Role of Membrane Transporters in Cell Function

Vocabulary practice flashcards covering cell membrane structure, fluid mosaic model properties, passive and active transport mechanisms, vesicular transport, and body fluid compartments.

Cell Membrane Thickness

The structural dimension of the plasma membrane, measuring between 75\text{--}100\text{ \text{\normalfont\normalcolor\r{A}}} ($10\text{ nm}$).

Fluid Mosaic Model

A membrane model where phospholipids form a thin, flexible sheet with a fluid hydrophobic core (resembling salad oil), in which discrete globular proteins float in a mosaic pattern and carbohydrates extend outward.

Plasma Membrane Composition

The gross weight distribution of the cell membrane, consisting of approximately $50\%$ lipid and $50\%$ protein by weight, along with $2\text{--}10\%$ ($\sim 3\%$) carbohydrates.

Cholesterol (Membrane)

A lipid comprising approximately $13\%$ of the membrane that links fatty acid chains together in animal cells, thereby stabilizing and strengthening the membrane.

Integral Proteins

Intrinsic membrane proteins that span from one side of the phospholipid bilayer to the other (often multiple times) and function as transporters, receptors, or anchors to the cytoskeleton.

Peripheral Proteins

Extrinsic membrane proteins that do not span the bilayer but are loosely bound to the membrane surface, performing structural or enzymatic functions and occupying approximately $20\%$ of the membrane surface.

Glycocalyx

A carbohydrate coat covering the outer surface of the cell membrane formed by the oligosaccharide chains of glycolipids and glycoproteins.

Tight Junctions

Intercellular connections between epithelial and endothelial cells that form natural barriers to separate two extracellular environments and regulate the passage of molecules.

Gap Junctions

Intercellular connections formed by cylindrical proteins that create channels between adjacent cells, allowing the passage of dissolved substances (ions, $\text{cAMP}$, glucose) and enabling electrical synapses in cardiac and smooth muscle.

Simple Diffusion

Passive net movement of lipid-soluble substances (e.g., steroids, alcohol, urea) and small non-polar molecules ($O_2$, $CO_2$, $H_2O$) directly through the lipid bilayer from high to low concentration without energy input.

Fick's Law of Diffusion

A principle stating that the rate of diffusion is determined by: $\text{Rate of diffusion} = \frac{\text{conc. gradient} \times \text{surface area}}{\text{membrane resistance} \times \text{membrane thickness}}$.

Facilitated Diffusion

Passive, carrier-mediated transport that moves substances down their concentration gradient without energy expenditure, characterized by specificity and saturation.

Channel Proteins

Transport proteins that form water-filled pores across the membrane for rapid, less selective movement of small molecules such as water and ions.

Carrier Proteins

Highly selective transport proteins that possess specific binding sites and undergo conformational changes to transport solutes across the membrane at relatively slower rates without directly connecting ECF and ICF.

Gated Channels

Pore-forming channel proteins whose open or closed state is regulated by specific internal or external signals, such as membrane potential (voltage gating) or ligand binding (chemical gating).

Uniport

A type of carrier-mediated transport that moves a single substance across the membrane in one direction.

Symport

A form of cotransport wherein two or more different molecules or ions are transported across the membrane in the same direction.

Antiport

A form of cotransport wherein two or more different molecules or ions are transported across the membrane in opposite directions.

Osmosis

The net diffusion of water molecules across a semi-permeable membrane down its concentration gradient, moving from a solution of low solute concentration to one of high solute concentration.

Tonicity

The ability of solutes within a solution to exert an osmotic pressure and generate a driving force for the net movement of water molecules across a membrane.

Isotonic Solution

A solution that has an equal osmotic pressure (or concentration) relative to the intracellular fluid of a cell, resulting in no net movement of water.

Hypertonic Solution

A solution that has a higher osmotic pressure (or solute concentration) than the interior of a cell, driving water out of the cell.

Hypotonic Solution

A solution that has a lower osmotic pressure (or solute concentration) than the interior of a cell, driving water into the cell.

Active Transport

The carrier-mediated pumping of substances against their concentration gradient (from low to high concentration) utilizing metabolic energy in the form of $ATP$.

$Na^+/K^+\text{-ATPase}$ (Sodium-Potassium Pump)

A primary active antiport pump found in all animal cells that expels $Na^+$ and imports $K^+$, utilizing up to $30\%$ of total cellular $ATP$ ($70\%$ in nerve cells) to maintain ionic gradients.

Secondary Active Transport

Transport of a solute against its gradient powered indirectly by the potential energy stored in an ion concentration gradient (such as the $Na^+$ gradient created by primary active transport), exemplified by the $Na^+/\text{glucose}$ symporter (SGLT).

Endocytosis

The energy-dependent process of surrounding extracellular substances with the cell membrane to form an internalized vesicle.

Pinocytosis

A nonselective form of endocytosis that internalizes extracellular fluid droplets and small dissolved molecules.

Phagocytosis

An active endocytic process whereby specialized immune cells (neutrophils, macrophages, dendritic cells) engulf large particles into a phagosome via pseudopod extension, which then fuses with lysosomes.

Exocytosis

The energy- and $Ca^{2+}$-dependent process of exporting materials out of a cell through the fusion of intracellular vesicles (derived from the RER and Golgi apparatus) with the plasma membrane.

Constitutive Exocytosis

A continuous, unregulated pathway of vesicle fusion and secretion that takes place constantly in all cells.

Regulated Exocytosis

A secretory pathway coupled to external triggering signals, such as the release of insulin, histamine, or neurotransmitter molecules.

Transcytosis

A vesicular transport pathway in which large intact proteins cross an endothelial or epithelial cell by undergoing endocytosis at one surface and exocytosis at the opposite surface.

Transepithelial Transport

The directional transport of solutes across an epithelial cell layer mediated by the asymmetric distribution of transport proteins between the apical and basolateral membrane surfaces.

Body Fluid Distribution

The division of total body water ($40\text{ L}$, $60\%$ of body weight) into intracellular fluid ($25\text{ L}$, $40\%$ of body weight) and extracellular fluid ($15\text{ L}$, $20\%$ of body weight, consisting of $12\text{ L}$ interstitial fluid and $3\text{ L}$ plasma).

Extracellular Fluid Ion Profile

The compartment solution characterized by $Na^+$ as the principal cation and chloride ($Cl^-$) and bicarbonate ($HCO_3^-$) as the principal anions.

Intracellular Fluid Ion Profile

The compartment solution characterized by $K^+$ as the principal cation and proteins, amino acids, and phosphates as the principal anions.