Photoautotrophs (Greek for "self-feeding" organisms) can use the sun's energy to build complex organic molecules from carbon dioxide and water. The overall chemical reaction can be written as:

6 H2O + 6 CO2 -> C6H12O6 + 6 O2

This reaction is only superficially correct and the underlying mechanisms are extremely complex. The many reactions involved in photosynthesis can be divided into two separate processes; the light reaction and the Calvin cycle. In this simulation, we will focus on the light reaction — the conversion of sunlight into chemical energy in the form of ATP and the reducing agent NADPH.


In eukaryotic organisms, photosynthesis takes place in the chloroplast. These cell organelles consist of three membranes. The innermost membrane forms stack-like structures called thylakoids. More specifically, photosynthesis takes place in the thylakoid membrane.

Diagram of a chloroplast. An outer boundary membrane encases an inner boundary membrane, forming the exterior structure of the chloroplast. The space inside the inner boundary membrane is labelled as the stroma. Within the stroma, disk like structures are stacked together, these are labelled as thylakoids. The thylakoids structure is formed by the thylakoid membrane and the space inside the membrane is labelled as the thylakoid lumen.

Figure 1: Structure of a chloroplast.

There is an interesting story to the two surrounding membranes: Chloroplasts used to be free-living photosynthetic cells/bacteria that were ingested by early eukaryotic cells. Instead of being digested, they turned out to be useful in providing energy for the cell. The two symbiotic cells became dependent on each other, and the photosynthetic one became what we now call an organelle. The two membranes are remnants from the vacuole of the eukaryote (outer membrane) and the plasma membrane of the photosynthetic cell (inner membrane).