Dear Katrina,

Cells are important for everything that all living things do. When we run and play, our movements are caused by muscle cells and nerve cells working together. When we speak, get an idea or decide what to say to somebody, brain cells are working. When we digest our food, cells in our mouth, stomach, and intestines are doing their jobs. Our beating heart, breathing, eyesight, hearing, feeling and even sleeping all require cells. All plants and animals are made of cells, and different kinds of cells perform special types of work. Cells in the leaves of green plants use sunshine, water and carbon dioxide in the air as food to help the plants grow. Cells also produce fish scales and bird feathers. Yeast cells make bread rise before it is baked, and bacterial cells can make us sick, help clean up oil spills and turn milk into yogurt.

Cells do many more things than the largest factory on earth, yet most of them are too small to see without a microscope. There are about 100 trillion cells in your body. This is 200 times greater than the number of stars in the Milky Way galaxy. If every cell in your body were the size of a small drop of water you would be 20 miles tall, and your waist would be as big around as your house. As it is, most cells are so small that several hundred could sit on the head of a pin without lying on top of each other. A few unusual cells are very large though. For example, the yellow part of a chicken’s egg is one cell, and some nerve cells in the giant squid are several yards long. In the human body, there are 252 different kinds of cells. Each of these performs particular jobs in particular organs of our body. For example, our blood contains more than one dozen different kinds of cells. Some blood cells carry oxygen throughout the body, some form blot clots when we get cut, and others protect us from infections and diseases.

Scientists who study how cells work are called cell biologists. Here are some of the things they have learned. Particular jobs inside a cell are done by tiny cell parts called organelles which means “little organs.” One organelle called the mitochondrion uses energy in the food we eat to make chemical fuel needed to keep the cell alive and working well. One cell contains hundreds of mitochondria. Another organelle called a ribosome manufactures thousands of different proteins. Proteins are the chemical workhorses for the cell. They make our muscles contract, allow us to think and form communication networks between cells. One cell contains thousands of ribosomes. Organelles called microtubules are like tiny soda straws inside the cell. Like bones in our body, they form a mini-skeleton that gives a cell its shape. The largest organelle is the nucleus. In humans the nucleus contains two copies each of about 30,000 genes, one copy from our mother and the other from our father. Genes carry chemical information that makes a tree a tree and a human a human. Genes determine the color of our eyes and our hair. They may also help give us our personalities, but just as important for this are our experiences at home, in school, with friends and while reading books.

One final thing about cells that is important to remember is that every person in the world began life as one single cell that grew and divided many times to make the trillions of cells in our bodies. In this way and in many other ways, we are all alike.

Thanks for your question,
Aubie and Dr. Bradley