In complex multicellular organisms, eukaryotic cells become specialised for specific functions. Specialised cells are organised into tissues, tissues into organs and organs into systems.
In embryonic stem cells, at the very early embryo stage of development, all cells are identical.This is because cells are produced from mitotic division from the fertilised egg and they contain the exact same genes.
Ever cell contains the genes needed for it to develop into any one of the many different cells in an organism however, only some of the genes are expressed (switched on) in any one cell. For each type of specialised cell, different genes are switched on and the rest are switched off.
The expressed gene does not only change the shape of the cell that it varies but also the organelles. Some cells such as epithelial cells have more mitochondria where as bone cells have very few.
Cells of a multicellular organism is adapted for its specialised function which makes it perform effectively therefore the whole organism functions effectively.
These cells are grouped together for working efficiency to form tissues an example are epithelial tissues to exchange gas in alveoli and xylem which is used to transport water and mineral in plants.
An organ is a combination of different tissues that are grouped together to perform a variety of specific function for example the stomach which consists of muscle tissues to churn up the food as it contracts and relaxes and epithelium to protect the stomach wall and produce secretion.
A leaf is also an organ which is made up of palisade mesophyll which carry out most of the photosynthesis; spongy mesophyll for gaseous diffusion; epidermis to protect the leaf and allow gaseous diffusion; xylem to transport water and ions; and phloem to transport organic material.
Several organs may work together as an organ system to make the body more efficient. For example the circulatory system consists of the heart, arteries and veins.