Tag: photosystems

Photosynthesis is a redox reaction in which carbon dioxide is reduced to sugars and water is oxidised to release oxygen.

Photosynthesis is chemically a redox reaction in which carbon dioxide is reduced and water is oxidised. During light reaction phase of photosynthesis, the electron that has been donated to the electron transport chain is replaced through photolysis (light dependent splitting) of water, a process that not only yields electrons but also is the source of almost all the oxygen in Earths atmosphere. 

Photolysis, the chemical process by which molecules are broken down into smaller units through the absorption of light.

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.