Difficulty: Introductory | Prerequisites: None (this is foundational material for the course)
Cells are studied using light microscopy (live cells, resolution down to ~200 nm) and electron microscopy (fixed cells, resolution down to ~1 nm). Inside, cells are divided into membrane-bound organelles (nucleus, mitochondria, ER, Golgi, lysosomes, and others) and nonmembrane-bound structures (cytosol, ribosomes, cytoskeleton, proteasomes). Knowing which compartment does what is the foundation for everything else in cell biology.
Light microscopy
A technique using visible light and optical lenses to observe cells and larger structures, with a resolution limit of roughly 200 nm. In simple terms, this is the standard microscope you use in lab to watch living cells in real time.
Electron microscopy (EM)
A technique using electron beams instead of light, achieving resolution down to about 1 nm, allowing visualisation of ultrastructural detail. Think of it as the high-powered alternative you turn to when you need to see membranes, ribosomes, or the internal architecture of organelles.
Transmission electron microscopy (TEM)
A form of EM that passes electrons through a thin specimen to produce cross-sectional images of internal cell structure. In simple terms, TEM gives you a slice view through the cell.
Scanning electron microscopy (SEM)
A form of EM that bounces electrons off a specimen's surface, producing detailed three-dimensional surface images. Think of it as the version that shows you what the outside of a structure looks like.
Membrane-bound organelle
A subcellular compartment enclosed by at least one lipid bilayer membrane, separating its internal environment from the cytosol. In simple terms, these are the organelles wrapped in their own membrane: the nucleus, mitochondria, ER, Golgi, lysosomes, endosomes, peroxisomes, and secretory vesicles.
Nonmembrane-bound structure
A cellular component that lacks its own enclosing membrane. Think of it as the parts of the cell that sit freely in the cytoplasm, such as ribosomes, the cytoskeleton, proteasomes, and the cytosol itself.
Resolution limit of approximately 200 nm
Uses visible light and optical lenses
Can observe living cells in real time (live-cell imaging)
Useful for general cell morphology, cell shape, and gross features
Cannot resolve fine ultrastructural details such as individual organelle membranes or ribosomes
Resolution down to about 1 nm, roughly 200 times better than light microscopy
Uses electron beams rather than visible light
Specimens must typically be fixed (not alive), stained with heavy metals, and sectioned
Two main types:
TEM (transmission electron microscopy): electrons pass through thin specimen slices, producing cross-sectional images of internal structure
SEM (scanning electron microscopy): electrons scan the surface, producing three-dimensional surface detail
Membrane-bound organelles (mitochondria, lysosomes, ER) appear with far greater clarity under EM
Nonmembrane-bound structures (ribosomes, cytoskeleton) are also better resolved with EM
Light microscopy remains the go-to for live-cell work and quick morphological assessment
Cells divide their internal work across specialised compartments. The two broad categories are membrane-bound organelles and nonmembrane-bound structures.
Nucleus: houses DNA, controls gene expression
Mitochondria: ATP production via oxidative phosphorylation (the "powerhouse")
Peroxisomes: lipid metabolism, detoxification of reactive oxygen species
Lysosomes: degradative enzymes for breaking down biomolecules and pathogens
Endosomes: sorting and trafficking of endocytosed material (subtypes: early, late, recycling, transport)
Endoplasmic reticulum (ER): rough ER synthesises proteins; smooth ER synthesises lipids and handles detoxification
Golgi apparatus: modifies, sorts, and packages proteins and lipids for secretion or delivery
Secretory vesicles: transport cargo to the plasma membrane for exocytosis
Cytosol: the aqueous matrix where many metabolic reactions occur (including glycolysis)
Nucleolus: site of ribosomal RNA synthesis, located within the nucleus
Centriole: organises the mitotic spindle during cell division
Cytoskeleton: network of protein fibres (microtubules, microfilaments, intermediate filaments) providing structure and enabling transport
Ribosomes: sites of protein synthesis, free in the cytosol or attached to rough ER
Proteasomes: degrade ubiquitinated (tagged-for-destruction) proteins, maintaining protein quality control
Electron microscopy is the workhorse of diagnostic pathology when tissue ultrastructure matters, for example identifying viral particles, characterising kidney biopsy samples, or diagnosing ciliary dyskinesia. Light microscopy remains central to histology labs, blood smear analysis, and any situation where live-cell behaviour is the question.
Students often assume electron microscopy can be used on living cells. It cannot; specimens must be fixed and typically stained with heavy metals.
The cytosol and the cytoplasm are frequently confused. The cytosol is just the fluid portion; the cytoplasm includes the cytosol plus all the organelles and structures suspended in it.
Students sometimes list the nucleolus as a membrane-bound organelle. It is not; the nucleolus has no enclosing membrane of its own.
Ribosomes are sometimes called organelles. Strictly, they are nonmembrane-bound macromolecular complexes, not organelles in the membrane-bound sense.
Expect questions asking you to classify structures as membrane-bound or nonmembrane-bound. This is a very common exam format.
Know the resolution limits: light microscopy ~200 nm, EM ~1 nm. These numbers come up frequently.
TEM vs SEM: cross-sectional internal images (TEM) versus surface detail (SEM). Exams often give you an image and ask which technique produced it.
Be able to match each organelle to its primary function in a single sentence. Table-format questions are common.
True or false: SEM produces cross-sectional images of a cell's interior.
False. SEM produces surface images. TEM produces cross-sectional images.
True or false: The nucleolus is a membrane-bound organelle.
False. The nucleolus lacks its own enclosing membrane.
Fill in the blank: Light microscopy can resolve features down to approximately ______ nm.
200 nm.
True or false: Ribosomes can be found either free in the cytosol or attached to the rough ER.
True.
Fill in the blank: The aqueous fluid matrix of the cell, where glycolysis occurs, is called the ______.
Cytosol.
Q: A researcher wants to observe the detailed internal membrane structure of mitochondria. Which imaging technique should they use, and why?
A: Transmission electron microscopy (TEM). TEM achieves resolution down to about 1 nm and produces cross-sectional images, making it suitable for visualising the inner membrane folds (cristae) of mitochondria. Light microscopy cannot resolve these fine details.
Q: Classify the following as membrane-bound or nonmembrane-bound: lysosome, ribosome, Golgi apparatus, proteasome, peroxisome, cytoskeleton.
A: Membrane-bound: lysosome, Golgi apparatus, peroxisome. Nonmembrane-bound: ribosome, proteasome, cytoskeleton.
Q: A student claims that the cytosol and the cytoplasm are the same thing. Is this correct? Explain.
A: No. The cytosol is the aqueous fluid component only. The cytoplasm is broader: it includes the cytosol plus all the organelles and other structures suspended within it.
Q: Name the four subtypes of endosomes and briefly state the role of each.
A: Early endosomes receive endocytosed material. Late endosomes mature and fuse with lysosomes. Recycling endosomes return materials to the plasma membrane. Transport endosomes move cargo within the cell.
Q: What is the approximate resolution of electron microscopy, and how does it compare to light microscopy?
A: Electron microscopy resolves down to about 1 nm. Light microscopy resolves to about 200 nm. EM therefore offers roughly 200 times finer resolution.
This material connects directly to membrane structure and transport (covered in the next set of notes), because understanding which organelles are membrane-bound sets the stage for understanding how membranes work and why selective permeability matters. It also connects to the endomembrane system (ER, Golgi, vesicles, endosomes, lysosomes), which is a major exam topic in its own right. Clinical correlations in later units, such as lysosomal storage diseases and mitochondrial disorders, depend on knowing these compartments and their functions.
Light microscopy, electron microscopy, TEM, SEM, transmission electron microscopy, scanning electron microscopy, resolution limit, membrane-bound organelles, nonmembrane-bound structures, nucleus, mitochondria, powerhouse of the cell, peroxisomes, lysosomes, endosomes, early endosomes, late endosomes, recycling endosomes, transport endosomes, endoplasmic reticulum, ER, rough ER, smooth ER, Golgi apparatus, secretory vesicles, cytosol, cytoplasm, nucleolus, centriole, cytoskeleton, microtubules, microfilaments, intermediate filaments, ribosomes, proteasomes, cell compartments, cellular organisation, cell biology, University of Florida