Nearly all food ultimately comes from the sun, and living organisms fall into one of two categories, based on specifically how they obtain that nutrition.
What Are Autotrophs?
Plants, and some forms of bacteria, are considered self-feeders. They are able to synthesize their food from scratch, through a process called photosynthesis. The raw materials that photosynthetic autotrophs use to create their own organic molecules are:
- carbon dioxide (the stuff we breathe out)
- and inorganic elements obtained from the environment
In order for autotrophs to obtain energy from sunlight, the light energy must first be absorbed by chemical pigments that are located within their cells. The sunlight energy is then ultimately converted into food energy in the form of a sugar called glucose. Glucose can then be broken down (metabolized), to release energy needed to run the cell.
Because they are able to make their own food, autotrophs are considered producers. These organisms directly or indirectly produce the food for other living things that can’t photosynthesize.
What Are Heterotrophs?
Whether you know it or not, you are a heterotroph; an organism that must obtain its energy from the organic molecules (proteins, carbohydrates and fats) made by autotrophs.
If a heterotroph is a herbivore (plant eater), it obtains the energy directly from the plant that originally captured that energy. Carnivores, (meat eaters) and generalists like omnivores (organisms which eat both plants and meat), still obtain their energy from captured sunlight, but that energy has already been transformed from the light energy originally captured by the plant, into food energy within the organism that ate the pant.
Example of Energy Transformation: Sun > Autotroph > Heterotroph
When people head to the grocery store, unless they are vegetarians, they typically purchase both plant and animals products as food. The lettuce and other vegetables that you purchase for your salad is food energy captured from sunlight while those vegetables were growing and photosynthesizing. The cow, which provided the beef, milk and other dairy products at the store, ate grains and grasses (plants), and turned that energy that was stored in the plants it ate into muscle and milk. So when you eat the beef or dairy product, it is kind of like getting second-hand sunlight energy.
More Information on Biological Energy Flow
To learn more about biology and the flow of energy through living systems, see the following websites:
FT Exploring: The Flow of Energy Through Plants and Animals
Campbell, N. & Reece, J. (2004) Biology Seventh Edition. Pearson Benjamin Cummings.