Photosystems
Okay. Got the diagram in your mind? Let's start from the beginning. A photosystem is a protein complex, a group of two or more proteins, that is essential for the photochemistry of photosynthesis. In particular, it carries out the absorption of light photons and the transfer of electrons (e- in our diagram). There are two photosystems, Photosystem I and Photosystem II. Photosystem II is first in the process of the light-dependent reactions of photosynthesis, but it has a 'II' after its name because it was discovered second. The photosystems are found in the thylakoid membrane of the chloroplast in plants and algae and in the cell membrane of photosynthetic bacteria.
Reaction Centers & Electron Transfer
The reaction center is a complex of protein and pigments (such as chlorophyll) that is the functional center of the photosystem. The light reactions of photosynthesis begin when a photon, or small bundle of light, of the correct wavelength hits the pigment in Photosystem II. The photon excites an electron, raising it to a higher energy level. This is a very unstable state for the electron; its natural tendency is to drop back down to its previous energy level. The chloroplast takes advantage of this, passing the electron from molecule to molecule down the electron transport chain (Number 2 on the diagram). Each step in the electron transport chain releases a little bit of the electron's energy until it is back to its happy, stable state. We will see why the chloroplast does this in just a moment.
In order for the light reactions to continue, the electron that moved on to the electron transport chain needs to be replaced, otherwise the pigment in the reaction center would eventually run out of electrons. The electron is reclaimed by breaking apart a water molecule, releasing the oxygen that we breathe as a byproduct.
A similar process occurs in Photosystem I. A pigment in the reaction center absorbs a photon, which excites an electron. In this case, however, the electron is not sent down an electron transport chain, but rather it is transferred to the electron acceptor NADP, creating NADPH. The NADPH is used later in the light-independent reactions of photosynthesis to create carbohydrates such as glucose. This is the food that we eat.
Let us briefly return to the electron transport chain and discuss its function in the chloroplast.
Even though the reactions in Photosystem II and Photosystem I occur simultaneously, Photosystem II is said to occur first in the process. This is because the electron from Photosystem II, the one that moved down the electron transport chain, is the electron that refills Photosystem I. As before, the electron that is excited in Photosystem I needs to be replaced, or the system will eventually run out of electrons and come to a stop. There is a second function for the electron transport chain. We mentioned that when the electron is transferred from molecule to molecule, it releases its energy a little bit at each step. This energy is used to create ATP. Similar to NADPH, ATP will be used later to create glucose.
Lesson Summary
Plants are essential for the survival of humans and other animals on our planet. This is because, through the process of photosynthesis, they create two of the products essential for animal life: food and oxygen. The photosystems are the functional centers of photosynthesis.
The pigments, such as chlorophyll, in the photosystems absorb light energy, which excites electrons. These electrons are transferred to electron-accepting molecules, eventually producing ATP and NADPH. ATP and NADPH are used in the light-independent reactions of photosynthesis to create carbohydrates, which are a source of energy for both plants and animals. The electrons that are passed on need to be replaced so the process can continue. This is done by breaking a water molecule, producing the oxygen that we breathe as a byproduct.
