Plant organs and transport

A plant's organs (roots, stems and leaves) work together to take in water, mineral ions and carbon dioxide, make food and move it around. Water travels up from the roots in the xylem; sugars made in the leaves travel in the phloem. Water evaporating from the leaves pulls more water up behind it: transpiration.

Key facts

  • A leaf is made of tissues: the upper epidermis (clear, with a waxy cuticle), the palisade mesophyll (most photosynthesis), the spongy mesophyll (air spaces for gas exchange), xylem and phloem, and the lower epidermis with stomata and guard cells.
  • Xylem is made of dead cells with no end walls, strengthened by lignin. It carries water and mineral ions from the roots to the stems and leaves.
  • Phloem is made of living cells with pores in their end walls (sieve plates). It carries dissolved sugars to the rest of the plant, up and down, for immediate use or storage: translocation.
  • Transpiration is the loss of water vapour from the leaves through the stomata. It pulls water up the xylem in the transpiration stream.
  • Transpiration is faster when it is warmer, brighter, windier or less humid. Guard cells close the stomata when water is short and in the dark.
  • A potometer estimates the rate of transpiration from the rate of water uptake: rate = distance the bubble moves ÷ time. Volume taken up = πr² × distance moved.
  • Meristem tissue at the tips of shoots and roots makes new cells, so the plant can grow.

Worked examples

Example 1

The diagram shows a cross-section of a leaf. Name the tissue where most photosynthesis happens, and say why it is near the top.

  1. The top layer is the upper epidermis: thin and clear, with a waxy cuticle.
  2. Below it are tall cells packed with chloroplasts: the palisade mesophyll.
  3. They are at the top, where there is the most light, so most photosynthesis happens there.
  4. Below them, the spongy mesophyll has air spaces that let carbon dioxide reach the cells.

Answer: the palisade mesophyll; it is near the top surface, where it gets the most light

Example 2

In a potometer the bubble moved 36 mm in 4 minutes. Calculate the rate of water uptake.

  1. Rate = distance moved ÷ time. 36 ÷ 4 = 9
  2. The rate of water uptake is 9 mm/min.

Answer: 9 mm/min

Example 3

The capillary tube of a potometer has a radius of 0.5 mm. The bubble moved 40 mm in 5 minutes. Calculate the volume of water taken up per minute.

  1. Volume = πr² × distance. π × 0.5² × 40 = 31.4 mm³
  2. Divide by the time. 31.4 ÷ 5 = 6.3 mm³/min

Answer: 6.3 mm³/min

Example 4

Explain why a plant transpires faster on a warm, windy day.

  1. Warmer water molecules have more energy, so more evaporate inside the leaf.
  2. Wind blows the water vapour away from the leaf surface.
  3. The air outside the stomata stays dry, so water vapour diffuses out faster.

Answer: water evaporates faster when warm, and wind blows water vapour away, keeping the concentration gradient steep