Here's why:
* Basic unit of life: Cells are the smallest unit of life that can carry out all the functions necessary for life, such as metabolism, reproduction, and response to stimuli.
* Unicellular organisms: Some organisms, like bacteria and amoebas, are single-celled, meaning they are made up of only one cell.
* Multicellular organisms: More complex organisms, like humans, plants, and animals, are made up of many cells working together.
There are a few exceptions to this rule:
* Viruses: Viruses are not considered living organisms because they cannot reproduce on their own and require a host cell to survive. They are not made of cells but rather consist of a protein coat surrounding genetic material.
* Prions: Prions are misfolded proteins that can cause disease. They are not considered living organisms because they lack genetic material and do not have a cellular structure.
However, the vast majority of living things, from the smallest bacteria to the largest trees, are all composed of cells.
