Photosynthesis

Photosynthesis

Photosynthesis is a process of capturing light energy and converting into chemical energy. The chemical energy produced in the form of ATP and NADPH is used to synthesise organic compounds (carbohydrates); to be used as food. This ability makes photosynthesis, a significant process taking place on earth.

Eukaryotes (plants and algae) and prokaryotes (cyanobacteria and purple, green bacteria) are capable of carrying out photosynthesis. Cyanobacteria perform photosynthesis in a similar manner to plants.

Process of photosynthesis in cyanobacteria

The process of Photosynthesis is divided into

1.     Light reaction

2.     Dark reaction

In light reaction, light energy is captured by photosynthetic pigments and converted into chemical energy (ATP and NADPH).

In dark reaction, the chemical energy is used to fix CO₂ to construct organic compounds (carbohydrates); to be used as food. This reaction is also called CO₂ fixation or Calvin’scycle.

Light reactions in cyanobacteria

• Photosynthetic pigments

All photosynthetic organisms contain pigments to observe light. Chlorophyll is the main pigment involved in the absorption of light. In cyanobacteria, chlorophyll a is the predominant pigment present and it absorbs red light at a wavelength of 665nm. Cyanobacteria also contain accessory pigments like phycobiliprotein (Phycoerythrin and phycocyanin) which help to absorb light at a broader wavelength (470-630nm) and makes photosynthesis more efficient. These pigments are located in the cytoplasmic membrane of cyanobacteria.

Photosystems

The pigment molecules are arranged in highly organized arrays called reaction center or Photosystems. Cyanobacteria and green plants have two photosystems namely Photosystem I (P700) and Photosystem II (P680) through which the electrons, excited by the capture of photons, flow.

When P700 (Photosystem I) absorbs energy and gets excited, the excited electron is transferred through a series of proteins like ferrodoxins and are eventually used to reduce NADP⁺ to produce NADPH⁺. (When a compound accepts electrons, it is said to be reduced). When P680 absorbs light and gets excited, the electron passes through pheophytin, plastaquinone, cytochromes and plastacyanin to be donated to the oxidised P700 in order to replenish the lost electron. It is during this process, ATP (Adenosine Tri phosphate) is generated (Figure 6.2). Oxidised P680 obtains electron when water is split (Photolysis) into oxygen atoms (1/2 O₂) and hydrogen ions (2H⁺) which results in evolution of oxygen. Hence it is called oxygenic photosynthesis.

This process of electron flow where two photosystems are involved in the generation of ATP is called non cyclic photophosphorylation.

Photosynthesis in Bacteria

There are four groups of photosynthetic bacteria. They are green sulphur bacteria (Example: Chlorobium) and green non sulphur bacteria (Example: Chloroflexus) purple sulphur bacteria (Example: Chromatium) and purple non sulphur bacteria (Example: Rhodospirillum). These photosynthetic bacteria can fix atmospheric CO₂ in a similar fashion like cyanobacteria but using only one photosystem and using H₂S as the electron donor instead of H₂O.

Process of photosynthesis in bacteria

The electron transport system in purple and green bacteria consists of only one Photosystem PSI (P870). They do not possess photosystem II. When P870 gets excited upon capture of light energy, it donates the electron to bacteriopheophytin. Electrons flow through quinones and cytochromes and are reverted back to P870. This process is cyclic (since the electron excited from P870 comes back to P870) and generates ATP. 

A reversed electron flow operates in purple bacteria to reduce NAD⁺ to NADH. Electrons are extracted from external electron donors like hydrogen sulphide, hydrogen, elemental sulphur and organic compounds to synthesise NADH. Since H₂O is not used as electron donor, oxygen is not evolved which explains the anoxygenic nature of the organisms involved (Figure 6.3). The sulphur evolved during this reaction is deposited as sulphur globules either outside or inside the cells. CO₂ + H₂S →  (CH₂O)_n + S. Table 6.3 compares the photosystheic process in plants, algae and bacteria.