Let's first dissect the light reactions. As I said, the goal of the light reactions is to convert solar energy into usable chemical energy. So the beginning of these reactions has to be the capture of the light. This is done by the pigment chlorophyll, which is found in chloroplast, which is actually how the chloroplast gets its name. Within the chloroplast, the chlorophyll pigment molecules are found embedded within the thylakoid membranes. Here, they are found connected to large clusters of proteins. Chlorophyll has to be associated with the membranes, because as you remember, chlorophyll is a hydrophobic molecule. That is it can't be dissolved in water. The role of chlorophyll in these protein complexes is to directly absorb the light energy. Incidentally, because chlorophyll absorbs light from photosynthesis, it is a green pigment, and chlorophyll is what gives thylakoids and plants their green color. Now, let's look for a second at how light interacts with the pigment to give a particular color. What we refer to as light is actually a small part of the electromagnetic spectrum. This is the spectrum of energy waves, which span very short waves, such as gamma or x-rays, and very long waves such as radio waves. The actual size of an x-ray, for example, is about one nanometer. And a radio wave is huge, it can be over a kilometer in size. We actually can see a very small part of the spectrum, what we call visible light. And that's somewhere between 400 nanometers, which we see as blue, and 650 nanometers, which we see as red. So I hope you realized that the progression from shorter to longer visible wave lengths gives us the order of colors that we see in a rainbow. With the shorter blue at the bottom, and the light waves getting longer from green to yellow, to red at the top of the rainbow. When sunlight, which contains wavelengths of all the colors hit the surface, some of the waves are absorbed and others are reflected. For example, we see a ripe tomato is red, because it absorbs all wavelengths, except the red light. And these red waves are bounced off the fruit to our eye, and then we see it or define it as red. Similarly, when sunlight hits a leaf, the chlorophyll absorbs all the waves, except for the green and yellow ones, which are reflected back to our eyes and seen as green. Just to emphasize this point, chlorophyll absorbs primarily red and blue light, while green and some yellow are reflected back to our eyes. In other words, it's the blue and red that are most important for photosynthesis, not the green. Though, this is actually what we see. This is clearly shown in this graph, which shows what's called an absorption spectrum for different types of chlorophyll. The bottom of this graph progresses from blue light at 400 nanometers all the way though to red light. The vertical axis shows how much light is absorbed by chlorophyll for each wavelength. We can see here that the two types of chlorophyll measured here, chlorophyll A and chlorophyll B, each absorb lights in the blue parts and the red parts of the spectrum. But they don't absorb the green light, here it's flat. While these two type of chlorophyll are slightly different, for this class, they are essentially identical.