Light Reaction

We are all aware that the process of photosynthesis requires sunlight. But did you know chloroplast only absorb the blue and red light wavelengths from the sunlight? That is correct. Let us learn about Light Reaction and how it functions.

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Photosynthesis

Photosynthesis is the process by which autotrophic plants make their own food. Co2, water, chlorophyll, and sunlight are four important requirements for this process. Photosynthesis occurs in two steps: Light reaction and Dark Reaction.

• Light Reaction – It is a light dependent reaction
• Dark Reaction– It is a light-independent reaction. Learn about Dark Reaction here in more detail.

Let’s us learn more about the light reaction of photosynthesis.

Light Reaction

The light reaction of light dependent reaction occurs in the chloroplast of the mesophyll cells of the leaves. The chloroplasts are double-membraned cell organelles that are comprised of stacked disc-like structures known as thylakoids. The pigment, chlorophyll, which is required for the process is present on the membrane of these thylakoids and this is where the light reaction occurs.

Browse more Topics under Photosynthesis In Higher Plants

• Dark Reaction and Photorespiration
• Factors Affecting Photosynthesis
• Introduction To Photosynthesis

The Steps Involved in the Light Reaction

The main purpose of the light reaction is to generate organic energy molecules such as ATP and NADPH which are needed for the subsequent dark reaction.

• Chlorophyll absorbs the red and blue segment of the white light and photosynthesis occurs most efficiently at these wavelengths.
• When the light falls on the plant, the chlorophyll pigment absorbs this light and electrons in it gets excited.
• This process occurs in a complex protein system which is collectively called as a photosystem. There are two closely linked photosystems known as PSI and PSII.
• The chlorophyll pigments which are excited give up their electrons and to compensate for the loss of electrons, water is split to release four H+ ions and four electrons and O2. The electrons that are lost from the PSII enter into an electron transfer chain or ETC.
• The electrons finally reach the reaction centre where they combine with NADP+ and reduce it to NADPH
• While the electrons are taken care of, the built up of H+ ions inside the thylakoid lumen is of equal importance.
• The hydrogen ions building up inside the lumen creates a positive gradient and in the presence of the enzyme ATP synthetase, these H+ ions combine with the ADP in the nearby region to form ATP.
• The oxygen that is a waste product is released by the plant into the atmosphere and some of it is used in photorespiration if the plant needs to.

To summarise the light reaction, we can write it down as the following reaction:

2H₂O + 2NADP+ + 3ADP + 3Pi → O₂ + 2NADPH + 3ATP

For any plant performing photosynthesis, four factors influence this process. CO2, water, light, and chlorophyll are the raw materials for photosynthesis. But, in case of light dependent reaction or light reaction of photosynthesis, it is most influenced by presence or absence of light. The other three factors do not play a critical role in it.

Solved Example for You

Q: Which colour light are chlorophylls most sensitive to?

Sol: Blue and Red are the colour which chlorophylls are most sensitive to. Leaves are green and so they reflect the green wavelength of white light. The chlorophyll pigment most efficiently absorbs those wavelengths which lie in the red and blue light regions of white light.