Chapter 7 : Nutrition in Plants

7.2 Leaf Structure and function

External Structure of Leaf

• Network of Veins – Veins carry water, mineral salts and manufacture food.

• Lamina – Large flat surface so as to obtain maximum light for photosynthesis. Lamina allow carbon dioxide to rapidly reach inner cells.

• Petiole – Holds

lamina away from stem

. So as to obtain sufficient sunlight and air.

Internal Structure of Leaf

1. Upper Epidermis  Waxy Cuticle:

Prevent loss of water

,

transparent allow sunlight to pass through

to  Single layer of

closely packed cells

.

Internal Structure of Leaf

2. Palisade Mesophyll   Closely packed, long and cylindrical cells Contains

numerous chloroplast

:

Absorb maximum sunlight for photosynthesis.

Internal Structure of Leaf

3. Spongy Mesophyll   Irregularly shaped cells Contains

numerous large intercellular spaces

: for

diffusion of gases

 Fewer chloroplast as compared to palisade mesophyll.

Internal Structure of Leaf

3. Spongy Mesophyll  Cells are layered with a

thin film of moisture

:

Allow Carbon Dioxide to dissolve in

 Presence of vascular bundle (xylem + phloem)

Internal Structure of Leaf

4. Lower Epidermis    Single layer of closely packed cells Cuticle Many minute opening known as

stoma gaseous exchange

: Important of

Guard Cells

• Bean-shaped • Contains chloroplast.

Difference between Guard Cells and Epidermis Cells

Shape Chloroplast Cell Wall Function

Guard Cells

Bean-shaped √ Cell wall near the stoma is thicker than elsewhere.

Controls diffusion rate

Epidermis Cell

Irregular shaped X Uniform cell thickness Does not control diffusion

Internal Structure of Lamina: Guard Cells

• During Day, 1. Water potential of guard cell decreases.

2. Water from neighbouring cells would start to enter guard cells by osmosis 3. Guard cells become turgid.

Internal Structure of Lamina: Guard Cells

• During Night, 1. Water potential of guard cell increases.

2. Water from neighbouring cells would start to leave guard cells by osmosis 3. Guard cells become flaccid.

How does Carbon Dioxide and Water enter the leaf?

• Carbon dioxide enter the leaf through the stomata.

• Water is transported in the leaf through xylem (vascular bundle)

How does Carbon Dioxide enter the leaf?

• Through

stomata

• Carbon dioxide is used for photosynthesis • During the day,

carbon dioxide is rapidly used up

during photosynthesis • As a result, there is a

lower concentration of carbon dioxide in leaf

.

• Carbon dioxide from air would then

diffuse into leaf through stomata and goes into mesophyll cells.

How does Water enter the leaf?

• Vascular bundle – xylem and phloem • Xylem transports water and dissolved minerals from roots to leaf • Water and mineral salts move from xylem to mesophyll cells.

• Water is used for photosynthesis.