Photosynthesis Intro and Light-Dependent Reactions

In photosynthesis, light is used to make organic compounds, especially sugars, in plants. This is extremely important for virtually all life on earth because these organic compounds are what we, and our food are all made of! So, how does this process work?

In short, when light strikes a leaf, this light is absorbed and the light energy is used to drive a series of chemical reactions that ultimately make sugars and other organic molecules. Overall carbon dioxide and water are taken in by the plant and used to make glucose and oxygen gas. Photosynthesis can be divided into two sets of reactions: 1.

The light-dependent reactions and 2. The Calvin Cycle The light-dependent reactions, as you might have guessed, depend on light. To understand how these work, let’s zoom in on a leaf so we can see them happen.

Leaves are made of plant cells, and inside of these cells are special organelles called chloroplasts that do photosynthesis. Each chloroplast has many disks called thylakoids with pigments (like chlorophyll) that absorb light. Think of thylakoids as solar panels in the chloroplast.

They absorb the light energy from the sun. Now let’s zoom in on the thylakoid and focus on the thylakoid membrane. This is where pigments such as chlorophyll are found and is where light gets absorbed.

The pigments are arranged in clusters called photosystems. Let’s start with photosystem II. When Photosystem II absorbs light, electrons in chlorophyll gain energy (or get excited) from the light.

These excited electrons leave chlorophyll and move to something called an electron transport chain. But, since chlorophyll lost electrons, those electrons need to be replaced. To replace them, water is split and its electrons go to chlorophyll.

When water is split, electrons, hydrogen ions, and oxygen are produced. You probably know that plants make oxygen when they do photosynthesis. This is where that oxygen comes from!

Now, back to the electron transport chain. The electron transport chain transports electrons. As it does, the energy in the electrons is used to pump hydrogen ions (or protons) across the thylakoid membrane, into the thylakoid.

This creates a high concentration of hydrogen ions inside the thylakoid. These ions would be trapped inside the thylakoid, but a special enzyme called ATP synthase allows them to passively diffuse from high to low concentration. This flow of hydrogen ions through ATP synthase causes ATP synthase to spin and produce ATP, similar to how water flowing through a turbine produces power at a hydroelectric dam.

This ATP is a key product of the light-dependent reactions. When the electrons reach the end of this first electron transport chain, they go to photosystem I where light excites them once again. They travel down a second, shorter electron transport chain where they are accepted by a molecule called NADP+.

When it accepts the electrons, it also accepts hydrogen and becomes NADPH. NADPH is an electron carrier and is another key product of the light-dependent reactions. It carries electrons and hydrogens to the next set of reactions in photosynthesis, the Calvin Cycle.

Both ATP and NADPH are critical products of the light-dependent reactions that are needed to make sugar in the Calvin cycle, which we will examine in our next video. If you’d like to try the light dependent reactions yourself, check out the link for the Photosynthesis Interactive at BioMan Biology and don’t forget to subscribe if you’d like to be notified when I release new videos!