Functional Organization of the Cell

Vocabulary flashcards covering the major topics from the lecture notes on membrane structure, organelles, trafficking, cytoskeleton, and epithelial cell organization.

Phospholipid bilayer

A double layer of phospholipid molecules forming the basic structure of cellular membranes; hydrophilic heads face water, hydrophobic tails face inward.

Monolayer

A single layer of phospholipid molecules; forms at air–water interfaces.

Glycerol

The three-carbon backbone of phospholipids to which fatty acids and a phosphate-containing headgroup attach.

Phosphate group

Hydrophilic head group of many phospholipids; interacts with the aqueous environment.

Fatty acid tails

Nonpolar hydrocarbon chains that form the hydrophobic core of membranes.

Hydrophobic

Water-fearing property of lipid tails that avoids contact with water.

Hydrophilic

Water-loving property of polar head groups facing the aqueous environment.

Cholesterol

A sterol lipid intercalated in membranes; modulates fluidity and stability.

Sphingolipid

A class of membrane lipids including sphingomyelin and glycosphingolipids that contribute to membrane structure and signaling.

Phosphatidylcholine

A common outer-leaflet phospholipid with a choline headgroup.

Phosphatidylserine

An inner-leaflet phospholipid; its externalization signals apoptosis and marks inner surface.

Lipid asymmetry

Unequal distribution of lipids between the outer and inner leaflets of a membrane.

Lateral mobility

Diffusion of phospholipids and cholesterol within the same leaflet of the bilayer.

Peripheral membrane protein

Proteins bound noncovalently to membrane surfaces or to integral proteins; not embedded in the bilayer.

Integral membrane protein

Proteins embedded in the lipid bilayer; often span the membrane.

Transmembrane domain

Membrane-spanning region of an integral protein, typically an alpha helix.

Alpha helix

A common membrane-spanning secondary structure of proteins.

Receptor

Membrane protein that binds signaling molecules and initiates cellular responses.

Adhesion protein

Proteins that mediate attachment to extracellular matrix or adjacent cells (two types: ECM-binding and cell–cell adhesion).

Transport protein

Proteins that move ions or molecules across membranes; includes channels, carriers, and ATP-driven pumps.

Enzyme (membrane)

Membrane-bound enzymes that catalyze reactions at or near the membrane.

Endoplasmic reticulum (ER)

Organelle where membrane and secreted proteins are synthesized, folded, and initially glycosylated.

Signal sequence

A short amino acid sequence that directs ribosome targeting to the ER.

Signal recognition particle (SRP)

Cytosolic complex that recognizes the signal sequence and pauses translation to target the ribosome–nascent chain to the ER.

SRP receptor

ER membrane receptor that binds SRP to dock the ribosome–nascent chain at the translocon.

Translocon

Protein-conducting channel in the ER membrane through which polypeptides enter the ER lumen.

Stop-transfer sequence

A hydrophobic segment that halts translocation and anchors the protein in the membrane.

Glycosylation

Addition of carbohydrate groups to proteins; occurs in the ER and Golgi; includes N- and O-linked forms.

N-linked glycosylation

Attachment of sugars to asparagine residues, typically initiated in the ER.

O-linked glycosylation

Attachment of sugars to serine or threonine residues, mainly in the Golgi.

Golgi apparatus

Organelle where proteins are further processed, glycosylated, and sorted for trafficking.

Vesicle

Membrane-bound carrier that transports proteins between organelles and to the plasma membrane.

Constitutive secretion

Continuous, unregulated secretion of secretory proteins to the plasma membrane.

Regulated secretion

Secretion that occurs in response to specific hormonal or neural signals.

SNARE

Family of proteins that drive vesicle fusion with target membranes; work with SNAPs and Rab GTPases.

Rab GTPase

Small GTPases that regulate vesicle trafficking and docking to specific membranes.

Clathrin

Coat protein that forms vesicles during endocytosis and certain trafficking steps.

SNAP

Soluble NSF attachment protein; cooperates with SNAREs to mediate vesicle fusion.

Mannose-6-phosphate

Carbohydrate tag added in the Golgi that targets lysosomal enzymes to lysosomes.

M6P receptor

Receptor recognizing M6P-tagged enzymes to direct them to lysosomes.

Lysosome

Acidic organelle containing hydrolytic enzymes for degradation of proteins

Nuclear pore complex

Large protein complex spanning the nuclear envelope that regulates transport between nucleus and cytosol.

Nuclear envelope

Double membrane surrounding the nucleus; contiguous with the ER.

Nucleolus

Nuclear substructure where ribosomal RNA is synthesized and ribosome assembly begins.

Chromatin

DNA–protein complex within the nucleus that forms chromosomes.

Mitochondrion

Organelle producing most cellular ATP; Ox-phos, maintains a proton gradient across the inner membrane. double membrane

Rough endoplasmic reticulum

ER region studded with ribosomes; site of synthesis for secreted and membrane proteins.

Smooth endoplasmic reticulum

ER region involved in lipid synthesis and detoxification; lacks ribosomes.

Brush border

Microvilli-rich surface of intestinal epithelial cells supported by actin filaments.

Actin

Thin filament protein (G-actin polymerizes to F-actin); involved in motility and structure.

Myosin

Molecular motor that interacts with actin to generate contractile force.

Transepithelial transport

Coordinated transport across apical and basolateral membranes in epithelial cells.

Tight junction

Junctional complex formed by claudins that seals spaces between cells.

Adhering junction

Junction with cadherins linking cells to the actin cytoskeleton.

Gap junction

Intercellular channel formed by connexons that allows direct cytoplasmic exchange.