AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2024: Question 4

✅ Correct answer

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

💡 Key knowledge

• Carbon dioxide and water are the reactants.
• Glucose and oxygen are the products.
• The numbers in front must balance the atoms on both sides.

🧠 Exam technique

In a multiple-choice question, check both the substances and the balancing. A common quick check is:

• 6 carbons on each side
• 12 hydrogens on each side
• 18 oxygens on each side

❌ Common errors

• Choosing the respiration equation instead.
• Putting oxygen and glucose on the wrong side.
• Forgetting the big 6 in front of CO₂, H₂O, and O₂.

✅ Correct answer

A full-mark answer could be:

As temperature increases, the rate of photosynthesis increases and then decreases. The fastest rate is between 30 °C and 35 °C. Up to 30 °C, the rate doubles for each 5 °C increase (1, 2, 4, 8, 16 cm³).

💡 Key knowledge

• The volume of gas produced in 1 hour is being used as a measure of rate.
• More gas in the same time means a faster rate of photosynthesis.
• The results rise from 10 °C to 30 °C, stay highest at 30–35 °C, then fall sharply.

🧠 Exam technique

• Use the word rate, not just “more gas”. The mark scheme says unqualified references to volume alone should be ignored.
• Quote data to support your point.
• Describe the whole pattern: increase → highest → decrease.

📐 Using the data step-by-step

1. At 10 °C, gas volume is 1 cm³ in 1 hour.
2. At 15 °C, it is 2 cm³ — this is double.
3. At 20 °C, it is 4 cm³ — doubles again.
4. At 25 °C, it is 8 cm³ — doubles again.
5. At 30 °C, it is 16 cm³ — doubles again.
6. At 35 °C, it stays at 16 cm³ — still the fastest rate.
7. Then it drops to 2 cm³ at 40 °C and 0 cm³ at 45 °C.

❌ Common errors

• Saying the rate keeps increasing all the way to 45 °C.
• Only saying “the volume increases” without linking it to rate.
• Missing the fact that the highest rate is between 30 °C and 35 °C.
• Ignoring the decrease after 35 °C.

✅ Correct answer

A full 2-mark answer could be:

At 45 °C, enzymes involved in photosynthesis become denatured. Their active site changes shape, so the substrate no longer fits or binds.

💡 Key knowledge

• Photosynthesis is controlled by enzymes in the plant/chloroplast.
• At high temperatures, enzymes become denatured.
• When the active site changes shape, the substrate cannot bind, so the reaction stops.

🧠 Exam technique

For 2 marks, make two linked points:

1. What happens to the enzyme.
2. How that stops the reaction.

This is what stronger answers did in the mark scheme.

❌ Common errors

• Do not say “the enzymes are killed”. The mark scheme does not accept this.
• Do not say “the plant is denatured” or “cells are denatured”.
• “The plant died” is too vague and would not get the full explanation marks.

✅ Correct answer

Tick:

Repeat the investigation, measuring the volume of gas to 0.5 cm³

💡 Key knowledge

Accuracy means being closer to the true value. Measuring to a smaller scale division, such as 0.5 cm³, gives a more precise reading and can improve the recorded result.

🧠 Exam technique

Read the key word carefully:

• Accuracy = better measurement/detail
• Reliability = repeats and averages

Here, the best option is the one using a finer measurement scale.

❌ Common errors

• Choosing “repeat the investigation for 24 hours” — this changes the method rather than improving measurement accuracy.
• Choosing a different plant — this adds variation.
• Choosing temperatures of 5 °C and 50 °C — this gives more data points, not more accurate measurements.

✅ Correct answers

Any two from the mark scheme:

• There may be too many tomatoes to eat or sell.
• Other factors could limit photosynthesis.
• The optimum temperature might be above or below 25 °C.
• Greenhouses are poorly insulated.
• Pollution from the heater.
• Increase in the rate of transpiration.
• Increased spread of plant disease(s).

💡 Key knowledge

• Temperature is only one factor affecting photosynthesis.
• Light intensity and carbon dioxide concentration may still limit the rate.
• Warmer conditions can also increase water loss by transpiration.

🧠 Exam technique

Because the question says against, give disadvantages only.

Good 2-mark responses gave two separate reasons, not one reason explained twice.

Examiner insight: top answers moved beyond “it gets hotter” and linked the temperature to biological or practical disadvantages.

❌ Common errors

• Talking about cost — the question explicitly says not to.
• Saying “25 °C increases photosynthesis” — that is a reason for, not against.
• Assuming 25 °C must be the optimum temperature.
• Giving vague answers like “it is bad for plants” without a clear biological reason.

✅ Correct answer

Meristem

💡 Key knowledge

• Meristem tissue contains unspecialised cells.
• These cells can divide and differentiate into new plant cells.
• This is why roots and shoots can grow from meristem tissue.

🧠 Exam technique

In plant tissue questions, look for the tissue linked with growth and differentiation. That is meristem.

❌ Common errors

• Epidermis protects the plant.
• Mesophyll is mainly for photosynthesis in leaves.
• Phloem transports sugars.

04.2 model answer

As temperature increases, the rate of photosynthesis increases and then decreases. The fastest rate is between 30 °C and 35 °C. Up to 30 °C, the rate doubles every 5 °C increase.

04.3 model answer

At 45 °C the enzymes involved in photosynthesis are denatured. Their active site changes shape so the substrate no longer fits, so no photosynthesis happens and no gas is produced.

04.5 model answer

Other factors, such as light intensity or carbon dioxide concentration, could limit photosynthesis, so 25 °C may not increase yield much. Also, the higher temperature could increase transpiration, causing more water loss from the plants.

🧠 How to score well

• Use exact scientific terms: rate, denatured, active site, meristem.
• When asked to use data, actually quote numbers or a clear pattern from the table.
• For “suggest” questions, give distinct points, not repeated versions of the same idea.

❌ Biggest traps in this question

• Mixing up photosynthesis and respiration equations.
• Describing gas volume instead of the rate of photosynthesis.
• Using incorrect enzyme language such as “enzymes are killed”.
• Ignoring command words like “against” and “do not refer to cost”.