AQA GCSE Biology Biology Paper 1 (Higher), June 2023: Question 6

✅ Mark Scheme Answer

LHS (Reactants): CO₂ and H₂O (in either order)

RHS (Products): O₂

Full balanced equation:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

❌ Common Errors

• Writing names instead of chemical symbols (e.g. writing "carbon dioxide" instead of CO₂ ).
• Incorrect capitalisation or subscript formatting: writing Co2 or h2o will cost the mark.
• Writing numbers in front of the molecules when balancing numbers are already provided.

✅ Correct Selection

Tick the box: From 0 to 5 000 lux

💡 Key Knowledge

The equation y = mx + c represents a straight line. Looking at Figure 8:

• From 0 to 5 000 lux, the curve is essentially a straight line passing through the origin (directly proportional).
• Above 5 000 lux, the gradient decreases as the curve bends and eventually levels off (plateaus) around 22 000 lux.

✅ Any ONE valid method:

• Use different coloured light bulbs or LEDs.
• Place different coloured filters (or cellophane/plastic) in front of the lamp / between lamp and beaker.
• Wrap the beaker with different coloured transparent cellophane.

🧠 Exam Technique

Be precise. Stating "use a coloured filter" is much safer than just saying "change the screen". If naming a material, ensure it is transparent or translucent so light can still pass through.

✅ Correct Identifications

Independent variable (1 mark): Colour of light (or wavelength of light)

Dependent variable (1 mark): Time taken for 10 leaf discs to reach the surface of the solution

❌ What Examiners Ignored / Rejected

• Independent variable: Stating "colour of filter", "colour of lamp", or "bulb" was ignored. You are testing the light itself, not the equipment.
• Dependent variable: Stating just "rate of photosynthesis" was insufficient because the question asked for the specific variable measured in this investigation. Always write the exact measurement taken!

✅ Any ONE reason:

• So that the leaf discs would sink to the bottom of the beaker at the start (so they do not already float).
• To ensure that any gas that makes the discs rise is produced by photosynthesis (not pre-existing trapped air).
• To standardise starting air content (control variable).

💡 Mechanism Behind the Assay

Spongy mesophyll tissue contains air spaces. Drawing a vacuum replaces these air pockets with the liquid solution, increasing tissue density so discs sink. As photosynthesis proceeds, newly produced gas refills the intercellular spaces, decreasing density until the discs float.

✅ 2-Mark Structure

1. Sodium hydrogencarbonate provides / releases carbon dioxide (CO₂). [1 mark]
2. Carbon dioxide is required / used as a reactant for photosynthesis (ensuring CO₂ is not a limiting factor). [1 mark]

❌ Common Misconceptions

• Writing "it contains carbon dioxide" without stating that it releases/provides it into solution.
• Claiming it "provides water" or "controls pH" (both rejected for this mark).

✅ Required Points

• Oxygen gas was produced during photosynthesis. [1 mark]
• The oxygen/gas bubbles become trapped inside/around the leaf disc, causing it to become less dense than the solution (or buoyancy forces push it upwards). [1 mark]

🧠 Exam Technique

Always name the specific gas: oxygen. Saying "gas is produced" only gets partial credit unless explicitly identified as oxygen formed by photosynthesis.

✅ Correct Answer

To absorb / use a wider range of wavelengths / colours of light (or to increase the total rate of photosynthesis). [1 mark]

❌ Mark Scheme Trap

Do NOT write "to absorb all colours of light" or "all wavelengths of light". Chlorophyll reflects green light (neither absorbs green well), so it does not absorb all light!

✅ Full 4-Mark Model Answer

1. Blue absorption: Chlorophyll absorbs a high percentage / most of blue light (approx. 450–499 nm). [1 mark]
2. Green absorption: Chlorophyll absorbs very little / least amount of green light (approx. 500–570 nm) / reflects most green light. [1 mark]
3. Link to rate: Leaf discs in blue light rose faster (115 s) because they photosynthesised at a higher rate; in green light they took the longest (831 s) because the rate of photosynthesis was much slower. [1 mark]
4. Data quote: Quote figures from Figure 10 / Table 6 (e.g. Chlorophyll b absorbs ~80% of blue light around 460 nm, but absorption drops to under 5% in green light). [1 mark]

❌ Common Student Pitfalls

• Inverting the time-rate relationship: Confusing a smaller time (115 s) with a slower rate. 115 seconds means it rose faster, meaning a higher rate of photosynthesis!
• Forgetting data quotes: The question explicitly says "Use data from Figure 10 and Table 6". Missing numerical absorption percentages or wavelength ranges automatically caps marks at 3/4.