Tag: photosynthesis in plants

Photosynthetic bacteria are anaerobic and only one type of photosystem is found in them but cyanobacteria or blue-green algae have both systems PSI and PSII. In PS I contain only one chlorophyll that is chlorophyll a while PS II contains various yellowish carotenoids and phycocyanin that differ in colour in different species.

Photosystems are the functional and structural unit of photosynthesis that contains chlorophyll and various electron carriers that helps in energy liberation.

Photolysis of water is carried out to fulfill the need of electrons that are given by photosystem II to photosystem I in the Z scheme. The splitting of water is associated with PSII in which water is split into H$^+$, [O] and electrons and this also creates oxygen, one of the net product of photosynthesis.

The constituents of PS I is located in the thylakoid membrane of stroma and the outer surface of grana, contain a number of light-harvesting complexes and a reaction centre complex, which is a protein complex with two special chlorophyll a molecules and a primary electron acceptor.

Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae and cyanobacteria. Within the photosystem, enzymes capture photons of light to energize electrons that are then transferred through a variety of co-enzymes and co-factors to reduce plastoquinone to plastoquinol. The energized electrons are replaced by oxidizing water to form hydrogen ions and molecular oxygen.

Each photosystem is characterized by the wavelength of light at which it's reaction center absorbs maximum light i.e., 700 and 680 nanometers, respectively for PS I and PS II in chloroplasts. thus PS I is denoted by P$ _{700}$. Thus, the correct answer is 'P$ _{700}$'.

A) Electrons from excited chlorphyll molecule of photosystem II are accepted first by Quinone.

Water is split in chloroplasts in the light reaction of photosynthesis. Chlorophyll, acting as a photopigment, captures sunlight and transfers that energy to an electron pair of a water molecule. Water molecule is separated into 2 protons, molecular oxygen and a free electron pair, which then enters to PS II and accepted by plastoquinone and carried by variety of electron acceptors in a series of redox potential. The excited electrons are used for photolysis of water generating oxygen and protons which then used in synthesis of ATP and NADPH.

In the electron transport process, the excited electron from P$ _{680}$ is accepted by primary acceptor quinone, which sends them to an electron transport system consisting of plastoquinone, cytochrome complex and plastocyanin.

The excited electron absorbed by P$ _{700}$ reaction centre, then transfer to the ferredoxin (Fe), then the reduced Ferredoxin donates the electron to NADP$^+$.

The NADP$^+$ take an electron from ferredoxin, protons from the medium and get reduced to NADPH in presence of enzyme ferredoxin- NADP- reductase. So, the last electron acceptor of PS I is ferredoxin.

Hence, the correct answer is 'Ferredoxin'.

When the pigment in the reaction center of PS II (P$ _{680}$) absorbs a photon, an electron in this molecule gets excited and first transferred to electron acceptor namely pheophytin then to a series of redox molecules. The electron flow goes from Photosystem II to Photosystem I. It utilizes absorbed light for electron transport from plastocyanin on the lumenal side to ferredoxin on the stromal side of the thylakoid membrane to generate NADPH and ATP.