The Human Cell / The Human Cell	Navigate This Article Introduction Cell Structure Cell Structure - Part 2 Cell Function The Origins of Cells The Discovery & Study of Cells
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Cells fall into one of two categories: prokaryotic or eukaryotic.

In a prokaryotic cell, found only in bacteria and archaebacteria, all the components, including the DNA, mingle freely in the cell's interior, a single compartment. Eukaryotic cells, which make up plants, animals, fungi, and all other life forms, contain numerous compartments, or organelles, within each cell. The DNA in eukaryotic cells is enclosed in a special organelle called the nucleus, which serves as the cell's command center and information library. The term prokaryote comes from Greek words that mean “before nucleus” or “prenucleus,” while eukaryote means “true nucleus.”

A. Prokaryotic Cells

Prokaryotic cells are among the tiniest of all cells, ranging in size from 0.0001 to 0.003 mm (0.000004 to 0.0001 in) in diameter. About a hundred typical prokaryotic cells lined up in a row would match the thickness of a book page. These cells, which can be rodlike, spherical, or spiral in shape, are surrounded by a protective cell wall. Like most cells, prokaryotic cells live in a watery environment, whether it is soil moisture, a pond, or the fluid surrounding cells in the human body. Tiny pores in the cell wall enable water and the substances dissolved in it, such as oxygen, to flow into the cell; these pores also allow wastes to flow out.

Pushed up against the inner surface of the prokaryotic cell wall is a thin membrane called the plasma membrane. The plasma membrane, composed of two layers of flexible lipid molecules and interspersed with durable proteins, is both supple and strong. Unlike the cell wall, whose open pores allow the unregulated traffic of materials in and out of the cell, the plasma membrane is selectively permeable, meaning it allows only certain substances to pass through. Thus, the plasma membrane actively separates the cell's contents from its surrounding fluids.

While small molecules such as water, oxygen, and carbon dioxide diffuse freely across the plasma membrane, the passage of many larger molecules, including amino acids (the building blocks of proteins) and sugars, is carefully regulated. Specialized transport proteins accomplish this task. The transport proteins span the plasma membrane, forming an intricate system of pumps and channels through which traffic is conducted. Some substances swirling in the fluid around the cell can enter it only if they bind to and are escorted in by specific transport proteins. In this way, the cell fine-tunes its internal environment.

The plasma membrane encloses the cytoplasm, the semifluid that fills the cell. Composed of about 65 percent water, the cytoplasm is packed with up to a billion molecules per cell, a rich storehouse that includes enzymes and dissolved nutrients, such as sugars and amino acids. The water provides a favorable environment for the thousands of biochemical reactions that take place in the cell.

Within the cytoplasm of all prokaryotes is deoxyribonucleic acid (DNA), a complex molecule in the form of a double helix, a shape similar to a spiral staircase. The DNA is about 1,000 times the length of the cell, and to fit inside, it repeatedly twists and folds to form a compact structure called a chromosome. The chromosome in prokaryotes is circular, and is located in a region of the cell called the nucleoid. Often, smaller chromosomes called plasmids are located in the cytoplasm. The DNA is divided into units called genes, just like a long train is divided into separate cars. Depending on the species, the DNA contains several hundred or even thousands of genes. Typically, one gene contains coded instructions for building all or part of a single protein. Enzymes, which are specialized proteins, determine virtually all the biochemical reactions that support and sustain the cell.

Also immersed in the cytoplasm are the only organelles in prokaryotic cells—tiny bead-like structures called ribosomes. These are the cell's protein factories. Following the instructions encoded in the DNA, ribosomes churn out proteins by the hundreds every minute, providing needed enzymes, the replacements for worn-out transport proteins, or other proteins required by the cell.

While relatively simple in construction, prokaryotic cells display extremely complex activity. They have a greater range of biochemical reactions than those found in their larger relatives, the eukaryotic cells. The extraordinary biochemical diversity of prokaryotic cells is manifested in the wide-ranging lifestyles of the archaebacteria and the bacteria, whose habitats include polar ice, deserts, and hydrothermal vents—deep regions of the ocean under great pressure where hot water geysers erupt from cracks in the ocean floor.

B. Eukaryotic Animal Cells

Eukaryotic cells are typically about ten times larger than prokaryotic cells. In animal cells, the plasma membrane, rather than a cell wall, forms the cell's outer boundary. With a design similar to the plasma membrane of prokaryotic cells, it separates the cell from its surroundings and regulates the traffic across the membrane.

We have to start somewhere. Let's start on the outside. Around every cell is a CELL MEMBRANE. The membrane is like a big plastic bag with tiny holes in it. Scientists also call the cell membrane a PLASMA MEMBRANE.

What's it For?

The purpose of the cell membrane is to hold the cell together. It keeps all of the pieces, like the organelles and the CYTOPLASM, inside. The membrane also controls what goes in and out of the cell. It acts like a crossing guard and says "You better stop right there buddy. You aren't getting in here."

Cell Membrane Structure

Scientists have a theory about the way a cell membrane works. The theory is called the FLUID MOSAIC MODEL. The idea says that there are two layers of MOLECULES, a BILAYER. These two layers are made up of molecules called phospholipids. Take a look, it's like a sandwich with two pieces of bread and some alfalfa on the inside.

Each phospholipid has an HYDROPHOBIC and HYDROPHILIC end. They are big words, but they mean very simple things. HYDRO means water. PHOBIC means afraid. PHILIC means loving. So one end of the molecule is afraid of the water, and one end loves being in the water. Millions of these molecules line up together to form a cell membrane.

Cell Membrane Proteins

Throughout the membrane are proteins stuck inside the membrane. These proteins cross the bilayer and make the holes that let ions and molecules in and out of the cell. (That crossing guard thing again.)

When ions move through the cell membrane, it is called FACILITATED DIFFUSION. Facilitated means helped. Diffusion means moving from one area to another. So facilitated diffusion is a procedure where an ion is helped across the membrane. (Like helping an old lady across the street.)

The eukaryotic cell cytoplasm is similar to that of the prokaryote cell except for one major difference: Eukaryotic cells house a nucleus and numerous other membrane-enclosed organelles. Like separate rooms of a house, these organelles enable specialized functions to be carried out efficiently. The building of proteins and lipids, for example, takes place in separate organelles where specialized enzymes geared for each job are located.

The nucleus is the largest organelle in an animal cell. It contains numerous strands of DNA, the length of each strand being many times the diameter of the cell. Unlike the circular prokaryotic DNA, long sections of eukaryotic DNA pack into the nucleus by wrapping around proteins. As a cell begins to divide, each DNA strand folds over onto itself several times, forming a rod-shaped chromosome.

The nucleus is surrounded by a double-layered membrane that protects the DNA from potentially damaging chemical reactions that occur in the cytoplasm. Messages pass between the cytoplasm and the nucleus through nuclear pores, which are holes in the membrane of the nucleus. In each nuclear pore, molecular signals flash back and forth as often as ten times per second. For example, a signal to activate a specific gene comes in to the nucleus and instructions for production of the necessary protein go out to the cytoplasm.

The Nuclear Envelope

Around the nucleus is another membrane (different from the cell membrane). The nuclear membrane holds the nucleus together. Scientists call the membrane the NUCLEAR ENVELOPE. Just like in the other membranes and the cell wall, this one has tiny holes. Pieces of protein and RNA pass through these holes.

Chromatin

When the cell is just sitting around, there is something called CHROMATIN in the nucleus. We just talked about DNA. Well chromatin is made up of DNA, RNA and proteins. When the cell is going to divide, the chromatin becomes very compact, it condenses. When it comes together, you can see the things that scientists call chromosomes.
 

Nucleolus

If you see a picture of a nucleus, you might see a small dark area inside the nucleus, almost like a tiny nucleus inside the nucleus. This dark area is called the NUCLEOLUS. The nucleolus is made up of protein and RNA with very little DNA.

Looking for DNA

James Watson, American, biologist, scientist, Nobel prize winner. He worked with a guy named Crick and Wilkins and was one of the first scientists to come up with the structure of a DNA molecule. They used something called x-ray diffraction to see the shape of the molecule.

They took an X-ray of the DNA and when they looked at it, they saw it was in the shape of a DOUBLE-HELIX. That's like having a ladder and then twisting it. With their discovery, scientists were then able to figure out how DNA replicates in the cell.

Here is Francis Crick. He worked with Watson and Wilkins on figuring out the structure of DNA. He was born in England and then worked at Cambridge University. For his work, he won a Nobel Prize in 1962.

The big deal about coming up with the structure was that scientists could figure out how DNA duplicates. Crick helped discover that DNA uses something called codons (sets of three nucleic acids) when it duplicates and when it helps make proteins. Did you know that for every amino acid there is a specific codon that binds to it and carries it to a ribosome to make a protein?