Glossary: Metabolism and Enzymes

This chapter's terms in context

Chapter 5 is anchored by five terms that describe how cells control the speed and direction of chemical reactions: metabolism, enzyme, substrate, active site and denaturation. Metabolism is the sum of all the chemical reactions that occur inside a living organism, divided into two opposite categories, anabolism, which builds larger molecules from smaller ones and requires energy, such as protein synthesis from amino acids, and catabolism, which breaks larger molecules into smaller ones and releases energy, such as respiration breaking down glucose. Every reaction discussed in later chapters, from digestion to photosynthesis to respiration, is a metabolic reaction, which is why this chapter functions as a conceptual hub for the rest of the syllabus.

An enzyme is a biological catalyst, a protein that speeds up the rate of a specific chemical reaction without itself being used up or permanently changed by that reaction. Because an enzyme is a protein, its function depends entirely on its three-dimensional shape, and this is where the active site becomes essential: the active site is the specific region on an enzyme's surface, shaped to fit only one particular substrate, the substance an enzyme acts on, in what is described as a 'lock and key' or, more precisely at SPM level, an 'induced fit' relationship, where the active site changes shape slightly to mould around the substrate as it binds. This is exactly why one enzyme normally catalyses only one type of reaction, a property called specificity, and a common misconception is treating enzymes as generic reaction-speeders rather than substrate-specific proteins.

Denaturation describes what happens when an enzyme is exposed to a temperature too high or a pH too far from its optimum: the bonds holding the enzyme's three-dimensional shape together break, the active site's shape is permanently distorted, and the enzyme can no longer bind its substrate, a change that, once it has occurred, cannot be reversed by returning the enzyme to normal conditions. Students very frequently confuse denaturation with simple 'slowing down', but the two are different processes with different graphs: as temperature rises towards the optimum, enzyme activity increases because particles move and collide faster, but once the temperature exceeds the optimum, activity drops sharply and permanently because the enzyme has denatured, not merely slowed.

Both temperature and pH graphs typically show a curve rising to a peak at the optimum value and then falling steeply on the high side, and examiners often ask candidates to explain the falling side of the curve specifically in terms of denaturation rather than simply stating that 'the rate decreases'. This distinction, between a reversible slowdown caused by low temperature (where molecules simply move more slowly and can speed up again once warmed) and an irreversible loss of activity caused by denaturation, is one of the most heavily tested ideas to come out of this single chapter, and it resurfaces in Paper 3 practical questions on amylase and pepsin as well as in Paper 2 structured questions.

Paper 3 questions on enzymes typically ask a candidate to design or interpret an experiment varying temperature or pH while measuring the time taken for a reaction such as amylase digesting starch, and examiners specifically reward answers that state the controlled variables in full, substrate concentration, enzyme concentration and volume, for instance, rather than only the one being tested. A frequent scoring gap appears when a candidate correctly predicts that a very high temperature will stop the reaction but explains this using vague language such as 'the enzyme is damaged' instead of the precise term denaturation, or fails to state that this change is irreversible. Practising the exact vocabulary, active site, denaturation, optimum, in full sentences rather than as isolated keywords is what converts a plausible-sounding answer into one that matches the mark scheme.