KU

Cell Biology Notes

Learning Objectives

  • 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

Cells in Organisms

  • 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

Microscopy

  • 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.

Light Microscopes

  • 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.

Dissecting Microscopes

  • 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.

Electron Microscopes

  • 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.

Scanning Electron Microscope (SEM)
  • 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.

Transmission Electron Microscope (TEM)
  • 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.

Career Connection: Cytotechnologist

  • 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.

Cell Theory

  • 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.