Week 14 L7: C3 and C4 Photosynthesis Flashcards
What is the photosynthesis reaction?
6CO2 + 6H2O –> C6H12O6 + 6O2
Where do the light dependent reaction occur?
thylakoids
Where does the light independent reaction occur?
stroma
What is RuBisCo?
enzyme in the calvin cycle responsible for fixing CO2.
Binds CO2 and converts it to 2 molecules of 3-phospho-glycerate.
Why is RubisCo bad?
Cant differentiate between O2 and CO2
When does RuBiso become even less efficient due to climate change?
elevated temps, its affinity for O2 becomes even higher. (Carboxylase decrease vs oxygenase increase, CO2 solubility decreases.)
reduced pp CO2 vs O2.
not adapt well to climate change.
What does the plant produce when guard cells close?
increase O2
more futile Rubico reactions
lose energy even though they are photosynthesising
photorespiration reaction.
What happens in hot environments to C3 plants?
Water available but limited
Transpiration or water loss rates high
RuBisCo inefficient at high temps
Photorespiration increases
What happens in arid/dry environments to C3 plants?
Low water potential and/or availability
Shut stomata
Reduce water loss
But CO2 falls and O2 increases
What C3 plants can grow in dry enviroments?
cacti
pineapple
epiphytic orchids
rice wheat potato
What plants can grow in dry environments?
cacti
pineapple
epiphytic orchids
How did evolution mitigate photorespiration?
CO2 concentrating mechanism (CCM)
-help promote carboxylase activity
O2 exclusion or insensitivity - cant be poisoned, no futile reaction.
Adaptations present in C4 and CAM photosynthesis
Why did rubusco have oxygenase activity?
As CO2 levels were high billions of years ago andthere were no consequences to it oxygenase ability.
Why did C4 and CAM arise?
Increasing O2 levels.
Historical climate change
Falling CO2 levels
Increasing O2 levels
What is CAM?
Crassulacean Acid Metabolism (CAM)
survival adaptationgather sunlight during the day and fix carbon dioxide molecules at night.
When does CAM occur?
Take up CO2 during low temps (night)
Need to store CO2 until light is available (day)
Good for arid or low water availability areas
What plants use CAM?
vanilla
cacti
What limits CAM?
The size of its CO2 battery
When is CAM useful?
Really an adaptation to surviving drought rather than biomass accumulation
Why do not many crops use CAM?
Vacuole acidity unpalatable
Other adaptations make them less appealing
How can CAM be useful?
Open up new areas for farming?
Introduce facultative CAM to existing crops to improve drought survival.
How can CAM be useful?
Open up new areas for farming?
Introduce facultative CAM to existing crops to improve drought survival.
What does C4 plants separate?
separate chemical reaction
separate RuBisCo and O2
When would CAM be useful?
Achieves CO2 levels 25-50 higher than atmospheric
Good for areas with sufficient water and temps >20°C
At 25°C C4 plants can produce double the biomass of C3
Grow with high yield. very effective.
What is the difference between C3 rice and C4 maize?
location of mesophyll
Where does light dependent and light independent occur?
in all cells
Why does rubisco poison C3 plants?
Both light reactions happen in the same cells
What is the difference between mesophyll and bundle sheath cell?
sheath has no PS2 COMPLEX, no oxygen. Can fuel the rubico in the Calvin cycle. and is CO2 is refixed by rubisco
What is the difference between mesophyll and bundle sheath cell?
sheath has no PS2 COMPLEX, no oxygen. Can fuel the rubisco in the Calvin cycle. and is CO2 is refixed by rubisco
What do the thylakoids permit?
chlorophyll
light reactions
generate ATP and NADPH
Oxygen generating
Where and what uses ATP and NADPH?
stroma
used by RuBisCo
Where is CO2 fixed?
Stroma, where it can be fixed into these complex carbon skeletons. used for growth metabolism
What is the basis of C3 photosynthesis?
Rubisco binding CO2 and fixing it
Why are 25% of RuBisCo reactions wasted?
Due to a process called photorespiration. cant differentiate between CO2 and O2.
Plant wastes energy
When RuBisCo binds O2 instead of CO2, what is produced?
2-phosphoglycerate
How does RuBisCo waste energy generated from photosystems 1 and 2?
futile reaction.
To convert 2-phosphoglycerate to 3-phosphoglycerate, ATP and NADPH are needed.
How does RuB cost the plant even more?
in the conversion to 3-phospho-glycerate, the cell releases CO2, H2O2 and NH3, which are toxic.
The plant needs waste energy converting these to detoxify them.
Where are the adaptation to mitigate photosynthesis?
C4 and CAM photosynthesis
What is CAM photosynthesis?
gather sunlight during the day and fix carbon dioxide molecules at night.
What is the mesophyll?
the inner tissue (parenchyma) of a leaf, containing many chloroplasts
What is temporal separation?
CAM,
Separates photosynthesis into 2 stages,
Where is CO2 stored in the plant?
vacuole - malate acid
What is the carbon fixing enzyme in CAM?
PEPC - phosphoenolpyruvate carboxylase
What does PEPC convert CO2 to?
Malate acid
What are the disadvantages of CAM?
Yield penalty
CO2 fixation limited by [PEP]
Uses 3 more ATP than C3 per CO2 fixed
Where are the C4 biochemical reactions happening?
Separate parts of the plant. Rubisci and O2 are in different parts of the plant so dont get the futile photorespiration
What are the C4 plants?
maize
sugar cane
What is kranz anatomy?
The chloroplasts concentrated in the sheath cells
Why does C4 photosynthesis not poison Rub?
No phII, the O2 producing photosystem part
What enzyme fixes CO2 in the mesophyll of C4 photosynthesis?
PEPC
Is O2 insensitive
What happens to malate when it enters the sheath?
decarboxylated by malic acid
How does C4 increase WUE?
For a given CO2 conc you have, the stomata are more shut - less water loss. keep CO2 conc higher but have less water loss
How does C4 increase NUE?
Reducing rubisco reduces the demand for proteins, many proteins contain N
Where is NADP found?
chloroplast
Where is NAD found?
mitochondria
Why do you need extra light for C4?
It needs more energy to convert CO2 to glucose
needs more light per molecule of glucose fixed
What is the increased WUE likely a bi-preoduct of?
trying to increase rub efficiency. It works well in dry enviro - CAM
