Question

4. What are the key points of oxygenic photosynthesis (the light reactions) as performed by plants and cyanobacteria? 5. Referring back to the question above, compare how anoxygenic photosynthesis is done (with both the 'green and 'purple' bacterial methods)

Answer 1

4. The form of photosynthesis first comes in mind that is carried out by the plants is oxygenic photosynthesis because one of its products is molecular oxygen, resulting from the oxidation of water. This form of photosynthesis is also carried out by cyanobacteria and algae. The reactions of oxygenic photosynthesis in algae and plants take place within a special cell organelle, the chloroplast.

5. Photosynthesis by bacteria other than cyanobacteria, does not involve evolution of molecular oxygen. Instead of water (H₂O), other electron donors, for example, hydrogen sulfide (H₂S), are oxidized. This type of photosynthesis is called anoxygenic photosynthesis.

Anoxygenic photosynthesis is used mainly by the purple bacteria, the green sulfur and nonsulfur bacteria, the heliobacteria and the acidobacteria. Oxygenic photosynthesis is used by the cyanobacteria, the algae, and by plants.

In Oxygenic photosynthesis, electron flow occurs through two different electron transport chains, these electron transport chains are called the Z scheme. The stars of each chain are photosystem I (PSI) and photosystem II (PSII), each containing chlorophyll reaction centers surrounded by antenna pigments. The chlorophyll in PSI is called P700, and the chlorophyll in PSII is called P680, for the wavelengths of light each absorbs most efficiently.

In anoxygenic photosynthesis, many steps are the same as those for oxygenic photosynthesis as light excites the photosynthetic pigments, causing them to donate electrons to the electron transport chain and ATP is again generated from the proton motive force created by electron transport. Anoxygenic photosynthesis differs from oxygenic photosynthesis in the following main ways:

• Oxygen is not released because P680 of PSII is not present. Water is too electropositive to act as the electron donor for the photosystem.
• Depending on the species, the reaction center can consist of chlorophyll, bacteriochlorophyll, or other similar pigments. The reaction center in purple bacteria is called P870.
• Some of the carriers within the electron chain are different, including bacteriopheophytin, which is bacteriochlorophyll without its Mg²⁺ ion.
• Electrons cycle back to reduce P870, so this is a cyclic electron transport chain leading to generation of ATP through cyclic photophosphorylation.
• Unlike in oxygenic photosynthesis, where NADPH is the terminal electron acceptor, no NADPH is made because electrons are cycling back into the system.