From a metabolic point of view, life begins at the cellular level. This level is made up of the smallest unit of living matter, the cell. Individual cells may have some common functions but vary widely in size and shape.
Living creatures have various levels of organization that are extremely complex. Starting at the lowest level of organization, subatomic particles, such as protons, neutrons and electrons, make up atoms that can combine with other atoms to form a variety of molecules.
Atoms turn into molecules that get together and form cellular structures, including organelles. Many different organelles work together inside a cell. The cell is the most basic functional unit of life.
In a multicellular organism, a group of cells that all share the same tasks is called a tissue, and one or more tissues form an organ. Organs are organized into organ systems, and a whole organism has many organ systems working together.
An ecosystem includes all the living organisms in a certain area, in addition to the nonliving parts of their environment, which usually includes many different communities. Finally, the highest level of organization is the biosphere, encompassing all living things on Earth.
A single-celled individual lacks tissues, organs and organ systems but still has the same levels of organization below the cellular level: protons, neutrons, electrons, atoms and molecules.