AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2019: Question 5

💡 Key knowledge

• magnification = image size ÷ real size
• use consistent units before calculating
• a microscope is used safely by starting on the lowest power
• oxygen is needed for respiration, which provides energy for active transport
• nitrate ions are used to make amino acids and then proteins

🧠 Exam technique

• Always show your working for calculations.
• For “describe” questions, give several clear points from the table.
• For “explain” questions, link cause → process → result.
• Use the wording in the mark scheme: active transport, respiration, protein synthesis.

❌ Common errors

• Forgetting to convert mm to µm
• Writing the inverse formula the wrong way round
• Skipping from low power straight to high power on the microscope
• Saying nitrate ions are used to make glucose or starch
• Not linking oxygen to aerobic respiration and energy

✅ Correct answer

Real length = 860 µm

This is the full-mark answer from the mark scheme.

📐 Calculations step-by-step

1. Use the magnification equation:
   magnification = image size ÷ real size
2. Rearrange to find the real size:
   real size = image size ÷ magnification
3. Substitute in the values:
   real size = 43 mm ÷ 50
4. Calculate:
   43 ÷ 50 = 0.86 mm
5. Convert mm to µm:
   0.86 mm = 860 µm

🧠 How marks are awarded

• 1 mark for using the correct magnification relationship
• 1 mark for rearranging correctly
• 1 mark for getting 0.86 mm
• 1 mark for converting to 860 µm

The mark scheme says that an answer of 860 µm scores 4 marks.

❌ Common traps

• Using 43 × 50 instead of dividing
• Leaving the answer in mm when the question asks for µm
• Writing 86 µm because of a conversion error
• Not showing any working, which can lose method marks

💡 Key knowledge

Unit conversion reminder:

• 1 mm = 1000 µm
• So 0.86 mm × 1000 = 860 µm

✅ Correct answer

A strong 4-mark response could include:

• Place the slide on the stage and secure it.
• Start with the lowest power objective lens (for example ×4).
• Adjust the light / mirror so light passes through the slide.
• Use the coarse focus to move the slide close to the lens, then focus until the image appears.
• Use the fine focus to get a clear image.
• Change to the ×10 objective lens.
• Refocus carefully to reach a total magnification of ×50 with the ×5 eyepiece.

🧠 Examiner tip

To reach top marks, you must explain how to focus the slide/cells and how to achieve the required magnification of ×50.

The mark scheme accepts a sequence like:

• ×5 eyepiece + ×10 objective = ×50 total magnification
• Use the fine focus after the image is found

❌ Common errors

• Starting on high power
• Forgetting to mention focusing
• Saying “move the slide towards the lens” without describing careful focusing
• Not showing how you get from ×5 to ×50

✅ Correct answer points

• The rate is fastest in the first 0.5 hours.
• The rate decreases gradually after the first 0.5 hours.
• The rate is constant between 1.0 and 2.0 hours or between 2.0 and 3.5 hours.
• The rate becomes zero between 3.0 and 3.5 hours.

📐 Working from the table

You can support your answer with changes in absorbed mass over time:

• With oxygen: 0 to 100 in the first 0.5 h = fastest start
• 100 to 145 from 0.5 to 1.0 h = slower than the start
• 145 to 170 and 170 to 195 etc. = roughly steady increase for a time
• 250 to 265 from 3.5 to 4.0 h = very small increase, so rate is almost zero

🧠 Exam technique

For this kind of question, don’t just list numbers. Turn the numbers into statements about rate:

• “increases quickly”
• “slows down”
• “stays constant”
• “stops”

The mark scheme gives credit for any three correct observations.

✅ Correct answer

• More nitrate ions are absorbed in the presence of oxygen.
• This suggests nitrate ions are absorbed by active transport.
• Active transport needs energy from respiration.
• With oxygen there is more aerobic respiration, so more energy is available.
• Some nitrate ions may also be absorbed by diffusion.

💡 Key knowledge

Why oxygen matters:

• Oxygen allows aerobic respiration.
• Respiration releases energy in plant cells.
• That energy is needed for active transport of nitrate ions.
• Without oxygen, less energy is released, so uptake is lower.

🧠 How to gain all 4 marks

• Make a clear comparison between the two set-ups.
• Use the term active transport.
• Say where the energy comes from: respiration.
• Include a qualifier such as “some nitrate ions may still be absorbed by diffusion”.

❌ Common misconceptions

• Saying oxygen is used to “make energy” directly
• Writing “energy from oxygen” instead of energy from respiration
• Confusing active transport with diffusion
• Forgetting that some ions may be absorbed by diffusion or other routes

✅ A model 4-mark response

More nitrate ions are absorbed when oxygen is present, which suggests that nitrate ions are absorbed by active transport. Active transport requires energy from respiration. With oxygen, aerobic respiration can happen, so more energy is available for absorption. Some nitrate ions may also be absorbed by diffusion.

✅ Correct answer

• Nitrate ions are used with glucose.
• They are used to form amino acids.
• Amino acids are used to make proteins, which are needed for growth.

💡 Key knowledge

Plant growth needs proteins because they are used to make new cells and tissues. Nitrate ions are an essential source of nitrogen for making amino acids.

🧠 Examiner tip

This is a chain of ideas, so write it in order:

nitrate ions + glucose → amino acids → proteins → growth

💡 Must-remember facts

• magnification = image size ÷ real size
• 1 mm = 1000 µm
• Start microscopy on the lowest power
• Oxygen supports aerobic respiration
• Respiration provides energy for active transport
• Nitrates are needed to make amino acids and proteins

🧠 Final exam advice

• For calculations: write the formula, substitute numbers, calculate, convert units.
• For descriptions: quote or refer to the table data.
• For explanations: link oxygen → respiration → energy → active transport.
• Use scientific terms exactly for best marks.