Whether plant or animal, life begins with the cell. Single-celled animals, such as the amoeba, have all they need to survive contained within one flexible cell membrane. More complex plants and animals have several different types of cells organized into groups to perform certain functions.
Plant and animal cells are remarkably similar. Both have a nucleus that contains the nucleolus, the keeper of a cell's genetic material. Ribosomes are found in the cytoplasm and create proteins to nourish the cell. The Golgi apparatus packages the protein for distribution within the cell, stored in vacuoles, and to the main organism. Some proteins go to the mitochondria for the generation of ATP, the fuel for the cell.
Plant cells also have cell walls, located outside the cell membrane and made of cellulose. These act as exoskeletons for roots, stems and leaves and provide a conduit for water to flow within the plant. Chloroplasts contain chlorophyll, the green substance plants use to turn water and carbon dioxide into sugar and oxygen. The sugar is used for food and the oxygen is released into the atmosphere.
The amoeba is an example of life in miniature. It has all the elements of an animal cell as well as pseudopods that allow it to move and find prey. When it's time to reproduce, the nucleus divides the amoeba into two.