First 315 words of the document:
In multicellular organisms, cells are specialised to perform specific
functions. Similar cells are then grouped into, tissues into organs and
organs into systems for increased efficiency.
In multi cellular organisms the cells adapt to perform a particular role.
Initially all cells are identical. After some time, each cell adapts to suit its
function in order for the cell to change some genes are expressed.
This explains why cells differentiate when all cells in an organism have
the same genes. Because each cells is suited for a particular function
they are unable to perform other functions well, they therefore depend
upon other cells to perform these functions for them. These other cells
are adapted to this function and they perform it efficiently. The
advantage of cellular differentiation is that the whole multicellular
organism functions more efficiently than a single celled organism. This
allows for greater growth.
A collection of similar cells that perform a specific function is known as
a tissue. Tissues can consist of sheets of cells packed together or they
can be made up of number of different cells types.
Tissues are arranged into organs. An organ is a combination of tissues
that are coordinated to perform a variety of functions, although they
often have one main function. Arteries and veins are considered organs
as they consist of many tissues.
Organs work together as a single unit known as an organ system. Organ
systems allow for a particular function to be carried out more efficiently.
Examples of organ systems are the digestive system (consists of
salivary glands, oesophagus, stomach, duodenum, ileum pancreas and
liver), the reparatory system (consists of trachea bronchi and lungs) and
the circulatory system (consists of heart arteries and veins).