Cells form tissues, tissues form organs, and organs form organ systems which together make up an organism. Groups of similar cells perform shared functions as tissues, organs are made of multiple tissues working together for a specific function, and organ systems are groups of related organs that work together to perform body functions.

1. Levels of Organisation (from Year 9)
CELLS are the smallest structural and
functional unit of every organism.
Groups of cells with similar structures,
performing a shared function form a TISSUE.
An ORGAN is a structure made up of a group
of tissues, working together to perform a
specific function.
Several organs with related functions working
together to perform a body function form an
ORGAN SYSTEM.
Groups of organs systems together make up
an ORGANISM.