We learned that cells have a cycle of growth and division

•Skin cells like this one grow divide and eventually die over a period of about two weeks.

•These are the cells replaced the most frequently in the human body

Can you guess what cells are never replaced?

Neurons! Our brain cells

There are factors that control cell growth

•External Factors operate from outside the cell•They can be physical or chemical.•Some chemical factors are called growth factors•External chemicals bind to the cell membrane•This causes an internal cellular response•Internal factors include enzymes and proteins that help a cell go through the cell cycle

Cells also die, this is called Apoptosis

Cell Division Is Uncontrolled in Cancer

Cells differentiate to do different things

Cells working together make tissues

Tissues working together make organs

Organs working together make organ systems

Organ systems working together make organisms