F4 Biology teaching notes Ch 10: 2nd Term NE 17

Structure	Feature and function
root cap	n   protects the tip of the root n   continually rubbed away and replaced by new cells.
(6) Root hair (根毛)	n   provide a large (7) surface area for the absorption of water and minerals.
(8) Epidermis               (表皮)	n   not covered with (9) cuticle n   made up of thin-walled cells n   protects the inner tissues from (10) infections (感染) n   some cells are differentiated into root hairs
(11) Cortex	n   Made up of several layers of thin-walled cells n   Stores food in the form of (12) starch / sugar n   Allows the passage of water and minerals across the root n   Cells become turgid after water absorption which provides (13) support for the root.
Vascular bundle (維管束)
(14) Xylem	n   transports water and (15) minerals
(16) Phloem	n   transports food

Ø  Adaptive features of roots for the absorption of water and minerals:

Feature		Adaptation
Root hairs	numerous	Provide a large (18) surface area for the absorption of water and minerals
	highly (17) branched
	(19) long and fine so that they can easily grow between soil particles	Help absorb water and minerals as they are in close contact with soil particles
Epidermis	not covered with (20) cuticle and thus permeable to water	n  No need of water-proof cuticle due to the high humidity in soil n  Allows water and minerals to pass through the epidermis into the root easily
	formed by a layer of thin-walled cells	Reduces the diffusion distance of minerals and the distance of osmosis of water

B

How are water and minerals absorbed by roots? (Book 1B, p. 10-20)

1      Absorption of water      

ŒŒŒŒ  1      In leaves, water is lost continuously due to (21) transpiration.

v     Water is drawn up the (22) xylem vessel by the (23) transpiration pull from the roots to the leaves.

In roots:

w     Water in the cortex cells near the xylem vessels of the roots enters the xylem vessels. This decreases the (24) water potential of the cytoplasm of the cortex cells.

x     Water moves from the neighbouring cells to these cells by (25) osmosis.

A (26) water potential gradient set up across the cortex causes water to move inwards from cell to cell by osmosis.

Some water moves inwards along the (27) cell wall without entering their cytoplasm.

y     As water is drawn into the inner cells, the water potentials of the root hair cells become lower.

The water potential of the soil is now (28) lower than those of the root hair cells.

Therefore, water in the soil enters the root hair cells by (29) osmosis.

2      Absorption of minerals

n  The concentrations of minerals in the soil are usually lower than that in the root hair cells.

n  Minerals are mainly absorbed against the (27) concentration gradient by (26) active transport.