IB Biology HLEnzymes & MetabolismPaper 1 & 2~12 min read
Metabolism: Enzymes & Reactions
A cell is a warm, watery bag at about 37°C with a near-neutral pH. Those are terrible conditions for chemistry — and yet thousands of reactions run inside you every second. Enzymes are how the cell gets away with it.
📚 What you need to know
Metabolism is every chemical reaction happening inside a cell or organism. The molecules taking part are called metabolites.
Enzymes are globular proteins that act as biological catalysts. They speed reactions up and are not used up, so a cell needs only a small amount of each one.
A lab chemist speeds reactions up with heat, pressure or extreme pH. A cell cannot do any of those, so it uses enzymes instead.
The active site is the small pocket where the substrate binds. Its shape and chemistry are complementary to the substrate — not identical to it.
Because each enzyme is specific, an organism needs thousands of different ones, and it can control metabolism by controlling which enzymes it makes.
Anabolic reactions build big molecules from small ones, use condensation, and are endergonic (energy in).
Catabolic reactions break big molecules into small ones, use hydrolysis, and are exergonic (energy out).
Both types are enzyme-catalysed and both are linked to ATP, the cell’s energy carrier.
Why cells need enzymes at all
Most reactions either will not start on their own or crawl along far too slowly to be useful. In a factory you fix that with brute force: crank up the temperature, raise the pressure, use a strong acid, or pile in a huge concentration of reactants.
A cell cannot do any of that. Heat it past about 40°C and its proteins fall apart. Push the pH to an extreme and the same thing happens. So the cell needs a way to make reactions go quickly without changing the conditions. That way is an enzyme.
Definition
An enzyme is a globular protein that acts as a biological catalyst: it speeds up a specific chemical reaction and is not changed or used up by it.
Two consequences follow straight from that definition, and both are worth marks:
Because an enzyme is not used up, one molecule works again and again. A cell only needs a small number of each enzyme.
Because it is unchanged, the enzyme you find at the end of the reaction is the same enzyme you started with, ready for the next substrate.
The collision problem
For two reactant molecules to react, they have to bump into each other at the right angle and with enough speed. Left to chance in a cell, the odds of that happening are so poor that the reaction would essentially never occur.
An enzyme fixes the odds. It grabs the substrate molecules, holds them in the correct orientation, and pulls them close together. The lucky collision stops being lucky — it becomes the normal outcome.
Think of two people trying to shake hands while blindfolded in a crowd. Possible, but unlikely. The enzyme is the person who takes both of their hands and puts them together. Nothing about the handshake changed — only the chance of it happening.
The active site and specificity
An enzyme is a big protein, but the part that does the work is tiny. The active site is a pocket made from only a few amino acids, held in exactly the right positions by the protein’s overall 3D shape.
The active site and the substrate are complementary. The pocket has a shape and a set of chemical properties — charges, polar groups — that match the substrate the way one jigsaw piece matches its neighbour. This is called enzyme–substrate specificity.
⚠ Read this before you write “same shape”
Complementary does not mean the same shape. Two jigsaw pieces fit because they are different in matching ways. Writing “the active site and substrate are the same shape” loses the mark.
Specificity has a big consequence. One enzyme handles one reaction, so an organism needs thousands of different enzymes to run all its chemistry. That sounds wasteful, but it is exactly what gives the cell control: by switching the production of one enzyme on or off, the cell switches one reaction on or off, without touching anything else.
Specificity is what turns a bag of chemicals into a controlled system: nothing reacts unless the cell has made the enzyme for it.
Anabolism and catabolism
Metabolism is a huge word for a simple split. Reactions either build things up or break things down.
Anabolic reactions build
Small molecules are joined into larger ones.
They usually involve condensation reactions, which release a molecule of water each time a bond forms.
They are endergonic: energy has to be put in. The products end up storing that energy.
Examples: photosynthesis building sugars from carbon dioxide and water, amino acids joined into a protein, glucose molecules linked into glycogen.
Catabolic reactions break
Large molecules are broken into smaller, simpler ones.
They usually involve hydrolysis reactions, which use a molecule of water to split a bond.
They are exergonic: free energy is released, and the cell uses it or loses it as heat.
Examples: respiration oxidising sugars to carbon dioxide and water, deamination of proteins producing urea, digestion breaking macromolecules into monomers.
The enzyme itself does not care which direction it works in. What changes is whether bonds are being made or broken.
🧠
Keeping the two words apart
Anabolic — think of an anabolic athlete building muscle. Building up. Catabolic — think of a catastrophe. Things fall apart.
Comparing them side by side
Feature
Anabolism
Catabolism
Direction
Builds large molecules from small ones
Breaks large molecules into small ones
Energy
Requires an input (endergonic)
Releases free energy (exergonic)
Typical reaction
Condensation
Hydrolysis
What it is for
Growth, repair, energy storage
Digestion, excretion, supplying energy
Energy in the products
Stored in chemical bonds
Released as usable energy and heat
Enzymes involved?
Yes, every step
Yes, every step
Linked to ATP?
Yes — ATP is spent
Yes — ATP is made
Metabolism runs in stages. Most metabolic reactions do not happen in one leap. They run as a series of small steps, and each step has its own enzyme. A linked series like this is a metabolic pathway — there is a whole page on those later in this topic.
Worked examples
WE 1
Explain why cells need enzymes
Explain why chemical reactions in cells would be too slow without enzymes. (3 marks)
Point 1: the conditions in a cell are mild
Cells cannot use high temperature, high pressure or extreme pH to speed reactions up, because those conditions would damage their proteins.
Point 2: random collisions are rare
Reactants must collide at the correct orientation and with enough speed, and under cell conditions this happens far too rarely to support life.
Point 3: what the enzyme does about it
An enzyme holds the substrates in the correct orientation and close together, so successful collisions happen often enough for the reaction to run at a useful rate.
Mild conditions + rare collisions = enzymes are the only optionsay what the enzyme does to the substrate, not just that it “speeds things up”
WE 2
Classify reactions as anabolic or catabolic
State whether each reaction is anabolic or catabolic, and justify your answer: (a) glycogen formed from glucose, (b) protein digested into amino acids. (4 marks)
(a) glycogen from glucoseAnabolic — many small glucose molecules are joined into one large molecule by condensation, and energy is stored in the bonds formed.
(b) protein into amino acidsCatabolic — one large molecule is broken into many small ones by hydrolysis, releasing free energy.
Build + condensation = anabolic; break + hydrolysis = catabolicthe justification carries the marks, not the label on its own
💡 Exam tips
Define an enzyme as a globular protein that is a biological catalyst. Both halves of that phrase get used in mark schemes.
Always write complementary, never “the same shape”.
If a question asks why cells cannot just use heat, say that proteins denature — do not just say “it is bad for the cell”.
Learn one clean example each way: glycogen synthesis (anabolic), digestion (catabolic).
Link the words in pairs: anabolic–condensation–endergonic, catabolic–hydrolysis–exergonic.
Remember that enzymes are reusable and unchanged — that is often a whole mark on its own.
⚠ Common mistakes
Saying enzymes are used up. They come out of the reaction unchanged and go straight into the next one.
Calling an enzyme a “chemical” or a “molecule”. Be specific: it is a protein.
Mixing up endergonic and exothermic. Endergonic and exergonic are about total free energy; endothermic and exothermic are about heat only.
Saying enzymes make reactions possible that could never happen. The reaction could happen — the enzyme changes how fast, not whether.
Writing that the active site is the whole enzyme. It is a small pocket made of only a few amino acids.
Forgetting the water. Condensation releases water, hydrolysis uses water. Examiners like that detail.
Up next: How Enzymes Work. Now that you know what the active site is for, the next page follows a substrate all the way through it — binding, the induced fit, the products leaving, and what happens when heat wrecks the whole thing.
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