The Units of Life

Would do you like to know the units of life?

What is the Microscope?

The Microscope: A Tool Cells are the smallest units that carry out the activities of life in organisms. Yet, as important as they are, you certainly don't see individual cells when you look at most plants or animals. You need a device, such as a magnifying glass or a microscope, to see most cells.

 Trying to see separate cells in a large plant, this is like trying to see individual bricks in a wall from three blocks away. If you start to walk toward the wall, it becomes easier to see individual bricks. When you get right up to the wall, you can see each brick in detail. A microscope performs a similar function. A micro-scope has one or more lenses that make an enlarged image of an object. Through these lenses, you are brought closer to the leaf, and you see the individual cells that carry on life processes. 

Microscopes are simple or compound, depending on how many lenses they contain. A simple microscope is similar to a magnifying glass. It has only one lens. In 1590, a Dutch maker of reading glasses, Zacharias Janssen, put two magnifying glasses together in a tube. The result was the first crude compound microscope. By combining two lenses he got an image that was larger than an image made by only one lens. These early compound micro-scopes weren't satisfactory, however. The lenses would make a large image, but it wasn't always clear.

 In the mid 1600s, Anton Van Leeuwenhoek, another Dutch scientist, made a simple microscope with a tiny glass bead for a lens. With it, he reported seeing things in pond water that no one had ever imagined before. His microscope could magnify up to 270 times. Another way to say this is that his microscope could make an image of an object 270 times larger than its actual size. Today we would say his lens had a power of 270X. 

The microscope you will use in studying life science is a compound light microscope like the one in the image. A compound light microscope lets light pass through an object and then through two or more lenses. The lenses enlarge the image and bend the light toward your eye. It has an eyepiece lens and an objective lens. An eyepiece lens usually has a power of 10X. An objective lens may have a power of 43X. Together, they have a total magnification of 430X (10X times 43X). Some compound microscopes have more powerful lenses that magnify an object up to 2000 times (2000X) its original size. 

Your classroom may have a stereoscopic (stereo) light microscope that gives you a three-dimensional view of an object. Stereo microscopes are used to look at thick structures that light can't pass through, such as whole insects or leaves, or your fingertips. Things that are too small to be seen with a light micro-scope can be seen with an electron microscope. Instead of using lenses CO bend beams of light, an electron micro-scope uses a magnetic field to bend beams of electrons. Electron microscopes can magnify images more than 300 000 times. The left image shows the kind of detail that can be seen with an electron microscope. There are several kinds of electron microscopes. One is the transmission electron microscope (TEM), which is used to study parts inside a cell. The object has to be sliced very thin and placed in a vacuum. There is no air in a vacuum. Asa  result, only dead cells can be observed this way. A scanning electron microscope (SEM) is used to see the surfaces of whole objects. With an SEM, you can view and photograph living cells. From the time of Van Leeuwenhoek until the present, the microscope has been a valuable tool for studying cells. You will sec how it was used to develop the cell theory. 

Do you all know what is the cell theory?

During the centuries when explorers like Columbus and Magellan set out to find new lands, scientists were busy observing everything they could about the smaller world around them. Curiosity made them look through their microscopes and lenses at mud from ponds and drops of rainwater. They examined blood and scrapings from their own teeth. Cells weren't discovered until the microscope was improved. In 1665, Robert Hooke, an English scientist, made a very thin slice of cork and looked at it under his microscope. To Hooke, the 

cork seemed to be made up of little empty boxes, which he called cells. The drawing of cork cells reproduced in image, was made by Robert Hooke more than 300 years ago.

Actually, Hooke was not aware of the importance of what he was seeing. In 1838, Matthias Schleiden, a German scientist, used a microscope to study plant parts. He concluded that all plants were made of cells. Just a year later, another German scientist, Theodor Schwann, after observing many different animal cells, concluded that all animal were made up of cells. Together, they became convince( that all living things were made of cells. About 15 years later, a German doctor, Rudolpt Virchow, hypothesized that new cells don't form on their own. Instead, cells divide to form new cells. This was a startling idea. Remember that at that time, people thought earthworms fell from the sky when it rained. They thought that life came about spontaneously. What Virchow said was that every cell that is or has ever been came from a cell that already existed. The observations and conclusions of Schleiden, Schwann, Virchow, and other scientists became known as the cell theory. The major ideas of the cell theory are: 

1. All organisms are made up of one or more cells. 

2. Cells are the basic units of structure and function in all organisms.

3. All cells come from cells that already exist. 

The cell theory is one of the major theories in science. It is not based on the hypotheses and observations of only one person, but is result of the discoveries of many scientists. Today it serves as the basis for scientists who study the parts of cells, how cells are organized, and how cells and organisms reproduce and change through time.