Week 8 Johnson Lecture 2

Question 1

How does electron transfer occur in photosynthesis?

Answer 1

Using 2 light driven reactions in complexes PSII and PSI. This uses a chain of acceptors ranging from water to NADP+, with oxygen released as a by-product.

Question 2

How does ATP synthesis happen in photosynthesis?

Answer 2

The electron transfer is coupled to the formation of a proton gradient which is utilised to make ATP.

Question 3

What is the basic structure of a photosystem?

Answer 3

There are 2 main sites;

• The reaction centre; contains the special pair chlorophylls which can receive and donate electrons to molecules
• Light harvesting antenna complex; which contains many antenna chlorophylls which absorb light energy

Question 4

How does a photosystem work?

Answer 4

Light harvesting antennas gather the excitation energy and transfer it to the special chlorophyll pairs.

Question 5

What is chlorophyll?

Answer 5

Its a light absorbing pigment

Question 6

What is the structure of chlorophyll?

Answer 6

The chlorophyll molecule is formed of two parts;

• a tetrapyrrole ring, coordinated by Mg2+ ion
• hydrophobic phytyl tail region (used to stick to surfaces)
• -The conjugated π-electron system in the tetrapyrrole ring is responsible for light absorption

Question 7

Why do plants have a variety of pigments?

Answer 7

Different pigments absorb different light at different wavelengths. The combination of having a variety of pigments broadens the spectrum of light energy being absorbed

Question 8

Examples of other pigments?

Answer 8

Many types;

• carotenoids
• chlorophyll b
• chlorophyll a

Question 9

How are light complex antennas usually structured?

Answer 9

pigments are bound to antenna proteins non-covalently to ensure a high concentration for efficient light absorption

Question 10

What does each light harvesting complex include?

Answer 10

Contains:

• 4 x carotenoids
• 6 x chlorophyll b
• 8 x chlorophyll a

Question 11

What is the antenna structure of photosystem II?

Answer 11

PSII forms a dimeric supercomlex with Light Harvesters II. There are around 157 chlorophylls per reaction centre. There are multiple antenna proteins to provide a large spatial cross-section for light absorption. This system is modular, so there will be more LHII’s when growing under low light conditions

Question 12

What is the antenna structure of photosystem I?

Answer 12

PSI forms a monomeric supercomplex with Light Harvesters I. There are around 155 chlorophylls per reaction centre. There are multiple antenna proteins to provide a large spatial cross-section for light absorption. This system is modular, so there will be more LHII’s when growing under low light conditions.

Question 13

How does absorption and emission work?

Answer 13

Molecules only absorb photons with energy equal to the energy gap between electron orbitals

• Absorption uses energy, from ground state to excited state
• Emission releases energy as light, from excited to ground state

Question 14

What does the jablonksi diagram show?

Answer 14

Chlorophyll has two major excited states;

• Red photons match the S0—>S1 (first excited state) energy gap,
• blue photons match the S0—>S2 (second excited state) energy gap.
• -Light absorption happens on a femtosecond timescale (10-15s)