Photosystem: Definition & Overview

Photosystem: Definition & Overview
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  • 0:05 Plants & Photosynthesis
  • 1:01 Phtosystems
  • 1:52 Reaction Centers &…
  • 4:33 Lesson Summary
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Lesson Transcript
Instructor: Joanne Abramson

Joanne has taught middle school and high school science for more than ten years and has a master's degree in education.

Learn how plants and photosynthesis are a requirement for animal life on our planet. Discover the role of photosystems in this process, and see how plants use light and water to make oxygen and food.

Plants & Photosynthesis

Plants are a beautiful and essential element of our earth's ecosystem. Without them, we and other animals would not be able to exist. The reason for this is photosynthesis. Photosynthesis, the process of capturing light energy and storing it in carbohydrates, creates two of the requirements for animal life: food and oxygen. The photosystems are the protein structures in plant chloroplasts that absorb light energy.

Photosynthesis Diagram

Take a deep breath of that wonderful plant-produced oxygen. Now brace yourself: the reactions of photosynthesis are notorious for being complicated and confusing. To help make the process a little bit easier, we'll be relying heavily on this diagram. We'll be focusing on the numbered steps. Take just a second to look at it before we move on.


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.

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