By the end of this section, you will be able to:
Describe the roles of cells in organisms
Compare and contrast light microscopy and electron microscopy
Summarize the cell theory
A cell is the smallest unit of a living thing.
A living thing is called an organism.
Cells are the basic building blocks of all organisms.
In multicellular organisms:
Cells of a particular kind interconnect to form tissues (e.g., muscle, connective, nervous).
Tissues combine to form an organ (e.g., stomach, heart, brain).
Organs make up an organ system (e.g., digestive, circulatory, nervous).
Systems functioning together form an organism (e.g., elephant).
Two broad categories of cells:
Prokaryotic
Eukaryotic
Eukaryotic cells:
Animal cells
Plant cells
Fungal cells
Protist cells
Prokaryotic cells:
Bacteria
Archaea
Cells vary in size.
Individual cells are generally too small to be seen without a microscope.
A microscope is an instrument that magnifies an object.
Images taken with a microscope are called micrographs.
A typical human red blood cell is about 8 micrometers (µm) in diameter, or 8 \times 10^{-6} meters.
The head of a pin is about 2 millimeters (mm) in diameter, or 2 \times 10^{-3} meters.
Approximately 250 red blood cells could fit on the head of a pin.
Optics of lenses of a light microscope changes the orientation of the image.
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.
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.
Microscopes use two sets of lenses to magnify the image.
This produces an inverted image.
Binoculars and dissecting microscopes work similarly but include an additional magnification system that makes the final image appear to be upright.
Most student microscopes are light microscopes.
Visible light passes through and is bent by the lens system.
Advantageous for viewing living organisms.
Individual cells are generally transparent, so their components are not distinguishable unless stained.
Staining usually kills the cells.
Commonly magnify up to approximately 400 times.
Two important parameters in microscopy:
Magnification: the degree of enlargement of an object.
Resolving power: the ability of a microscope to allow the eye to distinguish two adjacent structures as separate.
The higher the resolution, the closer those two objects can be, and the better the clarity and detail of the image.
When oil immersion lenses are used, magnification is usually increased to 1,000 times for the study of smaller cells, like most prokaryotic cells.
Light entering a specimen from below is focused onto the eye of an observer.
The sample must be thin or translucent for light to pass through.
Lower magnification (20 to 80 times the object size) than light microscopes.
Can provide a three-dimensional view of the specimen.
Thick objects can be examined with many components in focus at the same time.
Designed to give a magnified and clear view of tissue structure as well as the anatomy of the whole organism.
Most modern dissecting microscopes are binocular.
Two separate lens systems, one for each eye.
Provide a sense of depth.
Make manipulations by hand easier.
Optics correct the image so that it appears as if being seen by the naked eye and not as an inverted image.
The light illuminating a sample typically comes from above the sample, but may also be directed from below.
Use a beam of electrons instead of a beam of light.
Allow for higher magnification and more detail.
Provide higher resolving power.
Preparation of a specimen for viewing under an electron microscope will kill it.
Live cells cannot be viewed using this type of microscopy.
The electron beam moves best in a vacuum.
Impossible to view living materials.
A beam of electrons moves back and forth across a cell’s surface.
Renders details of cell surface characteristics by reflection.
Cells and other structures are usually coated with a metal like gold.
The electron beam is transmitted through the cell.
Provides details of a cell’s internal structures.
Electron microscopes are significantly more bulky and expensive than light microscopes.
Cytotechnologists study cells through microscopic examinations and other laboratory tests.
They are trained to determine which cellular changes are within normal limits or are abnormal.
Specimens come from all organs.
They consult a pathologist (a medical doctor who can make a clinical diagnosis) when they notice abnormalities.
They play vital roles in saving people’s lives because when abnormalities are discovered early, a patient’s treatment can begin sooner, which usually increases the chances of successful treatment.
Pap smear: 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 cervical cancer or a microbial infection.
Antony van Leeuwenhoek (1600s):
Dutch shopkeeper with skill in crafting lenses.
Observed the movements of protists (single-celled organisms) and sperm, termed “animalcules”.
Robert Hooke (1665, Micrographia):
Coined the term “cell” (from the Latin cella, meaning “small room”) for the box-like structures he observed when viewing cork tissue through a lens.
Van Leeuwenhoek (1670s):
Discovered bacteria and protozoa.
Later advances enabled scientists to see different components inside cells.
Matthias Schleiden (botanist) and Theodor Schwann (zoologist) (late 1830s):
Studied tissues and proposed the unified cell theory.
All living things are composed of one or more cells.
The cell is the basic unit of life.
All new cells arise from existing cells.
These principles still stand today.