Transport in Plants

1. Transport in Plants

2. Learning Objectives • Features of effective transport systems in plants. • Nature of waste products and excretory mechanisms and systems in plants. • Features of effective surfaces of gaseous exchange; mechanisms and systems of gaseous exchange in plants; process of diffusion.

3. Structure of a Stem

5. Phloem Cambium Vascular bundle Xylem Pith Cortex Epidermis

6. Phloem • Food substances such as sucrose and amino acids travel in the phloem. • The sucrose is made in the leaves, through photosynthesis, and travels to the rest of the plant to be used as food for cellular respiration. • The cells in the phloem tissue form elongated tubes called sieve tubes. • The cells in the phloem remain alive, although they lose their nuclei. • The end walls of each cell become perforated to allow substances to pass through. These are called sieve plates.

10. Xylem • Water and mineral salts are transported in the xylem from the roots to the rest of the plant. • The cells become vessels – long cells joined end to end (like a straw). The end walls break down and the cells die when they are mature. • Cell walls in xylem vessels become strengthened with a substance called lignin. This makes the cell walls impermeable. • Xylem vessels are further strengthened by many elongated, lignified cells called fibres.

12. Questions • What are the differences between xylem and phloem: • in structure? • in function?

13. Roots • Internal structure with vascular bundles are set out differently compared with stems. • Vascular bundle is in centre of the root. • Xylem carries salts and water from roots to stem. • Phloem will bring food from the stem to the roots.

14. Root Hairs • The cells of the outer layer of the root just above the root cap produce tiny, tube-like outgrowths called root hairs. • Root hairs grow between soil particles and stick closely to them. • Root hairs take up water by osmosis. • They absorb mineral salts by active transport. • The large number of root hairs greatly increases the surface area for absorption.

17. Transpiration • Evaporation of water from leaves of a plant. • Water moves from inside the cell walls of the cells in the leaf to the air spaces between the cells. • The water evaporates from here and passes by diffusion through the air spaces in the mesophyll and out of the stomata. • This creates a force that draws water up from the soil through the plant – it causes a tension up the stem, which ‘sucks’ water molecules along. • The transpiration stream is the flow of water up the vascular bundles.

18. Rate of Transpiration • Can be altered by reducing or increasing evaporation from leaves. • What processes might affect the rate of transpiration? • Light intensity • Humidity • Air movement • Temperature

19. How does the water move from roots to leaves? • Draw a flowchart or annotated diagram to illustrate and summarise how water moves through the plant.