IB Biology HLEnzymes & MetabolismPaper 1 & 2~14 min read
Enzyme Activity (Skills)
Three factors change how fast an enzyme works: temperature, pH and substrate concentration. You need to be able to sketch all three graphs from memory, explain every part of their shape, and describe an experiment that would produce them.
📚 What you need to know
Rate rises with temperature up to an optimum, then falls sharply as the enzyme denatures.
Every enzyme has an optimum pH. It is not always near neutral: pepsin works best at about pH 2, and some bacterial enzymes at pH 9–10.
Rate rises with substrate concentration, then plateaus once the active sites are saturated.
Past saturation, adding more substrate does nothing. To go faster you must add more enzyme.
Reaction progress can be followed by measuring the rate of product formation or the rate of substrate disappearance.
Catalase breaks hydrogen peroxide into water and oxygen — measure the volume of oxygen released.
Amylase hydrolyses starch to maltose and glucose — test samples with iodine solution over time.
A colorimeter measures absorbance or transmission, so it can follow any reaction that involves a colour change.
You must be able to identify the independent, dependent and control variables in any of these investigations.
Temperature
This curve has two halves that are caused by two completely different things. Get that separation right and the question is yours.
Notice how much steeper the right-hand side is. Cooling a cold enzyme back down restores the rate; cooling a denatured one does not.
Why the rate rises
Molecules have more kinetic energy, so they move faster.
There is a higher frequency of successful collisions between substrate and active site.
More enzyme–substrate complexes form per second.
Collisions also carry more energy, making it more likely that bonds are broken or formed.
Why the rate falls
Above the optimum, the increased vibration starts breaking the weak bonds that hold the enzyme’s 3D shape. The active site changes shape, substrate can no longer bind, and the rate drops sharply. This is denaturation, and it is permanent.
Low temperature and high temperature are not mirror images. Cold enzymes are inactive but intact — warm them and they work again. Hot enzymes are ruined. Say “slowed down” for cold and “denatured” for hot, and never swap them.
pH
pH is a measure of hydrogen ion concentration. A low pH means a high concentration of hydrogen ions (acidic); a high pH means a low concentration (alkaline). The scale is logarithmic, so a tenfold rise in hydrogen ion concentration lowers the pH by exactly 1 unit.
Extremes of pH interfere with the hydrogen bonding inside an enzyme, so the protein loses its shape and denatures irreversibly. Every enzyme has an optimum pH where its active site holds its best shape — and that optimum matches where the enzyme normally works.
The curves are roughly symmetrical, unlike the temperature curve. Both sides are caused by the same thing: disrupted bonding.
🧠
Optimum pH follows the address
Pepsin lives in the stomach → pH 2. Trypsin lives in the small intestine → about pH 8. If you forget a number, ask where the enzyme works.
Substrate concentration
Start adding substrate to a fixed amount of enzyme and the rate climbs quickly — more substrate means more collisions with active sites. But active sites are occupied while they are working, so as more of them fill up, fewer are free for the next substrate molecule.
Eventually every active site is busy all the time. The enzyme is working flat out, and the curve flattens. This is the point of active site saturation.
The plateau is set by the amount of enzyme, not the amount of substrate. That single sentence answers most questions on this graph.
A supermarket helps here. Substrate molecules are shoppers, active sites are checkouts. More shoppers means more people served — until every checkout has a queue. After that, more shoppers only makes the queue longer. To serve people faster you need more checkouts, not more shoppers.
Designing the experiment
Whatever factor you are testing, the plan has the same skeleton.
🧩 Planning checklist
Choose the independent variable: temperature, pH or substrate concentration.
Decide the intervals you will use and how many values — enough to show the shape of the curve.
Decide how the dependent variable will be measured, and in what units.
List the control variables and say how each is kept constant. Temperature must be controlled when you are testing pH, and pH must be controlled when you are testing temperature.
Choose apparatus that suits those decisions — a water bath, buffers, a gas collection set-up, a colorimeter.
Repeat each value and calculate a mean.
Following product formation: catalase
Hydrogen peroxide is a common but toxic by-product of metabolism, so cells have to remove it quickly. Catalase, found in the cells of most organisms, breaks it down into water and oxygen.
Because one product is a gas, this reaction is easy to follow: mix hydrogen peroxide with a catalase source such as potato cubes, and measure the volume of oxygen collected in a set time.
Two small details examiners like: the water bath level, and swirling the flask so the contents stay at one even temperature.
Following substrate disappearance: amylase
Amylase hydrolyses starch into maltose and glucose. It works best at about pH 7 and 37°C. Here you follow the substrate vanishing instead of a product appearing.
🧩 The iodine method
Mix amylase and starch, and start a stopwatch.
At regular intervals, remove a small sample of the mixture.
Add each sample to a drop of iodine in potassium iodide solution on a spotting tile.
Starch turns the iodine blue-black. When no starch is left, the drop stays yellow-brown.
Record the time taken for the blue-black colour to stop appearing — that is the time for the starch to be broken down.
Repeat at different starch concentrations and compare the rates.
The same set-up can be adapted to test pH, temperature or enzyme concentration — just change which variable you vary and control the rest.
Using a colorimeter
A colorimeter measures how much light a solution absorbs or how much light is transmitted through it. Any enzyme reaction with a colour change can be followed this way. As the colour fades, transmission goes up and absorbance goes down, and that change tracks the reaction.
🧩 Colorimetry with starch and amylase
Calibrate the colorimeter first — here a weak iodine solution is used to set the endpoint at 100% transmission.
Make a stock solution of starch of known concentration.
Produce a range of concentrations by serial dilution: take 1 cm3 of the previous solution and add 9 cm3 of water. Each step is a tenfold dilution, so 1% becomes 0.1%, then 0.01%, and so on.
Switch on the red filter to maximise the reading, then measure percentage absorbance or transmission for each tube.
Plot a calibration graph of starch concentration on the x-axis against absorbance or transmission on the y-axis.
Why serial dilution? It is far more accurate than trying to measure out five tiny different volumes. Each step uses the same easy measurement — 1 cm3 into 9 cm3 — and the errors do not stack up the way they would if you diluted each one from scratch.
Worked examples
WE 1
Explain the shape of the temperature curve
Explain the shape of a graph of enzyme activity against temperature between 0°C and 60°C. (5 marks)
Below the optimum
Rising temperature gives molecules more kinetic energy, so they move faster and collide more often with the active site.
Why that raises the rate
More successful collisions means more enzyme–substrate complexes form per second, and collisions carry more energy so bonds are more likely to break or form.
At the optimum
The rate is at its maximum, at about 37°C for a human enzyme.
Above the optimum
Vibration breaks the weak bonds holding the enzyme’s shape, the active site changes shape, and substrate can no longer bind.
The result
The rate drops sharply to zero because the enzyme has denatured, and this cannot be reversed.
Kinetic energy up to the peak, denaturation after ittwo halves, two different explanations — never use “denatured” for the left-hand side
WE 2
Identify the variables
A student investigates the effect of pH on catalase by measuring the oxygen released from hydrogen peroxide. State the independent and dependent variables, and two control variables. (4 marks)
Independent variable
The pH of the reaction mixture, set using buffer solutions.
Dependent variable
The volume of oxygen collected, in cm3, in a fixed time.
Control variables
Temperature, kept constant with a water bath; and the concentration and volume of hydrogen peroxide. The mass and surface area of the potato would also be accepted.
IV = pH, DV = volume of oxygen per unit timegive units for the dependent variable — a bare “oxygen” often misses the mark
WE 3
Explain a plateau
A student doubles the substrate concentration but the rate of reaction does not change. Explain this result and suggest one way to increase the rate. (3 marks)
Point 1: what is happening
All the active sites are already saturated — every one is occupied by substrate at any moment.
Point 2: why extra substrate does nothing
Additional substrate molecules cannot bind because there are no free active sites, so the rate cannot rise.
Point 3: the fix
Increase the enzyme concentration. That provides more active sites, so more complexes can form at once.
Saturated sites: add enzyme, not substrate“the enzyme is the limiting factor” is a good phrase to include
💡 Exam tips
Practise sketching all three curves with labelled axes. Rate always goes on the y-axis.
You may be asked to sketch a new curve for changed conditions — for example, more enzyme raises the plateau but keeps the same shape.
When explaining any rate change, mention successful collisions and enzyme–substrate complexes by name.
Always name the control variables and say how they are controlled.
Learn one product-formation method (catalase and oxygen) and one substrate-disappearance method (amylase and iodine).
For serial dilution, remember the numbers: 1 cm3 into 9 cm3 gives a dilution factor of 10 each time.
⚠ Common mistakes
Saying enzymes are denatured by low temperature. They are slowed, not denatured, and the effect reverses on warming.
Drawing a symmetrical temperature curve. The fall after the optimum is much steeper than the rise.
Assuming every optimum pH is 7. Pepsin is pH 2; some bacterial enzymes are pH 9–10.
Treating the pH scale as linear. One pH unit is a tenfold change in hydrogen ion concentration.
Explaining the plateau as “the enzyme is denatured”. It is not — it is working as fast as it can.
Forgetting to control temperature in a pH experiment (and the other way round). Examiners look for this exact point.
Up next: Enzyme Reaction Rates (Skills). You can describe the curves now — the next page shows you how to get an actual number out of one, including drawing a tangent and calculating an initial rate.
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