Module 3: Cell Membrane and Transport Mechanisms

Vocabulary practice flashcards covering cell membrane composition, lipid organization, fluid mosaic model, transport mechanisms (passive, active, transepithelial), endocytosis, and membrane potential.

Fluid Mosaic Model

A biological membrane model proposed by Jonathan Singer and Garth Nicolson in 1972 describing the membrane as a fluid phospholipid bilayer approximately $75\,\text{Å}$ thick in which proteins are embedded and capable of lateral movement.

Membrane Asymmetry

The structural difference between the inner and outer monolayers of the plasma membrane, characterized by glycoproteins and glycolipids located exclusively on the extracellular side and distinct distributions of lipids and proteins between leaflets.

Glycerophospholipids

A major class of amphipathic membrane phospholipids containing a glycerol backbone, two fatty acid chains, a phosphate group, and an alcohol-derived head group, such as phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol.

Phosphatidylserine

A specific glycerophospholipid found predominantly on the cytosolic face of the plasma membrane that contributes a net negative electrical charge to the inner surface of the cell.

Phosphosphingolipids

A class of phospholipids built upon a sphingosine backbone rather than glycerol, with sphingomyelin being the primary representative in animal cell membranes.

Cholesterol (Membrane Sterol)

A sterol comprising approximately $20\%$ of membrane lipids in animal cells; it embeds between phospholipids and decreases membrane fluidity at physiological temperatures due to its rigid planar steroid ring core.

Sitosterol

The plant sterol that functionally replaces cholesterol in the plasma membranes of plant cells.

Phospholipid Aggregates (Bilayer, Micelle, Liposome)

Three distinct supramolecular structures that form spontaneously in aqueous environments due to the amphipathic nature of membrane lipids: two-dimensional sheets (bilayers), single-layered spheres with hydrophobic cores (micelles), and spherical vesicles enclosing an aqueous core (liposomes).

Lipid Rafts

Specialized microdomains in the outer leaflet of the plasma membrane that exhibit low fluidity and are enriched in cholesterol, saturated phospholipids, and specific signaling proteins, serving as platforms for cell signaling and pathogen entry.

Caveolae

A specialized subset of lipid rafts visible under electron microscopy as flask-shaped invaginations of the plasma membrane that participate in endocytosis.

Glycocalyx

A carbohydrate-rich protective layer on the extracellular surface of the plasma membrane consisting of oligosaccharide chains from glycolipids and glycoproteins; functions include barrier protection, defense against mechanical/ionic stress, and immune recognition (such as the AB0 blood group system).

Integral Membrane Proteins

Proteins permanently associated with the lipid bilayer, including transmembrane proteins that span the bilayer and lipid-anchored proteins; most extracellular domains are glycosylated.

Peripheral Membrane Proteins

Proteins that do not penetrate the hydrophobic core of the bilayer but associate reversibly with membrane surfaces or other integral proteins, often connecting the membrane to the underlying actin cytoskeleton.

Frye and Edidin Experiment (1970)

A landmark fusion experiment between human and mouse cells labeled with red and green fluorescent antibodies that demonstrated the lateral mobility of membrane proteins, which completely intermixed across the hybrid cell surface within $40\,\text{minutes}$.

Polarized Epithelial Cells

Cells organized into distinct plasma membrane domains with different protein and lipid compositions: an apical surface specialized for nutrient absorption and a basolateral surface specialized for nutrient transfer to adjacent capillaries.

Simple Diffusion

The passive, non-carrier-mediated movement of solutes down their concentration gradient directly through the lipid bilayer; typical for lipophilic molecules, small uncharged molecules ($O_2$, $CO_2$), and limited amounts of $H_2O$.

Fick's Law of Diffusion

A physical law stating that the rate of diffusion across a membrane is directly proportional to surface area, concentration gradient, and membrane permeability, and inversely proportional to membrane thickness: $\text{Rate} \propto \frac{\text{Surface Area} \times \text{Concentration Gradient} \times \text{Permeability}}{\text{Membrane Thickness}}$.

Facilitated Diffusion

A form of passive transport across membranes down an electrochemical or concentration gradient mediated by selective integral proteins (channels or carriers) without cellular energy expenditure.

Porins

Transmembrane pore-forming channel proteins that permit the free passive diffusion of ions and small polar molecules across the outer membranes of bacteria, mitochondria, and chloroplasts.

Connexons (Gap Junctions)

Multimeric channel structures that bridge adjacent animal cells to allow direct intercellular passage of ions and small signaling molecules without exposure to the extracellular fluid.

Aquaporins

Specialized tetrameric water channel proteins (e.g., AQP1) that facilitate rapid, bidirectional passage of water molecules while completely excluding ions to preserve membrane electrochemical gradients.

Carrier Proteins (Transporters)

Membrane transport proteins that selectively bind specific solute molecules and translocate them across the bilayer via a cycle of alternating conformational changes, operating slower than open channels and never exposing both sides simultaneously.

Uniport, Symport, and Antiport Carriers

The three modes of carrier-mediated transport: uniport moves a single solute type; symport co-transports two or more substrates in the same direction; antiport moves two or more substrates in opposite directions.

Primary Active Transport

Energy-dependent transport that moves solutes against their electrochemical gradients directly powered by the hydrolysis of adenosine triphosphate ($\text{ATP}$) via membrane-bound ATPases.

$$Na^+/K^+\text{-ATPase Pump}$$

An electrogenic primary active antiport carrier that hydrolyzes one $\text{ATP}$ molecule to pump $3\,Na^+$ out of the cell and $2\,K^+$ into the cell, maintaining intracellular low $Na^+$ and high $K^+$ concentrations.

$$Ca^{2+}\text{-ATPase Pump}$$

A primary active transport pump powered by $\text{ATP}$ hydrolysis that maintains extremely low resting cytosolic $Ca^{2+}$ levels by pumping calcium into the extracellular fluid or into the sarcoplasmic/endoplasmic reticulum lumen.

ABC Transporters

A large superfamily of ATP-Binding Cassette transport proteins that utilize $\text{ATP}$ hydrolysis to translocate nutrients into prokaryotic cells or pump toxic compounds and drugs out of eukaryotic cells.

MDR Transporters (Multidrug Resistance)

ABC-family active efflux pumps overexpressed in many cancer cells that actively extrude chemotherapeutic agents across the plasma membrane, conferring multidrug resistance.

CFTR (Cystic Fibrosis Transmembrane Conductance Regulator)

An ABC transporter family member functioning as an $\text{ATP}$-regulated chloride channel; loss-of-function mutations cause defective chloride ion transport resulting in cystic fibrosis.

Secondary Active Transport

Transport of a solute uphill against its concentration gradient driven by the potential energy stored in an electrochemical gradient of another ion (typically $Na^+$) previously established by primary active transport.

SGLT (Sodium-Glucose Transporter)

An apical secondary active symporter that harnesses the inwardly directed $Na^+$ electrochemical gradient to co-transport glucose uphill into renal tubular and intestinal epithelial cells.

Transepithelial Glucose Transport

Polarized vector transport across an epithelial cell monolayer involving apical uptake of glucose against its gradient via SGLT symport, basolateral exit into blood via GLUT-mediated facilitated diffusion, and $\text{Na}^+/\text{K}^+\text{-ATPase}$ activity to maintain the driving $Na^+$ gradient.

Phagocytosis

A form of endocytosis discovered by Christian de Duve in which actin-mediated pseudopodia engulf large particles, cell debris, or intact bacteria into a phagosome, which subsequently fuses with a lysosome to form a phagolysosome.

Macropinocytosis

A non-specific endocytic mechanism involving the extension of actin-rich lamellipodia (sheet-like projections) to enclose and ingest large volumes of extracellular fluid.

Clathrin-Mediated Endocytosis

Selective vesicular uptake of receptor-bound cargo molecules via clathrin-coated pits, where cytosolic adaptor proteins recruit clathrin triskelions to generate an invaginating basket-like curved coat.

Dynamin

A large GTPase protein that assembles as a helical ring around the neck of budding clathrin-coated pits and hydrolyzes $\text{GTP}$ to induce membrane fission and liberate endocytic transport vesicles.

Receptor-Mediated Endocytosis of LDL

The endocytic pathway in which low-density lipoprotein (LDL) binds its specific plasma membrane receptor, undergoes clathrin-mediated internalization, uncoating, delivery to endosomes, and transport to lysosomes for free cholesterol release, while the receptor is recycled back to the cell surface.

Resting Membrane Potential

The steady-state electrical potential difference across the plasma membrane (typically $-60\,\text{mV}$ in squid axons) generated by high permeability to $K^+$ through leak channels and non-diffusible intracellular anionic proteins, making the cytoplasmic surface negative relative to the extracellular fluid.

Voltage-Gated Action Potential Cycle

The sequential electrical changes during an impulse along an excitable axon: resting state ($-60\,\text{mV}$), opening of voltage-gated $Na^+$ channels causing depolarization ($+30\,\text{mV}$), inactivation of $Na^+$ channels and opening of voltage-gated $K^+$ channels causing repolarization/hyperpolarization ($-75\,\text{mV}$), and return to resting potential.

Chemical Synapse

A junction between an excitable neuron and a target cell where depolarization promotes $Ca^{2+}$ entry, triggering the exocytosis of neurotransmitters (e.g., acetylcholine) into the synaptic cleft to bind postsynaptic receptors (ligand-gated channels or GPCRs).