Cells - Part 1

You've seen it before. Cells are the basic unit of life. Well, if you are studying life science, it's hard to think of a subject more basic than cells. Unless you have a deep understanding of cells and how they work, you can't really understand how they reproduce. If you can't understand how they reproduce, then you won't be able to understand genetics. And, if you don't understand genetics, then evolution will be a curricular abyss that you'll never find your way out of.

The good news about cells? They are everywhere! You are made out of them. I am made out of them. The ants crawling on the walls of our classroom are made out of them. And sometimes, they live all by their lonesome, like the unicellular organisms that are living in the pond water sample in our classroom.

The bad news about cells? They are tiny. You can't see them without a microscope, and you can't really see much detail inside of them without microscopes that cost thousands and thousands of dollars. Even with very expensive microscopes, you usually have to kill a cell before you can really examine it closely enough to see any of the details of its insides. That's because they don't have much color to them. In order to see the detail, you have to stain them. Certain stains are attracted to certain parts of their internal structure, so by picking and choosing the right stains, you can see the parts you are interested in. Unfortunately, we don't have the equipment needed to do that. So, you are going to have to do most of your learning about cells from pictures, photographs, and videos. Don't worry. You will get a chance to see some real, living cells under the microscope. It's just that you won't be able to see a lot of the important structures that are inside of cells and that you need to learn about.

Prokaryotes and Eukaryotes

These are two pretty tough words, aren't they? Even though you have already learned that cells can be broken down into the plant category and the animal category, there is another way to categorize them. The simplest cells are called prokaryotic cells, or prokaryotes. These cells are among the oldest forms of life on Earth. Even though they are organisms just like you or I, there are some differences between prokaryotic cells and eukaryotic cells, which are the kinds of cells you and I are made of.

Prokaryotic cells do not have a nucleus. They do not have membrane-bound organelles. That means that their organelles are not surrounded by a membrane. Finally, their DNA is a long, circular molecule. The prokaryotes are separated into two types. You've heard of one type: bacteria. You probably have not heard of the other type: archaea. Even though archaea have been around for a very, very long time, scientists don't know much about archaea except that they seem to be able to survive in very harsh environments.

All organisms that are not bacteria or archaea are made of eukaryotic cells and are called eukaryotes. Some eukaryotes are unicellular and others are multicellular.

The Discovery of Cells

Robert Hooke was the first person to ever describe cells. Way back in 1655, he built a microscope and started looking around for things to put under it. One of the things he tried was a slice of cork. Cork is a soft material that is found in the bark of cork trees. When Hooke looked at the cork under his microscope, he saw something that looked to him like little boxes. He named them cells. Cells means "little rooms" in Latin. The word cell is still used for a little room. Have you ever heard of a prison cell? They are not exactly spacious! The illustration below is the drawing that Hooke made of the cork cells he saw under his microscope.

Hooke was pretty excited by his discovery, and so he started looking at parts of other plants under his microscope. He noticed that it didn't seem to matter what kind of plant he looked at, he saw the little boxes. Some of the cells that Hooke saw were filled with what he called juice. These, of course, were living cells. Hooke looked at parts of animals under the microscope, too. When he looked at animal parts, though, he didn't see his little boxes, and so he didn't think they were made of cells. He was wrong, of course. The reason that he couldn't see the cells in animals is that animal cells do not have a cell wall the way that plant cells do. Cell walls are thick, and so it made it easier for Hooke to see plant cells.

Hooke was not the only person who made a microscope. About 10 years after Hooke noticed that plants all seemed to be made of cells, Anton Von Leeuwenhoek put some pond water under his microscope. He saw tiny organisms swimming around. He named them animalcules. Leeuwenhoek's animalcules were the unicellular organisms we now call protozoa, or protists.

Leeuwenhoek also looked at blood from different animals. He noticed that not all blood looked the same. He was the first person to see bacteria, and he was the first person to discover that the yeast used by bakers to make bread dough rise is a unicellular organism.

The Cell Theory

Hooke and Leeuwenhoek made their discoveries in the mid-1600s, but it took almost 200 years before someone managed to connect the dots. In 1838, Matthias Schleiden decided that all plants were made of cells. A year later, Theodor Schwann decided that all animals were made of cells. He put these two things together and wrote the first two parts of the cell theory:

• All organisms are made of one or more cells.
• The cell is the basic unit of all living things.

The third piece of the cell theory came about 20 years later when Rudolf Virchow added:

• All cells come from existing cells.

The Chemistry of Life

In order to understand cells, then you need to know about the chemical compounds that they are made of. Some of these compounds will be new to you, but some of them will be very familiar because we studied them during Health Science in first semester!

Proteins

Proteins are large molecules that are made of smaller subunits called amino acids. You can think of amino acids as letters in the alphabet. Have you ever played a game where you try to make as many words as you can using the letters contained in just one word? Well, that's what cells do with proteins. Organisms get their proteins from the food they ingest. They break down those proteins into the amino acids they are made from. Then, using the instructions contained in their DNA, cells are able to put those amino acids back together to make new proteins.

Enzymes

Enzymes are proteins, but they have a special job to do, so I am discussing them separately. Enzymes act as catalyst. A catalyst is something that allows a chemical reaction to occur more rapidly, allows a chemical reaction to occur at a lower temperature, or allows a chemical reaction to take place that would not otherwise take place.

There are a lot of examples of catalysts in everyday life. One common one is epoxy glue. The next time you're at Home Depot, go to the glue aisle. You'll see that epoxy glue comes in two separate containers. The contents of each container is a different chemical compound. Neither compound by itself will ever get hard. But when the catalyst in one container is mixed with the glue in the second, the result is a glue that will dry to produce a very strong bond.

Enzymes are important to cells. There are many different chemical reactions that need to take place inside of a cell in order for it to remain alive. Some of these reactions normally need high temperatures to take place. In some cases, the temperatures are high enough to cook the cell! But, enzymes allow these reactions to take place at lower temperatures. So, the cell is able to make the compounds it needs to survive without getting cooked.

Carbohydrates

Carbohydrates can be divided into two types: simple carbohydrates and complex carbohydrates. Simple carbohydrates are made of one or a few sugar molecules. Complex carbohydrates are made of hundreds or even thousands of sugar molecules. Cells use sugar (usually a sugar called glucose) for energy. Sometimes, cells have more sugar than they need. When this happens, they put the extra sugar molecules together to make starch. Starch is a complex carbohydrate. If a cell needs energy and has no available sugar, it can break down complex carbohydrates to obtain the sugar it needs.

Lipids

Lipids are compounds that will not mix with water. Two examples of lipids are fat and oil. Fat is normally solid at room temperature and is the lipid found in animal cells. Oil is normally liquid at room temperature and is the lipid found in plant cells. Fat and oil are used by cells for long term energy storage. If cells need energy and those energy needs cannot be met by the available carbohydrates, the cell will break down lipids. That is why humans will lose body fat when they go on a diet. Anytime the number of calories you burn through activity exceeds the number of calories you take in, your body will use your fat, your lipids, to meet its energy needs.

A special type of lipid is a phospholipid. Phospholipids are what cell membranes are made of. As you can see in the illustration below, a phospholipid has a "head" that is attracted to water and a "tail" that repels water. Phospholipid molecules are arranged in a double layer in the cell membrane. This arrangement of phospholipids helps cells to control what goes into and out of the cell.

Adenosine Triphosphate (ATP)

ATP is the fuel for the cell. You have already learned that cells obtain their energy from glucose. However, cells cannot use the energy that they release from glucose. Before they can use that energy, it needs to be transferred to a molecule of ATP. Then, the energy is available for use by the cell.

Nucleic Acids

Nucleic acids are made up of smaller subunits called nucleotides. DNA, the nucleic acid that is found in all eukaryotic cells, is made of four nucleotides: adenine, cytosine, guanine, and thymine. These nucleotides are commonly referred to by the first letters of their names, or A, C, G, and T. RNA is also made of four nucleotides. In RNA, though, the thymine is replaced with uracil, or U.

Nucleic acids contain the instructions for synthesizing (building) proteins. They are able to do this because each amino acid (the building blocks of proteins) has a "code" that consists of three nucleotides. For example, CCG tells a cell to use a specific amino acid and TAG tells it to use a different one. The order and arrangement of the nucleotides in the DNA molecule determines what amino acids will be put together, and in what order they will be put together. That is how DNA codes protein synthesis.

Extend the Lesson

You can see the actual work of Robert Hooke by following this link to the Gutenberg Project. The actual drawings that Hooke made of what he saw through his microscope are included.

You can read more about Anton von Leeuwenhoek by following this link. Included are the letters that he wrote to Robert Hooke about his discoveries.

Preparing for a Test

• Can you state the cell theory?
• Can you offer some ideas on where the first cell might have come from?
• Do you know the two types of cells?
• Do you know the differences between prokaryotic and eukaryotic cells?
• Do you know the two types of prokaryotic cells?
• Can you name some places where archaea live?
• Can you name some of the unique energy sources archaea utilize?
• Can you explain the difference between helpful and harmful bacteria?
• Do you know the two types of eukaryotic cells?
• Do you know who discovered and named cells?
• Can you explain why plant cells are easier to see than animal cells?
• Can you describe each of the important compounds that are contained in all cells?