As you have already learned, everything that lives is made up of cells. And the cells themselves are made up of many different parts – right down to their molecules.
In fact, what scientists call “the universal principle of life” is defined as the specific interaction of molecules with one another. Now let’s find out a little more about cells and molecules. There are many different cells that do many different things. But all of these cells fall into one of the two main categories: prokaryotic cells and
eukaryotic cells.
These cells are more alike than they are different. So first let’s talk about what prokaryotic and eukaryotic cells have in common.
What Prokaryotic and Eukaryotic Cells Have in Common
http://www.phschool.com/science/biology_place/biocoach/cells/common.html
- Both have DNA as their genetic material (it’s DNA that tells cells what kind of cells they should be).
- Both are covered by a cell membrane.
- Both contain RNA.
- Both are made from the same basic chemicals: carbohydrates, proteins, nucleic acid, minerals, fats and vitamins.
- Both have ribosomes (the structures on which proteins are made).
- Both regulate the flow of the nutrients and wastes that enter and leave them.
- Both have similar basic metabolism (life processes) like photosynthesis and reproduction.
- Both require a supply of energy.
- Both are highly regulated by elaborate sensing systems ("chemical noses”) that make them aware of the reactions within them and the environment around them.
That's what prokaryotic and eukaryotic cells have in common. But there are significant differences between them too. The two main differences are age and structure.
Prokaryotic and Eukaryotic Differences
Age Differences
Scientists believe that prokaryotic cells (in the form of bacteria) were the first life forms on earth. They are considered “primitive” and originated about 3.5 billion years ago. That's 2 billion years earlier than eukaryotic cells and billions of years before our earliest ancestors, the hominids.
You learned a little about this when we studied Early Earth in our lesson on The Solar System, but here is a brief timeline of the development of life on Earth:
- 4.6 billion years ago the Earth was formed
- 3.5 billion years ago the first life arose: prokaryotic bacteria
- 1.5 billion years ago eukaryotic cells arose
- 0.5 billion years ago the Cambrian explosion – multi-celled eukaryotes arose
- 3 million years ago our earliest ancestors, the hominids, appeared
There is strong data to suggest that eukaryotic cells actually evolved from groups of prokaryotic cells that became interdependent on each other. You’ll be learning more about this theory later.
Structural Differences
http://www.cod.edu/people/faculty/fancher/CellStructure.htm
Eukaryotic cells contain two important things that prokaryotic cells do not: a nucleus and organelles (little organs) with membranes around them.
DNA arrangement
Although both eukaryotic and prokaryotic cells contain DNA, the DNA in eukaryotic cells is held within the nucleus. In prokaryotic cells, the DNA floats freely around in a unorganized manner.
Presence of organelles
The organelles in eukaryotic cells allow them to perform more complex functions than prokaryotic cells, which don’t have these little organs. If you don’t know much about organelles, don’t worry – you’ll be learning more in an upcoming instruction.
Some of the organelles in eukaryotic cells are: 
- The Nucleus – the “brain” or control center of the cell. It contains DNA, which makes up genes. That DNA gets transcribed, or copied onto messenger RNA. That messenger carries a copy of the genes orders for certain protein production. These orders go to the protein factories.
- Ribosomes – These are the protein factories. They follow instructions from messenger RNA (remember that the messenger RNA got its orders from the DNA). The instructions tell the ribosomes to make specific proteins. Note, this particular organelle is found in prokaryotes too!
- Endoplasmic Reticulum (ER) – structures that modify proteins produced in the ribosomes. Not all of the proteins made by the ribosomes need changing, but those that do get “altered” here.
- Golgi Apparatus – This structure will make even more changes to the proteins that already got changed when they were in the E.R. Remember those proteins were made in the ribosomes, changed once in the E.R. and will be changed again in the Golgi Apparatus. The Golgi also acts as a post office by packaging and shipping proteins to other parts of the cell or out of the cell.
- Mitochondria – structures which produce the cell’s energy, a.k.a. powerhouses of the cell.
- Chloroplasts – structures which allow plants to trap sunlight and carry out photosynthesis.
There are some important differences between prokaryotic and eukaryotic cells.
Size
Eukaryotic cells are, on average, ten times larger than prokaryotic cells.
Cell Wall Differences
Prokaryotic cells have a cell wall composed of peptidoglycan (amino acid and sugar). Some eukaryotic cells also have cells walls, but none that are made of peptidoglycan.
Flagella Arrangement
The flagella in eukaryotic cells are different from the flagella in prokaryotic cells. Flagella are the structures that help cells move (scientists call it motility). The flagella in eukaryotic cells are composed of several filaments and are far more complex than the flagella in prokaryotic cells.
All cells have their genes arranged in linear chains called chromosomes. But eukaryotic cells contain two (or more) copies of every gene. During reproduction, the chromosomes of eukaryotic cells undergo an organized process of duplication called mitosis. You've learned about mitosis in several previous Lessons and you'll also hear more about it later.
New Discoveries
Until recently, it was thought that only eukaryotic cells existed in multi-cell groups like in organs and tissues. But recent discoveries suggest that some prokaryotic cells do too. This is just one more example of how new discoveries are always changing what we know – or think we know.
But that’s what makes science so exciting!
Cell Theory
http://fig.cox.miami.edu/~cmallery/150/unity/cell.text.htm
In studying biology, you will often hear references to Cell Theory. Different scientists have different ways of expressing it, but basically it goes like this:
- All living things are made up of cells and the products of those cells.
- All cells carry out their own life functions.
- New cells come from other living cells.
As with many things, however, there are a few exceptions -- most notably viruses, mitochondria and chloroplasts.
Viruses are not cells and there is a debate as to whether or not they are really alive. They are made up of protein and nucleic acid but have no membranes, nucleus or protoplasm. They appear to be alive, however, when they reproduce after infecting a host cell.
Mitochondria and chloroplasts are organelles (small structures inside cells) that have their own genetic material and reproduce independently from the rest of the cell.
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