Gas Exchange in Plants and Xerophytic Adaptations Be

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?

Reminder Photosynthesis:

Photosynthesis vs. Respiration Plants 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. Respiration Some 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 1 cm 2 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 Stoma Epidermal cells Guard 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 – 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.

Conserving water 23 of 26 © Boardworks Ltd 2008

Xerophytes

Marram Grass Thick cuticle with no stomata on upper surface – why? Also: • Rolled leaves • Hairy • Sunken stomata These features trap a layer of humid air near the stomata.

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 Reason Thick waxy cuticle Reduces evaporation from epidermis Small leaves/spines Smaller surface area for evaporation Few stomata Stomata sunk in pits in epidermis Less transpiration Layer of humid air trapped near stomata to reduce transpiration Hairs around stomata or over whole leaf surface

Exam practice…

Leafy Game • 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. First one to shout the correct word gets to leave!

Whole diagram

• Air Spaces

• No stomata

• Waxy Cuticle

• No palisade mesophyll