lecture 6 - plant structure and function - leaves and CO2 Flashcards
What is the role of cells in the epidermis?
Cells in epidermis have no photosynthetic capacity but help to refract light into the cells which do
Where does the Calvin Cycle take place?
chloroplast
What type of cell contains chloroplasts?
mesophyll cells
Describe proton transport and ATP synthesis in photosynthesis
Light energy from photons excites electrons (e-) that are moved along the photosystem.
The e- moved are replaced by splitting water: 2H2O O2 + 4H+ + 4e-
Protons (H+) accumulate in the lumen causing H+ movement back into the stroma through the H+/ATPase complex generating ATP in the stroma.
What is the role of NADPH?
NADPH is also produced in the stroma (through PS I complex) for reducing power
Why are NADPH and ATP unsuitable for long term storage?
reactive and costly to make (both in terms of P and energy)
What is the primary role of the Calvin Cycle?
to recycle these compounds and to use the energy within the bonds of the molecules to make compounds that can be accumulated (i.e. glucose, starch).
What is the role of the enzyme rubisco?
Ribulose 1,5- bisphosphate carboxylase (Rubisco)
catalyses the carboxylation (the addition of CO2 to another molecule) of RuBP to form 2 x
3 carbon molecules- 3-Phosphoglyerate
What is the issue with rubisco?
Rubisco also catalyses oxygenation of RuBP
The 2C compound (phosphoglycolate) produced cannot be used in the Calvin cycle
CO2 is released, hence this is called photorespiration
The function of this reaction is a matter of debate
The trouble with it is that it catalyses ANOTHER reaction, as we see here, and the C2 compound goes into the C2 cycle.
The oxygenase activity may be an evolutionary “accident”. When it evolved, the O2 concentration in the atmosphere was much lower, and thus had less impact.
Why it persists is a matter of debate: may be that carboxylase/oxygenase activity are functionally too closely linked, constraining evolution of carboxylation only Rubisco
What is the photorespiration problem?
Rubisco evolved in a high CO2 low O2 atmosphere
Oxygenation and carboxylation of RuBP compete for the same active site on RuBisco (plants can lose 50% of fixed C)
CO2 levels in the chloroplasts must be high to maintain carboxylation rates over oxygenation
Where water is limiting there is a major conflict between CO2 acquisition and water loss
This is made worse by the fact that oxygenase activity increases faster with temperature than carboxylase activity
How does water shortage affect photorespiration?
Stomata open CO2 moves in by diffusion.
But if WATER short, stomata shut, CO2/O2 ratio declines and photorespiration occurs
How do C4 plants prevent photorespiration?
Some plants (called C4 plants) use a mechanism that prevents photorespiration by providing Rubisco with saturating levels of CO2 (called C4 photosynthesis). Capacity for oxygenase reaction is still there -but high CO2 concentrations allow the carboxylase reaction of Rubisco to proceed at its maximal rate.
What is photorespiration?
a respiratory process in many higher plants by which they take up oxygen in the light and give out some carbon dioxide, contrary to the general pattern of photosynthesis.
How does C4 photosynthesis work?
Enzyme PEP carboxylase catalyses addition of ‘CO2’ (but note in the form of bicarbonate) to a 3C compound (PEP) to produce a 4C compound (oxaloacetate).
Hence, called ‘C4’ photosynthesis.
Oxaloacetate is converted into another C4 compound (malate or aspartate) and shuttled into the bundle-sheath cells.
In the bundle cells - malate is converted to 3C pyruvate and CO2 – the CO2 enters the Calvin Cycle (‘C3’ pathway, as before)
3C pyruvate diffuses back into the mesophyll cells and is converted into PEP thus the ‘C4 cycle’ and continue.
What are the bundle sheath reactions?
CO2 concentrating pump: CO2 produced in bundle sheath cannot diffuse out of bundle sheath (suberised lamellae coat cell walls). Results in very high conc of CO2 in bundle sheath (10 times that in ambient air - i.e. 3000-4000 ppm! – cf 200ppm in oleander)
CO2 is then fixed via C3 pathways to produce sugars
Result: almost total inhibition of photorespiration
