February 26, 2022
In this topic of IBDP Biology, we will learn about gas exchange in flowering plants.
Gas exchange in flowering plants
- Main function of leaf - photosynthesis
- This requires light, carbon dioxide and water
- In low light intensity – CO2 is given out due to respiration.
- As light increases – photosynthesis begins so some CO2 is used up so less is released.
- Compensation point – when carbon dioxide uptake = output.
- With further increase – CO2 in taken up to fuel photosynthesis.
- At light saturation (max rate of photosynthesis) – CO2 uptake levels off.
- When it comes to IBDP Biology, water is delivered by the transpiration stream.
Gross adaptations of the leaf
- Large flat surface to absorb maximum light.
- Thin to provide a short diffusion pathway for gases to reach the mesophyll cells.
- Veins to support the large flat thin blade.
Microscopic adaptations of the leaf
Upper epidermis
- Close fitting epidermal cells.
- When it comes to IBDP Biology, these secrete a protective waxy layer - the cuticle
- This reduces water loss.
- Few or no stomata
- Few or no chloroplasts in the cells – transparent to allow light to be transmitted to:
Palisade mesophyll
- Long cylindrical cells perpendicular to surface, packed with chloroplasts.
- Allows maximum light absorption.
- Large air spaces between cells allow effective gaseous exchange between cells and air spaces inside the leaf.
- Moist cell walls allow gases to dissolve.
Spongy mesophyll
- Even larger air spaces between cells.
- When it comes to IBDP Biology, it allows uninterrupted diffusion of gases between stomata and palisade.
- Less important for photosynthesis.
- Moist cell walls allow gases to dissolve.
- Cells have very large surface area exposed for gas exchange
Lower epidermis
- Similar structure to upper epidermis.
- Thinner waxy cuticle
- Tens of thousands of stomata per cm2.
- Stomata are pores are flanked by guard cells that contain some chloroplasts.
- Guard cells are specialised epidermal cells (only ones with chloroplasts).
- Kidney shaped
- Change shape to opens or closes the stomatal pore.
Stomatal control
- When it comes to IBDP Biology, there is a balance between requirements for photosynthesis and water loss.
- Leaves need carbon dioxide for photosynthesis but water evaporates through open stomata
- Stomata open and close according to the external conditions and the needs of the plant.
How stomata open and close?
- If water enters guard cells they become turgid and size of cell increases.
- They cannot expand widthways so are forced to elongate.
- The cells are fixed at each end so they bow outwards creating the stomatal pore.
Mechanism for changing water potential
- In light:
- When it comes to IBDP Biology, ATP is used to actively pump potassium ions (K+) into guard cells
- This creates a concentration gradient so water is drawn into the cell by osmosis
- The guard cells become turgid
- In dark:
- There is less ATP so pumping stops and ions leak out of cells.
- Water leaves by osmosis along gradient.
- Cells become less turgid and the stomatal pore closes.
- How do plants close stomata during daylight if they have a lack of water?
- A lack of water increases activates a potassium pump in the opposite direction
- Water follows and turgor is reduced, closing the pore.
That's the end of part 1.
You can now proceed to part 2!