©THEBRIGHTSTARS 2024
EEB 162 Final Exam Questions With Correct
Answers.
How do plants reduce photorespiration? - answer✔Concentrating CO2 around RUBISCO to
reduce oxygenation
What are 3 types of carbon concentrating mechanisms that plants use to improve carbon
assimilation and reduce photorespiration? - answer✔C4 photosynthesis
CAM
CO2 pumps at the plasma membrane
Explain C4 photosynthesis: - answer✔1) Atmospheric CO2 enters mesophyll cell and is
transformed into HCO3, which is fixed to 3-C PEP, by the enzyme PEP-carboxylase to produce
malate (4-carbon sugar). All CO2 taken up is passed to the bundle sheath cells in malate form
2) Malate diffuses passively into bundle sheath cells via plasmodesmata where it is
decarboxylated by NADP-malic enzyme to pyruvate (3-carbon sugar), which diffuses back to
mesophyll cells, and CO2, which is released to the bundle sheath chloroplasts where it enters the
full C3 Calvin Cycle reactions in the bundle sheath. The chloroplasts in the bundle sheath
experiences high saturation of CO2 and ZERO photorespiration. There is a separation of
pathways in space (compartmentalization) between mesophyll and bundle sheath cells.
3) High CO2 concentration in the chloroplasts of bundle sheath cells reduces photorespiration
and C4 increases water-use efficiency (photosynthesis/transpiration rate) since more CO2 can be
concentrated without losing too much water.
4) C4 plants have Kranz anatomy, where there are lots of chloroplasts in enlarged bundle sheath,
mesophylls cells close to bundle sheath, and many plasmodesmata connecting the mesophyll to
the bundle sheath. Plasmodesmata allows diffusion of malate to the bundle sheath.
Do all C4 plants have Kranz anatomy? - answer✔No - some operate C4 within single cells, by
separating the PEP carboxylase reactions from the Calvin cycle
Kranz Anatomy - answer✔Enlarged bundle sheath with lots of chloroplasts. Mesophyll cell is
close to bundle sheath with plasmodesmata connecting to it.
, ©THEBRIGHTSTARS 2024
Mesophyll cells fix CO2 into a 4-C sugar that gets sent into bundle sheath cells, which ends up
getting decarboxylated to 3-C sugar in order to create a high CO2 concentration in the bundle
sheath for C3 Calvin cycle.
Why does C4 photosynthesis have a cost? Please describe direct and indirect costs. -
answer✔Direct costs: regeneration of PEP carboxylase consumes 2 ATP per CO2
Indirect costs: extra NRG required to make Kranz anatomy, bundle sheaths, and enzymes and
metabolites required for the extra biochemical pathways
*If O2 was low and photorespiration did not occur, C4 plants require more light per CO2 than
C3 plants for the same CO2 fixation
What possible environmental conditions might promote the evolution of C4 photosynthesis
pathway? Why? - answer✔1) High Temperature
2) Low CO2
3) High Salinity
4) Mild-to-moderate drought
Explain why high temperature promotes C4 evolution. - answer✔C3 plants will photorespire
more at high temperatures because CO2 comes out of solution faster at high temperatures and O2
becomes more soluble and RUBISCO reacts quicker with O2.
Thus, C4 has an advantage
What happens to C3 plants if given high CO2? - answer✔It's photorespiration will eliminate
and C3 is favored
Explain why low CO2 promotes C4 evolution. - answer✔C4 plants can open stomata less for
the same amount of fixation under high irradiance, so have improved water use efficiency. (=
CO2 fixed per water loss). C4 don't need to open stomata much to fix CO2, so it experiences less
water loss while maintaining the same amount of photosynthetic rate
Explain why high salinity promotes C4 evolution. - answer✔C3 plants can't grow in high
salinity because there is low water potential causing the plant to experience a drought
Explain why mild-to-moderate drought promotes C4 evolution. - answer✔When soil is dry,
stomata closes. C4 loses less water and can keep their leaf water potential higher during
transpiration so they can operate effectively in dry soil
Where do C4 plants tend to originate? - answer✔Arid regions
What are important conditions promoting C4 evolution? - answer✔Heat, drought, salinity
, ©THEBRIGHTSTARS 2024
Advantage of C4 plants under high irradiance - answer✔C4 plants open stomata less, lose less
water, and keep their leaf water potential higher during transpiration and operate effectively on
drier soil
Under what conditions would C4 plants have an advantage over C3 plants? - answer✔Low
CO2, high temperature, high salinity, mild-to-moderate drought
Under what conditions would C3 plants have an advantage over C4 plants? - answer✔High
CO2, low temperature, low salinity, no drought
Explain CAM photosynthesis: using stomata, Rubisco, malate, PEP carboxylase, water-use
efficiency - answer✔At night, stomata is OPEN: CO2 is taken up and PEP carboxylase fixes
HCO3- to form malate (4-Carbon sugar), which is stored in the vacuole. By opening stomata at
night (allowing CAM to operate), CAM leads to high water-use efficiency (reduces water loss,
because at night, lower temperatures = lower VPD therefore lower transpiration)
During day, stomata is CLOSED: malate is released from the vacuole and breaks down to
pyruvate and CO2 in the chloroplast, leading to the light and C3 Calvin cycle (occurs under
saturating CO2 therefore no photorespiration)
Advantage of CAM - answer✔Reduces transpiration and water demand
Increases water-use efficiency
What factors can limit photosynthesis? - answer✔CO2 limitation = limitation to RUBISCO
activity
Light limitation = limitation to RUBP activity (need light to make NADPH)
Leaf anatomy for light capture - answer✔Palisade cells
Spongy mesophyll cells
How do palisade cells allow penetration of light? - answer✔It allows penetration of light via
vacuoles and intracellular air spaces: light channeling
How do spongy mesophyll cells allow penetration of light? - answer✔Honeycombed airspaces
reflect and refract light: light scattering
What is better for capturing direct light? - answer✔Palisade cells
What is better for capturing diffuse light? - answer✔Spongy mesophyll cells
What does the diffusion rate of CO2 through the stomata depend on? - answer✔STOMATAL
resistance
EEB 162 Final Exam Questions With Correct
Answers.
How do plants reduce photorespiration? - answer✔Concentrating CO2 around RUBISCO to
reduce oxygenation
What are 3 types of carbon concentrating mechanisms that plants use to improve carbon
assimilation and reduce photorespiration? - answer✔C4 photosynthesis
CAM
CO2 pumps at the plasma membrane
Explain C4 photosynthesis: - answer✔1) Atmospheric CO2 enters mesophyll cell and is
transformed into HCO3, which is fixed to 3-C PEP, by the enzyme PEP-carboxylase to produce
malate (4-carbon sugar). All CO2 taken up is passed to the bundle sheath cells in malate form
2) Malate diffuses passively into bundle sheath cells via plasmodesmata where it is
decarboxylated by NADP-malic enzyme to pyruvate (3-carbon sugar), which diffuses back to
mesophyll cells, and CO2, which is released to the bundle sheath chloroplasts where it enters the
full C3 Calvin Cycle reactions in the bundle sheath. The chloroplasts in the bundle sheath
experiences high saturation of CO2 and ZERO photorespiration. There is a separation of
pathways in space (compartmentalization) between mesophyll and bundle sheath cells.
3) High CO2 concentration in the chloroplasts of bundle sheath cells reduces photorespiration
and C4 increases water-use efficiency (photosynthesis/transpiration rate) since more CO2 can be
concentrated without losing too much water.
4) C4 plants have Kranz anatomy, where there are lots of chloroplasts in enlarged bundle sheath,
mesophylls cells close to bundle sheath, and many plasmodesmata connecting the mesophyll to
the bundle sheath. Plasmodesmata allows diffusion of malate to the bundle sheath.
Do all C4 plants have Kranz anatomy? - answer✔No - some operate C4 within single cells, by
separating the PEP carboxylase reactions from the Calvin cycle
Kranz Anatomy - answer✔Enlarged bundle sheath with lots of chloroplasts. Mesophyll cell is
close to bundle sheath with plasmodesmata connecting to it.
, ©THEBRIGHTSTARS 2024
Mesophyll cells fix CO2 into a 4-C sugar that gets sent into bundle sheath cells, which ends up
getting decarboxylated to 3-C sugar in order to create a high CO2 concentration in the bundle
sheath for C3 Calvin cycle.
Why does C4 photosynthesis have a cost? Please describe direct and indirect costs. -
answer✔Direct costs: regeneration of PEP carboxylase consumes 2 ATP per CO2
Indirect costs: extra NRG required to make Kranz anatomy, bundle sheaths, and enzymes and
metabolites required for the extra biochemical pathways
*If O2 was low and photorespiration did not occur, C4 plants require more light per CO2 than
C3 plants for the same CO2 fixation
What possible environmental conditions might promote the evolution of C4 photosynthesis
pathway? Why? - answer✔1) High Temperature
2) Low CO2
3) High Salinity
4) Mild-to-moderate drought
Explain why high temperature promotes C4 evolution. - answer✔C3 plants will photorespire
more at high temperatures because CO2 comes out of solution faster at high temperatures and O2
becomes more soluble and RUBISCO reacts quicker with O2.
Thus, C4 has an advantage
What happens to C3 plants if given high CO2? - answer✔It's photorespiration will eliminate
and C3 is favored
Explain why low CO2 promotes C4 evolution. - answer✔C4 plants can open stomata less for
the same amount of fixation under high irradiance, so have improved water use efficiency. (=
CO2 fixed per water loss). C4 don't need to open stomata much to fix CO2, so it experiences less
water loss while maintaining the same amount of photosynthetic rate
Explain why high salinity promotes C4 evolution. - answer✔C3 plants can't grow in high
salinity because there is low water potential causing the plant to experience a drought
Explain why mild-to-moderate drought promotes C4 evolution. - answer✔When soil is dry,
stomata closes. C4 loses less water and can keep their leaf water potential higher during
transpiration so they can operate effectively in dry soil
Where do C4 plants tend to originate? - answer✔Arid regions
What are important conditions promoting C4 evolution? - answer✔Heat, drought, salinity
, ©THEBRIGHTSTARS 2024
Advantage of C4 plants under high irradiance - answer✔C4 plants open stomata less, lose less
water, and keep their leaf water potential higher during transpiration and operate effectively on
drier soil
Under what conditions would C4 plants have an advantage over C3 plants? - answer✔Low
CO2, high temperature, high salinity, mild-to-moderate drought
Under what conditions would C3 plants have an advantage over C4 plants? - answer✔High
CO2, low temperature, low salinity, no drought
Explain CAM photosynthesis: using stomata, Rubisco, malate, PEP carboxylase, water-use
efficiency - answer✔At night, stomata is OPEN: CO2 is taken up and PEP carboxylase fixes
HCO3- to form malate (4-Carbon sugar), which is stored in the vacuole. By opening stomata at
night (allowing CAM to operate), CAM leads to high water-use efficiency (reduces water loss,
because at night, lower temperatures = lower VPD therefore lower transpiration)
During day, stomata is CLOSED: malate is released from the vacuole and breaks down to
pyruvate and CO2 in the chloroplast, leading to the light and C3 Calvin cycle (occurs under
saturating CO2 therefore no photorespiration)
Advantage of CAM - answer✔Reduces transpiration and water demand
Increases water-use efficiency
What factors can limit photosynthesis? - answer✔CO2 limitation = limitation to RUBISCO
activity
Light limitation = limitation to RUBP activity (need light to make NADPH)
Leaf anatomy for light capture - answer✔Palisade cells
Spongy mesophyll cells
How do palisade cells allow penetration of light? - answer✔It allows penetration of light via
vacuoles and intracellular air spaces: light channeling
How do spongy mesophyll cells allow penetration of light? - answer✔Honeycombed airspaces
reflect and refract light: light scattering
What is better for capturing direct light? - answer✔Palisade cells
What is better for capturing diffuse light? - answer✔Spongy mesophyll cells
What does the diffusion rate of CO2 through the stomata depend on? - answer✔STOMATAL
resistance
