Light Reaction In Photosynthesis (For Class 10 and 11)  By P K Verma

NCERT(Class 10) Says  this, Now to expend the Knowledge....

When light strikes chlorophyll (or an accessory pigment) within the chloroplast, it energizes electrons within that molecule.

These electrons jump up to higher energy levels; they have absorbed or captured, and now carry, that energy. High-energy electrons are “excited.” 

The excited electrons leave chlorophyll to participate in further reactions, leaving the chlorophyll “at a loss”; eventually they must be replaced. That replacement process also requires light, working with an enzyme complex to split water molecules.

 In this process of photolysis (“splitting by light”), H2O molecules are broken into hydrogen ions, electrons, and oxygen atoms. The electrons replace those originally lost from chlorophyll. Hydrogen ions and the high-energy electrons from chlorophyll will carry on the energy transformation after the light reactions are over.

The oxygen atoms, however, form oxygen gas, which is a waste product of photosynthesis.

To summarize, chloroplasts “capture” sunlight energy in two ways. Light ‘‘excites’’ electrons in pigment molecules, and light provides the energy to split water molecules, providing more electrons as well as hydrogen ions.

Light Energy to Chemical Energy

Excited electrons that have absorbed light energy are unstable. However, the highly organized electron carrier molecules embedded in chloroplast membranes order the flow of these electrons, directing them through electron transport chains (ETCs). 

At each transfer, small amounts of energy released by the electrons are captured and put to work or stored. Some is also lost as heat with each transfer, but overall the light reactions are extremely efficient at capturing light energy and transforming it into chemical energy.

(1) First, they pass down an ETC, which captures their energy and uses it to pump hydrogen ions by active transport into the thylakoids. These concentrated ions store potential energy by forming a chemiosmotic or electrochemical gradient – a higher concentration of both positive charge and hydrogen inside the thylakoid than outside. (The gradient formed by the H+ ions is known as a chemiosmotic gradient.) 

 Like water flowing through a hole in the dam, hydrogen ions “slide down” their concentration gradient through a membrane protein which acts as both ion channel and enzyme. As they flow, the ion channel/enzyme ATP synthase uses their energy to chemically bond a phosphate group to ADP, making ATP.

(2) Light re-energizes the electrons, and they travel down a second electron transport chain (ETC), eventually bonding hydrogen ions to NADP+ to form a more stable energy storage molecule, NADPH. NADPH is sometimes called “hot hydrogen,” and its energy and hydrogen atoms will be used to help build sugar in the second stage of photosynthesis.