Chapter 2: The Cytoplasm and Cellular Structures

Vocabulary practice cards focusing on cell biology concepts including cell membrane transport, organelles, protein sorting pathways, and cytoskeletal structures.

P face (Protoplasmic face)

The internal leaflet of the plasma membrane lipid bilayer that directly faces the cytoplasm, as visualized using freeze-fracture techniques.

E face (Extracellular face)

The outer leaflet of the plasma membrane lipid bilayer that faces the extracellular environment, observed in cryofracture preparations.

Fluid mosaic model

The structural concept describing the plasma membrane as a dynamic lipid bilayer containing proteins, glycolipids, and cholesterol that move laterally within the membrane.

Phagocytosis

A form of vesicular transport in which pseudopodia extend from the plasma membrane to ingest large particles into a phagosome for digestion.

Pinocytosis

A vesicular transport process involving the non-specific ingestion of extracellular fluid through small invaginations of the plasma membrane.

Transcytosis

The vesicular movement of fluids or molecules across a cell from one surface membrane domain to the opposite surface domain.

Receptor-mediated endocytosis

A highly specific cell transport process triggered by the binding of extracellular ligands to surface receptor proteins, internalized via clathrin-coated pits.

Clathrin

A coat protein that forms a cage-like lattice on the cytoplasmic side of coated pits to facilitate vesicle budding during receptor-mediated endocytosis.

Dynamin

A protein enzyme that forms a ring around the neck of an invaginated coated pit to pinch off the coated vesicle from the cell membrane.

Free polyribosomes

Cytoplasmic clusters of ribosomes that synthesize cytosolic and cytoskeletal proteins, as well as proteins imported into mitochondria, peroxisomes, and the nucleus.

ER-bound polyribosomes

Ribosomes attached to the cytosolic surface of the rough ER that synthesize membrane proteins, lysosomal enzymes, and secreted proteins.

Proteasome

A protein complex responsible for degrading misfolded or denatured proteins that have been conjugated to ubiquitin.

Signal Recognition Particle (SRP)

A cytosolic signal-binding complex that binds to the initial signal peptide of a nascent polypeptide chain and targets the ribosome to the RER membrane.

COP-II

A coat protein that promotes forward (anterograde) vesicle transport from the rough endoplasmic reticulum to the cis Golgi network.

COP-I

A coat protein that mediates retrograde vesicle transport from the cis Golgi network back to the rough endoplasmic reticulum.

Mannose-6-phosphate

A molecular signal added to newly synthesized proteins within the cis Golgi network that targets them for transport to lysosomes.

cis Golgi network (CGN)

The receiving face of the Golgi apparatus where vesicles from the RER fuse, N-linked oligosaccharides are trimmed, and lysosomal targeting tags are added.

trans Golgi network (TGN)

The exit face of the Golgi apparatus where final oligosaccharide modifications, such as sialic acid addition and sulfation, occur before sorting into specific destination vesicles.

Secretory vesicles

Membrane-bound structures carrying condensed secretory products from the Golgi network to the plasma membrane, typically appearing as intensely eosinophilic granules.

Lysosomes

Acidic membrane-bound organelles containing hydrolytic enzymes responsible for intracellular digestion that do not stain clearly with standard H&E stains.

Autophagosome

A double-membrane vacuole formed around damaged organelles, such as mitochondria, which subsequently fuses with primary lysosomes for degradation.

Microtubules

Cytoskeletal elements structured as hollow tubes ($25\,nm$ diameter) made of 13 parallel protofilaments of $\alpha\beta$-tubulin heterodimers ($54\,kDa$).

Dynamic instability

The rapid alternation between growth and shrinkage phases in microtubules regulated by GTP hydrolysis and free GTP-tubulin monomer concentration.

Centrosome

The principal microtubule-organizing center of the cell, composed of two orthogonal centrioles consisting of microtubule triplets connected by protein links.

Microfilaments

Cytoskeletal filaments ($5-7\,nm$ diameter) composed of two intertwined strands of F-actin built from globular G-actin monomers ($42\,kDa$).

Actin treadmilling

The steady-state assembly mechanism where G-actin monomers add to the plus (+) end while simultaneously dissociating from the minus (-) end of a microfilament.

Intermediate filaments

Stable, non-polar cytoskeletal fibers ($8-10\,nm$ diameter) formed by antiparallel tetramers of rod-like protein dimers (~$55\,kDa$) that provide structural stability to tissues.