Biology 2601A/B Lecture Notes - Light-Independent Reactions, Stoma, Electron Transport Chain
Lecture 10 – Plant Gas Exchange and Fluid Transport
Metabolism and photosynthesis are dependent on CO2 and O2 exchange with the environment
- Plants need CO2 for the Calvin Cycle, you have oxygen requirements for respiration, and
you have oxygen being formed in photosynthesis in the electron transport reactions
- All these gas have to get in and out in some way
Gas Exchange in photosynthesis
• Getting CO2 out of the external medium (air) and into the leaf
- Usually via Fickian diffusion through the stomata (or a stoma individually)
- This happens through diffusion, which is fixed equation that enters the leaf through
the stomata
• But this allows diffusion of other gases too
- To open up these stomata, it allows you to get into the plant to drive photosynthesis
but it also allows other gases to move in and out as well
• The most important is water…
Water vapour is also a gas
• Water vapour is a gas, where you can see it in certain circumstances
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• From the leaf, you have respiration where there is the uptake of oxygen and the loss of
carbon dioxide and photorespiration where you have the same thing with the uptake of
oxygen by rubisco and the release of carbon dioxide in that process
• For photosynthesis, you have the uptake of carbon dioxide and the release of oxygen
• GP = gross photosynthesis
• For all those processes for the gas to get in and out, you open up pores on the surface of
the leaf and that allows water to diffuse out of the leaf and into the atmosphere
• This is just a cross section of a leaf, showing basic anatomy
• On the very top you have a cuticle, and that cuticle is a very waxy substance and it’s
main purpose is to prevent water loss so that the cells that are underlining it are covered
by this waxy, relatively impermeable surface that minimizes water loss from the leaf’s
surface
• Underneath it, you have an epidermis layer and underlining that are the mesophyll cells
where photosynthesis occurs
- Mesophyll cells are kind of spongey
• You have air spaces in between the cells
• At the bottom, you have another layer of epidermal cells and another cuticle to prevent
loss
• These little pore holes at the bottom here are the stomata, and what this figure is showing
is that inside the leafs, all the cells are saturated with water
- These are living tissues
• There’s very high water vapour inside the leaf when you’re fully saturated with 100% of
humidity
• Outside the leaf, the air is dryer so that means there’s a driving force for water to diffuse
out of the leafs through the pores
• Plants don’t necessary want to lose water, but it is a requirement when you open the
stomata and there is a higher CO2 outside the leaf diffuse inside the leaf, where the CO2
concentration is lower because that CO2 is
• being consumed continuously by photosynthetic processes for the plants to fix that CO2
and reducing the concentration inside the leafs
- To get CO2 in, they have no choice where if they open the pores to get CO2 in and
water will leave
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• On average, how many molecules of water a plant loses to gain a 1 molecule of CO2?
- 400 molecules of water for every molecule of CO2
• There are huge movements of water within the system
Diffusion of gases: The Fick equation
- Fixed equation for diffusion
- There is the rate of net movement of a gas, where Q is a flux, so it is a movement
- Movement over time is equal to the diffusion coefficient x the area available for
diffusion x the concentration gradient
- When you want to increase diffusion, you can increase the area available for diffusion
to occur across (e.g. more stomata or a wide open; more CO2 to come in and more
water to be lost by the plant)
▪ It is also dependent on that concentration gradient, so if the air outside is very
dry then the plant loses a lot of water
▪ Whereas, when the air is foggy and saturated with water, the plant actually
loses less water
How do gases get in and out of leaves?
• Stomata
- Little pores on the leaf surface that allows access to air spaces underneath that are in
between those mesophyll cells
• Pores in the leaf surface that allow access from the outside to air spaces inside the leaf
- >90% of all gas exchange in a plant occurs through these pores
- For some species that have thick cuticles, it can be almost 100%
• Stomata are dynamic, where they can open and close depending on certain conditions
• Open is...
- Good for CO2 uptake, where photosynthesis can fix a lot of carbon and growfast
- Bad for water loss
• You want to have methods for minimizing your water loss or you have to be very good at
accessing water and getting it to your roots
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Lecture 10 plant gas exchange and fluid transport. Metabolism and photosynthesis are dependent on co2 and o2 exchange with the environment. Plants need co2 for the calvin cycle, you have oxygen requirements for respiration, and you have oxygen being formed in photosynthesis in the electron transport reactions. All these gas have to get in and out in some way. Gas exchange in photosynthesis: getting co2 out of the external medium (air) and into the leaf. Usually via fickian diffusion through the stomata (or a stoma individually) This happens through diffusion, which is fixed equation that enters the leaf through the stomata: but this allows diffusion of other gases too. To open up these stomata, it allows you to get into the plant to drive photosynthesis but it also allows other gases to move in and out as well: the most important is water .
