Light-dependent reaction

Light-dependent reactions (also known as light reactions) are the first stage of photosynthesis, the process by which plants convert energy from the sun into chemical energy in the form of glucose. These reactions occur in the thylakoid membranes of the chloroplasts in plants. The light-dependent reactions are so named because they require sunlight to occur.

Overview

During the light-dependent reactions, energy from sunlight is absorbed by chlorophyll and other photosynthetic pigments. This energy is used to convert water (H2O) into molecular oxygen (O2), and to produce adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH), which are energy-rich compounds.

Process

The process of light-dependent reactions can be divided into two systems: Photosystem II and Photosystem I.

Photosystem II

In Photosystem II, the energy from sunlight is used to extract electrons from water. This process releases oxygen as a byproduct. The electrons are then passed along a series of proteins known as the electron transport chain.

Photosystem I

In Photosystem I, the electrons are re-energized by sunlight and then used to produce NADPH. ATP is also produced during this process through a method known as chemiosmosis.

Significance

The light-dependent reactions are crucial for the survival of plants, as they provide the energy necessary for the production of glucose during the second stage of photosynthesis, the light-independent reactions or the Calvin cycle. They also produce oxygen, which is released into the atmosphere and is essential for the respiration of most living organisms.

See also

• Photosynthesis
• Chloroplast
• Thylakoid
• Calvin cycle