Photosynthesis Lecture Part I

The story of photosynthesis can be summarized briefly in three sections which should be memorized by anyone wishing to master this topic. Those sections are:

Below is a slightly more comprehensive treatment of:

The Light Reactions

In photosynthesis the light reactions begin with the absorption of electromagnetic radiation in the blue and red wavelengths. The light is absorbed by one of the numerous light harvesting complexes found in the thylakoid membranes.

The light energy causes a molecule of chlorophyll to be oxidized by exciting one of its electrons (in particular one of the outer electrons of a magnesium atom). Each chlorophyll a molecule is part of an antenna or light harvesting complex embedded in the thylakoid membrane of a chloroplast. After chlorophyll is oxidized by light, its excited electron becomes part of an electron transport chain composed of cytochromes, proton pumps, and mobile electron carriers.


For a bit more information about chloroplasts visit John Brown's site "What the Heck is a Chloroplast."

Water is split in order to resupply the electron lost by magnesium in the chlorophyll molecule. Oxygen is released as a byproduct and the hydrogen ions are carried by NADPH to the Dark Reactions.

Review the structure and function of chloroplasts. Site 1, Site 2, Site 3

As the electron is transferred from molecule to molecule its energy may be used to:

  • pump protons into the lumen of the thylakoid sac. The resulting free energy may be used to produce ATP
  • be transferred to the next molecule in the chain
  • or reduce NADP+ to NADPH -- the final electron acceptor

The final products of the light reactions are oxygen gas, ATP, and NADPH.


Some additional important details


Two photosystems are involved in the light reactions:

When coupled these systems absorb light and can both reduce NADPH and build a proton gradient in order to make ATP in what is called non-cyclic photosynthesis. The electrons lost by chlorophyll in PS-680 must be replaced by electrons removed from water. Protein X, which is part of Photosystem II, is responsible for the photolysis of water by taking electrons from none other than oxygen (the oxidation of water).

In cyclic photosynthesis light is absorbed by Photosystem I and the excited electron eventually returns back to its origin. This process can only activate proton pumps to maintain the proton gradient and contribute to the formation of ATP by chemiosmotic phosphorylation.

Photosynthesis Lab

Links to other Lectures about Photosynthesis:

Modified Nov 14, 2002