EEB 162 Final Test With
Complete Solution
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.
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
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
Intracellular airspace resistance
Liquid phase resistance
What reduces the internal resistance of diffusion of CO2 through the stomata? - Answer
Major part of leaf is airspace (so diffusion is mostly in air)
Having large internal cell surface area for CO2 uptake, which keeps diffusion distance
from stomata to chloroplast small
How are leaves specialized for internal CO2 uptake? - Answer Airspaces
Complete Solution
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.
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
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
Intracellular airspace resistance
Liquid phase resistance
What reduces the internal resistance of diffusion of CO2 through the stomata? - Answer
Major part of leaf is airspace (so diffusion is mostly in air)
Having large internal cell surface area for CO2 uptake, which keeps diffusion distance
from stomata to chloroplast small
How are leaves specialized for internal CO2 uptake? - Answer Airspaces
