Human Cell

A human cell is the smallest living unit that makes up your body. Every part of you, your skin, your bones, your blood, your brain, is built from cells working together. Most cells are far too small to see without a microscope. About 10,000 of them could fit on the head of a pin.

Scientists think the human body has about 37 trillion cells. That number is hard to picture. If you counted one cell every second, it would take more than a million years to count them all. Cells come in many shapes and sizes. Red blood cells are round and flat. Nerve cells, called neurons, have long branches that stretch across your body. Muscle cells are long and stringy so they can pull and stretch.

Even though cells look different, they share the same basic parts. The outside of a cell is a thin wall called the cell membrane. It decides what gets in and what stays out. Inside the cell is a jelly-like fluid called cytoplasm. Floating in the cytoplasm are tiny working parts called organelles, which means "little organs."

The most important organelle is the nucleus. The nucleus is the control center. It holds your DNA, the long instructions that tell each cell what to do. Other organelles do other jobs. Mitochondria turn food into energy and are sometimes called the cell's power plants. Ribosomes build proteins, the tools the cell uses to grow and repair itself.

Different cells specialize in different jobs. Red blood cells carry oxygen. White blood cells fight germs. Skin cells form a tough barrier. Bone cells build a strong frame. Neurons send fast signals so you can think and move. Cells working together form tissues. Tissues working together form organs like the heart and lungs. Organs working together form whole body systems.

Cells do not last forever. Skin cells live only a few weeks. Red blood cells live about four months. To replace them, cells split in half in a process called mitosis. One cell becomes two, two becomes four, and so on. Some cells, like most neurons in the brain, almost never split. That is one reason brain injuries are so hard to heal.

Every cell in you started from a single cell, the one made when you began. From that one cell, your whole body was built.