Lecture 13 part 1 Flashcards
what is one of the major problems facing terrestrial plants?
dehydration
what is the major route for gas exchange?
in a leaf
stomata
why would plants close the stomata?
they would close it on hot, dry days to conserve water which causes problemes for photosynthesis.
what are C3 plants?
plants that use rubisco (enzyme used in phase 1 of the calvin cycle - phase 1 = carbon fixation)
why are they called C3 plants?
they are called C3 because the first organic product of fixation is a 3-carbon compound (3-PGA)
what are the three different types of C3 plants?
1) mesophytes:
- live in temperature climates, where the amount of water is neither too little nor too much
2) hydrophytes:
- water plants
3) Xerophytes:
- plants adapted to arid conditions
the location of stomata (upper/lower epidermis), thickness of the cuticle, and water storage mechanisms vary between these groups
other than gas exchange, what does the stomata help with?
they also are the major route for evaporative H20 loss = dehydration
where does water flow to?
flows to the hypertonic side of the membrane
how does the stomata reduce photosynthetic yield?
on hot or dry days the stomata would partially or fully close in order to reduce water loss which would reduce the photosynthetic yield by limiting CO2.
- this results in plants switching to photorespiration
when would photorespiration occur?
occurs in C3 plants when they are in hot and dry conditions and their stomata partially or fully close as a result:
- CO2 levels decrease (carbon fixation in phase 1 of calvin cycle)
- O2 levels increase (splitting of H2O in PSII) inside the chloroplasts
how is photorespiration a wasteful pathway for plants?
- consumes ATP
- does not produce sugar
what is the direct competitor for rubisco’s active site?
O2
when CO2 levels decrease and O2 levels increase, rubisco starts to interact with O2.
in photorespiration what is made instead of two molecules of 3-PGA?
one 3-PGA and 2C molecules are formed
the 2C molecule can later be split into two molecules of Co2 by peroxisomes and mitchondria in the plant
*the 2C molecule will only be transformed into CO2 and used in the calvin cycle if the concentrations are high enough to outcompete O2.
what are possible protective effects of photorespiration?
plants with genetic defects can’t switch to photorespiration often suffer damage from excess light
give three ways in which photorespiration differs from the calvin cycle?
- no sugar produced during photorespiration
- decreased amount of RuBP available since carbon is lost from the process as 2C intermediate, then as Co2
- ATP and NADPH is used in the conversion of the 2C product into CO2 instead of being used to make sugar (G3P)
evolutionary explanation for photorespiration
what is the “evolutionary baggage” hypothesis?
- that it may be a metabolic relic of the times when Co2 levels of air were much higher than O2 levels
- since O2 levels were so low, there was no significant selective pressure against the competition of O2 for RuBisCO’s active site
- evolutionary adaptations have evolved for plants living in hot and arid climates to prevent such waste:
C4 plants & CAM plants
what were the reasons for adaptations of C4 and CAM plants that were living in hot and arid climates?
they adapted to minimize/prevent photorespiration
what two pathways do C4 and CAM plants use?
although they do it in slightly different ways
1) C4 pathway makes 40carbon molecule oxaloacetate
2) C3 pathway (calvin cycle): exactly same as in C3 plants
- make the 3-C molecule: 3-phosphoglycerate (3-PGA)
what are the two pathways used for carbon fixation in C4 plants separated by?
separated by location:
- C4 occurs first in the buncle sheath cell
- C3 pathway occurs in the mesophyll cell
both the mesophyll and bundle sheath cell have chloroplasts
describe the steps/pathway of carbon fixation for a C4 plant
1) CO2 (1C) is added to phosphoenolpyruvate (PEP) (3C) for form oxaloacetate (4C)
- enxyme = PEP caboxylase, has a higher affinity for CO2 than rubisco and no affinity for O2, therefore C4 plants would still fix carbon under conditions where rubisco would not be able to (hot and dry)
- location = mesophyll cell
- there is an extra cost of ATP associated with the C4 pathway
2) the 4C product oxaloacetate is converted to malate and exported out of the mesophyll cell to bundle-sheath cells
- move through the plasmodesmata (gap junction equivalent)
3) in the bundle sheath cells, malate releases the CO2
4) CO2 is incorporated into organic compounds by rubisco in the bundle-sheath cells and the calvin cycle can occur
5) pyruvate is regenerated and exported back to mesophyll cells for conversion to PEP
- ATP must be used to regenerate PEP and continue the cycle.
what are the enviornments that C4 plants thrive?
hot regions with intense sunlight, where the stomata partially close during the day ( carbon fixation adaptation)
C4 synthesis spends ATP energy to minimize photorespiration and enhance sugar production
what the differences between C3 and C4 plants?
describe the background of CAM plants
- CAM= crassulacean acid metabolism
- part of the family crassulaceae (family of plants in which CAM mechanism was first found)
- present in many cacti, pineapples…
- keep stomata open at night (the reverse of other plants)
- they have lead anatomy similar to xerophytes (C3)
CAM plants
what are the gains of keeping the stomata closed during the day?
conserves water but prevents CO2 from entering
what are the gains of keeping the stomata open at night?
allows CO2 to enter where it can be stored for use during the day following light reactions
how are the two pathways used for carbon fixation in CAM plants separated?
separated temporally ( by time)
- C4 occurs at night in mesophyll cells
- C3 occurs during the day in mesophyll cells
descrive the pathways CAM plants for carbon fixation
C4 pathways occurs at night:
- stomata are open
- CO2 is fixed into a variety of organic acids in mesophyll cells
- these organic acids are then stored in vacuoles untill morning
C3 pathways occurs during the day:
- stomata are closed
- the light reactions supply ATP and NADPH to the calvin cycle
- co2 is released from the organic acids
compare C4 plants versus CAM plants
**C4 plants: **
- two pathways used for carbon fixation (C4 and C3 pathways) which are separated by location
- C4 occurs first in mesophyll cells
- C3 pathway occurs in the bundle sheath of cells
**CAM plants: **
- two pathways used for carbon fixation (C4 and C3 pathways) which are separated by time
- C4 occurs at night in mesophyll cells
- C3 occurs during the day in mesophyll cells
summary of plants
summary of photosynthesis
photosynthesis versus respiration review
