Why?
Oxygen and carbon dioxide are essential reactants in respiration and photosynthesis (respectively).
These gases do not need to be circulated to new parts of the plant via a transport system
Instead, gases are exchanged via passive diffusion.
These gases do not need to be circulated to new parts of the plant via a transport system
Instead, gases are exchanged via passive diffusion.
How?
- The leaves are well adapted to acheive passive diffusion oxygen and carbon dioxide.
- Leaves are almost always flat - this is to maximise the surface area to volume ratio.
- Leaves have open air spaces within them to provide even greater surface area for diffusion.
- Gases enter and exit leaves via open stomatal pores.
- Stomatal pores also allow for evaporation of water
- This means in the hottest part of the day, water can leave the plant and the plant will dehydrate.
- Stomata have the ability to open and close.
- Most stomata are found on the lower epidermis (where they are shaded).
- E.g. Australian Eucalypts have vertically hanging leaves to limit direct sunlight on stomata.
How do stomata work?
- Each Stoma (stomatal pore) is flanked by two guard cells
- Guard cells are crescent-shaped and readily absorb water.
- When they have a high water conent they become more turgid and more curved in shape, opening the stoma.
- When Guard cells lose water they become more flacid and lose their curved shape - closing the stoma.
- When open, oxygen and carbon dioxide enter the pore and diffuse accross the moist cell wall and cell membranes.
Timing of stomata opening/closing:
Daybreak/Nightfall:
Daybreak/Nightfall:
- Typically we observe that stomata open at daybreak and close at night.
- It is therefore likely that light is a initiator of stomatal opening/closing.
- On warm days we observe stomata closing.
- It is likely that this is more to do with the % humidity in the air as opposed to the temerapture.
- Water evaporates in warm and dry conditions.
- If water loss exceeds water intake then cell water levels decline and the guard cells become flacid.
- High humidity = water saturation and open stomata.
- This will increase carbon dioxide levels in the leaf (as they are not being used up)
- This is another trigger for stomata to close.
- A decline in carbon dioxide will result in a decline in photosynthesis.
- This will cause the stomata to open in order to gain more carbon dioxide.
Now you should:
- Read 307-309 of textbook and take down any additional notes.
- Complete Question Set 12.3
- WACE Study Guide Questions page 123-125 "RESPIRATORY SYSTEMS"
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