Organization of Cells in Organisms:

 

Cellsàtissuesà organsà organ systemsàorganism

 

A cell may exist alone (unicellular, single-celled), or it may be part of a larger organism made up of many cells (multi-cellular).

 

Unicellular organisms are able to carry on all the life processes.  They synthesize and obtain nutrients, break them down for energy, synthesize new materials, reproduce, and perform all the other activities that organisms do. 

Examples of unicellular organisms: bacteria, protozoa, many algae and some fungi.

 

Multicellular organisms can consist of hundreds to billions (or even trillions) of cells of many different types. In multicellular organisms, the cells are specialized.  They cannot function as independent, single celled organisms, each performing all life functions. Instead, in multicellular organisms, each cell carries out only some of its own life processes. Each type of cell depends on all the other types of cells.  In multicellular organisms, neighboring cells stick to one another and communicate through special patches called intercellular junctions.  In animals, junctions hold sheets of cells tightly together to form tissues.

 

Tissues:

A group of cells that are structurally similar and perform the same function form a tissue.  Each cell in a tissue carries on many life processes at the same time.

 

Organs:

A group of tissues that work together to perform a specific function forms an organ.

Examples: the stomach, the heart, the kidneys, the lungs, etc…

 

Organ systems:

A group of organs that work together to perform a specific function forms an organ system.

Examples: the digestive system, the circulatory system, the excretory system, the reproductive system,

 

All these parts: cells, tissues, organs and organ systems, must work together for the organism to carry on its life processes.