The Basic Concepts

Certain key needs must be satisfied in order for organisms to survive. Once you get beyond the single celled animal, cells must organize in some way to function as a single organism. The most primitive way of doing this is to form a colony in which all cells do the same thing; they just live together. Corals are a good example of this.

In higher animals, however, cells become specialized (scientists call it "differentiated") into certain type of tissue. Examples include skin, muscle, bone, blood, and so on. Then tissues are formed during embryonic development into certain organs. Examples include stomach, liver, brain, lungs, and so on.

Organ systems of a dog, as seen when body is sliced in half

Key processes:

• Gas exchange: needed to provide the oxygen needed for energy production in all cells and to remove the carbon dioxide that all cells produce as a waste product of energy production.
  
• Nutrients: needed to provide chemical sources for energy and for building and maintaining the parts of a cell.
  
• Defense: against undue loss of water and against foreign invaders, such as viruses and bacteria
  
• Detoxication: neutralize or destroy toxins that get into the body or are normal products of cellular chemical reactions
  
• Coordination and Control: needed for consciousness, learning and memory, movement, emotions, and to detect environmental changes.