Cell Membrane Transport and Tissue Organization

Flashcards covering membrane transport mechanisms (pumps, carriers, channels), membrane potentials, tissue organization, and types of cell junctions.

Passive Transport

Movement of molecules from high to low concentration without requiring energy, including simple and facilitated diffusion.

Simple Diffusion

The process where small, nonpolar molecules like $CO_2$ and $O_2$ move directly across the lipid bilayer down their concentration gradient.

Facilitated Diffusion

Passive transport through specific carrier or channel proteins, such as glucose transporters or ion channels, moving substances down their concentration gradient.

Active Transport

Movement of molecules against their concentration or electrochemical gradient, requiring energy usually provided by ATP hydrolysis.

Primary Active Transport

A mechanism that uses ATP directly to transport ions or molecules, such as the $Na^+/K^+-ATPase$ or $Ca^{2+}-ATPase$.

Secondary Active Transport

Utilizes energy stored in an ion gradient established by primary transporters to drive the movement of another molecule against its gradient.

Sodium-Potassium Pump ($Na^+/K^+-ATPase$)

A primary active transporter that pumps $3\,Na^+$ out and $2\,K^+$ into the cell per ATP molecule to maintain ion gradients and cell volume.

Sodium-Glucose Symporter (SGLT)

An example of secondary active transport where $Na^+$ moves down its gradient into the cell to drive the transport of glucose against its gradient.

Osmosis

The diffusion of water molecules through a selectively permeable membrane from an area of high water concentration to an area of low concentration.

Membrane Potential

The electrical potential difference across a cell membrane caused by differences in the concentration of inorganic ions.

Electrochemical Gradient

The combined influence of a chemical gradient (concentration difference) and an electrical gradient (charge difference) on ion movement.

Resting Membrane Potential (RMP)

The electrical potential difference across the plasma membrane when a cell is not signaling; typically between $-60\,mV$ and $-70\,mV$ in animal cells.

Aquaporins

Specialized channel proteins in the plasma membrane that facilitate the rapid diffusion of water molecules.

P-type ATPases

Pump proteins that autophosphorylate during the transport cycle to move cations like $Na^+$, $K^+$, and $Ca^{2+}$ across membranes.

V-type ATPases

Multi-subunit complexes that pump protons ($H^+$) into organelles like lysosomes and vacuoles to create an acidic environment.

F-type ATPases

Also called ATP synthase, these primarily use proton gradients to synthesize ATP, though they can also function in transport.

Carrier Proteins (Transporters)

Integral membrane proteins that move specific substances by undergoing conformational changes to transfer molecules from one side of the membrane to the other.

Ion Channels

Transmembrane proteins forming hydrophilic pores that allow specific inorganic ions to pass rapidly and selectively across the lipid bilayer.

Selectivity Filter

The part of an ion channel pore, lined with carbonyl oxygen atoms, that determines which specific ions can pass through.

Voltage-gated Channels

Channels that open or close in response to changes in the membrane potential, such as the $Na^+$ channels involved in nerve impulses.

Ligand-gated Channels

Channels that open when a specific molecule (ligand) binds to the extracellular or intracellular side of the protein.

Depolarization

The process where the resting membrane potential becomes less negative (more positive), often caused by an influx of $Na^+$ ions.

Repolarization

The return of the membrane potential to its negative resting value following depolarization, driven by the efflux of $K^+$ ions.

Extracellular Matrix (ECM)

A complex network of proteins and polysaccharides secreted by cells that provides structural support and biochemical signaling to tissues.

Collagen

The most abundant ECM protein, providing tensile strength to tissues; Type I is common in connective tissue, while Type IV is found in the basal lamina.

Basal Lamina

A thin, tough sheet of ECM consisting of type IV collagen and laminin that supports the basal surface of epithelial sheets.

Tight Junctions (Occluding Junctions)

Junctions composed of claudins and occludins that form a seal between epithelial cells to prevent molecule passage between them.

Adherens Junctions

Anchoring junctions that connect the actin cytoskeleton of one cell to the actin cytoskeleton of a neighbor using cadherin proteins.

Desmosomes

Junctions that provide strong adhesion by linking the keratin intermediate filaments of adjacent cells through cadherin family proteins.

Gap Junctions

Communicating junctions formed by connexons that allow direct intercytosolic exchange of ions and small molecules under $1\,kDa$.

Hemidesmosomes

Structures where integrins anchor the keratin filaments of an epithelial cell to the underlying basal lamina.

Integrins

Transmembrane proteins that facilitate cell-extracellular matrix adhesion and transmit signals between the environment and the cell interior.