The modern cell theory states the following:

  1. All organisms are made up of cells.
  2. The cell is the basic unit of structure and function in living organisms.
  3. All cells come from other cells by cell reproduction.
  4. Cells contain hereditary information, which is passed from cell to cell during cell division.
  5. All cells are the same in chemical composition.
  6. All energy flow (metabolism and biochemistry) of life occurs within the cell.

In this section, you will look more closely at the third and fourth statements of the cell theory.

All living things are made of cells. It is estimated that there are 1 billion cells in every gram of tissue. That means there are 1 trillion cells in one kilogram of tissue.

If you weigh 45Kg (100 pounds), then you have approximately 45 trillion cells in your body!

The 45 trillion cells are continually dying and being replaced by new cells. In fact, in the time that you took to read this section of the lesson, you lost approximately 90 million cells! Fortunately, in that same amount of time about 90 million new cells were produced, replacing the cells that just died. Cells, just like all living organisms, have a life cycle. The length of a cell’s life varies depending on the type of cell. The chart below shows the average length of life for some of the cells of the human body.

Type of Cell
Average Length of Life
Liver cells 18 months
Red blood cells 4 months
Epidermal cells of skin 1 month
White blood cells 13 days
Stomach lining cells 2 days

All organisms pass through different stages in life. Similarly, cells pass through different stages in their life cycle. The life cycle of a cell is known as the cell cycle.

There are two main phases of the cell cycle: interphase and the mitotic phase or M phase. The cell spends most of its time in interphase. Some cells, such as nerve and muscle cells no longer divide and are always in interphase. The phase is divided into two steps: mitosis and cytokinesis.

Interactive exercise. Assistance may be required. To learn more about the cell cycle, click on the image map below.

In summary, the cell cycle produces two daughter cells that are identical to the original cell. Each new nucleus has the same number of chromosomes. The chromosomes are made of DNA and contain hereditary information. Every cell in your body, except sex cells, has a nucleus containing 46 chromosomes. Each specific type of cell goes through the cell cycle to make more of its own type of cell. For example, skin cells produce new skin cells.

Cell divisions allow for growth and repairs damaged cells in organisms. You have grown since you were born because of cell division. Every wound you have had was healed because of cell division.