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A2 Level · Biology 6 (IAL) WBI16

Measuring Rates: Enzymes, Growth & Photosynthesis

Revise Measuring Rates: Enzymes, Growth & Photosynthesis for Biology 6 (IAL) WBI16 (A2 Level) — revision notes, 2 practice questions and instant AI marking.

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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

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

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