lec 16- photosynthesis-coping with photorespiration Flashcards
how do plants avoid photorespiration?
by using CO2 concentrating mechanisms:
1. C4 carbon fixation
2. Crassulacean acid metabolism (CAM)
3. CO2 pumps in algae and cyanobacteria
what is C3 and C4 photsynthesis?
most plants have C3 carbon from radio labelled CO2, but tropical hot summer plants have C4 carbon from radio labelled CO2 as a measure to prevent photorespiration by having active transport move C4 into the bundle sheath cells and release CO2, but it requires more energy to do so
what are the characteristics of C3 and C4 plants?
C3- most common, radio labelled CO2 first incorporated into 3-phosphoglycerate (3-carbon, fix carbon in mesophyll cells, no chloroplast in bundle sheath cells
C4- minority of plants, found in hot arid climates and prairies, use four-carbon dicarboxylic acids, bundle sheath and mesophyll cells arranged in wreaths, few chloroplasts in mesophyll cells, lots of chloroplasts in bundle sheath cells, bundle sheath cells shielded from gas diffusion O2 by specialized cell wall
where does fixation/calvin cycle occur in C3 and C4 plants?
C3- calvin cycle occurs only in mesophyll cells
C4- fixation occurs in mesophyll cell where C4 is made, then C4 transported to bundle sheath cells to release CO2 to be used in calvin cycle
steps of the C4 cycle:
1a. carbonic anhydrase converts carbon dioxide into bicarbonate
1b. PEP carboxylase uses bicarbonate to carboxylate phosphoenol pyruvate into OAA C4 acid (no competition between O2 and CO2, different from RUBISCO)
2. OAA is converted into malate or aspartate in the chloroplast, malate C4 in the mesophyll cell
3. Malate and aspartate diffuse into bundle sheath cell via plasmodesmata
4. Malate is decarboxylated to release CO2 and pyruvate
5. RUBISCO uses CO2 to carboxylate 1,5-ribulose biphosphate in the Calvin cycle in bundle sheath cells
6. Pyruvate diffuse back to mesophyll cell for regeneration into phosphoenol pyruvate
how are mesophyll and bundle sheath cells connected?
by many plasmodesmata
how is O2 blocked from entering bundle sheath cells?
by a shield (gas-impermeable suberin layer), spacial separation of carbon uptake and fixation. C4 cycle raises CO2 in bundle sheath cells
what are the advantages and disadvantages of the C4 cycle?
advantage- reduce photorespiration, reduce water loss
disadvantage- more energy consumed
ex. of C4 plants- sugarcane, maize, sorghum, and millet
what is the differences in CO2 concentration between C3 and C4 plants?
C4 plants fix more CO2 in low CO2 concentration than C3, C3 plants fix more CO2 in high CO2 concentrations than C4
what is the differences of performance between C3 and C4 at different temps?
C4 performs better at temp higher than 32 degrees than C3
what do CAM plants do to reduce photorespiration?
keep stomata open in night when temp is cool and water loss is low which also allows gas exchange of CO2 into C4 malate, close it in the day to reduce water loss which also prevents gas exchange but malic acid moves from vacuole to chloroplast, CO2 is released from malic acid raising CO2 concentration
what are the advantages and disadvantages of CAM plants?
advantage- reduce photorespiration, reduce water loss
disadvantage- energy cost increased
ex. cactus
what are the water loss rates of C3, C4, and CAM plants?
C3- 400-500 g
C4- 250-300 g
CAM- 50-100 g
