Photolysis and the Light Reactions: Definitions, Steps, Reactants & Products

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  • 0:05 Photosynthesis
  • 1:09 Photosystems
  • 3:21 Photolysis
  • 4:00 Photosystem I
  • 4:37 Lesson Summary
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Lesson Transcript
Instructor: Kristin Klucevsek

Kristin has taught college Biology courses and has her doctorate in Biology.

In this lesson, we'll learn how electrons get excited during the light-dependent reactions of photosynthesis, jumping off photosystem reaction centers like they were trampolines! In addition, we'll learn how the splitting of water creates reactants for this part of photosynthesis while creating a little fresh air for us.


When plants start to get a little hungry, they need to only turn their leaves to the sun to start photosynthesis, the process that turns energy from sunlight into food. In this lesson, we'll start to break down how this process takes place.

Photosynthesis consists of two parts. The first part of this process consists of the light reactions, while the second is called the dark reactions. This might seem like two sides of something like Star Wars' good versus evil, but we promise that everything is good when it comes to photosynthesis. It's just that the light-dependent reactions use sunlight, and the dark reactions don't use sunlight.

Before we begin on how the light reactions work, let's remind ourselves of the products and reactants for photosynthesis. In this process, remember that light energy, carbon dioxide, and water are used to produce glucose, or sugar, and oxygen. In this lesson, we'll learn how two of these reactants are used. Specifically, we'll learn how light energy is used to split water, a process known as photolysis.


Inside a plant cell, inside a chloroplast, and within a thylakoid membrane, there are photosystems, which are the sites of the light-dependent photosynthesis reactions. There are two types of photosystems: photosystem I and photosystem II. Photosystem II is actually used before photosystem I, but they are numbered for the order they were discovered.

Light energy is absorbed at the reaction center of photosystems
Reaction center

Photosystems contain an antenna complex. The antenna complex contains the pigment chlorophyll a, and it's what makes them green. Photosystems also have accessory pigments of other colors. Pigments, you'll remember, absorb specific wavelengths of light. In photosystem II, chlorophyll molecules are called P680 because they optimally absorb light at this wavelength, while in photosystem I, they are P700.

In these photosystems, pigments pass the energy absorbed from light to the reaction center - this is in the middle of the photosystem. The reaction center contains a special pair of chlorophyll molecules coupled with proteins. Here comes the exciting part, literally. When the light energy hits the reaction center, electrons from the chlorophyll molecules are excited to a higher energy level. Imagine light energy jumping on a trampoline along a photosystem and then boom, landing straight in the center, sending electrons up in the air, handed to a primary electron acceptor!

Now, what goes up must come back down. These energized electrons come back down in energy levels by moving through an electron transport chain. You might remember the electron transport chain from cellular respiration. The electron transport chain in photosynthesis is similar to this. Electron acceptor proteins reside within the thylakoid membrane. These proteins also become reduced as they accept electrons and oxidized as they pass electrons down this chain. The primary electron acceptor passes the electrons down to these membrane proteins as they decrease in energy level until they reach photosystem I. Meanwhile, the electron transport chain of photosynthesis, just like in cellular respiration, creates a proton concentration gradient that makes ATP, or chemical energy, as protons move through an ATP synthase.

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