Gas exchange in the leaf of a plant 6.4

What is the gas exchange in plants ?

Photosynthesis happens only during the day

Carbon dioxide and water = glucose and oxygen

Respiration happens continuously

Glucose and oxygen = carbon dioxide and water

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Some carbon dioxide comes from respiration of cells most of it is obtained from the external air .

Some oxygen is used in respiration but most diffuses out of the plant.

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How do we increase the rate of diffusion ?

1. Thin = short diffusion pathway

2. large SA:V ratio ➤ absorb more sunlight

3. arranged in a manner that prevents overlapping ➤ prevents shadow forming on the surface

4. stomata on it's surface ➤ allow gases to enter/exit the leaf

5. numerous air spaces ➤ within which gases can be exchanged

6. large network of xylem and phloem

➤ xylem to transport water

➤ phloem to transport sugars formed in photosynthesis.

7. High concentration gradient.

Describe how guard cells work (5)

➤ guard cells actively pump in K+ ions
➤ reduces ψ
➤ water enters by osmosis down a water potential gradient
➤ guard cells become turgid
➤ stomata pore opens

How do dicotyledonous plants exchange gases?

At the surface of mesophyll cells
They are adapted to have a large surface area
Gases move in and out of many holes in the epidermis called stomata
Stomata are able to open and close to control the exchange of gases
Stomata can close if the plant is losing too much water

How do plants adapt if they find themselves losing too much water?

Guard cells lose water and become flaccid
This causes the guard cells to close the stomata

what is the function of epidermal tissue?

\-Covers the surfaces of the plant for protection

\-Covered with a waxy cuticle=waterproof, Helps to reduce water loss by evaporation as prevents water being able to pass through

Palisade mesophyll function?

The cells are packed with chloroplasts it is the site of most of the photosynthesis

Spongy mesophyll function?

Has lots of air spaces,This allows gases to diffuse in and out of cells.

Xylem function+phloem ?

Form vascular bundles and transport water, mineral ions and glucose around the plant

What is the role of the guard cells?

To open and close to let gases diffuse in and out through the stoma.

Stomata

Stomata are mostly found on the underneath of
leaves.

Each stoma consists of a pair of guard cells that
surround the stomatal aperture/pore.

These guard cells contain chloroplasts and swell
up to open the stomata.

Gas exchange at stomata

Oxygen diffuses out of the stomata

carbon dioxide diffuses in through the stomata

to reduce water loss by evaporation, stromata close at night, when photosynthesis wouldn’t be occurring.

what are xerophytes?

xerophytes are plants that are adapted to living in areas where water is in short supply.without these adaptations these plants would become desiccated and die.

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what are the adaptations?

Thick, waxy cuticle= Less water loss via cuticle

2)Fewer stomata=Fewer pores for water loss

Sunken stomata=Traps moist air - reduces diffusion gradient

Fine hairs on leaf surface=Traps moist air - reduces diffusion gradient

Stomata open at night=Less water loss during the day

Rolled leaves=Reduced surface area

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