Cells provide structure and function for all living things, from microorganisms to humans. Scientists consider them the smallest form of life. Cells house the biological machinery that makes the proteins, chemicals, and signals responsible for everything that happens inside our bodies.
Cells come in different shapes—round, flat, long, star-like, cubed, and even shapeless. Most cells are colorless and see-through. The size of a cell also varies. Some of the smallest are one-celled bacteria, which are too small to see with the naked eye, at 1-millionth of a meter (micrometer) across. Plants have some of the largest cells, 10–100 micrometers across. The human cell with the biggest diameter is the egg. It is about the same diameter as a hair strand (80 micrometers).
The trillions of cells that make up a human are organized into about 200 major types. All of a person’s cells contain the same set of genes (see more on genes). However, each cell type “switches on” a different pattern of genes, and this determines which proteins the cell produces. The unique set of proteins in different cell types allows them to perform specialized tasks. For instance, red blood cells carry oxygen throughout the body. White blood cells kill germ invaders. Intestinal cells release molecules that help digest food. Nerve cells send chemical and electrical messages that produce thoughts and movement. And heart cells contract in unison to pump blood.
When putting cells into categories, scientists can tell eukaryotic cells apart from prokaryotic cells because they look different. Eukaryotic cells make up animals, plants, fungi, and some single-celled organisms. And they have a number of structures inside them, called organelles. The most prominent organelle is the nucleus, which contains the cell’s genetic material, or DNA (see more on DNA). Prokaryotic cells don’t have a nucleus or other organelles. They are single-celled microorganisms that tend to be smaller than eukaryotic cells. There are two types of prokaryotic cells—bacteria and archaea.
In addition to the nucleus, the most prominent organelles include the following:
Many types of cells can move. Single-celled organisms move to find food. And even cells inside multicellular organisms may need to get around. For example, immune system cells must move toward invaders. And sperm needs to “swim” to fertilize eggs.
Cells move in several ways. Some simply float through water or other liquids. Some push themselves along using long, thin proteins, called flagella, and cilia that stick outside the cell membrane and wave around. Some “crawl” along, using what’s called amoeboid movements, in which cytoplasm-filled protrusions scoot the cell forward.
Within cells, nutrients and organelles move around to carry out various cellular functions. This kind of internal movement is called cyclosis, or cytoplasmic streaming. The internal structure of cells, which is called the cytoplasm, creates a directional flow that pushes the contents of the cells around.
Scientists study cell movement to better understand how cells work, including how cancer cells move from one tissue to another and how white blood cells move to heal wounds and attack invaders.
Cell biologists rely on an array of tools to peer into the body and examine cells. Imaging techniques magnify organelles and track cells as they divide, grow, interact, and carry out other vital tasks. Biochemical or genetic tests allow researchers to study how cells respond to environmental stressors, such as rising temperatures or toxins. These tests can also label specific proteins using fluorescent tags and other chemicals that allow scientists to visualize proteins at work inside cells. Sophisticated computational tools then integrate and analyze all the data.
One cell divides into two in a process called mitosis. Mitosis produces two genetically identical “daughter” cells from a single parent cell. Another type of cell division, meiosis, creates four daughter cells that are genetically distinct from one another and from the original parent cell. Only a few special cells can perform meiosis: those that will become eggs in females and sperm in males.
Cells come equipped with what they need to self-destruct. This is called programmed cell death, or apoptosis. And it serves a healthy and protective role in our bodies. For example, it helps shape our fingers and toes before birth, and it kills off diseased cells during our lives. Another kind of cell death, called necrosis, is unplanned and not protective. Necrosis can happen after a sudden traumatic injury, infection, or exposure to a toxic chemical.
Stem cells can renew themselves millions of times. Other cells in the body, such as muscle and nerve cells, cannot do this. Embryonic stem cells are undifferentiated, meaning they can turn into any type of cell in the body. Tissue-specific stem cells (sometimes called adult or somatic stem cells) arise later in development. They also can replenish cells. The primary role of tissue-specific stem cells is to maintain and repair the tissue in which they’re found.
Changes to the genes inside a cell, called mutations, can alter the cell’s ability to divide, make proteins, remove waste, or perform other tasks. These genetic mutations can lead to birth defects, cancer, and other diseases. Cells that are damaged through physical trauma or infection can, in extreme cases, contribute to harmful inflammation and organ malfunction.
Learning about how cells work—and what happens when they don’t work properly—teaches us about the biological processes that keep us healthy. It also uncovers new ways to treat disease. Cellular research has already led to cancer treatments, antibiotics, medicine that lowers cholesterol, and improved methods for delivering drugs. However, much more remains to be discovered. For example, understanding how stem cells and certain other cells regenerate could offer insight on how to repair damaged or lost tissue.
NIGMS is a part of the National Institutes of Health that supports basic research to increase our understanding of biological processes and lay the foundation for advances in disease diagnosis, treatment, and prevention. For more information on the Institute’s research and training programs, visit
Content revised August 2017
This page last reviewed on
4/15/2019 3:10 PM
Connect With Us: