Right now, inside every one of your cells, thousands of chemical reactions are running at once — and almost none of them would happen fast enough on their own. Metabolism is the whole set of those reactions, and enzymes are what make them possible at body temperature.
📚 What you need to know
Metabolism is all the chemical reactions that happen in a cell or organism. The molecules taking part are called metabolites.
Enzymes are globular proteins that work as biological catalysts: they speed reactions up without being used up.
An enzyme is unchanged at the end, so the same molecule can be used over and over. A cell only needs a small amount.
Anabolic reactions build big molecules from small ones. They take energy in, so they are endergonic.
Catabolic reactions break big molecules into small ones. They give energy out, so they are exergonic.
Most metabolism happens in pathways — a chain of small steps, each one controlled by its own enzyme.
Because the cell decides which enzymes to make, it controls which reactions actually run. That is how metabolism is regulated.
What metabolism actually means
The word sounds grand, but the idea is simple. Metabolism is just a name for everything chemical that a living thing does. Digesting a sandwich is metabolism. Building muscle protein is metabolism. So is respiring glucose, making urea and joining glucose molecules into glycogen.
Because that is a huge list, biologists sort it in two ways: by direction (building up or breaking down) and by pathway (which chain of steps a molecule travels along). Get those two ideas straight and the rest of this topic sits on top of them neatly.
Definition
metabolism = all the chemical reactions taking place inside cells and organisms
Why cells cannot manage without enzymes
In a school lab, if a reaction is too slow you have three obvious options: heat it, squeeze it under pressure, or push the pH to an extreme. A cell can use none of these. Heat above about 40°C wrecks its proteins, it cannot generate high pressure, and extreme pH would destroy the very molecules it is trying to protect.
There is a second problem. For two molecules to react they must bump into each other at the right speed and, just as importantly, at the right angle. Left to chance, the odds of that happening are tiny — so tiny that the reaction would be nowhere near fast enough to keep a cell alive.
Enzymes solve both problems at once. They grab hold of the reacting molecules, hold them in exactly the right position, and hold them close together. What was a lucky accident becomes a near certainty.
Think of an enzyme as a workbench with a clamp. The parts were always able to fit together — the clamp just stops them wobbling and lines them up so the join happens first time, every time.
Two words examiners like to see: enzymes are reusable and remain unchanged by the reaction they catalyse. That is the whole reason a cell can get away with making only small amounts of each one.
Anabolic and catabolic reactions
Every metabolic reaction is going in one of two directions. Either small things are being joined into something bigger, or something big is being taken apart.
Notice the small blue units are the same on both sides — the only thing that changes is whether they are being joined or separated.
Anabolic reactions (building up)
Small units are joined into larger ones, so a condensation reaction is usually involved and a water molecule is released at each join.
They need an energy input, which is why they are called endergonic.
The products store that energy in their bonds.
Examples: making protein from amino acids, making glycogen from glucose, and photosynthesis building sugars from carbon dioxide and water.
Catabolic reactions (breaking down)
Large molecules are split into smaller, simpler ones, usually by hydrolysis — water is used to break the bond.
Energy stored in the bonds is released, so they are exergonic.
That released energy powers everything else the cell does, and some escapes as heat.
Examples: respiration oxidising sugars, digestion breaking macromolecules into monomers, and deamination of amino acids to make urea.
Compare
Anabolism
Catabolism
Direction
Small molecules joined into large ones
Large molecules split into small ones
Energy
Takes energy in (endergonic)
Gives energy out (exergonic)
Type of reaction
Condensation — water is made
Hydrolysis — water is used up
What the cell gets
Growth, repair and energy stores
Usable energy, plus waste ready for removal
Example
Glucose joined into glycogen
Glucose oxidised in respiration
Both are made of enzyme-catalysed steps, and both are coupled to ATP. Catabolism makes ATP; anabolism spends it. That link is what stops the two halves of metabolism from being separate stories.
Metabolic pathways
Cells almost never turn A straight into Z. Instead a molecule is passed along a metabolic pathway: a chain of small reactions, each one catalysed by a different enzyme. The product of one step becomes the substrate of the next.
Because each step is a separate enzyme, the cell can switch a pathway on or off just by changing how much of one enzyme it makes.
This is the bit students skip and examiners love. The cell controls metabolism by controlling its enzymes — no enzyme, no reaction, however much substrate is sitting there.
Worked examples
WORKED EXAMPLE
Classify each reaction as anabolic or catabolic, and give a reason. (a) amino acids joined into a polypeptide (b) starch broken into maltose in the gut
Part (a): look at the size change
Many small units (amino acids) become one large molecule.
small → large = building upAnabolicit is a condensation reaction and it needs energy in, so it is endergonic tooPart (b): same test
One large molecule (starch) becomes smaller ones (maltose).
large → small = breaking downCatabolichydrolysis, and energy is released, so exergonic
WORKED EXAMPLE
A cell contains plenty of substrate A but produces no D at all. The pathway is A → B → C → D. Tests show that B is present in high amounts and C is absent. Suggest what has gone wrong. [2]
Step 1: find where the chain stops
B is piling up, C never appears. So the block is at the step B → C.
Step 2: say what controls that step
That step has its own enzyme, and only that enzyme can catalyse it.
Enzyme 2 is missing or non-functionalB builds up because it can still be made but cannot be used
WORKED EXAMPLE
Explain why a cell needs only a very small quantity of each enzyme. [2]
Step 1: what happens to the enzyme in the reaction
The enzyme is not part of the product, and it is unchanged when the reaction finishes.
Step 2: what that allows
The same enzyme molecule is released and can bind another substrate straight away.
Enzymes are reusable, so one molecule catalyses many reactions“used up” is the word to avoid here — that is what happens to substrate, not enzyme
💡 Exam tip
To classify a reaction, ignore the names and just ask: did the molecules get bigger or smaller? Bigger is anabolic, smaller is catabolic.
Pair the words up as you learn them: anabolic goes with condensation and endergonic; catabolic goes with hydrolysis and exergonic.
Write “biological catalyst” in any definition of an enzyme, and add that it is a protein. Both are creditable.
If a question asks how a cell controls a reaction, the answer is almost always about controlling the enzyme, not the substrate.
“Speeds up the reaction” is worth more if you add “without being used up” or “and is unchanged at the end”.
Learn one clear example of each direction. Glycogen from glucose (anabolic) and respiration of glucose (catabolic) will cover most questions.
⚠ Common mix-up
Saying enzymes are “used up”. They are not. Substrate is used up; the enzyme is released ready to go again.
Mixing up endergonic and exergonic. Endergonic takes energy in; exergonic sends energy out. Anabolism is the one that costs energy.
Confusing condensation and hydrolysis. Condensation makes water and builds. Hydrolysis uses water and breaks.
Thinking metabolism means only respiration. Metabolism is every reaction in the cell, building and breaking.
Calling enzymes “living” or saying they are “killed”. They are molecules. If they stop working they are denatured.
Forgetting the pathway. Most reactions happen in stages, so a question about a “blocked” pathway is usually asking about one missing enzyme.
Up next: How Enzymes Work — the active site, the enzyme–substrate complex, induced fit, and what actually happens when an enzyme is denatured.
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