Unit 8: Transport in Plants

Xylem

Carry water and minerals from the roots to the leaves, supporting the stem and strengthening it

Phloem

Carry sugar and other organic nutrients made by plants from the leaves to the rest of the plant

Structure of Xylem Tissue

• Dead cells

• Thick strengthened cellulose cell wall

• Hollow lumen

• End walls of the cells have disappeared

• Contains hole (pits)

Structure of the Phloem tissue

• Long narrow tube

• Perforated sieve plates

• Made of columns of living cells, which contains a cytoplasm but no nucleus

• It’s activities control by a companion cell next to it which has a nucleus

Distribution of xylem and phloem in roots

Xylem and phloem are in the centre to withstand stretching forces

Distribution of xylem and phloem in the stem

They are arranged in bundles near the edge to resist compression and bending forces

Distribution of xylem and phloem in the leaves

They are grouped together into veins and vascular bundles

Water uptake

1. At the very tip is a root cap which protects the roots as it grows through the soil.

2. The rest of the root is covered by a layer of cells called the epidermis

3. The root hairs are a little way up from the root tip. Each root hair is a long epidermis cell.

Functions of root hair cells

• Increase the external surface area of the root for absorption of water and mineral ions

• Provide anchorage for the plant

Transpiration

• water enters root hair cells by osmosis . This happens when the water potential in the soil surrounding the root is higher than in the cell a water diffuses from the soil into the root hair, down its concentration gradient

Factors affecting transpiration rate

Temperature, air movement, humidity, light intensity

Temperature affecting transpiration

The kinetic energy of water molecules → diffuse faster

Air Movement affecting movement

Removes water molecules as they pass out of the leaf → maintaining a steep concentration gradient for diffusion

Humidity affecting air movement

The concentration of water molecules outside the leaf → steeper concentration gradient for diffusion

Light intensity affecting transpiration

Stomata open to allow gas exchange for photosynthesis→ water vapor can diffuse out of the leaf