Tag: modern concept of photosynthesis

Photolysis means splitting of water in presence of sunlight. During photosynthesis, light reaction occurs in chloroplasts. In light reaction the water is oxidised and the electrons from water are passed on through a series of electron acceptors and ultimately used to reduce NADP. Since electrons from water molecule are never returned or cycled back, this process is called as non cyclic flow of electrons. It involves PS-II and PS-I both.

Chloroplasts contain light trapping chlorophyll molecules. The chlorophyll molecules are an essential requirement for light reaction. The light energy is trapped by chlorophyll molecules for carrying out photolysis of water i.e., light dependent split of water molecules and photophosphorylation i.e., light dependent synthesis of adenosine triphosphate (ATP) by phosphorylation of adenosine diphosphate (ADP).

Experiments with isolated chloroplasts, in presence of oxidants like ferricyanides and benzoquinone (Hill reagents) were conducted by Hill in 1937 and 1939. By placing the isolated chloroplasts in sunlight, he noticed evolution of oxygen from water. Hill concluded that if there was suitable hydrogen acceptor in the medium it would have been reduced. Arnon 1951 and 1954 repeated the experiment of isolated chloroplast and concluded that, the hydrogen acceptor present in plants is NADP (Nicotinamide adenine dinucleotide phosphate). It is reduced to $NADPH _2$ and an ADP molecule is converted to ATP in light by utilizing inorganic phosphate. This phenomenon is called as photophosphorylation.

The process of photolysis is light dependent split of water molecule releasing oxygen as a by product. The essential minerals for this process are calcium, manganese and chlorine.

Hill reaction occurs in light or can be called as light reaction. Hill first of all demonstrated that isolated chloroplasts can release oxygen in presence of suitable inorganic electron acceptors. The Hill reaction is the light-driven transfer of electrons from water to Hill reagents (non-physiological oxidants) against a chemical potential gradient. 

Photolysis of water refers to the light dependent split of water, which occurs during the light phase of photosynthesis in the chloroplast. The minerals essential for the process of photolysis are calcium, chlorine and manganese.

Photolysis of water occurs during non cyclic electron flow. During non cyclic electron flow, the ultimate source of electrons is water. The water molecule is split in a process, called as photolysis. The electrons are ultimately used in reduction of oxidised NADP to form reduced $NADPH _2$. The oxygen is released as a byproduct of photosynthesis. The mineral ions essential for photolysis are manganese, chlorine and calcium.

The process of photosynthesis occurs in two stages light reaction and dark reaction. The light reaction involves oxidation of water (photolysis i.e., light dependent splitting of water), which releases oxygen as a by product. The photolysis of water also releases electrons, which are used to drive electron flow in the thylakoid membrane ultimately this electron flow results in the synthesis of ATP and $NADPH _2$. These products ATP and $NADPH _2$ are used up subsequently in the process of dark reaction.

As we know that the PS-II or photosystem II constantly supplies electrons during the light reaction.