Lecture 23

Question 1

1. What is photosynthesis?
2. What is the energy from PMF used for?
3. What are the ATP and NAD(P)H then used for?
4. Why is this process important?

Answer 1

1. The use of sunlight to generate proton motive force (PMF).
2. The energy from PMF is used to make ATP and also to reduce NAD to NADH (or NADP to NADPH)
3. The ATP and NAD(P)H are then used to fix CO₂ into organic molecules with which to make cellular material.
4. Phototrophic CO₂ fixation generates almost all of the organic carbon present in organisms.

Question 2

1. How do phototrophs obtain their energy?
2. Where do photoautotrophs get their carbon from? Where do they get their electron and protons for reducing NAD?
3. Where do photoheterotrophs get their carbon from? Where do they get their electron and protons for reducing NAD?
4. What reducting is used for anoxygenic photosynthesis? What does it yield?
5. Give an example of an organism that carries out this process?

Answer 2

1. Obtain their energy for ATP synthesis from sunlight.
2. CO₂ as a carbon source, H₂S or H₂O as the source of electrons and protons for reducing NAD.
3. Organic molecules as a carbon source and as a source of electrons and protons for reducing NAD.
4. H₂S (or other reduced compounds, but not H₂O) as the reductant; yields sulfur (S⁰) or sulfate (SO₄^2-).
5. Carried out by several kinds of Bacteria; example, Rhodobacter (Alphaproteobacteria).

Question 3

1. What is used as a reductant in oxygenic photosynthesis? What does it yield?
2. What organisms carry out this process?

Answer 3

1. H₂O is used as a reductant; yields O₂
2. Carried out by Cyanobacteria (and Eukarya phototrophs - plants, algae, and certain protist).

Question 4

1. What is Chlorophyll?
2. What is chlorophyll similar to?
3. What kinds of chlorophyll are there?
4. How do these chlorophyll differ?
5. What type of Chlorophyll does green alga have? What kind of light does it absorb?
6. What type of Chlorophyll does purple bacterium have?
7. What makes these types of chlorophyll different?

Answer 4

1. Pigments that function to absorb light and then release that energy as electrons.
2. Chlorophyll is similar in structure to the heme portion of cytochromes, but with Mg in place of Fe.
3. There are several types of chlorophylls. Chl a is in oxygenic phototrophs. Bchl a (bacterial chlorophyll a) is in anoxygenic phototrophs.
4. Different kinds of chlorophyll differ in the wavelengths of light they absorb.
5. Chl a is present in green alga. Best absorbs red light (680 nm) and blue light (430 nm). The peak at 480 nm is absorption by carotenoids (and it transmits green light, so it looks green).
6. Contains Bchl a, the peaks at 870, 805, 590, and 360 nm are due to Bchl a, and the peaks at 525 and 475 nm are due to carotenoids.
7. There are slight differences in Bchl (a, b, c, c_s, d, e, g) in different bacteria, with small chemical differences that result in differences in the wavelengths they best absorb light at (absorption maxima).

Question 5

1. What is embedded in cytoplasmic membrane?
2. Many bacteria have extensive internal photosynthetic membranes, what are they an extension of?
3. What would be the benefit of having one of these?

Answer 5

1. Bchl a is embedded in the cytoplasmic membrane.
2. These are extensions of the cytoplasmic membrane and are seen as vesicles or membrane stacks.
3. The benefit of this is having a larger surface area allows for more light to be picked up.

Question 6

1. What are reaction centers?
2. What do some of these reaction centers participate in?
3. What are antenna pigments?
4. What are reaction centers effecient in?

Answer 6

1. Complexes of membrane-associated chlorophyll attached to proteins.
2. Some of these reaction centers (RC), participate directly in conversion of light energy to ATP.
3. Others are antenna pigments (light harvesting complexes, LH), collect light and channel it to the RC.
4. Efficient collection of low levels of light.

Question 7

1. What are Accessory pigments?
2. What are their functions?
3. Where are they found?

Answer 7

1. Carotenoids - give cells colors (red, purple, brown), closely associated with chlorophyll.
2. Functions:
   
   1. help transfer light energy to the reaction center.
   2. protect photosynthetic apparatus, by absorbing excess light. Strong light can drive photooxidation reactions that generate toxic oxygen products; these products are very reactive and can damage the photosynthetic pigments.
3. Found in all photosynthetic bacteria.

Question 8

1. What are Phycobilins?
2. What are phycobiliproteins?
3. How are they formed?
4. What do they help collect?

Answer 8

1. Accessory pigments associated with proteins, phycobiliproteins.
2. Phycobiliproteins are major light harvesting systems in cyanobacteria and chloroplast of red algae.
3. Form as aggregates called phycobilisomes composed of protein plus red or blue bilins, making phycocyanin and allophycocyanin.
4. Help collect additional wavelengths of light and transfer that light energy to the RC (Chl a of photosystem II)

Question 9

1. What is anoxygenic photosynthesis?
2. What is the main component of the reaction center?
3. What does the absorbance of light by P870 do?
4. What is released during this process?
5. What is the order of the flow of electrons in this system?

Answer 9

1. Bchl-based light-mediated formation of PMF, via a series of electron carriers in the membrane; the PMF then drives ATP synthesis.
2. P870 (a complex of chlorophylls and protein).
3. Absorbance of light energy by P870 (directly or transferred from LH complex) converts P870 to a strong electron donor, P870^* (excitation by absorption of the energy of a proton).
4. This excitation energy is released as a flow of electrons (a series of reduction/oxidation reactions) via electron carriers that is very similar to the flow of electrons in respiration.
5. An electron of P870^* goes to Bchl to bacteriophaeophytin (Bph = Bchl without the Mg). reducing it.
   
   1. then to quinones
   2. to cytochrome bc1, an iron-sulfur complex.
   3. to cytochrome c2
   4. and then back to P870, which returns P870 to its original state

Question 10

1. What does cyclic electron flow through the membrane drive?
2. What does this establish?
3. How is ATP then made? What is this process called?

Answer 10

1. Cyclic electron flow through the membrane drives protons across the membrane.
2. Establishes a proton gradient (PMF).
3. via a membrane associated proton-translocating ATPase, ATP is formed. This is called cyclic photophosphoylation.

Question 11

1. How is the reducted (NADH) that is needed to fix CO₂ formed?

Answer 11

1. External electron doners (e.g., H₂S) are oxidized and the electrons go to the quinone pool. Then, using PMF to drive the reverse electron flow, the electrons are used to reduce NAD to NADH. So, some of the PMF generated by absorbance of light energy is consumed to make the reductant for CO₂ fixation.

Question 12

1. What is unique about oxygenic photosynthesis?
2. What kind of Chl a does PS I have?
3. What kind of Chl a does PS II have?
4. What is formed during this process?

Answer 12

1. Has two seperate but linked reactions, each has a distinct form of reaction center chlorophyll a.
2. Chl a absorbs longer wavelengths of light.
3. Chl a absorbs shorter wavelengths of light and releases oxygen.
4. PMF is formed, and electrons to reduce NAD(P) come from the splitting of water.

Question 13

1. What is autotrophy?
2. What is the first key enzyme (specific to fixation of CO₂)?
3. What is the second key enzyme?
4. What does “to biosynthesis” mean?

Answer 13

1. Assimilation of CO₂ as a carbon source (fixation of CO₂) - Calvin cycle use of ATP, use of reductant (NADPH)
2. Ribulose bisphosphate carboxylase
3. Phosphoribulokinase
4. Means some Glyceraldehyde 3-P can be tapped off for the formation of glucose by a reversal of reactions of glycolysis.

Question 14

The Calvin Cycle:

1. What is produced from CO₂?
2. What is hexose used in?
3. What is the energy of ATP then used for?

Answer 14

1. One molecule of hexose
2. Hexose is used in biosynthesis.
3. Energy from APT is then used ruduce CO₂ via electrons and protons from NAD(P)H

Question 15

Kinds of Photosynthetic Bacteria:

1. Describe Purple Phototrophic Bacteria
2. Describe Purple Sulfur Bacteria. What is an example?
3. Describe Purple Nonsulfur Bacteria. What is an example?

Answer 15

1. anoxygenic photosynthesis, bacteriochlorophyll; and various different carotenoid pigments.
2. use H₂S as an electron donor - produces granules of elemental sulfur in the cell, found in illuminated anoxic zones of lakes where H₂S accumulates. Chromatium is an example.
3. can use H₂S but at lower concentrations than purple sulfur bacteria, some can grow anaerobically in the dark using fermentation and anaerobic respiration, most are photoheterotrophs; found in muds, lake water. Ectothiorhodospira is an example.

Question 16

Green Phototrophic Bacteria

1. Describe Green Sulfur Bacteria: What is an example?
2. Describe Green Nonsulfur Bacteria: What is an example?
3. Give an example of Gram-Positive Bacteria
4. Cyanobacteria?

Answer 16

1. anoxygenic, obligately anaerobic, utilize H₂S as an electron donor, excrete sulfur as globules outside the cell. Example: Chlorobium
2. anoxygenic, filamentous, thermophilic, photoautotrophic or (better) photoheterotrophic; example: Chloroflexus (can grow aerobically in the dark as a chemoorganotroph).
3. Heliobacteria - anoxygenic phototrophs; strictly anaerobic soil bacteria; fix nitrogen and can grow by fermentation.
4. Oxygenic Photosynthesis; very large and diverse group of microbes; found in lakes, ponds, streams, ocean; some fix nitrogen; Example: Anabaena.