Cell Membrane Structure, Passive Transport, and Active Transport

A comprehensive vocabulary review of plasma membrane structure, historical models, passive and facilitated transport, osmosis, water potential, and active bulk/coupled transport mechanisms.

Active Transport

An energy-requiring cellular process where a transport protein pumps a substance across a biological membrane against its concentration gradient (from $[Lo]$ to $[Hi]$), which is energetically uphill and requires $ATP$.

Directly Coupled Active Transport

A mechanism of active transport involving the direct binding of a transport protein with both $ATP$ and the transported substance, triggering a conformational shape change as $ATP$ is hydrolyzed into $ADP + P_i$.

Uniport

A transport protein that carries only one specific substance or ion across the membrane in a single direction, such as a proton pump.

Symport

A cotransport protein that moves two different substances across the plasma membrane simultaneously in the same direction.

Antiport

A transport protein that moves two different substances across the membrane simultaneously in opposite directions, exemplified by the sodium-potassium pump.

Sodium-Potassium Pump

An active transport antiport carrier protein that expels $3\,Na^+$ ions out of the cell and imports $2\,K^+$ ions into the cell per cycle using $ATP$ phosphorylation, maintaining ionic gradients and cell resting membrane potential.

Resting Membrane Potential (RMP)

The steady electrical charge difference across a resting cell membrane, characterized by a positive exterior relative to the negative interior in neurons, maintained by the sodium-potassium pump.

Cotransport

A coupled active transport mechanism where a primary active pump uses $ATP$ to generate potential energy in the gradient of substance 1 (e.g., $H^+$), and a secondary transport protein couples the downhill diffusion of substance 1 with the uphill transport of substance 2 (e.g., sucrose).

Endocytosis

A bulk transport process involving the infolding and invagination of the plasma membrane to enclose extracellular materials within an intracellular vesicle or vacuole.

Phagocytosis

A form of endocytosis ('cell eating') wherein a cell uses pseudopodia to engulf large solid particles, such as food or pathogens, enclosing them in a phagosome or food vacuole.

Pinocytosis

A form of endocytosis ('cell drinking') during which the plasma membrane invaginates to take up droplets of extracellular fluid containing dissolved solutes into small vesicles.

Receptor-Mediated Endocytosis

A selective form of endocytosis where specific extracellular ligands bind to receptors concentrated in coated pits on the plasma membrane, pinching inward to form coated vesicles.

Exocytosis

A bulk transport process involving the fusion of an intracellular vesicle or vacuole with the plasma membrane to discharge its luminal contents to the exterior of the cell.

Egestion

A form of exocytosis specifically responsible for expelling undigested, non-metabolic solid waste materials from a cell or organism.

Excretion

The removal and discharge of metabolic waste substances from an organism, including urine (water, salts, urea), sweat, and exhaled $CO_2$.

Secretion

The exocytic export of functionally synthesized materials by a cell, such as saliva, pancreatic enzymes, mucus from goblet cells, and oil from sebaceous glands.

Contractile Vacuole

A specialized osmoregulatory organelle in freshwater unicellular organisms (such as Paramecium) that collects excess cytoplasmic water via radial canals and contracts to expel it through a pore.

Cystic Fibrosis (CF)

An autosomal recessive genetic disorder caused by mutations in the $Cl^-$ transport protein gene, resulting in thick mucus buildup, impaired $O_2$ diffusion in the lungs, and increased susceptibility to infections.

CFTR Protein

The chloride transport protein located on the surface of epithelial lung membranes that normally pumps $Cl^-$ to establish an osmotic gradient pulling water onto the surface to thin the mucus.

Charles Overton (1894)

Researcher who observed that lipid and lipid-soluble molecules enter cells more rapidly than water-soluble substances, proposing that the cell boundary is composed of lipids based on 'like dissolves like'.

Irving Langmuir (1917)

Physical chemist who demonstrated that amphipathic phospholipids spread on an aqueous surface form an artificial monolayer with hydrophilic heads immersed in water and hydrophobic tails facing outward.

Gorter and Grendel (1925)

Scientists who extracted lipids from red blood cells and found their surface area was twice that of the cells, concluding the plasma membrane is organized as a phospholipid bilayer.

Davson-Danielli Model (1935)

The 'sandwich model' of membrane structure proposing that a central phospholipid bilayer is surrounded on both surfaces by continuous sheets of hydrophilic globular proteins.

Trilaminar Appearance

The distinct three-layered profile of the plasma membrane revealed by electron microscopy in the 1960s, showing a total thickness of $7\text{--}8\,nm$ with two electron-dense outer layers and a central electron-lucent layer.

Fluid Mosaic Model (1972)

The structural membrane model proposed by Singer and Nicolson depicting the plasma membrane as a fluid matrix of amphipathic phospholipids interspersed with dispersed, embedded globular proteins.

Freeze-Fracture Technique

A microscopic preparation method in which frozen cells are cracked along the interior hydrophobic plane between the two phospholipid leaflets, demonstrating that proteins are embedded within the bilayer.

Amphipathic Molecule

A chemical structure containing both a hydrophilic (polar, water-soluble) region and a hydrophobic (nonpolar, water-insoluble) region, characteristic of phospholipids and integral membrane proteins.

Phospholipid Bilayer

The fundamental two-layered matrix of biological membranes, oriented such that polar phosphate heads face aqueous compartments and nonpolar hydrocarbon fatty acid tails face inward toward one another.

Cholesterol

A steroid lipid found exclusively in animal cell membranes that modulates membrane fluidity, preventing crystallization and packing of fatty acid tails at low temperatures.

Glycocalyx

The carbohydrate-rich outer surface of animal cell membranes made of oligosaccharides attached to lipids (glycolipids) and proteins (glycoproteins), functioning in cell recognition and protection.

Integral Proteins

Amphipathic membrane proteins that are at least partially embedded within the hydrophobic core of the phospholipid bilayer, frequently spanning it entirely as transmembrane alpha-helices.

Peripheral Proteins

Extrinsic membrane proteins that do not enter the hydrophobic interior of the bilayer, instead binding loosely to integral proteins or cytoplasmic cytoskeletal elements.

Aquaporins

Specialized channel transport proteins in the cell membrane that selectively facilitate the rapid passive transport of water molecules across the lipid bilayer.

Primary Cell Wall

A thin, flexible layer secreted by growing plant cells that consists of cellulose microfibrils embedded in a matrix of hemicellulose and pectin.

Middle Lamella

The sticky, pectin-rich intercellular layer that cements together the primary cell walls of adjacent plant cells.

Secondary Cell Wall

A thick, rigid structural layer deposited between the primary cell wall and the plasma membrane in mature plant cells, commonly reinforced with lignin to support against gravity.

Selective Permeability

The property of cellular membranes that allows small nonpolar molecules to diffuse freely across the bilayer while restricting or regulating the entry of ions and polar substances via transport proteins.

Simple Diffusion

The spontaneous, exergonic ($-\Delta G$) net movement of solute particles down their concentration gradient directly through the phospholipid bilayer without the involvement of transport proteins.

Facilitated Diffusion

The passive, spontaneous movement of ions or polar molecules down their concentration gradient through the membrane facilitated by specific channel or carrier transport proteins without metabolic energy input.

Transport Protein Saturation

The functional limit reached in facilitated diffusion when all available binding sites on transport proteins are occupied, causing the rate of transport to plateau at its maximum.

Competitive Inhibition of Transport Proteins

A reduction in transport rate occurring when an inhibitor molecule structurally mimics the normal substrate and binds to the carrier protein's active binding site, preventing normal solute transport.

Osmosis

The net passive movement of water molecules across a selectively permeable membrane from an area of higher water concentration (low solute) to an area of lower water concentration (high solute).

Osmotic Concentration

The total solute concentration of a given solution, typically expressed in terms of percentage concentration or molarity, that determines the osmotic movement of water.

Hypertonic Solution

A surrounding solution having a higher solute concentration (and lower free energy of water) than the interior of a cell, driving water out of the cell.

Hypotonic Solution

A surrounding solution having a lower solute concentration (and higher free energy of water) than the interior of a cell, causing a net influx of water into the cell.

Isotonic Solution

A solution that has a solute concentration equal to that of the cell interior, resulting in dynamic equilibrium with no net movement of water across the membrane.

Plasmolysis

The process occurring in plant cells in a hypertonic environment where water exits the central vacuole, causing the protoplast to shrink and the plasma membrane to pull away from the cell wall.

Osmotic Pressure

The measure of a solution's tendency to take up water by osmosis, defined as the hydrostatic pressure needed to prevent water influx from pure water across a semipermeable membrane (pure water = $0$).

Osmotic Potential

The measure of the tendency of a solution to donate water to another solution; assigned a value of $0$ for pure water and values less than zero ($< 0$) for all solute-containing solutions.

Water Potential ($\Psi$)

The measure of the free energy of water, calculated as the sum of solute potential and pressure potential ($\Psi = \Psi_s + \Psi_p$), where the solute potential is determined by $\Psi_s = -iCRT$.