Why are cells called building blocks?

All living things are made up of cells, which are the basic units of life. The human body contains about 100 trillion cells, which live in our brain, bones, muscles, nerves, skin, and blood. Humans have about 200 different types of cells that come in different shapes and sizes, and each has a specific job to do in the human body. However, no matter what job they perform, all cells have a similar structure. Holding the cell together around its outside is the cell membrane. Openings in the membrane allow certain chemicals to travel in and out. The membrane is made up of a watery fluid called cytoplasm, which contains structures called organelles, specialized subunits that play specific roles in making the cell work. The nucleus is the cell’s control center, which sends the organelles chemical instructions and which also contains chromosomes, the packaging for our genetic material, or DNA (deoxyribonucleic acid).