Basics of Biology: Photosynthesis

The first set of reaction will take place in the thylakoid, the thin, stacked penny-like membranes in the chloroplast. These are shown in green in the figure below, and they are black on the picture above. The goal of the light-dependent reactions is to make NADPH and ATP, which are energy rich compounds that will power the light-indemendent reactions. The first step of photosynthesis is to break the water down into individual atoms, using the energy of the sun captured in Photosystem II. The oxygen is not let in the thylakoid, but the hydrogen ions do get in, and are very important later in the process. The electrons captured in PSII travel along the Electron Transport Chain to Photosystem I. As they travel they pump hydrogen ions out side of the thylakoid into the thylakoid. When the electrons get to PSI, they are re-energised so they can bind hydrogen ions outside of the thylakoid, to NADP+, creating NADPH. NADPH is an election carrier molecule that will provide some of the energy for the light-independent reactions. The hydrogen ions that the ETC pumps into the cell, make up the hydrogen ion gradient, which powers the ATP Synthase protein located in the thylakoid membrane. Click here to watch the ATP Synthase protein in action. Now the thylakoid has created all the necessary ingredients for the light-independent reactions, or Calvin Cycle to occur.

The second set of reactions takes place outside of the thylakoid, but inside the chloroplast. This set of reactions is called the Calvin Cycle. The Calvin Cycle takes the NADPH and ATP of the light-dependent reactions and combines them with CO^2 let into the plant through stoma, or pores on the underside of a leaf. The Calvin Cycle is a series of reactions that converts ATP, NADPH and CO^2, into two 3-carbon sugars, that then combine to form the six-carbon sugar, glucose. The result of the Calvin Cycle is that chemical energy in the form of glucose is produced, ATP and NADPH are converted to ADP and NADP+, which go back into the thylakoid to be converted back into ATP and NADPH.