Plant organs and transport · Year 12
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.