Tag: photosynthesis in higher plants

• The photolysis of water (H$ _2$O) in the light reactions of photosynthesis occurs in the water-splitting complex of photosystem II.
• photosystem II  in thylakoid membranes of chloroplasts where light is used to split water molecules as represented by the following equation: 2H$ _2$O → 4H+ + 4e- + O$ _2$.
• Oxygen (O$ _2$) is liberated to the atmosphere as a waste product in this process and the electrons (e-) are gained to replace those lost through photoexcitation from chlorophyll a in the reaction center of photosystem II which begins noncyclic photophosphorylation. Hence four electrons produced during photolysis of water will enter PS  II.
• So, the correct answer is 'PS  II'.

Photolysis is part of the light-dependent reactions of photosynthesis. The general reaction of photosynthetic photolysis can be given as 

H2AH2A + 2 photons (light) → 2 e−e− + 2 H+H+ + A

So, the correct option is 'Option D'.

A) Mg is responsible for the activation of many enzymes, but is not involved in photolysis. Cl ions help in the photolysis of water.

Non-cyclic phosphorylation involves both Photosystem I and Photosystem II. These two photosystems work in series, first PS II and then PS I. The two photosystems are connected through an electron transport chain. $NADPH$ is synthesized by this kind of electron flow. Electrons are transferred from the PS II and then passed on to the pigments of PS I. Then, electrons get excited and transferred to another acceptor. These electrons are then moved downhill again to a molecule of $NADP^{+}$. The addition of these electrons results in the formation of $NADPH$. Extrusion of the electron from reaction centre of PS II leaves a hole which is filled by electron released from $H _{2}O$. This is achieved by electrons available due to the splitting of water. The water splitting complex is associated with PS II, which itself is physically located on the inner side of the membrane of the thylakoid. Water is split into $H^{+}$, $O$ and electrons. The protons and oxygen formed by splitting of water is released within the lumen of the thylakoids. The oxygen produced is released as one of the net products of photosynthesis.

A) $ _{ \quad  }^{ 16 }{ O }$ is not used to study photolysis.
B) $ _{ \quad  }^{ 15 }{ O }$ is not used to study photolysis.

A) Mn and Ca are involved in the photolysis of water.
B) Mn, Cl and Ca are involved in the photolysis of water.

A) Absorption of light is carried out by photosynthetic pigments.

During ATP synthesis in the chloroplast, the proton gradient is formed. There are several processes that take place during activation of electrons and their transport which lead to the development of a proton gradient. Photolysis of water is the first process of this activation. The splitting of the water molecule takes place on the inner side of the thylakoid membrane and so the hydrogen ions (protons) that are produced, they accumulate within the lumen of the thylakoids. Oxygen-evolving complex called Z-enzyme is attached to the inner surface of the thylakoid membrane. The enzyme has four Mn ions. Light energized  Mn and removes electrons from $OH^{-}$ component of water forming oxygen. So, the essential element required for water splitting in photosynthesis leading to release of oxygen is Mn.

In oxygenic photosynthesis, the first electron donor is water, creating oxygen as a waste product. In anoxygenic photosynthesis, various electron donors are used. Cytochrome b6f and ATP synthase work together to create ATP. This process is called photophosphorylation.