Tag: photosystems

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.

The process of water dissociation during photosynthesis is called as photo-oxidation of water. It requires the presence of Mn++, Ca+ and Cl- , a water oxidising enzyme and an unknown substance Z.

Water has high specific heat due to hydrogen bonds amongst molecules. Due to high specific heat, water minimizes changes in temperature. Hydrogen bonds are broken when it absorbs heat. Similarly, hydrogen bonds are formed when water releases energy in the form of temperature. This decreases the temperature of water.

When the chlorophyll molecule is excited by light, the energy level of an electron in its structure is boosted to a higher energy level and this excited chlorophyll moves rapidly the reaction center of the photosystem I where it transfers its extra energy to an electron which is then expelled from the reaction center and is accepted by the first member of a chain of electron carriers and ultimately reaches NADP$^+$, reducing it to NADPH. The reaction center has lost an electron and this electron hole is filled by stripping electrons from water which leaves hydrogen ion (H$^+$) and molecular oxygen (O$ _2$). The pathway of electrons from water to NADP$^+$ has Z shape called as Z scheme.