Lecture 4 Flashcards
(21 cards)
What happens in photorespiration?
RuBP + oxygen -> 2-phosphoglycerate + 3-phosphoglycerate
Why is it a wasteful process?
2-phosphoglycerate must be converted to 3-phosphoglycerate as it is not useful in carbon fixation and requires energy (1ATP) and results in a net loss of carbon.
What is the process of conversion from 2-phosphoglycerate to 3-phosphoglycerate?
2-phosphoglycerate -> Glycine -> Serine -> Glycerate -> 3-phosphoglycerate.
Where does it take place?
In the peoxisome and the mitochondria.
What is an issue with it?
Produces CO2 and ammonia which is energetically costly to remove from the cell.
How does O2 and CO2 concentration affect photorespiration?
They both compete for RuBisCO so high CO2 favours photosynthesis and high O2 favours photorespiration.
Therefore there will always be some photorespiration.
How does RuBisCO structure affect photorespiration?
Molecule structure between species varies in ability to distinguish between CO2 and O2. There is a trade off between specificity and rate. The more specific the enzyme is, the slower it is.
Give some examples of different specie’s RuBisCO.
Red algae is specific but very slow.
Cyanobacteria is non-specific but faster.
How does temperature affect photorespiration?
High temperature reduces the specificity of RuBisCO due to protein folding and increased rate of reaction. Also gasses are less soluble at higher temperatures and CO2 leaves solutions before O2.
How do water levels affect photorespiration?
Low water levels cause stomata to close to conserve water which prevents gas exchange and causes CO2 to be used up while O2 builds up.
What percentage of plants accept photosynthetic losses?
85%: rice, wheat, trees etc. as they are in a favourable niche.
What are the 4 strategies to reduce photorespiration?
Dissolved inorganic carbon pump
C2 photosynthesis
C4 photosynthesis
CAM
How does the dissolved inorganic carbon pump work?
Converts CO2 into bicarbonate and pumps it into pyrenoid structure using ATP.
Simplest method, used in algae.
How does C2 photosynthesis work?
Shuttles CO2 produced by photorespiration back to RuBisCO using glycine.
Only in 40 angiosperm species.
How does C4 photosynthesis work?
Instead of fixing CO2 directly into the Calvin cycle, it is first fixed into a 4Carbon molecule (Malate) which is broken down in bundle sheaths to form CO2.
In 3% of plant species but makes up 25% of primary productivity.
What does C4 photosynthesis achieve?
Concentrates carbon to 3-10 times ambient level around RuBisCO. Spatially separates initial carbon fixation from the Calvin cycle.
How does the process differ between C4 species?
Enzyme that catalyses the decarboxylation of malate in the bundle sheath cells differs between species.
What are the advantages of C4 photosynthesis?
They prefer higher temperatures and use water more efficiently so grow better in drier environments.
How did C4 evolve?
In monocots 21-35 mya. In dicots 15-21 mya.
Evolved independently 62 times, most recently in low, hot, dry areas. C4 genes can be found in a number of different lineages.
What are the advantages of C4 rice?
Rice is C3 so has low efficiency of light energy conversion due to photorespiration and has high N requirements.
C4 could increase yields by 50% and allow growth in more conditions over larger areas.
Staple for 50% of humans.
How could mutagenic methods achieve this?
Because vascular bundles are too far apart in rice and malate has to travel to far, the solution is to decrease the distance between the vascular bundles.
