Photosynthesis: Definition & Reactions

Instructor: Elizabeth Basa
Some people may falsely assume plants to be weaker than animals. However, plants and a few other organisms harness light energy from the sun to feed the rest of us. This lesson will introduce you to how they do it.

Making Sugar Molecules

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 molecules already made, mostly by plants. This process of assembling food molecules fueled by the power of sunlight is called photosynthesis. Keep in mind, 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. These single sugar molecules can be combined to make some (not all) other types of food molecules. In particular, the food molecules generated by photosynthesis provide energy to living things when eaten and broken down.

The Reactants of Photosynthesis

The prefix 'photo' means light, and 'synthesis' means to bring together. In other words, photosynthesis is the bringing together of things with light. Just what does 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 the light energy of 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 sugar is produced along with six molecules of oxygen as a by-product.

Molecule View of Photosynthesis

The Equipment

If given only eggs and cheese, you would have a hard time making an omelet without the pan, flame and spatula. The same goes for photosynthesis. Having carbon dioxide and water alone is not enough. As mentioned, sunlight is the energy source for this process. In terms of hardware, plants and other organisms capable of carrying out photosynthesis must have certain specialized components.

One in particular, to capture the light of the sun, a photosynthetic organism requires 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.

Green Leaves Containing Chlorophyll

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.

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.

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