AQA GCSE Biology Biology Paper 2 (Higher), June 2024: Question 4

✅ Correct Answer

• To prevent light (or phototropism) from affecting the direction of root growth / results.
• OR So that only gravity affects the direction of root growth.

❌ Common Errors & Examiner Tips

• Vague responses: Writing just "as a control variable" or "to make it a fair test" scores 0 marks. You must state the specific biological factor (light).
• Wrong organ: Do not mention shoot phototropism; the seedlings are being examined for root response.

✅ Correct Selections

The two correct ticked boxes are:

• ☑ To prevent water affecting the direction of root growth [1 mark]
• ☑ To provide enough water for root growth [1 mark]

🧠 Exam Technique

• Roots respond to water gradients (hydrotropism). If one side was wetter than another, the root would bend towards water rather than purely responding to gravity.
• Surrounding the seedlings completely with damp paper ensures moisture is uniform on all sides and sufficient for cellular elongation.

✅ Correct Answer

A full 2-mark answer requires two key parts:

1. Comparison: To act as a comparison (to compare the growth in B with apparatus A) [1 mark].
2. Isolating gravity: To show that gravity caused the bending in apparatus A OR because the rotation ensures gravity acts equally on all sides of the seedling (cancelling out any one-sided directional pull) [1 mark].

❌ Common Errors

• Saying apparatus B "cancels out gravity" or "creates zero gravity". Gravity is still present; it acts equally in all directions because of the rotation.
• Mentioning auxin here: the question asks why the physical apparatus is needed, not for a biochemical explanation.

✅ Marking Criteria (3 distinct features)

1. Direction: Root tip clearly curved/bent downwards towards gravity [1 mark].
2. Growth: Overall root is visibly longer than at the start [1 mark].
3. Elongation zone: The ink marks have spread further apart in the curved region [1 mark].

🧠 How to Draw This Diagram

Do not draw ink marks spread out near the base of the seed before the bend; cell elongation happens in the zone just behind the tip!

✅ Correct Answer

• The root grows straight out / horizontally (it does not bend downwards).

💡 Key Knowledge

Because the seedling is rotating slowly, auxin does not accumulate on any one side. As a result, cells on all sides elongate equally, leading to straight, horizontal growth.

✅ Correct Answer

1. Auxin distributes unevenly / there is more auxin on the lower side of the root [1 mark].
2. In roots, auxin inhibits cell elongation, so the upper side grows/elongates faster than the lower side, causing it to bend downwards [1 mark].

❌ The #1 Plant Hormone Trap

Students often confuse roots with shoots:

• In shoots: high auxin concentration stimulates cell elongation (shoots bend up).
• In roots: high auxin concentration inhibits cell elongation. The bottom grows slower, so the top side bends the root down!
• Never say "auxin makes the bottom side grow more in roots" — this loses the second mark immediately.

✅ Correct Answers (Any TWO)

• As a weed killer (selective herbicide causing rapid, uncontrolled growth).
• As a rooting powder (rooting hormones to grow plants from cuttings).
• To promote cell division / growth in tissue culture.

🧠 Exam Tip

Keep your recall simple and precise. Writing "rooting powder" and "weedkiller" is fast and scores both marks without ambiguity.

✅ Correct Answers (Any TWO)

• To promote / trigger flowering (flower initiation).
• To produce more fruit.
• To produce bigger fruit / increase fruit size.
• Also accepted: To make fruit ripen simultaneously (or increase overall yield).

💡 Summary of Gibberellin Uses

• End seed dormancy: Trigger early germination.
• Promote flowering: Induce flowers regardless of day-length.
• Increase fruit size: Commonly used on seedless grapes and apples to bulk them up.