By Robert Hazen, George Mason University
All living things are made up of one or more cells which control the chemical reactions essential to life. Cells, the chemical factories of life, were discovered by British physicist Robert Hooke, who lived from 1635 to 1702.
Prokaryotes
Biologists recognize two fundamentally different structural types of cells. The first are prokaryotes—meaning “before the nucleus”. These cells are small, simple objects, without any well-defined internal structure, such as a nucleus.
All prokaryotic cells are bacteria, and they all belong to the kingdom of monera, at least now; we may be splitting monera into two. There may also be archaea, but all archaea are also prokaryotes. They’re often extremophiles; they’re often organisms that live in an extremely high temperature, or at high pressure, and other extreme environments.
Eukaryotes
The second kind of cell is called eukaryotes, meaning “true nucleus”. These cells are generally larger. They have a nucleus and other discrete structures inside the cell which are bounded by their own membranes.
All plants, animals, fungi, and many kinds of protists and single-celled organisms are made of eukaryotic cells. Biologists now believe that these two kinds of cells represent different evolutionary pathways that diverged very early in the earth’s history.
Besides the prokaryotes and the eukaryotes, it’s possible to group cells in lots of other ways, as cells are remarkable, not only for their complexity but also for their ability to specialize—in higher animals and in plants.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Specialized Cells in Multicellular Organisms
Adult humans have tens of trillions, perhaps as many as 100 trillion cells, and there are several hundred different kinds of cells in our bodies. Each of these cells performs a specific kind of function, a chemical function or a physical function; each of these cells produces a different suite of enzymes that is characteristic of that particular function.
Under most circumstances, bacterial cells all look and function alike, but multicellular animals are different. They require these specialized structures. Indeed, that’s what it is to be a multicellular organism, which is to have specialized structures that allow you to function. You have a much larger volume, therefore you have to have internal structures and external structures.
Nerve Cells
We have nerve cells, or neurons. They act as wires in the body. They convey electrical impulses from one part of the body to another.
The human body contains more than 100 billion neurons, each of which has several important features. It has an array of fibers, called dendrites, which receive incoming information from a variety of points. Then, it has a cell body that processes the signal, and it has a long body called an axon. Here the nerve cell serves the function of a wire connecting two points in the body.
Muscle Cells
A second kind of cell forms muscles. They’re composed of many elongated cells that are basically what you call muscle fiber. As much as 90 percent of that muscle mass comes from two different proteins, called actin and myosin. These elongated proteins can slide over each other, which cause muscle contractions. That contraction occurs like a lever process.
When you plug in an ATP molecule, it causes a change in shape in the lever, and it causes two long molecules to slide over each other, which is the contraction that occurs when you contract your muscle.
Blood Cells
Blood features several different kinds of cells. You have red blood cells, which transport oxygen from the lungs to all the other cells in the body. These are the most simple, and they’re also the most abundant cells in the body. In fact, they’ve abandoned some of the internal structures that are common to eukaryotic cells.
For example, they don’t have a nucleus. They don’t have their own mitochondria. They then behave as little more than containers for oxygen that move around the body, but there are incredible numbers of these cells. A healthy human produces new red blood cells in their body at the staggering rate of 2.5 million cells per second.
Then we have white blood cells, a very different kind of blood cells that come in several different types. They combat infections; for example, they consume cellular debris if there’s leftover junk in the bloodstream. They also produce antibodies that attack foreign invaders.
Endocrine Cells
There are many different kinds of specialized cells that are part of the endocrine system. This regulates bodily functions through the action of chemicals called hormones.
For example, cells in the pancreas, the thyroid, and other glands manufacture specialized chemicals that travel throughout the bloodstream, and turn on and off other cellular functions, making other chemicals. So, we see a huge variety of cells in the human body.
Common Questions about the Different Types of Cells
Eukaryotes mean “true nucleus”. These cells are generally larger. They have a nucleus and other discrete structures inside the cell which are bounded by their own membranes. All plants, animals, fungi, and many kinds of protists and single-celled organisms are made of eukaryotic cells.
Red blood cells transport oxygen from the lungs to all the other cells in the body. These are the most simple, and they’re also the most abundant cells in the body. In fact, they’ve abandoned some of the internal structures that are common to eukaryotic cells; for example, they don’t have a nucleus.
Prokaryotes mean “before the nucleus”. These cells are small, simple objects, without any well-defined internal structure, such as a nucleus. All prokaryotic cells are bacteria, and they all belong to the kingdom of monera. They’re organisms that live in an extremely high temperature, or at high pressure, and other extreme environments.
