Plant Optimisation Flashcards
Why might water loss be useful to a plant?
Cooling
Transpiration
Describe stomatal structure in dicots and monocots
- Pore flanked by guard cells
Dicots - ‘sasuage shaped’ guard cells
Monocots
- dumbell shaped guard calls
- subsidary cells
What environmental cues may cause stomata to open/close
Light
Water availibility
Carbondioxide
How do stomata open?
Actication of P-type ATPase that pumps H+ out of the guard cell
Passive uptake of potassium ions due to hyperpolarisation by the ATPase
Uptake of chloride ions by H+Cl- symporter
Water moves into guard cells and they gain turgour
Malate ions also accumulate due to metabolism
How do stomata close?
Calcium ions activate a S-type anion channel that mediates Cl- efflux
This membrane depolarisation leads to K= efflux
Calcium signalling also reduces activity of the P-type ATPase
How is stomatal opening regulated by light?
Blue light receptor: phototropins
Increase in blue light activates the H+ pump:
due to phosphoylation of the C-terminus of the pump a regulatory protein binds to it and maintains its action
How does ABA contibute to stomatal opening/closure?
ABA accumulates in times of drought and inhibits stomatal opening while promoting stomatal closure
ABA binds to a receptor: PYR1
- PYR1 activates a MAP kinease protein which releases internal calcium stores from the vacuole
- Kinease also stimulates Cl- channals opensing and so chloride efflux
How does ABA reach guard cells?
ABA is released from plastids in the roots in response to drought and enters the transpiration stream to be targetted at guard cell complexes
Example of stomatal hjacking?
The pathogen Fusicoccum
Activates the H+ ATPase causes stomata to open in unfavourable conditions such as in drought
Response: leaves wilting
What factors may influence stomatal development?
High light and sufficent water in young years would promote more stomatal development
- high stomatal density and larger stomata
Low CO2 in early years leads to less stomata
What limits photosynthesis at high light intensities?
Carbon dioxide concentration
What is the light compensation point?
The light intensity at which there is 0 net CO2 uptake
How are photosytems organised within chloroplasts
Photosystems are not distributed evenly within a cell
Photosystem I is usually found in non-appressed outer granal lamelle
Photosystem II is usually found in the inner, appressed granum
Why are PSII more likely to be found in apressed membranes?
Increases their efficentcy of light capture, as they are generally less efficent than PSI
What is the solution for photosystems being relativley seperated from each other?
Mobile quinones transport electrons between photosystems
What is a problem assosiated with ROS in photosystems?
Proteins are often damage my reactivate oxidants
Constant renewal of the D1 protein is required (half life is only an hour)
PSII faces more difficulty because it is harder to access in the stacks
What is photoinhibtion
Occurs when damage to the D1 protein of PSII exceeds its rate of repair
What might a plant do to avoid photo bleaching?
Migrate pigments out of thylakoid stacks to reduce light absorption
Dissapate absorbed photon energy
How do shade and sun leafs differ?
Sun leaves are generally thicker (taller mesophyll cells) and have fewer air spaces than shade leaves. May have vertical organisation of chloroplasts.
More air spaces in shade leaves increases light skattering
How may the ultrastructure of chloroplasts differ in shade and sun leaves?
Sun chloroplasts may have thinner granum stacks and more stromal lamelle which stops photosytnthesis being ‘too’ efficent and damaging. Minimise photon absorption
Shade leaves will have more chlorophyll b molecules, may be 4-5x greater (chlorophyll b is better at light absorption)
What is the different between the light curves of sun and shade leaves?
Shade leaves have a lower light compensation point (more efficent at low light levels)
Shade leaves have an overall lower photosynthetic capacity as they are more suseptible to damage at high light.
How might entire plants acclimitise to changing light conditions?
Ajusting growth and metablism
Where does CO2 have to go after entering a plant/what phases?
Stomata -> Mesophyll Cells -> Chloroplast stroma
Must pass through gas and liquid
- diffusion through liquid is much slower (liquid phase resistance)
Some important rubisco facts
Most abundant protein on earth
8 large and 8 small subunits
Several steps are needed to assemble the complex enzyme
Relativley slow activity: about 3CO2 per second
Evolved when CO2 pressure was much greater and before oxygen
Bad at differentiating between CO2 and O2
What are the reactants and products for 1 fixation of 3CO2 by rubisco
3 ATP and 2 NADPH needed
1 GAP (3C) is released
What is produced from oxygenation reaction by rubisco?
Phosphoglyclolate, which is toxic to plant metabolism
How is phosophoglycolate removed by plants?
Photorespiration cycle. 2ATP and 2NADPH need to regenerate RuBP
CO2 is released
What does low O2 concentration do to the efficentcy of rubisco?
Increases its efficentcy becuase less photorespiration is taking place
What are two carbon concentrating mechanisms used by plants and alage?
C4 photosynthesis
Crassulacean acid metabolism (CAM)
What is PEPC
Phosphoenolpyruvate carboxylase
- Fixes CO2 to produce oxaloacetate which is converted to malate (4C compound)
Where does PEPC and rubisco act in C4 plants?
PEPC: In the mesophyll cells
Rubsico: In the bundle sheath cells
What is overall effect of C4 photosynthesis
Spacially seperated photosynthesis reactions which concentrated carbon around rubsico and reduces oxygen reactions
Describe the anatomy of C4 plants
Kranz anatomy
Bundle sheath cells contain chloroplastas and surround vascular bundles
Each mesophyll cell is connected to a bundle sheath cell by plasmodesmata
Much larger bundle sheath cells than C3 plants
What are some advantages of supressing photorespiration?
Energetics: C4 metabolism has a higher energtic cost and so C4 generally only gives energtic advantages at higher temperatures, when the rate of photorespiration increases
C4 photosynthesis has a lower CO2 compensation point and so photosynthesis is faster at low CO2 concentrations
Higher efficentcy of light use (more CO2 fixed per photon)
Lower stomatal conductance at same CO2 assimilation rate of C3 plant ie. PEPC fixes CO2 at low concentrations, increasing the CO2 gradient into leaf
How did the C4 pathway evolve?
Around 20million years ago when CO2 levels first fell a lot
Evolved over 60 times independently - convergent evolution
All evolution is in angiosperms
