R2101 2.5 Leaf Anatomy

Question 1

How is leaf adapated for photosynthesis and gaseous exchange?

Answer 1

Large surface area – absorb more light

Thin – short distance for carbon dioxide to diffuse into leaf cells

Chlorophyll – absorbs sunlight to transfer energy into chemicals

Network of veins – to support the leaf and transport water, mineral ions and sucrose (sugar)

Stomata – allow carbon dioxide to diffuse into the leaf and oxygen to diffuse out

Waxy cuticle – to stop the water vapour escaping through the epidermis, e.g. Sedum acre

Question 2

How is internal structure of the leaf adapted to promote efficient photosynthesis?

Answer 2

Thin cuticle made of wax – to protect the leaf from infection and prevent water loss without blocking out light

Epidermis is thin and transparent – to allow more light to reach the palisade cells

Palisade cells contain chloroplasts to absorb all the available light; at top of leaf – to absorb more light and increase the rate of photosynthesis

Spongy layer – spaces allow gases to diffuse through the leaf

Stomata – allow gaseous exchange

Xylem and phloem

Question 3

How does gas exchange work?

Answer 3

CO2 moves from the air into the leaf by diffusing through small pores called stomata.

Oxygen moves out of the leaf through the stomata. Movement of gases in opposite directions = gas exchange

Water vapour also diffuses out of the stomata. The stomata are surrounded by guard cells, which control their opening and closing.

Question 4

What is translocation?

Answer 4

Xylem moves water and mineral ions from the roots to the leaves.

Phloem moves food substances such as sucrose (sugar) and amino acids from leaves to the rest of the plant.

Question 5

What do root-hair cells do?

Answer 5

Provide large surface area to increase the rate of absorption of water and mineral ions

Absorb water and minerals.

Water is transported through the roots to the rest of the plant to:

• be used as a reactant in photosynthesis
• support leaves and shoots by keeping the cells rigid
• cool the leaves by evaporation
• transport dissolved minerals around the plant

Question 6

What do stomata do?

Answer 6

Tiny holes in the underside of leaves.

Control water loss and gas exchange by opening and closing. Allow water vapour and oxygen out of the leaf and CO2 into the leaf.

Drier conditions: small numbers of tiny stomata and only on lower leaf surface.

Plants regulate the size of stomata with guard cells. In bright light the guard cells take in water by osmosis and become plump and turgid. In low light the guard cells lose water and become flaccid, causing the stomata to close.

The size of the stomatal opening is used by the plant to control the rate of transpiration and therefore limit the levels of water loss from the leaf. This helps to stop the plant from wilting.

Question 7

What is transpiration?

Answer 7

When the plant opens its stomata to let in CO2, water on the surface of the cells of the spongy mesophyll and palisade mesophyll evaporates and diffuses out of the leaf.

Question 8

Describe ONE leaf adaptation to prevent water loss, giving a NAMED plant example.

Answer 8

1. Thick waxy cuticle, e.g. Sedum acre
2. Rolled leaf to maintain relative high humidty around the leaf and hence the stomata, e.g. Ammophila arenaria
3. Hairs on leaves and stems – e.g. plants in hot dry climates often have a silver grey appearance due to the presence of tiny fine hairs. These trap the water vapour leaving the leaf and increase the relative humidity, thus preventing water loss. e.g. Lavandula angustifolia and Stachys byzantina
4. Needles – create a smaller surface area for water loss. E.g. Larix decidua
5. Sunken stomata – in some plants, such as Pinus sylvestris