Leaves, water uptake and extreme environments — Edexcel GCSE Biology

Revision notes

4 short notes, in the order of the specification. Each one in short:

1. Leaf structure

   Triple only

   A leaf is adapted to absorb light for photosynthesis and to allow gas exchange. It is broad, flat and thin, giving a large surface area and a short diffusion distance. Palisade mesophyll cells near the top have many chloroplasts, air spaces in the spongy mesophyll let gases diffuse, and the waxy cuticle reduces water loss.

2. Factors affecting water uptake

   The rate of water uptake by a plant depends on the rate of transpiration, which is increased by higher light intensity, more air movement and higher temperature. Light opens the stomata, moving air carries water vapour away to keep a steep concentration gradient, and warmth makes water evaporate and diffuse faster. A potometer measures the rate.

3. Rate calculations for transpiration

   A rate of transpiration or water uptake is the change in a quantity divided by the time taken, for example the distance a bubble moves in a potometer per minute. Percentage change in mass equals change in mass divided by original mass, times 100. Calculate a mean from repeats first, and always give the unit, such as mm per minute.

4. Plants in extreme environments

   Triple only

   Plants in extreme environments, which are hot, dry or windy, have adaptations that reduce water loss by transpiration. They may have small, needle-like or rolled leaves or spines, a thick waxy cuticle, and fewer or sunken stomata. The cost is that less carbon dioxide enters, so these plants often photosynthesise and grow slowly.