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EEB 162 Final Exam Latest Update
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EEB 162 Final Exam LatestUpdate
1. Please explain C4 photosynthesis, using the following terms: HCO3-, PEP
carboxylase, mesophyll, Kranz Anatomy, plasmodesmata, malate, separation of
pathways in space, water use efficiency - Answer C4 photosynthesis reduces
photorespiration by concentrating CO2 near rubisco. In mesophyll cells, CO2 is
transformed to HCO3-, which is fixed to 3-carbon PEP by PEP carboxylase. This reaction
will produce a 4 carbon compound, often malate. Malate then diffuses through
plasmodesmata to bundle sheath cells. C4 plants have Kranz anatomy, an enlarged
bundle sheath with lots of chloroplasts. In the bundle sheath cells, malate is
decarboxylated to 3-carbon pyruvate and CO2, which is released to the bundle sheath
chloroplasts. The CO2 enters the full C3 Calvin cycle reactions in the bundle sheath. C4
plants that do not have Kranz anatomy use separation of pathways in space (separating
the PEP carboxylase reactions from the Calvin cycle reactions within the cell). C4 plants
have improved water use efficiency (almost double C3 plants). C4 plants open stomata
less for the same amount of CO2 fixation under high irradiance, allowing them to lose
less water.
2. a. Why does C4 photosynthesis have a cost? Please describe direct and indirect
costs. - Answer (Direct)-Regeneration of PEP consumes 2 ATP
(IDirect)-If O2 was low and photorespiration did not occur, C4 plants require more
quanta of light per CO2 than C3 plants for the same CO2 fixation.
2.b. Under what conditions would C4 plants have an advantage over C3 plants? -
Answer Higher temps, C3 plants increase photorespiration strongly, because Rubisco
reacts quicker with 02 and 02 becomes more soluble at higher temps.
C4 plants can lose less water, and so keep their leaf water potentials higher during
transpiration, and so operate effectively on drier soils.
2.c. Under what conditions would C3 plants have an advantage over C4 plants? Please
explain why. - Answer As CO2 levels increase, it is possible that C3 species may be
favored and replace C4 grasses in the world's arid zones. However temp. increase
along with CO2, not clear if photorespiration would really be lower overall in C3 plants.
What three possible environmental conditions might promote the evolution of the C4
photosynthetic pathway? Why? - Answer C4 tends to originate in arid regions,
implicating heat, drought and or salinity as
important conditions promoting C4 evolution.
, 1)Implicating heat
2)Drought
3)Salinity
-Competitive advantage when resources are limited
-When hot, CO2 comes out of solution faster.
-C4 can maintain photosynthesis in drier soils, even when CO2 levels are low. As
stomata closes, CO2 rate in declines. C4 have an enzyme with high affinity, which can
work efficiently in low [CO2], an advantage in drier soils.
4. Please explain CAM photosynthesis, using the following terms: stomata, Rubisco,
malate, PEP carboxylase, water-use efficiency - Answer CAM is like C4 in that PEP
carboxylase fixes HCO3-, and then releases CO2 to C3. CAM is like C4 where
photosynthesis reduces photorespiration by concentrating CO2 near rubisco. But CAM
allows increased CO2 concentration by fixing HCO3- with PEP carboxylase at night
(when the stomata open in CAM plants). The malate is stored in the vacuole. The PEP is
generated by the breakdown of starch from the chloroplast. During the day (when
stomata are closed), the malate is released from the vacuole, and breaks down to
pyruvate and CO2 in the chloroplast, and the light reactions and C3 Calvin cycle run.
CAM leads to a huge water use efficiency (10x higher than C3 plants) b/c stomata open
only at night when CAM operates. CAM plants often have very water-resistance cuticles,
CAM photosynthesis is very expensive, due to additional metabolic costs and slow
photosynthetic rates
5. How are sun leaves adapted to capture light and CO2 effectively? Please name six
different features of their cells and tissues. - Answer 1)Epidermis is transparent, with
convex cells that can act as lenses to focus light to chloroplasts within the leaf.
2)Palisade cells allow penetration of light via vacuoles and intercellular airspaces (light
channeling).
3)Spongy mesophyll has honeycombed airspaces that reflect and refract light (light
scattering).
4)Sun leaves are thicker, with more layers of palisade mesophyll to allow direct light to
penetrate the leaf. Intercellular area is maximized for CO2 capture.
5)Under low light, chloroplasts spread out in plane of leaf. Under high irradiance,
chloroplasts stack up (self shading).
6)Major part of leaf is airspace and large internal cell surface areas for CO2 uptake,
keeping diffusion distance from stomata to chloroplasts small.
7)Light is absorbed and CO2 fixed right into center of leaf. Light absorbed and CO2 fixed
right into center of leaf
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