Difficulty: Introductory | Prerequisites: None
This chapter introduces the tools biologists use to see cells and the foundational ideas about what cells are. It covers the invention of microscopes, the difference between magnification and resolution, the three tenets of cell theory, and the distinction between prokaryotic and eukaryotic cells. Everything else in the course builds on these basics.
Microscopes let biologists see cells, and three types (light, SEM, TEM) each offer different trade-offs between magnification and resolution. Cell theory states that all living things are made of cells, cells are the basic unit of life, and all cells come from pre-existing cells. Cells fall into two broad categories: prokaryotic (no nucleus, e.g. bacteria) and eukaryotic (nucleus present, e.g. animals, plants, fungi).
Light microscope (LM)
A microscope that uses visible light and glass lenses to magnify specimens. It is the most common type used in teaching labs and can magnify up to roughly 1,000x.
Scanning electron microscope (SEM)
An electron microscope that scans the surface of a specimen with a focused beam of electrons, producing detailed three-dimensional images of the outer structure. Think of it as a tool for seeing the surface texture of a cell.
Transmission electron microscope (TEM)
An electron microscope that passes electrons through a very thin specimen to reveal internal structures at extremely high resolution. Think of it as a way to look inside a cell at its organelles.
Magnification
The degree to which an image of a specimen is enlarged compared to its actual size. In simple terms, it is how much bigger something appears.
Resolution
The ability to distinguish two nearby objects as separate. Higher resolution means finer detail. In simple terms, magnification makes it bigger, but resolution makes it clearer.
Cell theory
The foundational principle of biology, comprising three tenets: (1) all living things are made of cells, (2) cells are the basic unit of life, (3) all cells come from pre-existing cells.
Prokaryotic cell
A cell that lacks a membrane-bound nucleus or membrane-bound organelles. Bacteria and Archaea are prokaryotes. They are generally smaller than eukaryotic cells.
Eukaryotic cell
A cell with a membrane-bound nucleus and membrane-bound organelles. Protists, fungi, plants, and animals are all eukaryotes. Eukaryotic cells are generally larger than prokaryotic cells.
Robert Hooke (1665) observed cork cells under a simple microscope and coined the term "cell."
Anton van Leeuwenhoek improved lens-grinding techniques and was the first to observe living microorganisms, which he called "animalcules."
Light microscope (LM): uses visible light. Good for observing living specimens. Limited resolution compared to electron microscopes.
Scanning electron microscope (SEM): uses an electron beam to scan surfaces. Produces 3D images of cell exteriors. Specimens must be dead and coated.
Transmission electron microscope (TEM): passes electrons through thin slices of a specimen. Reveals internal cell structures at very high resolution. Also requires dead, specially prepared specimens.
Magnification is about size: how many times larger the image appears.
Resolution is about clarity: the minimum distance between two points that can still be distinguished as separate.
A blurry image can be highly magnified but have poor resolution. Both matter.
Three tenets:
All living things are made of cells.
The cell is the basic unit of life.
All cells arise from pre-existing cells.
These tenets underpin virtually every topic in biology.
Prokaryotic cells: no membrane-bound nucleus, generally smaller, include bacteria and archaea. DNA sits in a region called the nucleoid.
Eukaryotic cells: membrane-bound nucleus containing DNA, membrane-bound organelles (mitochondria, ER, Golgi, etc.), generally larger. Include protists, fungi, plants, and animals.
Students often confuse magnification with resolution. A cheap magnifying glass can make something very large but blurry. Resolution is about clarity, not size.
Some students think prokaryotic cells have no DNA. They do; it is simply not enclosed in a membrane-bound nucleus.
Students sometimes list viruses as an exception to cell theory. Viruses are not considered living organisms under most biological definitions, so cell theory does not need to account for them.
Expect a question asking you to match each microscope type (LM, SEM, TEM) to what it is best suited for. Know which one shows surfaces and which shows internal structures.
The three tenets of cell theory are a classic short-answer or multiple-choice item. Memorise all three.
Know the key differences between prokaryotic and eukaryotic cells: nucleus, size, and examples of each.
True or False: A scanning electron microscope shows the internal structures of a cell.
False. SEM shows surface structures. TEM shows internal structures.
Fill in the blank: The three domains that include prokaryotic organisms are Bacteria and ________.
Archaea.
True or False: Resolution refers to how large an image appears.
False. Resolution refers to the clarity or level of detail in an image. Magnification refers to size.
Fill in the blank: According to cell theory, all cells come from ________.
Pre-existing cells.
Q: State the three tenets of cell theory.
A: (1) All living things are made of cells. (2) The cell is the basic unit of life. (3) All cells arise from pre-existing cells.
Q: Compare and contrast SEM and TEM. What does each show, and what are their limitations?
A: SEM scans the surface of a specimen and produces 3D images of external features. TEM passes electrons through thin slices and reveals internal structures at high resolution. Both require dead, specially prepared specimens, so neither can observe living cells.
Q: A student claims that prokaryotic cells are "simpler" than eukaryotic cells. In what specific ways is this true?
A: Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles. They are generally smaller. Their DNA sits in a nucleoid region rather than being enclosed in a nuclear envelope. However, they are still capable of all the basic functions of life.
Microscope, light microscope, LM, scanning electron microscope, SEM, transmission electron microscope, TEM, magnification, resolution, cell theory, prokaryote, prokaryotic, eukaryote, eukaryotic, nucleus, bacteria, archaea, protist, fungi, plants, animals, Robert Hooke, Anton van Leeuwenhoek, organelle, membrane-bound, nucleoid, cell biology