Which cell is most transparent? Tick (✔) one box - AQA - GCSE Biology - Question 6 - 2021 - Paper 1

Chloroplasts are present in leaf mesophyll cells but are absent in root hair cells, which do not perform photosynthesis.

The cells that control the rate of water loss in a leaf are the guard cells surrounding the stomata.

This movement of water is described by the term 'transpiration stream.'

Closing the stomata would decrease the rate of water loss from a plant's leaves.

Structure

• Xylem: Made of dead cells, typically hollow with no cytoplasm; contains lignin in its walls.
• Phloem: Composed of living cells, has pores in the end walls, and contains cytoplasm.

Function

• Xylem: Transports water and mineral ions from roots to leaves and is involved in transpiration.
• Phloem: Transports dissolved sugars and other organic substances bidirectionally, involved in the process of translocation.

To determine the rate of water loss, take the tangent of the graph at 12:00. If, for example, the tangent gives a slope of 5.71 cm³ per hour:

• Convert to minutes: 5.71/60 = 0.095 cm³/min.
• In standard form: $9.52 \times 10^{-2}$ cm³/min.

At midnight, temperatures are generally lower and less light is available, leading to reduced transpiration rates. Stomata may also be closed or partially closed during the night, minimizing water loss.