The light and dark reactions of Photosynthesis.

Peter Miles
5 min readDec 5, 2020

Including Photosystems, I and II.

Photosynthesis is a two-stage process, the light reactions using photons of light energy to drive molecules to a higher energy state and dark reactions which use the energy molecules to fix carbon dioxide into 6 carbon sugars.

The dark reaction doesn’t have to be in the dark and is also called the light independent reaction.

These reactions are sometimes referred to as photosynthesis 1 (light) and photosynthesis 2 (dark), not to be confused with photosystems I and II, see below.

Photosynthesis is an autotrophic process fundamental to all life on earth, converting solar energy into chemical energy and sugars or carbohydrates.

Diagrammatic representation of chloroplasts. Image — Creative Commons.

Photosynthesis occurs in specialized organelles within the cell called chloroplasts. Chloroplasts contain chlorophyll pigments which are responsible for the green colour of leaves. Light is captured and used to convert carbon dioxide from the atmosphere into sugars for use by the plant.

In photosynthesis the carbon dioxide CO2 is split into carbon C and oxygen 02. This occurs in the presence of light by one CO2 molecule combining with two water molecules H2O. Oxygen atoms in the water are released, along with one H2O and carbohydrate, generally triose (3 carbon) sugars are produced.

The abundance of life depends on this chemical reaction:

3 CO2 + 6 H20 + light -> C3H6O3 + 3 O2 + 3 H20

Carbon dioxide + water + light ->sugar + oxygen + water

It is also often written thus:

6CO2 + 12H20 + light -> C6H12O6 + 6O2 + 6H2O

This reaction occurs in three stages:

The first is absorption of light photons by the chlorophyll pigment molecules, with the energy of the light transferred to the photosystem reaction centres.

In the second stage electrons are released by splitting water, producing protons and oxygen. Electrons are then transported (only the energy is transported not the electron) on the electron transport chain in the thylakoid membrane of the chloroplast. This produces the high energy compounds NADPH from NADP+ and ATP from ADP ready for the third stage.

Peter Miles

45 years in Environmental Science, B.Env.Sc. in Wildlife & Conservation Biology. Writes on Animals, Plants, Soil & Climate Change.