There are smaller pieces that make up cells such as macromolecules and organelles. A protein is an example of a macromolecule while a mitochondrion is an example of an organelle. Cells can also connect to form larger structures. They might group together to form the tissues of the stomach and eventually the entire digestive system. However, in the same way that atoms are the basic unit when you study matter, cells are the basic unit for biology and organisms.

In larger organisms, the main purpose of a cell is to organize. Cells hold a variety of pieces and each cell type has a different purpose. By dividing responsibilities among different groups of cells, it is easier for an organism to survive and grow.

If you were only made of one cell, you would be very limited. You don't find single cells that are as large as a cow. Cells have problems functioning when they get too big. Also, if you were only one cell you couldn't have a nervous system, no muscles for movement, and using the internet would be out of the question. The trillions of cells in your body make your way of life possible.

Plant cells are easier to identify because they have a protective structure called a cell wall made of cellulose. Plants have the wall; animals do not. Plants also have organelles such as the green chloroplast or large, water-filled vacuoles. Chloroplasts are the key structure in the process of photosynthesis.

Humans have hundreds of different cell types. You have red blood cells that are used to carry oxygen (O2) through the body and other cells specific to your heart muscle. Even though cells can be very different, they are basically compartments surrounded by some type of membrane.

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Biology4Kids.com: Cell Structure - BIOLOGY 4 KIDS.COM