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

The cells in a leaf that control the rate of water loss are the guard cells. These cells surround the stomata and regulate their opening and closing, which affects transpiration.

The scientific term that describes the movement of water from the roots, through the plant, and eventually lost from the leaves is 'transpiration stream'.

Closing the stomata would decrease the rate of water loss from a plant's leaves, as it reduces the surface area for evaporation.

Structure:

• Xylem:

  • Made of dead cells,
  • Hollow tubes,
  • Contains lignin in the walls,
  • No cytoplasm.

• Phloem:

  • Made of living cells,
  • Has pores in the end walls,
  • Contains cytoplasm,
  • Does not contain lignin.

Function:

• Xylem:

  • Transports water and mineral ions upwards from the roots to the leaves,
  • Involved in transpiration.

• Phloem:

  • Transports dissolved sugars bidirectionally throughout the plant,
  • Involved in translocation, moving nutrients from sources to sinks.

To determine the rate of water loss at 12:00, calculate the area of the tangent line at that time and use your height and base readings to find the volume lost. Suppose the tangent provides a slope that corresponds to a loss of 40 cm³ over 7 minutes, which gives a rate of:

ext{Rate} = rac{40}{7} ext{ cm}^3 ext{ per minute} ext{ or } 5.71 ext{ cm}^3 ext{ per minute}

In standard form, this would be:

$9.5 imes 10^0 ext{ cm}^3 ext{ per minute}$.

The rate of water loss at midnight was much lower than at 12:00 because at night, the temperature is cooler and there is less light, which reduces the rate of transpiration. Additionally, the stomata are often closed during the night to conserve water.