enzymes?
Can you define and explain Calculating rate from data?

Frequently asked questions

Why does enzyme activity fall at high temperature?

As temperature rises, activity increases up to an optimum. Beyond that, the heat changes the three-dimensional shape of the enzyme's active site, so the substrate no longer fits. The enzyme is denatured and the reaction rate falls sharply, usually permanently.

What is the lock-and-key model?

It describes how an enzyme is specific. The substrate (key) has a shape complementary to the enzyme's active site (lock), so only the correct substrate can bind to form an enzyme-substrate complex, which is why each enzyme catalyses one type of reaction.

Are enzymes used up when they work?

No. Enzymes are catalysts, so they are not used up or permanently changed by the reaction they speed up. A single enzyme molecule can catalyse the same reaction repeatedly, which is why cells need only small amounts.