Ch. 6

Question 1

Depending upon the photosynthetic system, light energy has one or both of the following properties:

Answer 1

1. In ALL: light energy can drive the phosphorylation of ADP to make ATP (photophosphorylation)
2. In SOME: light can also drive the transfer of electrons from H₂O to NADP⁺

Question 2

How are phototrophic prokaryotes categorized?

Answer 2

Physiological differences
- Whether of not they produce oxygen
- Source of electrons for biosynthesis

Question 3

List the photosynthetic microorganisms we went over in class.

Answer 3

Oxygenic phototrophs
- Cyanobacteria
- Procholorophytes

Anoxygenic phototrophs
- Purple bacteria (sulfur and nonsulfur)
- Green bacteria (sulfur and nonsulfur)
- Heliobacteria

Question 4

What are photosystems?

Answer 4

Organization of photosynthetic pigments within the membrane

Question 5

The _____ consists of multiple proteins and associated _____ that absorb light energy and become excited.

Answer 5

1. light-harvesting complex (antenna)
2. pigments (accessory pigments)

Question 6

The _____ contains a pigment molecule that can undergo oxidation, giving up an electron.

Answer 6

reaction center

Question 7

What replaces the electron after the pigment in the reaction center gives it up?

Answer 7

Electron donor

Question 8

What are the two outcomes of prokaryotic photosynthesis?

Answer 8

1. Proton motive force forms
   - Used to generate ATP
2. The electron is used to produce NADH or NADPH

Question 9

What type of reaction center do purple bacteria and green non-sulfur bacteria have? What does it use as terminal electron acceptors?

Answer 9

• Type II RC
• Uses quinones as terminal electron acceptors

Question 10

What are the eight steps of cyclic phosphorylation?

Answer 10

1. Dimer of bacteriochlorophyll (P₈₇₀) absorbs light energy
   - 1 electron is excited to a higher energy level (P*₈₇₀) [has very low reduction potential (E’_m)]
2. Electron donated to acceptor: Bpheo
3. Bpheo transfers electron to ubiquinone A (UQ_A)
4. UQ_A transfers electron to UQ_B –> UQ^-_B
   - Steps 1-4 repeated to add second electron –> UQ^2-_B
5. UQ^2-_B picks up 2 protons from the
   cytoplasm and is released from the reaction center as UQH₂
6. UQH₂ transfers the electrons to a bc₁ complex
7. bc₁ complex reduces cytochrome c₂ –> protons translocated –> generates ∆p
8. cytochrome c₂ returns electron to oxidized bacteriochlorophyll molecule in RC

Question 11

What is happening in steps 5-6 of cyclic phosphorylation?

Answer 11

Q cycle

Question 12

Why is UQ_B referred to as a two-electron gate?

Answer 12

Because 2 electrons are required before reduced quinone leaves the RC

Question 13

List the components of the reaction center of purple bacteria.

Answer 13

1. RC protein: serves as scaffolding for Bchl and Bpheo
   - Quinones and nonheme iron are bound
2. 4 bacteriochlorophyll (Bchl) molecules bound
   - 2 dimers, 2 monomers
3. 2 Bpheo (A and B)
4. 2 UQ (A and B)
5. 1 nonheme ferrous iron
6. 1 carotenoid molecule (protect the RC pigments from photodestruction)

Question 14

Describe the redox reactions in the RC of purple bacteria.

Answer 14

1. The RC and bc₁ cooperate to translocate protons to the outside
2. The RC generates a ∆Ψ and ∆pH
   - Electron moves from outside to inside by cyt c
   - 2 protons are taken from the cytoplasm and released in periplasm

Question 15

_____ and _____ are important sources of electrons for growth but serve separate functions.

Answer 15

1. NADH
2. NADPH

Question 16

NADH donates electrons to _____, resulting in _____. NADPH donates electrons to _____.

Answer 16

1. the respiratory chain
2. ATP synthesis
3. biosynthesis reactions

Question 17

What must purple bacteria do to generate NADPH?

Answer 17

Use the proton motive force (∆p) created by light energy to drive reverse electron transport

Question 18

What type of reaction center does green sulfur bacteria have?

Answer 18

Type I

Question 19

How do the RCs in green sulfur bacteria and heliobacteria different from the RCs in purple bacteria?

Answer 19

1. Possess iron–sulfur centers as terminal electron acceptors
2. Reduce NAD(P)⁺ instead of quinone

Question 20

What are the prosthetic groups in green sulfur bacteria?

Answer 20

• Primary electron acceptor (A₀) = bacteriochlorophyll 663
• A₁ = quinone-like molecule
• 2-3 Fe-S centers
• Fd is first reduced product outside RC

Question 21

What results in cyclic vs. non-cyclic flow in green sulfur bacteria?

Answer 21

• Cyclic: if the electron is donated from ferredoxin to MQ
• Non-cyclic: if the electron is donated from ferredoxin to NAD(P)

Question 22

How does photosynthesis in cyanobacteria and chloroplasts differ from photosynthesis in purple and green bacteria? (Hint: plants)

Answer 22

1. H₂O is the electron donor and oxygen is evolved
2. There are two light reactions in series occurring in two different RCs
3. Electron flow is primarily noncyclic, producing both ATP and NADPH

Question 23

How many reaction centers do cyanobacteria/chloroplasts have? What are they?

Answer 23

2 RCs
1. Photosystem I (RC I or P₇₀₀)
- Similar to Type I RC of green bacteria
2. Photosystem II (RC II or P₆₈₀)
- Similar to Type II RC of purple bacteria

Question 24

Diagram and explain the Z-scheme.

Answer 24

• PS II is connected to PS I by a short ETC that contains the b₆f complex (analogous to bc₁ complex)
• Electrons move from water –> PS II –> pheophytin –> electron carriers -> PS I
• ∆p is created by the b₆f complex via a Q cycle

Question 25

What are the different types of photosynthetic pigments? What are their functions?

Answer 25

1. Light-harvesting pigments - Absorb light at wavelengths different from RC chlorophyll - Transfer energy to the RC 2. Chlorophylls and bacteriocholorophylls - Harvest energy from light 3. Carotenoids - Absorb light energy and transfer it to the RC chlorophyll

Question 26

How are photosynthetic pigments regulated?

Answer 26

1. Transcriptional regulation based on: - Oxygen levels - Redox potentials - Light 2. Regulated according to Earth's rotation (circadian clock)

Question 27

Describe chlorophylls and pheophytin (general structure and what organisms have them).

Answer 27

1. Chlorophylls - General structure: 4 pyrrole derivatives with covalently bound Mg²⁺ - Different side chains from the pyrrole rings - Photosynthetic bacteria have Bchl - Note: narrow wavelength range 2. Pheophytin - Same structure as chlorophyll but with no Mg or Zn - In eukaryotes and cyanobacteria

Question 28

Describe carotenoids.

Answer 28

- Absorb light over wavelengths of 400-600 nm - Transfer light energy to chlorophylls - Expand wavelength range of light that is able to drive photosynthesis - Protect biological materials from photooxidation caused by reactive oxygen derivatives - Over 30 different types known

Question 29

Describe phycobiliproteins. 1. Are they soluble or insoluble? 2. General structure? 3. Found in what organisms (general)?

Answer 29

1. Soluble 2. Contain bilin (4 pyrroles in linear form) 3. Found in eukaryotes and cyanobacteria (not in other photosynthetic bacteria)

Question 30

Describe bacteriopheophytin (what organisms have it, type of RC).

Answer 30

- Found in photosynthetic bacteria - Use pheophytin-quinone type RC (but green sulfur bacteria and heliobacteria Fe-S type RC)

Question 31

Describe the photosynthetic apparatus in cyanobacteria. 1. What is it called? 2. Antenna molecules 3. RCs

Answer 31

1. Thylakoid - Similar to eukaryotes - Found in cytoplasm - Bilayer membrane 2. Antenna molecules - Found in cytoplasmic side - Chlorophyll a - Phycobiliproteins - Carotenoids 3. RCs - Chlorophyll a

Question 32

Describe the photosynthetic apparatus in green bacteria. 1. What is it called? 2. Antenna molecules? 3. RCs?

Answer 32

1. Chlorosome - Found on cytoplasmic membrane - Monolayered galactosyl diglyceride 2. Antenna molecules - Rod-shaped - Bchl c, d, or e - Carotenoids - Have baseplates bound to a RC 3. RCs - In cytoplasmic membrane - Contain Bchls and Fe-S proteins or Bpheo

Question 33

Describe the photosynthetic apparatus in purple bacteria.

Answer 33

A. Less well-developed structure B. Intracellular membrane structure: - Continuous with cytoplasmic membrane - Antenna molecules and RCs - Shape varies with strain C. Membrane is produced under anaerobic light conditions when pigments are produced