Photosynthesis Flashcards
Where does photosynthesis take place?
Chloroplasts
What do chloroplasts comprise of?
Many membrane-bound compartments like thykaloid membranes
Stroma and lumen
Describe thykaloid membranes.
Continuous
Grana: stacks of appressed membranes
Lamellae: unstacked non-appressed regions
What do the photosynthetic pigments (chlorophyll) harvest?
Light
At wavelengths of about 450-500nm (blue) and 650-700nm (red)
Describe the chemical structure of chlorophyll.
Chlorophylls a and b both in Porphyrin Ring structure surrounding a central Mg2+ ion, plus has a long hydrophobic carbon tail
What does the Porphyrin ring of the chlorophyll do? What does the hydrophobic tail do?
Absorbs light energy
Hydrophobic tail anchors the molecule in a membrane or protein complex
Why is cyclic phosphorylation used in chloroplasts?
When the precursors of NADPH become limiting, there is a potential back-log of high energy electrons that could build up
Switches to cyclic pathway to use up extra electron and build up H+ gradient
What is the Light Harvesting Complex (LHCII)?
Tetramer with high concentration of chlorophyll and carotenoid pigments
Embedded in thykaloid membrane
Can move laterally within thykaloid membrane and associated with either PSI or PSII
What is the LHC phosphorylated in response to?
Light/photosystem activity
What are the 3 steps of the Carbon Reactions? Also called the Calvin cycle
- Carboxylation
- Reduction
- Regeneration
Describe carboxylation stage of Calvin cycle
Fusion of carbon dioxide into carbon precursor, using enzyme RuBISCO
Describe RuBISCO structure
8 large subunits that are catalytic
8 small subunits that are structural
Describe the reduction stage of Calvin cycle
PGA (phosphoglyceric acid) is highly oxidised
Needs to be reduced to be converted into carb
1st, phosphorylation to BPG
Then reduced
What is triose phosphate?
Generic term for both compounds that the 3C product of the Calvin cycle can be
Enzyme triose phosphate isomerase switches the compound between the forms
What does the fate of triose phosphate depend on?
The needs of the plant
Insufficient carb for metabolic needs: sucrose synthesised in cytoplasm
Sufficient carb for metabolic needs: starch synthesised in chloroplast
Where do you get RuBP for the Calvin cycle from?
Regenerate it
Only one triose phosphate is used to make carb, other 5 regenerate RuBP
What are RuBISCO functions?
Carboxylation (favoured at lower temps)
Oxygenation (favoured at higher temps)
What is the Glyoxylate cycle?
Long, complicated process to regenerate RuBP from phosphoglycolate
What is CAM photosynthesis?
Gas exchange only at night to reduce water loss
Minimise photorespiration and save water by separating steps into night and day
What is C4 photosynthesis?
4 carbon product
Dissociated from gas exchange
Minimises photorespiration by doing initial carbon dioxide fixation and the calvin cycle in different cell types
