The Light-Dependent and Light-Independent Reactions

The light reactions, or the light-dependent reactions, are up first. We call them either and both names. The whole process looks a little like this:

Do not freak out or fill your head with all the complicated names in that diagram. No—stop right there. All in all, the process is simpler than it looks. In the light-dependent reactions of photosynthesis, the energy from light propels the electrons from a photosystem into a high-energy state. In plants, there are two photosystems, aptly named Photosystem I and Photosystem II, located in the thylakoid membrane of the chloroplast. The thylakoid membrane absorbs photon energy of different wavelengths of light.

Here again is our friend the chloroplast. All exposed the way he is, he kind of reminds us of a boat with green checkers in it:

Image source

Even though the two photosystems absorb different wavelengths of light, they work similarly. Each photosystem is made of many different pigments. Some of these pigments can be described as absorption pigments, and others are considered action pigments.

The absorption pigments transfer the energy from sunlight to another pigment; at each transfer, the absorption pigments pass the photon energy to another pigment that absorbs a similar or lower wavelength of light. Remember when we said that light is funky and acts like it has both particles and waves? A photon is what we call the particle-like aspect of light. In other words, a photon is the basic unit of light.

Anyway, eventually, the energy makes it to the reaction center, or action pigment. At this point, the photosystem loses a highly charged electron to adjacent oxidizing agents, or electron acceptors, in the electron transport chain. This transfer all occurs mind-bogglingly quickly at an estimated time of 200 × 10-12 seconds!

Weed killers called herbicides work by targeting enzymes used in the light reactions of photosynthesis. Come here, little chloroplasts.