Levels of Organization in Biology
What we study in Biology can be grouped into categories. Going from the very small to the very large
we start by talking about the cell and what it contains. All living things are made of cells. Cells contain
smaller parts that perform functions needed by the cell to sustain life such as synthesis (making) of
molecules. These small parts within a cell are called organelles. Plants have organelles called
chloroplasts that help them make food using energy from the sun. Animals have organelles called
centrioles that help the cell divide when it needs to make more cells.
Cells are the smallest functional unit of life. Cells may be simple like in a prokaryote (bacteria) and have
no organelles. Other cells called eukaryotes are more complex like a plant cell or animal cell.
If we put different cells together we form tissue. Plants have vascular tissue that carries food and water.
Animals have muscle tissue, nervous tissue, connective and epithelial tissue.
If we group together tissue we form organs. Organs in a plant include roots, stems and leaves. Organs
in an animal include your heart or your brain.
Groups of organs form organ systems. In a plant there is a vascular system that can move food from the
leaves to wherever it needs to go in the plant. In animals there are organ systems such as your digestive
system or your nervous system.
If you put all the organ systems together you get an individual organism. An example of a plant that is a
living organism is an oak tree, an example of an animal that is a living organism is a giraffe.
Groups of organisms of one species that live in the same area are called populations. For instance you
could have a group of pine trees or a herd of elephants.
If we put together different populations that live together in an area we get a community. This will have
more than one species. A community could be made of plants such as grass, shrubs and trees or it can
be made of animals such as bison, hawks and prairie dogs all living in an area together.
An ecosystem is all the members of the community (biotic) plus abiotic factors in the area. It takes all
the plant and animal and other organisms in the community plus air and water or what the ground is
like.
A biome is all the living (biotic) and non living (abiotic) factors in an area plus the climate of that area.
A biosphere is the crust of the Earth where life exists plus the atmosphere surrounding us.