Photosynthesis shoot 2 Flashcards
Where are ATP and NADPH produced in photosynthesis?
In the stroma of the chloroplast
What are the 3 stages of the Calvin cycle?
How much does the fixation of one CO2 cost in ATP and NADPH?
carboxylation, reduction and regeneration
3 ATP
2 NADPH
Why is carbon assimilation by the plant negative at low light intensity?
Respiration
Describe the sequence of events leading to increased carbon absorption with increasing light intensities.
More light means more energy coming from the photons being absorbed so more energy being absorbed in photosystems therefore more ATP and NADPH being produced and as part of the electron transport chain more proton gradient so more carbon is assimilated
What can be made and stored and even used at night?
G3P produced by the Calvin-Benson cycle can be used to make sucrose to be transported to other parts of the plant, or stored temporarily in starch granules.
The granules are hydrolysed into sucrose to power night metabolism.
What happens during the day and night in a plant?
The calvin cycle produces triose phosphates which can be stored as starch or sucrose.
At night starch is converted to maltose and glucose
Rubisco and photorespiration
Rubisco can catalyze oxygenation as well as carboxylation of RuBP.
Oxygenation produces one 3-PGA and one 2-phosphogylcolate (2-PG). which if in high quantities it can be toxic to the cell.
Recycling the oxygenation product 2-PG costs energy (needs to move to peroxisome) and releases fixed CO2: this is called photorespiration.
Oxygenation of RUBP increases with O2 to CO2 ratio and temperature
photorespiration wastes 6 ATP and 4 NADPH
Where do C4 plants fix CO2
(C3 plants use the C3 pathway or Calvin cycle for the dark reaction of photosynthesis. C4 plants use the C4 pathway or Hatch-Slack Pathway for the dark reaction of photosynthesis.)
C4 plants have different leaf structure called the Kranz anatomy, they don’t have palisade mesophyll and they have more bundle sheath cell and a bit less spongy mesophyll. C4 plants fix CO2 in mesophyll cells and carry out the Calvin-Benson cycle in bundle sheath cells.
Thanks to PEPC carboxylation and transport of C4 compound, CO2 is highly concentrated in bundle sheath cells, reducing photorespiration.
1) carbon fixation occurs in the mesophyll CO2 becomes bicarbonate (first carboxylation) then fixed with PEP by enzyme PEPC making a molecule of oxaloacetate
2)oxaloacetate then goes into the bundle sheath cells and into the chloroplast, it releases CO2 which concentrates CO2 inside, then the calvin benson cycle occurs. (second carboxylation)
CAM photosynthetic metabolism
CAM plants fix CO2 at night and carry out the Calvin-Benson during the day.
What does CAM stand for
Explain the cycle
CAM = Crassulean Acid Metabolism
Instead of separating the light-dependent reactions and the use of CO2, CAM plants separate these processes in time. At night, CAM plants open their stomata, allowing CO2 to diffuse into the leaves. This CO2 is fixed into oxaloacetate by PEP carboxylase (the same step used by C4 and is then converted to malate or another type of organic acid.)
The organic acid is stored inside vacuoles until the next day. In the daylight, the CAM plants do not open their stomata, but they can still photosynthesize. That’s because the organic acids are transported out of the vacuole and broken down to release CO2 which enters the Calvin cycle. This controlled release maintains a high concentration of CO2 around rubisco.
c4 acids accumulate during the night
Consequences of C4 and CAM metabolisms
(expand on this)
*Cost of C4 photosynthesis: ATP is needed for pyruvate transport back to mesophyll cells.
*The quantity of CO2 that can be fixed at night by CAM plants is not enough for a full day of Calvin-Benson cycle.
Where are C4 and CAM plants normally found?
Photorespiration increases with temperature so C4 and CAM metabolisms are advantageous in hot conditions.
Why is carbon assimilation lower at normal CO2 level and high temperatures for C3 plants compared to C4 plants ?
This is because of photorespiration that increases in higher temperatures therefore there’s more of a difference at those temperature
Why do C4 and CAM etabolisms exist in hotter climates?
*C4 and CAM metabolisms reduce water loss through stomata:
- C4 photosynthesis is more efficient at fixing CO2 and does not require long periods of stomatal aperture.
- CAM plants open their stomata at night when coolness and higher humidity reduce water evaporation.
C4 and CAM plants are better adapted to arid environments than C3 plants.
