Light reactions

Light reactions are the first stage of photosynthesis, the process by which plants, some bacteria, and some protists convert light energy into chemical energy. The light reactions take place in the thylakoid membrane of the chloroplast and convert solar energy to the chemical energy of ATP and NADPH, evolving oxygen in the process.

Overview

The light reactions are named so because they require light to function. During these reactions, light energy is converted into chemical energy, in the form of the energy carriers adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). The light reactions also produce oxygen as a byproduct from the splitting of water, a process known as photolysis.

Process

The light reactions occur in two stages: the photosystem II (PS II) and photosystem I (PS I). In PS II, light energy is absorbed by chlorophyll and other pigments, exciting their electrons. These high-energy electrons are then passed along an electron transport chain, releasing energy which is used to pump hydrogen ions across the thylakoid membrane, creating a concentration gradient. This gradient drives the synthesis of ATP via chemiosmosis.

In PS I, light energy is again absorbed, exciting electrons which are then used to reduce NADP+ to NADPH. This process also involves an electron transport chain and results in the pumping of hydrogen ions across the membrane.

Significance

The light reactions are crucial for the survival of plants and other photosynthetic organisms, as they provide the energy necessary for the Calvin cycle, the second stage of photosynthesis. The ATP and NADPH produced in the light reactions are used in the Calvin cycle to fix carbon dioxide into organic molecules, a process essential for life on Earth.