Cytology: The Study of Cells

Comprehensive question-and-answer flashcards covering cell structure, plasma membrane composition, membrane proteins, intercellular junctions, cytoskeleton, and organelle morphology based on cytology lecture notes.

Who first discovered cells in 1663, and what was he actually observing?

Robert Hook (Hooke); he observed the dead cell walls of plant cells (cork), noting they looked like small rooms or cellula.

Who was the first person to observe a live cell under a microscope?

Anton van Leeuwenhoek in 1674, who described the green algae Spirogyra.

What term refers to the scientific study of cell form and shape?

Morphology.

What is the diameter range of an average cell?

$5\text{--}10\,\mu\text{m}$.

What structural model describes the plasma membrane, and what is its typical thickness?

The fluid mosaic model, with a thickness of $5\text{--}7\,\text{nm}$.

Why are phospholipid molecules classified as amphipathic?

They possess two distinct regions: a polar, hydrophilic head (water-soluble) and a non-polar, hydrophobic tail (water-insoluble).

What occurs automatically when a phospholipid bilayer is disrupted in an aqueous environment?

It automatically reforms into a bilayer, with hydrophilic heads contacting water and hydrophobic tails sequestered inward.

What critical roles does cholesterol play within the plasma membrane?

It immobilizes the initial hydrocarbon groups of phospholipids to stabilize the bilayer, reduces deformability, decreases permeability to small water-soluble molecules, and prevents crystallization.

How do integral and peripheral membrane proteins differ structurally and functionally?

Integral proteins span the bilayer (transmembrane) to serve as transport gates or receptor components, whereas peripheral proteins attach to an inner or outer face to serve as anchors.

What is the glycocalyx and what biological processes does it mediate?

A sugary cell coat of carbohydrates linked to membrane proteins/lipids that mediates cell-cell recognition, sperm-egg recognition, phagocytic targeting signals, and ABO blood grouping.

Which two mechanical cell junctions are present between the fiber cells of the ocular lens?

Ball and socket junctions, and tongue and groove junctions.

How do occluding (tight) junctions (zonulae occludentes) function?

Adjacent proteins in opposing plasma membranes fuse completely, sealing the intercellular space and preventing the passage of molecules between cells.

Which specialized junction is illustrated here, featuring intracellular plaques linked to intermediate filaments and cadherins?

Desmosome (anchoring junction).

What distinguishes a belt desmosome, a spot desmosome, and a hemidesmosome?

A belt desmosome (zonula adherens) encircles a substantial portion of the cell, a spot desmosome (macula adherens) covers a localized point, and a hemidesmosome anchors the cell membrane to the extracellular substrate rather than to an adjacent cell.

What are connexons, and why are gap junctions crucial in the lens of the eye?

Connexons are transmembrane protein complexes forming gap junctions; because the lens lacks a blood supply, they provide cytoplasmic continuity for nutrients from the aqueous humour to diffuse throughout lens cells.

What four basic components comprise the cytoplasm?

Cytosol, cytoskeleton, membrane-bound organelles, and inclusions.

What is the structure, diameter, and cellular abundance of actin microfilaments?

They are approximately $7\text{--}8\,\text{nm}$ in diameter, composed of two helical strands of globular actin monomers, and comprise $10\text{--}30\%$ of total cell volume.

How do v-SNAREs and t-SNAREs mediate exocytosis?

Vesicle v-SNAREs bind target t-SNAREs on the plasma membrane, causing the lipid membranes to corkscrew together until the vesicle ruptures and discharges its contents outward.

What are the three forms of endocytosis?

Phagocytosis (cell eating), bulk-phase endocytosis (pinocytosis / fluid uptake), and receptor-mediated endocytosis (clathrin-dependent uptake).

What is the diameter and molecular composition of microtubules?

They are approximately $25\,\text{nm}$ in diameter, composed of $\alpha$- and $\beta$-tubulin heterodimers assembled into hollow dynamic cylinders.

Which organelle structure is depicted here, displaying a pair of perpendicular cylinders composed of 9 microtubule triplets?

Centrosome / centrioles.

What microtubule arrangement defines the axoneme of cilia and flagella?

A $9+2$ array, consisting of 9 outer microtubule doublets surrounding 2 central singlet microtubules.

Match the intermediate filament type to its primary cell type: keratin, vimentin, desmin, and glial fibrillary acidic protein.

Keratin in epithelial/epidermal cells; vimentin in fibroblasts and white blood cells; desmin in striated and smooth muscle; glial fibrillary acidic protein in astrocytes and Schwann cells.

Which cellular organelle is depicted in this cutaway diagram, showing cristae and an internal matrix?

Mitochondrion.

Why are mitochondria considered semi-autonomous organelles?

They possess their own circular DNA resembling bacterial genomes and their own translation machinery to synthesize some mitochondrial proteins independently.

How do free ribosomes differ functionally from membrane-bound ribosomes on the endoplasmic reticulum?

Free ribosomes synthesize proteins intended for intracellular use, whereas bound ribosomes synthesize proteins destined for incorporation into membranes or packaging for extracellular secretion.

What are the primary functional differences between rough and smooth endoplasmic reticulum?

Rough ER is ribosome-studded and manufactures secretory and membrane proteins; smooth ER lacks ribosomes, synthesizes lipids and steroids, and stores intracellular calcium (as sarcoplasmic reticulum in muscle).

Which organelle is depicted here, featuring flattened cisternae arranged into a cis receiving face and a trans shipping face?

Golgi apparatus (Golgi complex).

What are lysosomes, and what age-related insoluble pigment accumulates when lysosomal digestion is incomplete?

Membrane-bound digestive vesicles ($0.05\text{--}0.2\,\mu\text{m}$) containing acid hydrolases; indigestible residual products accumulate over time as lipofuscin.

What role do nuclear pores play, and what is their size and frequency per typical mammalian nucleus?

They allow transport of macromolecules (such as mRNA) between the nucleus and cytosol; they measure $30\text{--}100\,\text{nm}$ in diameter and number about $3000\text{--}4000$ per nucleus.

What is the primary function and composition of the nucleolus?

It manufactures ribosomes and is rich in ribosomal RNA (rRNA) and proteins.

What are cytoplasmic inclusions, and what are two common examples?

Non-living cellular substances or chemical reserves that vary by cell type; examples include melanin granules (in retinal pigment epithelium) and lipid/fat droplets (energy reserves).