Exchange in plants

Lesson objectives

• All will be able to state substances to be exchanged in plants

• Most will be able to describe and explain how plants maximise exchange

• Some will be able to suggest reasons why plants do not need to exchange carbon dioxide from the air continuously

Stick the leaf diagram in books and label

• Key words:

– Stomata

– Guard cell

– Upper epidermis

– Lower epidermis

– Waxy cuticle

– Palisade cells

– Spongy mesophyll

Exchange in plants

• What substances do plants need to exchange?

• Where do they exchange these substances?

How are leaves adapted for efficient gas exchange?

1. Surface area

• The flattened shape of a leaf (and internal air spaces) increases surface area for diffusion

• Thin to reduce the distance for diffusion

2. They only take in carbon dioxide when they need to

• Why don’t leaves need carbon dioxide all the time?

• All over the surface of a leaf are small openings known as stomata. They can be opened or closed (by guard cells). Only open when carbon dioxide is needed so prevents too much water loss.

Exchange in Plants

• Are there more stomata on top of the leaf or under it?

• Make a prediction

Exchange in plants

• Apply a thin layer of clear nail varnish to the lower surface of the leaf.

• Allow to dry• Carefully peel off the dry nail varnish with a pair of

tweezers• Place peel in a drop of water on a microscope slide• Cover with a coverslip • View sample under microscope first under low

magnification and then high.• Count and record the number of stomata you see.• Repeat - coating the upper surface of the leaf.

One Stoma

On the underside of leaves are small holes, or pores, called stomata. A single hole is called a stoma.

Each stoma is surrounded by two guard cells, which control the opening and closing of the stoma.

What would happen if the stomata were permanently open?

Exchange in plants

• Now look under the microscope at the cress roots – what do you notice

• How does this help the plant?

How are roots adapted for water/mineral uptake?

1. Thin

2. Divided into many tubes (increases surface area)

3. The cells of the roots are made up of tiny projections called root hair cells (increases SA)