Photosynthesis

Question 1

Structure of a Chloroplast

Answer 1

Stroma
Thylakoid
Lumen (Inside thylakoid)
Outer and Inner Envelope (Bilayer, Photosynthesis occurs across this bilayer)
Granal stacks (of thylakoids)

Question 2

Simple Overview of Photosynthesis

Answer 2

Conversion of light energy into chemical energy and reducing power.
Chemiosmosis makes ATP
NADP+ reductase forms NADPH

1. Excitation of antenna chlorophyll
2. Transfer of energy to reaction centre via FRET
3. Primary charge separation event displaces electron from water.
4. Electron transport chain forms electrochemical gradient to drive ATP synthesis and form NADPH
5. Water oxidised to replenish reaction centre electron from step 3.

Question 3

Chlorophyll a and b are similar but have small differences:

Answer 3

Main difference is in absorption maxima
Absorption maxima Chlorophyll b = ~650nm
Absorption maxima Chlorophyll a = ~670nm

Accessory pigments enhance their light harvesting capabilities.

Question 4

Förster resonant energy transfer (FRET)

Answer 4

Energy transfer between two light-sensitive molecules (chromophores). Transfer of energy is due to physical contact, works only when really close together.

Question 5

Detailed photosynthesis components in order of electron transfer:

Answer 5

Photosystem II aka P680 (680nm absorption maxima)
Pheophytin
Plastiquinone
Cytochrome B6F Complex
Plastocyanin
Photosystem I aka P700 (700nm absorption maxima)
Ferredoxin
Produces NADPH

8 protons produced along this mechanism are used to drive ATP Synthase.

Question 6

Photosystem II (molecule) Overview

Answer 6

Photosystem II – Oxygen producing

Water Splitting: 2x H2O —> O2 + 4H+ , Produces 4 e-
4 Mn complex forms active site of the enzyme.
4e- pass through the Mn complex, passed to Pheophytin, then Plastoquinone A then Plastoquinone B, then another Plastoquinone B that is outside the molecule.

Question 7

Photosystem I (molecule) Overview

Answer 7

Photosystem I – NADPH producing

Plastocyanin passes 4e- through photosystem 1, split into halves, 2e- go through the PsaA protein side, the other 2 through the PsaB protein side.
Then passed to the iron-sulfur center, coordinated by 2 cysteines from the PsaA and 2 from the PsaB.
Then passed to Ferredoxin, which passes them to Ferredoxin-NADP reductase, causing the formation of NADPH.

Question 8

What happens to the photosystems when there is too much light? and how can it be fixed?

Answer 8

PSII: Splits lots of water, passing on lots of protons. ATP synthase is the rate limiting step (especially in high light and low temperatures). Causes acidification of the lumen (since protons cant all be passed through ATP synthase). Resolved via non-photochemical quenching

PSI: Production of excess electrons. Resolved by the “Water-Water Cycle”.