Learning Objectives
In this section, you will explore the following questions:
- What is the role of cells in organisms?
- What is the difference between light microscopy and electron microscopy?
- What is the cell theory?
Teacher Support
An introduction to the principle of microscopy will facilitate the discussion of the various types of instruments. Explain to students the concepts of magnification, resolution, and contrast, which together contribute to the quality of microscopy. In particular, explain that resolution is determined by the smallest wavelength in visible light in the light microscope and the distance separating two atoms in solid material in a transmission electron microscope.
Review with students the tenets of the cell theory, because the cell, as the fundamental unit of life, is at the foundation of all biological classes. Life cannot be divided into pieces smaller than the cell and still be considered independent life. Therefore, viruses are considered particles, not cells, because viruses, for one, cannot replicate on their own. All living organisms are made of one or more cells. Those cells are similar in size. Why are cells so similar in size, no matter how large the organism itself grows? The main answer lies in how surface area and volume scale with cell radius and how these parameters affect exchange with the environment. This is covered in the next module.
Beginners consider magnification the most important quality of a microscope. Magnification without resolution yields a very large but blurry image which does not inform the viewer on the nature of the specimen. Illustrate the point by referring to the resolution of a photographic image or a YouTube video with which many students are familiar. Emphasize the importance of contrast. A cell is transparent unless it contains colored components or dense structures. Ask students for examples of such structures. Answers may include chloroplasts, chromoplasts, nuclei or other dense structures.
Connection for AP® Courses
A cell is the smallest unit of a living thing. A living thing, whether made of one cell (like bacteria) or many cells (like a human), is called an organism. Thus, cells are the basic building blocks of all organisms.
Several cells of one kind that interconnect with each other and perform a shared function form a tissue; several tissues combine to form an organ (your stomach, heart, or brain), and several organs make up an organ system (such as the digestive system, circulatory system, or nervous system). Several systems that function together form an organism (like a human being). Here, we will examine the structure and function of cells.
There are many types of cells, all grouped into one of two broad categories: prokaryotic and eukaryotic. For example, both animal and plant cells are classified as eukaryotic cells, whereas bacterial cells are classified as prokaryotic. Before discussing the criteria for determining whether a cell is prokaryotic or eukaryotic, let’s first examine how biologists study cells.
Microscopy
Cells vary in size. With few exceptions, individual cells cannot be seen with the naked eye, so scientists use microscopes (micro- = “small”; -scope = “to look at”) to study them. A microscope is an instrument that magnifies an object. Most photographs of cells are taken with a microscope, and these images can also be called micrographs.
The optics of a compound microscope’s lenses change the orientation of the image that the user sees. A specimen that is right-side up and facing right on the microscope slide will appear upside-down and facing left when viewed through a microscope, and vice versa. Similarly, if the slide is moved left while looking through the microscope, it will appear to move right, and if moved down, it will seem to move up. This occurs because microscopes use two sets of lenses to magnify the image. Because of the manner by which light travels through the lenses, this system of two lenses produces an inverted image (binocular, or dissecting microscopes, work in a similar manner, but include an additional magnification system that makes the final image appear to be upright).
Light Microscopes
To give you a sense of cell size, a typical human red blood cell is about eight millionths of a meter or eight micrometers (abbreviated as eight μm) in diameter; the head of a pin of is about two thousandths of a meter (two mm) in diameter. That means about 250 red blood cells could fit on the head of a pin.
Most student microscopes are classified as light microscopes (Figure 4.2a). Visible light passes and is bent through the lens system to enable the user to see the specimen. Light microscopes are advantageous for viewing living organisms, but since individual cells are generally transparent, their components are not distinguishable unless they are colored with special stains. Staining, however, usually kills the cells.
Light microscopes commonly used in the undergraduate college laboratory magnify up to approximately 400 times. Two parameters that are important in microscopy are magnification and resolving power. Magnification is the process of enlarging an object in appearance. Resolving power is the ability of a microscope to distinguish two adjacent structures as separate: the higher the resolution, the better the clarity and detail of the image. When oil immersion lenses are used for the study of small objects, magnification is usually increased to 1,000 times. In order to gain a better understanding of cellular structure and function, scientists typically use electron microscopes.
Electron Microscopes
In contrast to light microscopes, electron microscopes (Figure 4.2b) use a beam of electrons instead of a beam of light. Not only does this allow for higher magnification and, thus, more detail (Figure 4.3), it also provides higher resolving power. The method used to prepare the specimen for viewing with an electron microscope kills the specimen. Electrons have short wavelengths (shorter than photons) that move best in a vacuum, so living cells cannot be viewed with an electron microscope.
In a scanning electron microscope, a beam of electrons moves back and forth across a cell’s surface, creating details of cell surface characteristics. In a transmission electron microscope, the electron beam penetrates the cell and provides details of a cell’s internal structures. As you might imagine, electron microscopes are significantly more bulky and expensive than light microscopes.
Link to Learning
For another perspective on cell size, try the HowBig interactive at this site.
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Only electron microscopes can be used to view internal structures.
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Some electron microscopes allow visualization of external surfaces at very high magnification in a way that is not possible with standard light microscopes.
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Scanning electron microscopes can show internal structures clearly at very high magnifications.
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Electron microscopes are easier to use and less expensive than light microscopes.
Cell Theory
The microscopes we use today are far more complex than those used in the 1600s by Antony van Leeuwenhoek, a Dutch shopkeeper who had great skill in crafting lenses. Despite the limitations of his now-ancient lenses, van Leeuwenhoek observed the movements of single-celled organisms, which he collectively termed “animalcules.”
In a 1665 publication called Micrographia, experimental scientist Robert Hooke coined the term “cell” for the box-like structures he observed when viewing cork tissue through a lens. In the 1670s, van Leeuwenhoek discovered bacteria and protozoa. Later advances in lenses, microscope construction, and staining techniques enabled other scientists to see some components inside cells.
By the late 1830s, botanist Matthias Schleiden and zoologist Theodor Schwann were studying tissues and proposed the unified cell theory, which states that all living things are composed of one or more cells, the cell is the basic unit of life, and new cells arise from existing cells. Rudolf Virchow later made important contributions to this theory.
Career Connection
Cytotechnologist
Have you ever heard of a medical test called a Pap smear (Figure 4.4)? In this test, a doctor takes a small sample of cells from the uterine cervix of a patient and sends it to a medical lab where a cytotechnologist stains the cells and examines them for any changes that could indicate abnormal cell growth or a microbial infection.
Cytotechnologists (cyto- = “cell”) are professionals who study cells via microscopic examinations and other laboratory tests. They are trained to determine which cellular changes are within normal limits and which are abnormal. Their focus is not limited to cervical cells; they study cellular specimens that come from all organs. When they notice abnormalities, they consult a pathologist, who is a medical doctor who can make a clinical diagnosis.
Cytotechnologists play a vital role in saving people’s lives. When abnormalities are discovered early, a patient’s treatment can begin sooner, which usually increases the chances of a successful outcome.