Photosynthetic Organisms
There are three main classes of organisms which use light energy. The first two show many similarities. The Halobacteria are quite different and are often not considered as truly photosynthetic. They are dealt with separately.
A) Cyanobacteria and green plants (chloroplasts probably evolved from cyanobacteria-like endosymbionts). Oxygenic photosynthesis - O2 is produced and photosystems I and II are both present. Fix CO2. B) Photosynthetic Eubacteria. Anaerobic photosynthesis - no O2 is produced and only photosystem I is present. Sometimes fix CO2 but need an external reductant such as H2S or succinate.
C) Halobacteria. No chlorophyll, no photosynthetic electron transport. Possess a light-driven proton pump which produces energy but no reducing power. Need source of organic carbon. Cannot Fix CO2.
Three Main Stages of Photosynthesis:
I) Light is absorbed and used to generate proton motive force and reducing power (as NADH or NADPH).
II) Proton motive force is used to generate ATP.
Once the PMF has been generated by the photosystems, the mechanism of ATP synthesis is essentially the same as in respiration. An ATP synthetase is found in the photosynthetic membranes works just like the one in respiratory membranes.
III) ATP and NAD(P)H are used to fix carbon dioxide.
The supply of ATP and NAD(P)H do not have to come from light energy. Autotrophic bacteria which use inorganic reactions as a source of energy can often also fix CO2.
Trapping Light Energy
Light is absorbed by chlorophyll located in the photosynthetic membranes. When light is absorbed, an electron is released from the chlorophyll and travels down an electron transport chain. However, only a few special chlorophyll molecules in the reaction centre actually release electrons. Most of the chlorophyll is found in the antenna which absorbs light energy and funnels it to the reaction centre chlorophyll, where electron release occurs. For every reaction center chlorophyll there may be several hundred antenna molecules. The antenna and reaction center chlorophylls are chemically identical, they differ due to the proteins which bind them.
A typical antenna complex consists of a small to medium size protein which carries several each of chlorophyll a and chlorophyll b and a couple of carotenoids. Precise details vary between organisms. The two types of chlorophyll absorb light of different wavelength. The carotenoids absorb well in regions of the spectrum where chlorophyll absorbs poorly. In addition, carotenoids help protect against high light intensity. Bacteriochlorophyll (Bchl) is slightly different chemically from plant chlorophyll and absorbs light of longer wavelength (lower energy).
The reaction center consists of a cluster of proteins plus a pair of chlorophyll molecules and some electron carriers. Plants and cyanobacteria have two distinct reaction centers which operate in series whereas anaerobic photosynthetic bacteria have only a single reaction center. The electron ejected from the reaction center chlorophyll travels along a series of electron carriers and proton motive force is generated. In anaerobic bacteria, this electron cycles back to the reaction center. In cyanobacteria and chloroplasts, the electron continues on to reduce NADP after a second boost of energy from the second photosystem.
Purple Photosynthetic Bacteria
We will consider first the simpler anaerobic photosynthesizers which only possess one type of reaction center. The details of the reaction center complex varies among different bacteria. X-ray crystallography has been used to analyse the reaction center from Rhodopseudomonas viridis.
The components of the reaction center are:
bacteriochlorophyll b (Bchl) four (shared by L & M)
bacteriopheophytin (Bphe) two (shared by L & M)
ubiquinone (UQ) two (shared by L & M)
ferrous iron (non-heme) one (shared by L & M)
heme four (on protein C)
When chlorophyll absorbs a photon, an electron is excited to a higher-energy orbital. This energy may be released by any of:
b) Emit heat.
c) Transfer energy to neighboring molecule, as when antenna chlorophyll transfers energy to the reaction center. The energy may actually travel via many antenna chlorophylls before reaching P960.
d) Eject an excited electron, as occurs in the reaction center.
Sequence of Events:
b. Energy is transferred to P960 which is a pair of special Bchl b molecules in the reaction center. (P960 means pigment with absorption maximum at 960nm)
c. P960 becomes excited. The excited state, P960*, lasts less than one picosecond (pico = 10-15).
d. P960 loses an electron to one of the other Bchl b molecules in the reaction center.
e. The electron is rapidly passed on to Bphe. Bphe is a chlorophyll derivative where Mg2+ is replaced by 2H+. The transient existence of the biradical (Bchl)2+....Bphe- is detectable by its spectrum.
f. The electron is transferred from Bphe to firmly bound ubiquinone, UQA, to give the half-reduced form (the anionic semiquinone). [UQA is never fully reduced and never gains protons to become UQH2.]
g. The electron goes from UQA to the loosely bound ubiquinone, UQB. UQB waits for a second electron and also picks up two protons from water so becoming UQH2.
h. UQBH2 moves out of the reaction center, travels through the lipid bilayer, and transfers its electrons to the cytochrome bc1 complex.
i. Cytochrome bc1 transfers electrons to cytochrome c2 which is a soluble periplasmic protein.
j. The electrons are transferred from cytochrome c2 to reaction center protein C and travel successively via its four hemes back to P960+.
Generation of Proton Motive Force
Two protons are translocated across the membrane, from the cytoplasm to the periplasm, for each electron which goes around this loop. This creates the proton motive force which is used to make ATP. This is known as cyclic photophosphorylation by analogy to oxidative phosphorylation.
Generation of Reducing Power
Higher plants produce NADPH but purple bacteria often generate NADH during photosynthesis. They do so by reversed electron transport. Electrons from P960 flow to ubiquinone and then, instead of going via cytochrome bc1, they go to NADH dehydrogenase which reduces NAD+ to NADH. The NADH dehydrogenase is driven in reverse by the proton gradient (i.e. electrons are travelling uphill, energetically, and hence energy input is required).
Since the electrons are not returned to the P960 they must be replaced somehow. In practice purple bacteria need an external supply of electrons. For purple sulfur bacteria, the electron donor is H2S or thiosulfate. Non-sulfur bacteria use organic donors, e.g. malate or succinate.
Respiration in Purple Bacteria
Many purple bacteria can also respire aerobically in the dark. They use NAD for both the respiratory system and the photosynthetic system, depending on the circumstances. (Chloroplasts do not respire and use NADPH for only one purpose - fixing CO2) Oxygen represses the synthesis of chlorophyll and carotenoids but the rest of the electron transport system remains and cytochrome bc1 transfers electrons to cytochrome a/a3 which is induced by O2. The central part of the electron transport chain is shared by respiration and photosynthesis. During respiration the PMF is generated by ejecting protons across the membrane into the periplasmic space. There are three sites - between NADH dehydrogenase and ubiquinone, at cytochrome bc1 and between cytochrome a/a3 and O2.
Evolutionary Relationships
The order in which the ability to generate energy evolved is generally considered to be:
2. Anaerobic photosynthesis: light converted to PMF
3. Oxygenic phtosynthesis: light converted to PMF and oxygen evolved
4. Respiration: reverse electron transport chain uses O2
Ribosomal RNA sequence homology indicates that chloroplasts are related to gram-negative bacteria. Chloroplasts are prokaryotic in structure and probably derived from an ancestor related to cyanobacteria which was first symbiotic inside eukaryotic cells and finally became an organelle. Cyanobacteria lack chlorophyll b and instead possess phycobilins whereas green plants do not. However, Prochloron and its relatives are prokaryotic oxygenic photosynthesizers without phycobilins but which do have chlorophyll b . Hence they are classified as prochlorophytes not cyanobacteria, and may be closer to the chloroplast ancestor.
Oxygen Releasing Photosynthesis
Chloroplasts of higher plants and cyanobacteria both possess two photosystems with distinct reaction centres and antennas. Both operate at shorter wavelengths than photosynthetic bacteria. Electrons flow from H2O to photosystem II (which releases oxygen) to photosystem I to NADP.
PSI and PSII use red and green light respectively. PSI operates most efficiently when PSII is also working. Red light (700 nm) can be used efficiently even if green light is given as a flash before the red light is turned on. Hence the systems operate in sequence (PSII then PSI) and the electrons from PSII are needed for operation of PSI.
Components of Photosystem II
P680 is a pair of special reaction centre chlorophyll a molecules.
Phe is pheophytin (a chlorophyll derivative without the Mg).
QA and QB (= X320, in older diagrams) are plastoquinone which is similar in function to ubiquinone but found only in chloroplasts and cyanobacteria. As before, QA is fixed and QB is mobile. R is a long hydrophobic isoprenoid tail (40 to 50 carbons long).