Tag: photosystems

In order to split water, four photochemical reactions are needed to take out four electrons and four protons from two water molecules. This leads to the formation of one oxygen molecule. Protons form water are released into the lumen (donor side of PSII) and electrons are moved to the plastoquinone (QB) binding site. So, the correct option is "A" (2).

The light reaction, convert energy from the sun into chemical energy in the form of NADPH and ATP. These reactions must take place in the light and in chloroplasts of plants. Chlorophyll, which is a light-absorbing organelle in plant cells, is a very complex molecule that works in conjunction with the metal magnesium. There are primarily two types of chlorophyll: chlorophyll a and chlorophyll b. These chlorophyll molecules absorb light in the red and blue wavelengths, making the plants in which they are stored look green. Thus, the correct answer is option C.

Robin Hill in the year 1939 showed that all chloroplasts extracted from leaves of Stellaria media and Lamium album are suspended in a test tube containing suitable electron acceptor, e.g., potassium ferrioxalate and potassium ferricyanide, oxygen is released due to the photochemical splitting of water. Under these conditions, no CO$ _2$ is consumed and no carbohydrate is produced.

The photolysis of water occurs during light reaction. The site for occurrence of photolysis is thylakoid lumen. The photolysis of water releases protons which accumulate in the lumen and contribute to the electrochemical gradient of protons. The electrons released are used up to fill the electron hole in the reaction centre of PSII and oxygen molecule is released as by product of photosynthesis. The electrons released during photolysis of water are ultimately used to synthesize the reducing power for reducing carbon dioxide later on. The mineral ions required for photolysis of water are calcium, manganese and chlorine. 

The photolysis of water occurs during light reaction. The site for occurrence of photolysis is thylakoid lumen. The photolysis of water releases protons, which accumulate in the lumen and contribute to the electrochemical gradient of protons. The electrons released are used up to fill the electron hole in the reaction centre of PSII and oxygen molecule is released as by product of photosynthesis. The electrons released during photolysis of water are ultimately used to synthesize the reducing power for reducing carbon dioxide later on. The mineral ions required for photolysis of water are calcium, manganese and chlorine. 

Experiments with isolates 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 notices evolution of oxygen from water. Hill concluded that if there was suitable hydrogen acceptor in the medium it would have been reduced. In absence of light Hill reaction can not be possible. So, it can say it occurs only in day time.

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.