Growing and Changing

All humans cells started off the same but as you grow they develop to become specialized cells. Specialzed cells are cells which are specialized to a specific job and they are called tissues.

Plant cells are specialized too. Plant cells have walls and some have vacuoles.

As an organism grows groups of tissues arrange themselves into organs. Examples of organs include:

• Heart, brain, liver (in animals)
• roots, leaves, flowers (in plants)

All cells in a human body come from one cell which is the zygote (fertilized egg cell).

The zygote contains instructions for making all the organs in your body. These instructions are called DNA. DNA is the chemical that genes are made of.

As soon as the zygote is fertilized it starts growing. During the frst week of pregnacy it develops into a ball of 100 cells. In every one of these cells the nucleus contains the same DNA. As the embryo develops and grows some of the cells become specialized and then form tissues and after about two months most of the main organs have formed. The baby is now called a fetus. Adults have about 1000000000000000 cells all with the same DNA.

Identical twins are made when the embryo is a ball of cells. Sometimes this ball of cells will split into two and another embryo develops from this section. The cells in an early fetus are called embryonic stem cells because they have the ability to form new life.

For living things to grow bigger some of their cells must divide to make new cells. Flowering plants keep on growing all their lives. The parts of them that grow taller are their stems and roots.

Plants grow taller by making new cells at the tips of shoots. As well as this they have cells which increase their girth, these cells are called meristem cells.

Zygote - The cell made when a sperm cell fertilizes an egg cell in sexual reproduction.

Fetus - What a baby is called when it is about two months old and has all it's major organs.

Embryonic stem cells - Cells which have the ablity to form new life. Found in embryo's.

Meristem cells - unspecialied cells in plants which can develop into any kind of specilaized cell.