lec 15- photosynthesis II Flashcards
where do light reactions occur?
in the thylakoid membrane where the chlorophyll pigments are, this is all inside the chloroplast
where do carbon reactions occur?
carbon reactions occur in the chloroplast stroma (cytoplasm)
explain the Calvin cycle:
-carbon dioxide enters the plant through the stomata on leaves
-carboxylation occurs: where carbon dioxide is used to carboxylate ribulose biphosphate into 3-phosphoglycerate
-reduction occurs: where 3-phosphoglycerate is reduced to glyceraldehyde 3-phosphate
-regeneration occurs: where 5/6 glyceraldehyde 3-phosphate are regenerated into ribulose-1,5-bisphosphate and 1/6 3-carbon molecules become available as energy rich compound for metabolism or sucrose or starch synthesis
what enzyme carboxylates carbon dioxide?
carboxylase
what is carboxylation?
it is when a carboxylic acid is produced from a reaction with carbon dioxide, opposite is decarboxylation
what does a kinase do?
kinase transfers phosphate groups from molecules to substrates, AKA phosphorylation
what does a dehyrogenase do?
catalyzes the removal of a hydrogen from a particular molecule (ex. removes hydrogen from NADPH)
explain carboxylation phase of calvin cycle:
RUBISCO enzyme catalyzes carbon dioxide addition onto ribulose bicarbonate, the 6 carbon molecule (2-carboxy-3-ketoarabinitol-1,5-biphosphate) is unstable and is hydrolyzed turning into two 3-phosphoglycerates (3-PGA)
explain the reduction phase of calvin cycle:
-first kinase catalyzes the phosphorylation (adding of phosphate group) of 3-phosphoglycerate by ATP generating 1,3- biphosphoglycerate
-second dehydrogenase reduces 1,3- biphosphoglycerate by NADPH to glyceraldehyde 3-phosphate (extra phosphate group is replaced with hydrogen)
explain the regeneration phase of the calvin cycle:
five 3-carbon molecules regenerate into three 5-carbon molecules (glucose), ribulose 1,3-biphosphate is used as a substrate
what does RUBISCO consist of and how is it activated?
8 large catalytic subunits which are encoded by chloroplast gene and 8 small subunits encoded by nucleus gene (large protein complex), activated by light
what is photorespiration and why is it bad?
it is when RUBISCO uses O2 instead of CO2, considered wasteful process as it is less efficient and uses more energy (ATP and NADPH) due to it not producing enough 3-carbon molecules in one go, occurs alongside the calvin cycle
when is photorespiration more likely to occur?
the stomata is open in the day meaning more CO2 than O2, but in hot/dry air conditions the stomata is closed to prevent water loss which also causes more O2 and less CO2. This causes RUBISCO to do more photorespiration
what is the substrate ratio of RUBISCO?
3:1 (CO2 to O2)
what are the hypotheses to why RUBISCO has two competing reactions?
because it hasn’t evolved since times of high CO2 and low O2, active sites can’t discriminate between CO2 and O2, photorespiration helps get rid of reducing power when carbon cycle is limited
