Introduction

Question 1

What are the two main types of photosynthesis?

Answer 1

Rhodopsin homologs and Reaction centre based photosynthesis

Question 2

Where is most rhodopsin based photosynthesis done?

Answer 2

In archaea

Question 3

What colour is the rhodopsin pigment in archaea?

Answer 3

Rhodopsin give the membrane a purple colour.

Question 4

How is ATP generated following rhodopsin based photosynthesis?

Answer 4

Protons pumped across the membrane during photosynthesis flow back through ATP synthase to generate ATP.

Question 5

Describe the structure of rhodopsin.

Answer 5

GPCR fold - 7 TM helices.

Question 6

Describe the retinal cofactor of bacteriorhodopsin in the inactive state.

Answer 6

Trans retinal

Question 7

What happens to retinal following light activation of bacteriorhodopsin?

Answer 7

Photoisomerisation - to 13-cis retinal.

Question 8

Describe the key chemiosmosis experiment performed using bacteriorhodopsin.

Answer 8

Put bacteriorhodopsin and mitochondrial ATPase in a vesicle - spaced at a distance from one another. When light was shone on the membrane, ATP was produced.

Question 9

Describe features of photosynthetic reaction centres.

Answer 9

Involve 5 TM domain proteins, often involve pseudodimers, derived from a common ancestor. Usually have an antenna region fused to the RC protein or located close by.

Question 10

What are the reaction centre types? And what are they named after?

Answer 10

Type 1 (named after PSI) and Type 2 (named after PSII)

Question 11

What are the terminal electron acceptors for the two types of reaction centre?

Answer 11

Type 1 - FeS cluster

Type 2 - quinone

Question 12

Why does oxygenic photosynthesis require two enzymes, one of each reaction centre types?

Answer 12

Oxidising water requires a lot of energy - both reaction centres are needed to provide enough energy to split water and still have some left for carbon fixation.

Question 13

What is the general equation for photosynthesis?

Answer 13

CO2 + 2H2A -> [CH2O] + H2O + 2A (where A is oxygen or sulfur)

Question 14

Where does the free oxygen come from in the general equation for photosynthesis?

Answer 14

From the substrate water

Question 15

Where did oxygenic photosynthesis evolve from?

Answer 15

Cyanobacteria

Question 16

How can eukaryotes become photosynthetic?

Answer 16

By endosymbiosis with cyanobacteria.

Question 17

Is the photosynthesis reaction endogonic or exergonic?

Answer 17

Endogonic - requires light energy.

Question 18

What change in atomosphere was the carboniferous era responsible for?

Answer 18

More oxygen and high levels of coal formation.

Question 19

Why do carbon dioxide levels fluctuate throughout the year?

Answer 19

When carbon dioxide reduces, it is thought to be due to the photosynthesis of plants in the northern hemisphere in the summer.

Question 20

Describe the membranes in cyanobacteria.

Answer 20

Plasma membrane has normal respiratory chain and cyanobacteria also have a complex internal membrane system (like thylakoids).

Question 21

What is special about thermophilic bacteria?

Answer 21

Their proteins are very stable and tend to crystallise easily.

Question 22

What is the pH on either side of the thylakoid membrane in cyanobacteria?

Answer 22

pH 8 - outside of the thylakoid

pH 5 - inside the thylakoid membrane

Question 23

What are chloroplasts derived from?

Answer 23

Endosymbiosis of cyanobacterium - this was monophyletic, meaning it only happened once.

Question 24

Describe the structure of the chloroplast.

Answer 24

5μm ellipsoid structure. Outer membrane and inner membrane;

• continuous thylakoid membranes form stacks called grana
• grana linked by stromal lamellae
• stroma is the equivalent of cyanobacterial cytoplasm

Question 25

Where are PSI and ATP synthase located within the thylakoid membranes?

Answer 25

In the stromal lamellae because they project from the membranes more - cannot fit within grana.

Question 26

Where are PSII and Cytochrome b6f found within the thylakoid membranes?

Answer 26

PSII tends to be in the granal regions. Cytochrome b6f is distributed evenly throughout the membrane.

Question 27

What did EM tomography tell us about the structure of the thylakoid membrane?

Answer 27

Shows spiral staircase arrangement of continuous thylakoid membrane.

Question 28

Name some photosynthetic eukaryotes.

Answer 28

Higher plants, green algae, red algae and brown algae.

Question 29

What gave rise to the different types of algae?

Answer 29

Secondary and tertiary endosymbiosis events.

Question 30

Where are chloroplasts found and why?

Answer 30

Just underneath the mesophyll cell layer so that they can capture light easily.

Question 31

What is the minimum energy requirement per photon in photosynthesis?

Answer 31

Red photon - 680 nm.

Question 32

What happens if a blue photon is absorbed during photosynthesis?

Answer 32

The equivalent energy to a red photon is used and the extra energy provided by the blue photon is lost as heat.

Question 33

How do antennae regions increase the efficiency of light harvesting?

Answer 33

Couple thousands of pigments to the reaction centre so that the reaction centre can be excited many times per second.

Question 34

What happens to an antenna pigment when absorbing a photon?

Answer 34

This exciton makes downward energy jumps around the antenna region until it reaches the reaction centre where it can perform the chemistry of the reaction.

Question 35

What other pathways can excited state electrons undergo (if not absorbed by an antenna pigment)?

Answer 35

Photochemical reactions, red fluorescence and resonance transfer to an acceptor molecule.

Question 36

In decreasing energy levels, name the different coloured absorbing pigments.

Answer 36

highest energy is blue absorbing, then orange and red.

Question 37

What are the major light harvesting pigments?

Answer 37

Chlorophylls a and b

Question 38

Describe the structure of chlorophylls a and b.

Answer 38

Porin ring with a long hydrophobic phytol side chain. Lots of conjugated double bonds which absorb in the visible region of light.

Question 39

Why is it difficult to get chlorophyll in solution? How is this overcome?

Answer 39

It has a long phytol side chain. Can get into solution if within a membrane or bound to another protein.

Question 40

What are the accessory pigments?

Answer 40

Carotenoids and bilins

Question 41

Where are bilins found and what are they similar to?

Answer 41

Found in cyanobacteria. They are essentially an open chlorophyll molecule as they are linear tetrapyrroles.

Question 42

What is the main light harvesting protein in plants?

Answer 42

LHC2 - associated with PSII

Question 43

What are the major light harvesting proteins of cyanobacteria?

Answer 43

Phycobiliproteins which contain bound bilin pigments.

Question 44

Describe the phycobilisome.

Answer 44

Contains thousands of different coloured pigments that act as a funnel to the reaction centre. Fill in all gaps in the absorption spectrum - this makes a dense culture of cyanobacteria appear black.

Question 45

How are proteins in the thylakoid membranes connected?

Answer 45

By mobile electron carriers that can diffuse through the whole membrane system.