Phases of photosynthesis: description of the light and dark phases

Phases of photosynthesis: description of the light and dark phases

Photosynthesis is an example of solar energy that occurs naturally. It is the process by which plants, algae and certain bacteria convert sunlight into chemical energy, thus creating the base of the food chain and maintaining the balance of oxygen and carbon dioxide in the atmosphere.

This chemical process is divided into two interconnected phases: the light phase and the dark phase. In this article, we will explain these two critical phases in detail.

Luminous phase: Capturing solar energy

The light phase is the first stage of photosynthesis and takes place in the membranes of the thylakoids, which are disc-shaped structures within the chloroplasts of plant cells.

This phase gets its name from sunlight, which is essential for its functioning. 

Here the main stages of the light phase are broken down:

Sunlight Capture

At this stage, chlorophyll molecules and other photosynthetic pigments present in the thylakoids absorb the energy of photons of sunlight. Light is split into its different wavelengths, and this absorbed energy is converted into chemical energy.

Chlorophyll and other photosynthetic pigments such as carotene absorb light energy and use it to break water molecules, releasing oxygen as waste.

ATP generation

Light energy is used to pump protons across the thylakoid membrane into the thylakoid space, thus creating a proton gradient.

This gradient is used to drive the synthesis of adenosine triphosphate (ATP), a high-energy molecule that is essential for many cellular activities.

NADPH production

In addition to ATP, the light phase also generates another crucial molecule called NADPH (reduced nicotinamide adenine dinucleotide phosphate).

NADPH is an electron carrier that will be used in the dark phase to convert carbon dioxide to glucose.

Dark phase: the synthesis of glucose

The dark phase, despite its name, does not depend directly on sunlight, but uses the energy products of the light phase (ATP and NADPH) to carry out the synthesis of glucose and other organic compounds.

This phase takes place in the stroma of the chloroplasts and is made up of a series of complex chemical reactions:

CO2 fixation

In this stage, carbon dioxide (CO2) molecules are attached to a five-carbon compound called ribulose-1,5-bisphosphate (RuBP) by an enzyme called ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). .

This results in the formation of three-carbon molecules known as 3-phosphoglycerate (3-PGA).

3-PGA reduction

The 3-PGA molecules are reduced using the energy and electrons provided by the NADPH generated in the light phase. This results in the formation of glyceraldehyde-3-phosphate (G3P) molecules, which are precursors to glucose and other organic molecules.

RuBP regeneration

To maintain the dark phase cycle, some of the G3P molecules are used to regenerate RuBP. This process consumes ATP generated during the light phase.

Glucose production and energy storage

Finally, some of the G3P molecules are used for the synthesis of glucose and other carbohydrates. This glucose is stored in the plant as a source of energy and as a building material for its growth and development.


Light phase:

  • It occurs in the thylakoids of chloroplasts.

  • It captures sunlight through photosynthetic pigments such as chlorophyll.

  • It generates chemical energy in the form of ATP and NADPH.

  • It drives ATP synthesis via a proton gradient.

Dark phase:

  • It occurs in the stroma of chloroplasts.

  • It uses ATP and NADPH generated in the light phase.

  • Converts carbon dioxide (CO2) into glucose and other organic compounds.

  • It includes CO2 fixation , 3-PGA reduction and RuBP regeneration.

Together, these two phases of photosynthesis allow plants, algae, and some bacteria to capture solar energy and convert it into stored chemical energy in the form of glucose, while releasing oxygen into the environment.

This process is essential for life on Earth by supporting the food chain, regulating the carbon cycle, and providing oxygen.

Publication Date: October 6, 2020
Last Revision: August 28, 2023