Photosynthesis: Definition & Reactions

Instructor: Elizabeth Basa
In this lesson we'll cover the definition of photosynthesis, the equation for photosynthesis, as well as the reactants and products. Using diagrams of photosynthesis, we'll go over the formula and process.

What Is Photosynthesis?

Plants have a power we humans can only dream of - making food from sunlight. Sure, we can cook some savory dishes, but we use food already made, mostly by plants. This process of assembling carbohydrates fueled by the power of sunlight is called photosynthesis.

Keep in mind that the sun emits all sorts of different kinds of energy, such as heat and gamma rays, but photosynthesis specifically captures light energy. In addition, photosynthesis generates basic sugar molecules called glucose, which can be used to build more complex carbohydrates.

The Process of Photosynthesis

The prefix 'photo' means light, and 'synthesis' means to bring together. In other words, photosynthesis is the bringing together of molecules using light energy. Just what does the process of photosynthesis bring together to make sugar molecules? Two things: carbon dioxide and water.

How this process works can be understood from two overarching perspectives: matter and energy.

  • In terms of matter, photosynthesis takes smaller, simpler materials and assembles them into larger, more complex ones.
  • In terms of energy, photosynthesis transforms light energy from the sun into a form living things can access: the stored chemical energy in food molecules.

Technically, one round of photosynthesis requires six molecules of carbon dioxide and six molecules of water. From this, one molecule of glucose is produced along with six molecules of oxygen as a by-product.

  • carbon dioxide + water + sunlight -> oxygen and glucose, or more specifically:
  • 6CO2 + 6H2 O + sunlight -> C6 H12 O6 + 6O2

The formula for photosynthesis includes water, sunlight and carbon dioxide as reactants, and glucose and oxygen as products
Photosynthesis diagram showing the formula for reactants and products

Where Does Photosynthesis Occur?

The first part of the process of photosynthesis is the light dependent reactions. As mentioned, sunlight is the energy source for this process. In terms of cellular structures, plants and other organisms capable of carrying out photosynthesis must have certain specialized components.

One component that photosynthetic organisms require is a pigment molecule, which is a special-colored substance that acts like an antenna capable of absorbing light energy. Pigment molecules channel the light energy into the right spot where the cell is carrying out photosynthesis. The most common pigment is chlorophyll, which is responsible for the widespread green color we all associate with plants.

Plants contain their chlorophyll and pigments inside a type of organelle called a chloroplast, and it is within this structure that photosynthesis is carried out. Prokaryotic organisms that lack organelles simply carry out the food-making process in the cytoplasm, but still require pigment molecules to do so.

Chloroplasts are organelles inside plant cells where photosynthesis is carried out
the chlorophyll in chloroplasts is where photosynthesis occurs

Perhaps you have seen plants with red or yellow leaves, and not just in fall? These plants have other types of pigments in them instead of or in addition to chlorophyll.

The Products of Photosynthesis

One round of photosynthesis produces a single sugar molecule. The most common sugar made is glucose, which has the chemical formula C6 H12 O6. Like all molecules, glucose contains chemical potential energy.

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