Gas Exchange in Plants and Xerophytic Adaptations

- Be able to describe how a plant is adapted for efficient gas exchange with minimal water loss.

Plants – a Reminder

Write down the equation for respiration…

Write down the equation for photosynthesis…

Now try and label the cells on your sheet.

Gas Exchange in Leaves

• Plants, like animals, respire aerobically (all day and all night!).

What gas do they use?What gas do they produce?

• In the light plants also photosynthesise.What gas do they use?What gas do they produce?

ReminderPhotosynthesis:

Photosynthesis vs. RespirationPlants need oxygen for respiration all of the time.

Plants need carbon dioxide for photosynthesis during the day.

During the day, the rate of photosynthesis is around 6 times faster than that of respiration. Explain why.

Photosynthesis vs. RespirationSome of the oxygen produced in photosynthesis is used in respiration.Some of the carbon dioxide produced in respiration is used in photosynthesis.

During the day, plants need much more carbon dioxide than respiration can provide; carbon dioxide diffuses into the leaf through the stomata.

Key terms…

Dicotyledonous plants – flowering plants which have two cotyledons (embryonic leaves) in the seed.

Their leaves have branched veins and their leaves and stems contain vascular bundles of xylem and phloem arranged in circles.

Examples: roses, sunflowers, peas

Let’s examine a leaf

Use transparent nail varnish to paint a 1cm2 square on the upper and lower surface of a leaf.

When dry, use sellotape to remove the nail varnish and stick each piece of tape to a slide.

Examine under the microscope and draw a diagram for each one, describing any differences you see.

View of the epidermis of a cactus

StomaGuard cells

Epidermal cells

Structure of a leaf

Label the diagrams on your sheet.

Try to add the function of each tissue in the leaf.

Label your own

The gas exchange surface

• Surfaces of mesophyll cells in contact with air spaces in the leaf.

• Large surface area - cells in contact with air spaces.

• Short diffusion distance - leaf is thin and gases only need to diffuse through one cell membrane and wall.

Problem!

What substance could be represented by the arrows?

The problem with plants…

• Losing water by evaporation via the gas exchange surface.

• Water evaporates from the wet cell walls in contact with air spaces in the leaf and it is lost by transpiration.

FYI: Transpiration

Transpiration – evaporation of water from the leaves.

Transpiration stream – movement of water through the plant.

Reducing Water Loss

What factors might affect the rate of transpiration?

• Humidity• Temperature• Wind • Stomatal opening/closing

Stomata

• Carbon dioxide and oxygen diffuse in and out of leaves through small openings called stomata.

How stomata open and close

• Most stomata are underneath the leaf• A stoma is surrounded by 2 guard cells • Guard cells open and close stomata

– this controls the diffusion of gases and water vapour.

A side note…

Guard Cells

• Have chloroplasts• Cell walls are especially rigid.• When cells absorb water it expands and

becomes turgid.• Their rigid inner wall resists expansion and

they become more curved.• When they lose water and become flaccid and

collapse closing the stomata.

Adaptations for Dry Conditions

Xerophytes are plants that can live in dry conditions without losing too much water.

They are adapted to take up as much water as possible when it is available, and to reduce the loss of water through transpiration.

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Conserving water

Xerophytes

Marram Grass

Also:• Rolled

leaves• Hairy• Sunken

stomata

These features trap a layer of humid air near the stomata.

Thick cuticle with no stomata on upper surface – why?

Cactus• Stem stores water

(succulent)• Spines reduce SA, protect

against predators, trap humid air near stomata

• Long roots spread both deep (to take in water from deep underground) and wide (to take in water from rain before it soaks far into the ground).

Typical Xerophytic Features

Feature ReasonThick waxy cuticle Reduces evaporation from

epidermisSmall leaves/spines Smaller surface area for

evaporationFew stomata Less transpirationStomata sunk in pits in epidermis

Layer of humid air trapped near stomata to reduce transpiration

Hairs around stomata or over whole leaf surface

Layer of humid air trapped near stomata to reduce transpiration

Exam practice…

First one to shout the correct word gets to leave!

• Look at the simplified diagram of a cross section through a leaf

• You’ll then see a picture with something missing see if you can spot what it is.

Leafy Game

Whole diagram

• Air Spaces

• No stomata

• Waxy Cuticle

• No palisade mesophyll