The initial rate tells you how fast a reaction starts, and the total product tells you how much substrate was converted in the end. If you mix them up, your explanation of a flat graph will be wrong.
This lesson opens the set on interpreting enzymes and experimental rates. The next lesson is choosing a fair comparison.
How do you calculate a rate?
Rate is the change in product divided by the time taken. Write the unit as cm³ per minute or cm³/min.
Here is an original table for an enzyme breaking down a substrate at 30 °C.
| Time (min) | Total product (cm³) |
|---|---|
| 0 | 0 |
| 1 | 8 |
| 2 | 14 |
| 3 | 18 |
| 4 | 20 |
| 5 | 20 |
The initial rate is 8 cm³ per minute, since 8 cm³ forms in the first minute. Between minutes 3 and 4 the rate is 2 cm³ per minute (20 − 18). Between minutes 4 and 5 it is 0.
The total product is 20 cm³, which is the final reading.
Why does the rate fall?
The rate falls because substrate is used up. Early on, plenty of substrate molecules meet active sites, so collisions are frequent. Later, fewer substrate molecules remain, so collisions with active sites become rarer.
The enzyme itself has not been used up or damaged. The proof is that adding fresh substrate at minute 5 would make product form again.
A good answer to “Explain why the graph becomes flat” is: “Substrate has been used up, so no more product can form. The enzyme is still active.”
Worked example: same total, different rate
Two enzyme samples, X and Y, each work on 20 cm³ worth of substrate at the same temperature. Sample X forms 10 cm³ of product in the first minute. Sample Y forms 4 cm³ in the first minute.
Both reach a total of 20 cm³, but sample X gets there in about 3 minutes and sample Y in about 8 minutes.
Question: Which sample is more active, and does the total product show this?
Sample X has the higher initial rate (10 cm³ per minute against 4), so it is more active. The total does not show this, because both totals are equal. The total is fixed by the amount of substrate.
So a question that asks “which enzyme is faster” needs a rate, not a total, and a question that asks “how much substrate was converted” needs a total.
The mistake that costs marks
The common error is to say the enzyme “stopped working” when the graph levels. In most school experiments the real reason is that the substrate has run out.
| Student wrote | Problem | Better version |
|---|---|---|
| “The enzyme is used up” | Enzymes are reusable | “Substrate is used up, so the rate falls to zero” |
| “Rate is 20 cm³” | No time in the unit | “Rate is 8 cm³ per minute” |
| “X has more product so it is faster” | Totals are equal | “X has a higher initial rate of 10 cm³ per minute” |
If the question asks why the rate falls with time, name the substrate. If it asks why the rate is low from the start, look at temperature, pH or enzyme concentration.
Check yourself
Use the table above. Calculate the average rate between minute 1 and minute 3, with units.
Answer
Product at minute 1 is 8 cm³ and at minute 3 is 18 cm³. The change is 18 − 8 = 10 cm³.
The time taken is 3 − 1 = 2 minutes. Rate = 10 ÷ 2 = 5 cm³ per minute.
What to study next
Next, handle tables where two conditions change at once in choosing a fair comparison. The biological data interpretation trainer has extra tables.
If you want a teacher to give you paired tables and ask which quantity each question wants, see online one-to-one Biology tuition or the one-hour trial class (from RM50).