Unit C

Module 5 ~ Lesson 2

The Light Dependent Reactions

The electron transport chain that exists in the thylakoid membrane involves photosystem I and photosystem II. Photosystem II (PSII) absorbs all wavelengths of light allowed by its pigments. When light is absorbed, energy is transferred to a designated reaction-centre molecule. The reaction centre becomes "excited" and an electron leaves to catch the electron-transport chain.

PSII oxidizes water molecules to provide electrons to replace the electrons leaving the photosystem. Oxygen is produced at this step. This process also provides hydrogen ions for chemiosmosis. 

The electron from photosystem II travels down the electron-transport chain. Every encounter with an electron-carrying molecule releases energy. This energy pushes hydrogen ions (H+) from the stroma into the thylakoid space. A hydrogen ion concentration gradient is formed. This gradient will be used to produce ATP from ADP (adenosine diphosphate) and free phosphate groups.

The e- (electron) from photosystem II reaches photosystem I (PSI). Light has also been absorbed by PSI. The energy has been passed on to the reaction centre. A highly energized electron leaves photosystem I and is replaced by the e- from PSII at the end of its electron-transport chain. The highly energized electron from PSI moves down an electron-transport chain. The electron is used to reduce NADP+ to form NADPH. Both an electron and a hydrogen ion are required for this process.

The release of the electrons in the thylakoid membrane also causes a build-up of hydrogen ions. A process called chemiosmosis uses these excess hydrogen ions to produce ATP (see explanation below). 

Light-dependent reactions produce oxygen, which is released from chloroplasts to the atmosphere. Light-dependent reactions produce intermediate products of NADPH and ATP. These molecules are required in light-independent reactions for the production of glucose.


Watch the video explaining the light reactions. Pay careful attention to the redox reactions.


Chemiosmosis is the process by which ATP is generated through the movement of protons [H+] down a concentration gradient. It occurs in both the chloroplast and the mitochondrion.

During the electron transport chain reactions, hydrogen ions are building up in the thylakoid space. Once a hydrogen ion is in the thylakoid space, it cannot diffuse back to the stroma. However, ATP synthase embedded in the thylakoid membrane will move [H+] down the concentration gradient (high [H+] in the thylakoid space to low [H+] in the stroma). ATP is created using the energy from this concentration gradient by a process called chemiosmosis.


Read "The Light-Dependent Reactions of Photosynthesis" and "Mimicking Nature" on pages 170 to 176 of your textbook.