Snacking on sunlight
Written by: Heather Kropp and Angela Halasey
Illustrated by: Angela Halasey

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Carbon dioxide: a chemical compound (a gas) that has two oxygen atoms and one carbon atom (CO2); carbon dioxide is a greenhouse gas... more

Chlorophyll: the pigment that gives plants their green color and allows them to absorb sunlight... more

Chloroplast: a part of a cell found in plants that converts light energy into energy plants can use (sugar). Other living organisms such as algae also have cells that contain chloroplasts.



Plants Make Their Own Food

Venus fly trap

Some carnivorous plants seem to "eat" insects, but they only catch insects for nutrients, not energy. Click for more detail.

Do plants eat? Yes, but not like we do. Instead of going to the kitchen, the local store, or restaurant for a meal, they get food using a process called photosynthesis.

Take a quick look in a dictionary and you will see that “photosynthesis” is made from two words - “photo,” meaning light, and “synthesis,” which means to make something. Just knowing these root words gives a hint about the way plants get their food. They make it using light.  

Made From Light

Like animals, plants need energy to do work. Humans and other animals eat food to get energy. Your day might include cereal for breakfast, a sandwich for lunch and maybe a salad and fish for dinner. These foods and also many of the things we drink give us energy.

Basics of plant life

When plants make sugars, where do they get all the materials they need to grow? Click for more detail.


Plants use photosynthesis to make sugar. Just like eating sugar gives humans energy to work and play, sugar allows plants to grow and do other important work.

Snacking on Light

Like its name, photosynthesis can be split into two parts. The first part is the light-dependent reactions. In these reactions, plants convert sunlight energy into different forms of energy that are used in the second part of photosynthesis. In the second part, the Calvin cycle, carbon dioxide from the air and the energy from the light-dependent reactions are used to make a sugar called glucose (Greek gleukos = sweet wine). 

Where the Work is Done

Green plant

Why are most plants green? Click for more detail.

Inside plant cells there are little factories called chloroplasts. It is inside the chloroplasts where both parts of photosynthesis happen. These little factories also make a pigment that gives plants their green color. The pigment is called chlorophyll. Besides giving leaves and stems their green color, chlorophyll also helps plants soak up sunlight energy.

Do Plants Breathe?

Oxygen, the gas we breathe, is a product of the light-dependent reactions of photosynthesis. We breathe out carbon dioxide, which plants use in photosynthesis. Plants have tiny holes in their leaves called stomata. Stomata open to allow air to enter the leaf so that the plant can get carbon dioxide for photosynthesis.


This close-up view of the inch plant shows bright green stomata surrounded by dark red and green plant cells. Click for more detail.

The open stomata can allow a lot of water to leave the plant through evaporation, so stomata close when too much water exits. Plant cells also use oxygen when they break down sugars for energy in cellular respiration (Latin re = again, and spirare = to breathe). All cells respire, but only plants, algae, and some bacteria also have photosynthesis.

All of the carbon in our bodies came from photosynthesis. Just think about the foods you eat. They come from plants or from animals that eat plants. Our carbon comes from other living things, so we are called consumers or heterotrophs (Greek heteros = other, different, and trophos = feeder). Plants are producers or autotrophs (Greek auto = self, and trophos = feeder), which means that they get carbon from sources that are not living. Carbon dioxide from the air is the specific carbon source for plants, so you could say that plants are made out of “air”!

Images via Wikimedia Commons. Cyanobacteria image by BASF - The Chemical Company.

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Bibliographic details:

  • Article: Snacking on Sunlight
  • Author(s): Heather Kropp, Angela Halasey
  • Site name: ASU - Ask A Biologist
  • Date published: August 2, 2014
  • Date accessed:
  • Link:

APA Style

Heather Kropp, Angela Halasey. (2014, August 02). Snacking on Sunlight. ASU - Ask A Biologist. Retrieved from

American Psychological Association. For more info, see

Chicago Manual of Style

Heather Kropp, Angela Halasey. "Snacking on Sunlight". ASU - Ask A Biologist. 02 August, 2014.

MLA 2017 Style

Heather Kropp, Angela Halasey. "Snacking on Sunlight". ASU - Ask A Biologist. 02 Aug 2014. ASU - Ask A Biologist, Web. .

Modern Language Association, 7th Ed. For more info, see

Plants aren't the only photosynthetic organisms. Cyanobacteria also harness the sun's energy, as do most algae.

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