Improving Photosynthesis I Flashcards
(15 cards)
What happens in the Calvin cycle?
- Carboxylation
- Reduction
- Regeneration
RuBisco carboxylates RuBP using CO2, making 2 molecules of 3-phosphoglycerate
What happens in photorespiration?
RuBisco catalyses a wasteful reaction between RuBP and O2, creating phosphoglycolate, which is toxic and inhibits photosynthesis
What is RuBisco?
- Slow enzyme
- Catalytic rate in lab: 3 s-1
- Catalytic rate on land: 0.03s -1
- Relatively low affinity for CO2, hence high concentrations are necessary
When does photorespiration occur?
Photorespiration occurs 25% of the time
Higher photorespiration rates at low CO2, or high temperatures (e.g. tropical environments)
Why is RuBisco so inefficient?
- Specificity and activity are linked
- Faster RuBisco is less specific (e.g. cyanobacteria
- More specific RuBisco is slower (e.g. in plants)
- RuBisco was evolved before widespread oxygenation of the environment
How much does RuBisco make up of C3 and C4 plants?
RuBisco makes up 50% of soluble protein in C3 plants
RuBisco makes up 30% of soluble protein in C4 plants
What are the costs of photorespiration?
- Carbon and energy loss
- 2-PG is toxic
- One Co2 lost for every 2 O2 fixed, depleted calvin cycle intermediates
- One ATP used regenerating 3-PGA from glycerate
- One ATP used aminating glutamate to glutamine
- Two electrons used
How much is photosynthetic efficiency?
~1.5%
What are the natural strategies to avoid photorespiration on land?
Carbon concentration mechanisms (CCMs), such as C4 and CAM
Increases local CO2 proximity of RuBisco
What is the C4 pathway?
- Plants fix CO2 into malate in mesophyll cells and export it to bundle sheath cells, where it is decarboxylated to pyruvate, regenerating NADPH and CO2
- The high concentration of CO2 minimises photorespiration
- Usually occurs in warmer climates where photorespiration is high
- Requires specialised leaf anatomy (bundle sheath cells surrounding a vein and mesophyll cells surrounding them)
How many ATPs do C4 and C3 require?
C4 requires 15 ATP for each PGA synthesised
C3 requires 9 ATP
However, ATP is often not limiting in C4 plants due to the sunny tropical conditions
What is CAM?
- Typically occurs in succulents
- During the night, Co2 is absorbed and fixed into malate
- During the day, stomata close and malate is transferred to the vacuole where it is turned to ATP
What are the natural strategies to avoid photorespiration in water?
- Co2 diffusion in water is much slower than in air, restricting access for photosynthesis
- Algae and cyanobacteria can restrict outward CO2 diffusion by the stuctures; carboxysomes and pyrenoids
- CO2 is converted to bicarbonate in the cytosol and transferred to the carboxysome or pyrenoid
- Bicarbonate is then converted back to CO2, increasing the concentration and limiting photorespiration
What is a carboxysome?
A protein shell
What is a pyrenoid?
A protein/starch sheath
