An atom describes the smallest particle of matter that can exist and corresponds to chemical elements. Molecules represent at least two atoms bonded together, and can describe even up to large proteins.
The cell is the smallest unit of a living organism, describing a whole host of molecules working together to make a functioning machine. Cells are organised into cells with similar functions called tissues. These tissues can form complex machines called organs, such as the heart. Organs that work together collectively are called systems, such as the circulatory system or the nervous system.
All these systems work together to form an organism (i.e. one creature, such as a human). Organisms in the same species that occupy the same land and use the same resources are called populations. A number of populations of different species interact together to form an ecosystem.
