## PHOTOSYNTHESIS

### PHOTOSYNTHESIS

The process that involves harvesting of solar energy and its utilization by living organisms like plants and bacteria for production of carbohydrates through complex set of reactions is termed as photosynthesis.
Photosynthesis is the synthesis of carbohydrates in the presence of light. It includes oxidation of water and reduction of CO2 to form carbohydrates with the release of O2
6CO2 + 6H2O →  6C6H12O6 + 6O2
In plants, mesophyll cells are the actively photosynthesizing cells that contain green pigments, chlorophylls in the specialized organelles called chloroplast.
The process of photosynthesis starts in the internal membrane of chloroplast called thylakoids. The ATP and NADPH produced in the thylakoid reaction are used for carbon fixation in the stroma of the chloroplast.
In photosynthesis, the light energy is converted into chemical energy.
Light energy:- Light contain energy. The energy of light is given by

($\upsilon$ is the frequency of light).
4.1.    Pigments involved in photosynthesis
Light is absorbed by pigments molecules. Pigments capture the light energy e.g. chlorophyll. The chlorophyll a is primary pigment for photosynthesis in the plant. It present in all photosynthetic organisms except photosynthetic bacteria. It is a primary photosynthetic pigment because it performs the primary reaction of photosynthesis.
Chlorophyll-b:- The methyl group of chlorophyll is replaced by CHO group in chlorophyll-b.

Chlorophyll a and chlorophyll 'b' both are soluble in organic solvents, but chlorophyll a is more soluble in petrol and chlorophyll b is more soluble in methanol.
Head of chlorophyll is called porphyrin. The porphyrin has four pyrrole rings which are linked by methine bridges. Tail or chlorophyll is called phytol.
Phytol is a long chain of hydrocarbon that anchors the chlorophyll molecules with the thylakoid membrane. Phytol tail is connected to pyrrole by propionic acid residue. Chlorophyll without its mg+2– core is colourless and called pheophytin.

Chlorophyll is the major photosynthetic pigment that consists of polycyclic, tetrapyrrole ring and a central Mg2+ ion. Chloroplast also contains various other accessory pigments including carotenoids, phycobilins and phycoerythrin. These pigments absorb light in the wavelength region where chlorophyll does not absorb.
4.2.    Pigments participate in the absorption of light and transmission of energy
In chloroplasts, solar energy is absorbed by pigments that actively participate in photosynthesis. The chlorophylls and bacteriochlorophylls are the typical pigments of photosynthetic organisms. Chlorophylls a and b are found in green plants, and c and d are found in some protists and cyanobacteria. Each pigment molecule consists of two parts; a porphyrin ring that functions in light absorption and a hydrophobic phytol tail that keep the chlorophyll embedded on the photosynthetic membrane. The porphyrin ring is composed of tetrapyrrole rings which are linked by methane bridges having Mg atom at the core. The alternating single and double bonds along the edges of porphyrin ring delocalize electrons influences electron density on the absorption of light by the molecule.
Chloroplasts also contain pigments including carotenoids, phycobilins, phycocyanin and phycoerythrin. These pigments absorb light energy in the wavelength region where chlorophyll does not absorb and then transfer this light energy to chlorophyll. They protect the photosynthetic apparatus from damage caused by light. The majority of the pigments serve as an antenna complex, collecting light and transferring the energy to the reaction center complex, where the chemical oxidation and reduction reactions leading to long-term energy storage take place. Variation in antenna systems is observed among different classes of photosynthetic organisms, but the reaction centers, appears to be similar in even distantly related organisms.  Antenna systems contain chlorophyll and are membrane-associated units. The antenna pigments are associated with proteins to form pigment-protein complexes. The physical mechanism by which excitation energy is converted from the chlorophyll that absorbs light to the reaction center is fluorescence resonance energy transfer (FRET). By this mechanism, the excitation energy is transferred from one molecule to another by a non-radiative process. The photons absorbed by the antenna pigments transfer their energy to the reaction center for electric charge separation and then involve in redox reactions.
The response of a biological system with respect to a particular wavelength of light is termed as Action spectrum. For example, an action spectrum for photosynthesis can be constructed from measurements of oxygen evolution at a different wavelength. Action spectra were very important for the discovery of two distinct photosystems operating in O2-evolving photosynthetic organisms. Even on continuous illumination, a single chlorophyll molecule absorbs only a few photons each second. If there were a reaction center associated with each chlorophyll molecule, the reaction center enzymes would be idle most of the time, only occasionally being activated by photon absorption. However, if a reaction center receives energy from many pigments at once, the system is kept active a large fraction of time. Several hundred pigments are associated with each reaction center, and each reaction center must operate four times to produce one molecule of oxygen – hence the value of 2500 chlorophylls per O2. The reaction centers and most of the antenna complexes are integral components of the photosynthetic membrane. In eukaryotic photosynthetic organisms, these membranes are found within the chloroplast; in photosynthetic prokaryotes, the site of photosynthesis is the plasma membrane or membranes derived from it.
4.3.    Oxygen-evolving organisms have two photosystems
The quantum yield of photochemistry is nearly 1.0, the actions of about ten photons are required to produce each molecule of O2, so the overall maximum quantum yield of O2 production is about 0.1. Any photon absorbed by chlorophyll or other pigments is as effective as any other photon in driving photosynthesis. However, the yield drops dramatically in the far-red region of chlorophyll absorption (greater than 680 nm). Emerson discovered the enhancement effect. He measured the rate of photosynthesis separately with the light of two different wavelengths and then used the two beams simultaneously. When red and far-red light were given together, the rate of photosynthesis was greater than the sum of the individual rates. These and others observations were eventually explained by experiments performed in 1960 that led to the discovery that two photochemical complexes, now known as photosystem I and II (PSI and PSII), operate in series to carry out the early energy storage reactions of photosynthesis. Photosystem I absorbs far-red light, photosystem II absorbs red light. Another difference between the photosystems is that:
1.     Photosystem I produces a strong reductant, capable of reducing NADP+, and a weak oxidant.
2.     Photosystem II produces a very strong oxidant, capable of oxidizing water, and a weaker reductant than the one produced by photosystem I.
4.4.    The photosynthetic apparatus
In photosynthetic eukaryotes, the most active cell organelle that contributes to photosynthesis is chloroplast. The internal membrane of the chloroplast is organized into flattened membranous sacs, called thylakoids. Thylakoids are arranged in orderly stacks called grana. The space inside a thylakoid sac is the lumen, and the space outside the thylakoid and within the chloroplast envelope is the stroma, which contains the enzymes responsible for carbohydrate synthesis.

The PSI reaction center complex is a large multisubunit complex. Its reaction center core is made of 14 polypeptide sub-units absorbing light of wavelength 700nm. It is abundantly found on the exposed part of grana and stromal lamellae near ATPases. The core antenna and P700 are bound to two proteins, PsaA and PsaB. Electrons from PSI reaction center are transferred to ferredoxin (Fd), a small, water-soluble iron-sulfur protein.

Photosystem II is a large membrane protein complex located in the thylakoid membranes. It is composed of 20 sub-units. Out of them 17 are transmembrane and 3 are membrane peripheral extrinsic sub-units. It exists in dimeric form. The core of the reaction center consists of two membrane proteins known as D1 and D2 along with co-factors participating in electron transfer and water splitting oxygen-evolving complex(O.E.C). Surrounding the D1 and D2 proteins, there lies the CP47 and CP43 sub-units of six transmembrane helices each. They bind a number of chlorophyll molecules to serve an intrinsic light-harvesting function The primary donor chlorophyll, additional chlorophylls, carotenoids, phaeophytins, and plastoquinones are bound to the membrane proteins D1 and D2. It absorbs light of wavelength about 680nm and generally present on an appressed region of grana.