What Is a Cell?
Most cells are so small that they cannot be seen by the naked eye. So how did scientists find cells? By accident! The first person to see cells wasn’t even looking for them. A cell is the smallest unit that can perform all the functions necessary for life. All living things are made of cells. Some living things are made of only one cell. Others are made of millions of cells. Robert Hooke was the first person to describe cells. In 1665, he built a microscope to look at tiny objects. One day he looked at a piece of cork. Cork is found in the bark of cork trees. Hooke thought the cork looked like it was made of little boxes. He named these boxes cells, which means “little rooms” in Latin. The first cells that Hooke saw were from cork.
These cells were easy to see because plant cells have cell walls. At first,Hooke didn’t think animals had cells because he couldn’t see them. Today we know that all living things are made of cells.
In the late 1600s, a Dutch merchant named Anton van Leeuwenhoek studied many different kinds of cells. He made his own microscopes. With them, he looked at tiny pond organisms called protists. He also looked at blood cells, yeasts, and bacteria.
What Is the Cell Theory?
Since Hooke first saw cork cells, many discoveries have been made about cells. Cells from different organisms can be very different from one another. Even cells from different parts of the same organism can be very different. However, all cells have several important things in common. These observations are known as the cell theory. The cell theory has three parts:
1. All organisms are made of one or more cells.
2. The cell is the basic unit of all living things.
3. All cells come from existing cells.
What Are the Parts of a Cell?
Cells come in many shapes and sizes and can have different functions. However, all cells have three parts in common: a cell membrane, genetic material, and organelles
CELL MEMBRANE
All cells are surrounded by a cell membrane. The cell membrane is a layer that covers and protects the cell. The membrane separates the cell from its surroundings. The cell membrane also controls all material going in and out of the cell. Inside the cell is a fluid called cytoplasm.
GENETIC MATERIAL
All cells contain DNA (deoxyribonucleic acid) at some point in their lives. DNA is the genetic material that carries information needed to make proteins, new cells, and new organisms. DNA is passed from parent cells to new cells and it controls the activities of the cell. The DNA in some cells is found inside a structure called the nucleus. Most of your cells have a nucleus.
ORGANELLES
Cells have structures called organelles that do different jobs for the cell. Most organelles have a membrane covering them. Different types of cells can have different organelles.
What Are the Two Kinds of Cells?
There are two basic kinds of cells—cells with a nucleus and cells without a nucleus. Those without a nucleus are called prokaryotic cells. Those with a nucleus are called eukaryotic cells.
What Are Prokaryotes?
A prokaryote is an organism made of one cell that does not have a nucleus or other organelles covered by a membrane. Prokaryotes are made of prokaryotic cells. There are two types of prokaryotes: bacteria and archaea.
BACTERIA
The most common prokaryotes are bacteria (singular, bacterium). Bacteria are the smallest known cells. These tiny organisms live almost everywhere. Some bacteria live in the soil and water. Others live on or inside other organisms. You have bacteria living on your skin and teeth and in your digestive system. The following are some characteristics of bacteria:
no nucleus
circular DNA shaped like a twisted rubber band
no membrane-covered (or membrane-bound) organelles
a cell wall outside the cell membrane
a flagellum (plural, flagella), a tail-like structure that some bacteria use to help them move
ARCHAEA
Archaea (singular, archaeon) and bacteria share the following characteristics:
no nucleus
no membrane-bound organelles
circular DNA
a cell wall
Archaea have some other features that no other cells have. For example, the cell wall and cell membrane of archaea are made of different substances from those of bacteria. Some archaea live in places where no other organisms could live. For example, some can live in the boiling water of hot springs. Others can live in toxic places such as volcanic vents filled with sulfur. Still others can live in very salty water in places such as the Dead Sea.
What Are Eukaryotes?
Eukaryotic cells are the largest cells. They are about 10 times larger than bacteria cells. However, you still need a microscope to see most eukaryotic cells. Eukaryotes are organisms made of eukaryotic cells. These organisms can have one cell or many cells. Yeast, which makes bread rise, is an example of a eukaryote with one cell. Multicellular organisms, or those made of many cells, include plants and animals. Unlike prokaryotic cells, eukaryotic cells have a nucleus that holds their DNA. Eukaryotic cells also have membrane-bound organelles.
Why Are Cells So Small?
Your body is made of trillions of cells. Most cells are so small you need a microscope to see them. More than 50 human cells can fit on the dot of this letter i. However, some cells are big. For example, the yolk of a chicken egg is one big cell! Why, then, are most cells small? Cells take in food and get rid of waste through their outer surfaces. As a cell gets larger, it needs more food to survive. It also produces more waste. This means that more materials have to pass through the surface of a large cell than a small cell. As a cell’s volume increases, its outside surface area grows too. However, volume always grows faster than surface area. If the cell volume gets too big, the surface area will not be large enough for the cell to survive. The cell will not be able to take in enough nutrients or get rid of all its wastes. This means that surface area limits the size of most cells.