Difficulty: Introductory | Prerequisites: Basic chemistry (atoms, molecules, bonding).
This is the chapter where you meet the cell for the first time as a physical object, not just an idea. Before you can understand how cells divide, how genes are expressed, or how organisms develop, you need to know what is inside a cell, how we see it, and why prokaryotic and eukaryotic cells are built differently. Sections 6.1 and 6.2 cover the tools biologists use to study cells (microscopes, cell fractionation) and the fundamental divide between prokaryotic and eukaryotic organisation. If you have not reviewed basic chemistry (atoms, bonds, macromolecules), do that first.
Biologists use light microscopes and electron microscopes to see cells, each with trade-offs in resolution, magnification, and whether the specimen stays alive. All cells share a plasma membrane, cytosol, chromosomes, and ribosomes, but prokaryotes are smaller, lack a nucleus, and keep their DNA in a nucleoid, while eukaryotes compartmentalise their DNA inside a double-membraned nucleus and contain many membrane-bound organelles.
Magnification
The ratio of an object's image size to its real size. In simple terms, how much bigger the microscope makes something look.
Resolution
The minimum distance two points can be separated and still be distinguished as separate points. Think of it as the sharpness or clarity of the image. Higher resolution means finer detail.
Contrast
The difference in brightness between light and dark areas of an image. Without contrast, structures blend into the background and you cannot see them, which is why stains exist.
Light microscope (LM)
A microscope that passes visible light through a specimen and then through glass lenses, which refract the light to magnify the image. Stains are coloured molecules that bind to cell components to improve contrast.
Electron microscope (EM)
A microscope that focuses a beam of electrons through or onto a specimen. Electron beams have much shorter wavelengths than visible light, so EMs achieve far higher resolution. The trade-off: specimen preparation kills the cells. Stains are heavy metals.
Scanning electron microscope (SEM)
An EM that scans the surface of a gold-coated specimen. Secondary electrons are detected and translated into a 3D image of the surface topography.
Transmission electron microscope (TEM)
An EM that aims a beam through a thin section of the specimen stained with heavy metals. The image is formed from the electrons that pass through, revealing internal cell structure.
Cell fractionation
A technique that breaks cells apart (usually by centrifuge) and separates organelles and subcellular structures so researchers can study their individual functions.
Prokaryotic cell
A cell that lacks a membrane-bound nucleus. DNA sits in a region called the nucleoid. Prokaryotes are generally smaller and have a higher surface-area-to-volume ratio than eukaryotes. Think bacteria and archaea.
Eukaryotic cell
A cell with a true, double-membraned nucleus and membrane-bound organelles. Generally larger than prokaryotes. Think animal cells, plant cells, fungi, protists.
Nucleoid
The region in a prokaryotic cell where the DNA is concentrated. It is not enclosed by a membrane.
Cytoplasm
In prokaryotes, this refers to the entire interior of the cell. In eukaryotes, it means specifically the region between the nucleus and the plasma membrane.
Surface-area-to-volume ratio
As a cell grows, its volume increases faster than its surface area. Smaller cells therefore have a greater ratio of surface area to volume, which is important for efficient exchange of materials across the membrane.
Three things determine how useful a microscope image is:
Magnification – how much larger the image appears relative to the real object.
Resolution – how much fine detail you can distinguish. Resolution is inversely related to wavelength: shorter wavelength means higher resolution.
Contrast – the brightness difference between structures and background. Stains improve contrast.
Light microscope (LM)
Uses visible light and glass lenses.
Stains are coloured molecules that bind to cell components.
Cells can remain alive during observation.
Electron microscope (EM), general
Uses a beam of electrons instead of light.
Electrons have much shorter wavelengths than visible light, giving far higher resolution.
Specimen preparation kills the cells.
Stains are heavy metals.
Both SEM and TEM use electromagnets as lenses to bend the electron beam.
Scanning electron microscope (SEM)
Studies the surface topography of a specimen.
The sample is coated with gold. The electron beam excites surface electrons, and the pattern of secondary electrons is detected to produce a 3D image.
Transmission electron microscope (TEM)
Studies internal cell structure.
A thin section of the specimen is stained with heavy metals. The beam passes through the section. Where electron density is higher (more heavy-metal staining), fewer electrons are transmitted. The resulting image shows internal ultrastructure.
Cells are broken open and their contents separated by centrifuge.
Successive spins at increasing speeds pellet progressively smaller organelles.
This lets researchers isolate individual organelles and study their biochemical functions.
All cells have:
A plasma membrane (selective barrier)
Cytosol (jelly-like fluid that suspends cell components)
Chromosomes (carry DNA)
Ribosomes (complexes that build proteins)
Feature | Prokaryotic | Eukaryotic |
|---|---|---|
DNA location | Nucleoid (no membrane) | Double-membraned nucleus |
Cytoplasm | Entire interior of the cell | Region between nucleus and plasma membrane |
Size | Smaller (typically 1–5 µm) | Larger (typically 10–100 µm) |
Surface-area-to-volume ratio | Higher | Lower |
Membrane-bound organelles | Absent | Present |
When a cell grows, volume increases as the cube of the radius while surface area increases as the square. So larger cells have proportionally less membrane surface per unit of internal volume. This limits how efficiently a cell can exchange nutrients and waste across its membrane, which is one reason cells stay small or compartmentalise internally.
Light microscopy is the workhorse of clinical diagnostics: blood smears, Pap smears, and tissue biopsies are all examined under a light microscope. Electron microscopy is used in virology (visualising virus particles), materials science, and forensic analysis. Cell fractionation underpins modern biochemistry, letting researchers study isolated mitochondria, ribosomes, or membranes to work out what each component does.
Students often think higher magnification automatically means a better image. It does not. Resolution is what determines clarity. You can magnify a blurry image all you like and it stays blurry.
Students sometimes assume electron microscopes are simply "better" than light microscopes. They offer higher resolution, yes, but they kill the specimen during preparation, so you cannot observe living processes with an EM.
A common mix-up: SEM and TEM. SEM gives you the surface (3D topography). TEM gives you the interior (internal structure through thin sections). The names help: Scanning = surface, Transmission = through.
Students frequently confuse "nucleoid" with "nucleus." The nucleoid is just an unenclosed region where prokaryotic DNA concentrates. The nucleus is a membrane-bound organelle in eukaryotes.
⚠️ Expect a question asking you to compare SEM and TEM: what each is used for, how each produces its image, and what kind of image each generates (3D surface vs. internal cross-section).
⚠️ The relationship between wavelength and resolution (inverse) is a classic short-answer or multiple-choice target.
⚠️ You will almost certainly be asked to list features shared by all cells (plasma membrane, cytosol, chromosomes, ribosomes) and features that distinguish prokaryotes from eukaryotes.
⚠️ The surface-area-to-volume ratio concept appears repeatedly in biology. Understand why smaller cells are more efficient at membrane transport.
True or false: Electron microscopes can be used to observe living cells. (False – specimen preparation kills the cells.)
Resolution is ______ related to wavelength. (inversely)
True or false: Prokaryotic cells have a membrane-bound nucleus. (False – their DNA is in the nucleoid, which has no membrane.)
The two types of electron microscope are ______ and ______. (SEM and TEM)
True or false: As a cell gets larger, its surface-area-to-volume ratio increases. (False – it decreases.)
Q: What three parameters determine the quality of a microscope image?
A: Magnification, resolution, and contrast.
Q: Why do electron microscopes achieve higher resolution than light microscopes?
A: Electron beams have much shorter wavelengths than visible light, and resolution is inversely related to wavelength. Shorter wavelength means finer detail can be distinguished.
Q: Describe how a scanning electron microscope (SEM) produces an image.
A: The specimen is coated with gold. An electron beam scans the surface, exciting electrons there. The pattern of secondary electrons emitted from the surface is detected and translated into a 3D image of the surface topography.
Q: What is the key structural difference between prokaryotic and eukaryotic cells?
A: Eukaryotic cells have a membrane-bound nucleus (double membrane) and membrane-bound organelles. Prokaryotic cells lack a membrane-bound nucleus; their DNA is concentrated in a nucleoid region.
Q: Explain why smaller cells have a higher surface-area-to-volume ratio than larger cells.
A: As a cell grows, its volume increases as the cube of its linear dimension while surface area increases as the square. Volume outpaces surface area, so smaller cells have proportionally more membrane surface relative to their volume.
This material connects directly to Chapter 6.3–6.6, where you will learn about specific eukaryotic organelles (the nucleus, endomembrane system, mitochondria, chloroplasts, cytoskeleton). Understanding the prokaryote/eukaryote distinction here sets up the endosymbiont theory in Section 6.5. The surface-area-to-volume concept reappears in discussions of gas exchange (respiratory systems), nutrient absorption (intestinal villi), and thermoregulation later in the course.
microscopy, light microscope, electron microscope, SEM, TEM, scanning electron microscope, transmission electron microscope, magnification, resolution, contrast, cell fractionation, centrifuge, prokaryote, eukaryote, nucleoid, nucleus, cytoplasm, cytosol, plasma membrane, ribosomes, chromosomes, surface area to volume ratio, SA:V ratio, cell size, staining techniques, heavy metal stain