Tag: photosynthesis

Light reaction occur inside the thylakoids, especially those of grana region. It involves two types of reaction photosynthesis of water and production of assimilation power.  The phenomenon of breaking up of water into hydrogen and oxygen in the illuminated chloroplasts is called photolysis or photocatalytic , splitting of water, Water splitting complex is associated with the PSII, which itself is physically located on the inner surface of thylakoid  membrane. 
$4H _{2} O \rightleftharpoons 4H^{+} + 4OH^-$
$ 4OH^- \xrightarrow[Mn^{2+}, Ca^{2+}, Cl^-]{Oxygen \,\,evolving\,\, complex} 2H _{2} + O _{2}\uparrow + 4e^- $ , So, the correct answer is 'inner surface of thylakoid membrane.'.

Cycling flow of electrons during light reactions is known as cyclic photophosphorylation. It is performed by photosystem I independently. PS II is not involved in the process. In PS I, the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called $P _{700}$, while in PS II, reaction centre has an absorption maxima at 680 nm and is called $P _{680}$. As cyclic photophosphorylation requires only PS I, so it takes place only when light of wavelength beyond 700 nm is available for excitation. This process takes place in stroma lamellae membrane. An external source of electrons is not required. There is no evolution of oxygen takes place because it is not connected with photolysis of water. Cyclic photophosphorylation produces ATP only. It operates under low light intensity, anaerobic conditions or when $CO _{2}$ availability is low. When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs, due to cyclic flow of electrons. The membrane and lamella of the grana have both PS I and PS II, the stroma lamella membrane lack PS II as well as NADP reductase enzyme. The excited electron does not pass on to $NADP^{+}$ and is cycled back to the PS I complex through the electron transport chain.

Splitting up of water or photolysis is associated with Photosystem 2 and this photosystem 2 is present only in the membrane of thylakoids. 

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.