L7, photosynthesis 3

Question 1

why does cyclic photophosphorylation occur?

Answer 1

it occurs due to a demand for more ATP, which lowers the proton gradient in thylakoid

Question 2

what is the process of cyclic photophosphorylation?

Answer 2

1. electron flow is slowed down from PSII to PSI, and shifts energy input to PSI
2. cyclic electron flow is favored which provides extra ATP and prevents photoinhibition (occurs when there is severe light stress)
3. light energy is absorbed more by PSI and excited electrons travel through complexes to the Cyt b6/ f complex. no ATP is generated

Question 3

photosynthesis occurs in …

Answer 3

phases: light-dependent, carbon fixation

Question 4

what happens to water, and what happens to carbon in photosynthesis?

Answer 4

water is oxidized, donates electrons
carbon is reduced, gains electrons

Question 5

what happens with carbon dioxide in plants? what about algae and cyanobacteria?

Answer 5

for most land plants, CO2 enters leaves and photosynthetic cells via stomata (gas exchange). for algae or cyanobacteria, it is dissolved in the water

Question 6

what do light reactions provide?

Answer 6

3 ATP: cyclic photophosphorylation
6 ATP: non-cyclic photophosphorylation
6 NADPH: non-cyclic electron flow

feeds the calvin cycle

Question 7

explain the first stage of the calvin cycle (fixation)

Answer 7

CO2 is fixated to a molecule of RuBP, RuBP carboxylase/oxygenase aka Rubisco catalyzes the reaction, 6-carbon intermediate is unstable and rapidly hydrolyzed, generated 2 molecules of 3-phosphoglycerate

Question 8

what is the second stage of the calvin cycle?

Answer 8

1. 3-phosphoglycerate (PGA) is phosphorylated to 1,3-biphosphoglycerate
2. 1,3-biphosphoglycerate is reduced/gains an electron
3. 1 phosphate is removed
4. 6 molecules of PGAL produced

Question 9

what is rubisco, and what does it do?

Answer 9

Ribulose-1,5 biphosphate carboxylase/oxygenase

attaches CO2 to RuBP making a 6-carbon intermediate

Question 10

what is the downside of rubisco despite being so important?

Answer 10

its inefficient even though it is the most abundant enzyme on the planet, it lacks specificity

Question 11

what is photorespiration? why does it happen?

Answer 11

done by RuBisCO, consumes oxygen and releases carbon dioxide, yielding neither ATP nor NADPH
photorespiration occurs because earth’s atmosphere had little/no oxygen when rubisco evolved

Question 12

in what conditions does photorespiration occur?

Answer 12

in high temperature, when the stomata is closed to conserve water, and when there is no supply of CO2