HL Bio - Unit #5.3: Gas Exchange in Plants

Waxy Cuticle

A thick upper layer that has very low permeability to prevent the loss of water - Contains pores called stomata (stoma - singular)

• They are clear to allow for more photosynthesis by collection more energy

• The stomata allow for carbon dioxide and oxygen to pass through

  • Water proof

Epidermis

The upper layer of tissue on the leaf that contains guard cells that swell/shrink in order to allow access to the stomata

Palisade Mesophyll

The layer with an abundance of chloroplast and where photosynthesis takes place

Spongy Mesophyll

• A large and spacious layer through which oxygen and carbon dioxide can diffuse through

• A large surface area for gas exchange

• Permanently moist walls that allows CO2 to dissolve and diffuse through the mesophyll cells

Concentration gradient within the leaf

• When photosynthesis uses CO₂ a concentration gradient is formed between the air outside to leaf to the gasses within the chloroplasts in the mesophyll cells (Takes more CO₂ in due to the lower concentration within the plant)

• Photosynthesis produces O₂ which further creates a concentration gradient allowing O₂ to diffuse out of the leaf due to its larger concentration

Vessel

Supplies water through the xylem to the mesophyll

• Water is necessary as it’s one of the reactants during photosynthesis

Lower Epidermis

ANother layer containing guard cells found at the underside of the leaf

Stomata

Pores found throughout the upper/lower layers of the leaf that allow for gas/water to pass through (enter or leave)

Guard cells

Cells found in the epidermis layers which swell/shrink in accordance to how much water is given to the leaf

The shrinking or swelling of the guard cells allow for different molecules to pass through the stomata or keep them inside

When do guard cells close

• Guard cells close at night time when less photosynthesis is occurring and less CO2 is needed

• Guard cells also close when there is water stress and the plant needs to preserve it’s water reserve from being passed out of the plant

Gas exchange in leaves and Transpiration

Essentially water that is used during photosynthesis needs to be replenished. Water to the leaves are transported using the xylem tube

Transpiration

The process of water being transported to the different parts of the plant through the xylem vessels

Measurement of Transpiration rates

Done using a potometer

They measure the amount of water uptake within the shoot and the rate can be determined by how long it took for the plant to uptake as much as it did

Transpiration Rates affected by:

1. Wind

2. Light

3. Temperature 

4. Humidity

Transpiration Rates affected by: Wind

Higher movement of air around the plant will increase transpiration rates

• It basically just sweeps away the water vapor molecules that are accumulating outside of the stomata

• Concentration different between the water vapor within the plant and outside increases (gradients) 

Transpiration Rates affected by: Light

The stomata open under the presence of light to continue the photosynthesis process

• The stomata opening allows for more water vapor to be lost 

Transpiration Rates affected by: temperature

Positive Correlation: As water molecules within the plant receives more energy (increased temperature), evaporation rates is also increase 

Transpiration Rates affected by: Humidity

Negative Correlation: As humidity levels increase, transpiration rates will decreases

• The pockets of air around stomata become saturated with H2O molecules which decreases the concentration gradient

Stomatal Density

The number of stomata per unit area of the leaf

Stomatal Density (mm^-2) = (Mean number of stomata)/ (Area of the field of view (mm²))

Methods of viewing the stomata

1. Peel off the epidermis layer

2. Use clear nail varnish on smooth non-hairy leaves and peel the varnish off to see the groves of the stomata