Revise Measuring Rates: Enzymes, Growth & Photosynthesis for Biology 6 (IAL) WBI16 (A2 Level) — revision notes and instant AI marking. Free to start.
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Edexcel IAL Biology • Unit 6: Practical Skills in Biology II
Measuring Rates: Enzymes, Growth & Photosynthesis
⏱️ Big idea: A rate is always a change divided by a time — but which change, measured when, decides whether the number means anything. Most biological rates fall as the experiment proceeds, so a rate taken over the whole run underestimates the true one. The initial rate is usually the only fair comparison.
Summary — What This Topic Covers
Calculating a rate, and why initial rate is usually the right one
Using 1 ÷ time when you measure how long something takes
Measuring rates for enzymes, photosynthesis and microbial growth
Reading a rate off a curve using a tangent
Choosing what to measure so the rate reflects the biology
1. What a Rate Is
The definition
rate = change in quantity ÷ time taken
The unit is always the unit of the quantity followed by the unit of time with a negative index — cm³ min⁻¹, mg s⁻¹, bubbles min⁻¹.
18 cm³ of oxygen collected in 3 minutes
rate = 18 ÷ 3 = 6 cm³ min⁻¹
mass falls from 4.50 g to 4.05 g in 30 minutes
rate = (4.50 − 4.05) ÷ 30 = 0.015 g min⁻¹
Common mistake
Leaving the unit off. A rate without a unit is not a rate, and the unit is usually a mark in its own right.
2. Why Initial Rate
Nearly every biological rate falls with time. In an enzyme reaction the substrate is used up, so collisions with active sites become less frequent; in photosynthesis, CO₂ in a closed vessel is depleted.
volume
of gas │ ● ● ● ● ← plateau: substrate exhausted
│ ●
│ ● ← rate falling
│ ●
│ ● ← steepest here: INITIAL RATE
└────────────────────── time
average over the whole run = underestimates the true rate
gradient of the tangent at t = 0 = the fair comparison
Why it matters
If you compare two conditions using the total gas collected in ten minutes, and one reaction finished after three, you are comparing a rate with a final amount. Initial rates compare like with like.
What mark schemes look for
The phrase "initial rate" with the reason — substrate concentration is highest and has not yet begun to limit the reaction.
3. When You Measure Time: Use 1 ÷ t
Some practicals measure how long something takes — for the cross to disappear, for the iodine to stop turning blue-black, for a disc of catalase-soaked paper to rise. Time is inversely related to rate, so plotting time directly gives a falling curve that is hard to interpret.
time taken / s 100 50 25 20
rate = 1/t / s⁻¹ 0.010 0.020 0.040 0.050
plotting t → a curve falling steeply, then flattening
plotting 1/t → proportional to rate; higher = faster, as expected
Easy marks
If the dependent variable is a time, the examiner almost always wants 1/t. Label the axis "1/time / s⁻¹" and the mark is yours.
4. Rates in the Three Common Systems
SYSTEM MEASURE RATE UNIT
────────────────────────────────────────────────────────────────
enzyme (catalase) volume of O₂ in a gas syringe cm³ min⁻¹
enzyme (amylase) time for iodine to stop s⁻¹ (as 1/t)
turning blue-black
photosynthesis volume of O₂, or bubbles cm³ min⁻¹
per minute from Elodea
respiration O₂ uptake in a respirometer cm³ min⁻¹
microbial growth optical density on a AU h⁻¹
colorimeter, or colony count
plant growth change in height or dry mass mm day⁻¹
Counting bubbles is poor practice — bubble size varies, so the count is not proportional to volume. Collect and measure the gas instead
Optical density measures turbidity, so it counts dead cells as well as living ones — a viable count on agar measures only the living
Dry mass is a better measure of growth than fresh mass, because water content fluctuates independently of growth — but it destroys the sample
5. Reading a Rate from a Curve
The method
Draw a tangent at the point of interest — a straight line touching the curve at that point only. Take two widely separated points on the tangent and calculate gradient = Δy ÷ Δx.
tangent drawn at t = 0 passes through (0, 0) and (2.0 min, 14.0 cm³)
initial rate = (14.0 − 0) ÷ (2.0 − 0)
= 7.0 cm³ min⁻¹
draw and label the triangle — it is marked separately
Worth remembering
Never take the gradient from two plotted data points on a curve. That gives the average rate between them, not the rate at either.
Practice Questions
Practice Question 1
Explain why the initial rate of an enzyme-controlled reaction is used to compare different temperatures, rather than the total product formed after ten minutes.
Practice Question 2
A student records the time for a solution to lose its blue-black colour: 80 s at 20 °C and 20 s at 40 °C. Calculate the rates and comment on the relationship.
Practice Question 3
Explain why counting oxygen bubbles from pondweed is less reliable than collecting the gas in a syringe.
Practice Question 4
Explain why dry mass is a better measure of plant growth than fresh mass, and state the drawback.
What to Memorise
rate = change ÷ timeUnit always has a negative time indexInitial rate = tangent at t = 0Substrate highest at the startTime measured → plot 1/tBubbles vary in size — collect the gasOptical density counts dead cells tooDry mass beats fresh mass, but destroys the sampleTangent, not two data points
Concepts Checklist
Exam Tips
What mark schemes look for
"Initial rate, because substrate concentration is highest and is not yet limiting." The reason is worth as much as the phrase.
The trap
Taking a gradient between two plotted points on a curve. That is the average rate over that interval, not the rate at either point. Draw a tangent.
Easy marks
Units. cm³ min⁻¹, s⁻¹, mg day⁻¹ — a rate without a unit rarely scores.
Worth remembering
Whenever the dependent variable is a time, expect to plot 1/t. It converts an inverse relationship into a proportional one.