Vocab Membranes & Transport

Membrane structure

Biological membranes consist of lipids and proteins assembled into a thin film that separates the aqueous solution outside a cell from the aqueous solution inside it; phospholipids and sterols are the two major membrane lipid types.

Phospholipid

A membrane lipid composed of glycerol (a 3-carbon polyalcohol), two nonpolar/hydrophobic fatty acids, and a polar/hydrophilic phosphate group; spontaneously forms a bilayer with fatty acids on the inside and phosphate groups on both surfaces.

Sterol

A membrane lipid with dual solubility, having nonpolar carbon rings with a nonpolar side chain at one end and a single polar (—OH) group at the other; aligns with the nonpolar bilayer interior, polar end facing the surface.

Cholesterol

The main sterol in animal cell membranes; helps regulate membrane fluidity depending on concentration and temperature.

Fluid mosaic model

The model proposing that a membrane consists of a fluid phospholipid bilayer in which proteins are embedded and float freely; "fluid" refers to phospholipids moving and exchanging places within a layer, "mosaic" refers to the scattered arrangement of membrane proteins.

Integral protein

A membrane protein embedded within the phospholipid bilayer.

Peripheral protein

A membrane protein held to membrane surfaces by noncovalent bonds, not embedded in the bilayer.

Lipid-anchored protein

A membrane protein anchored to the membrane by means of a lipid molecule.

Membrane fluidity

The mobility of membrane components; increased by unsaturated fatty acid chains at low temperatures, and modulated by cholesterol, which prevents stiffening at low temperatures and stabilizes/decreases fluidity at higher temperatures.

Frye and Edidin experiment

A study fusing human and mouse cells with differently fluorescently labeled membrane proteins, showing that after 40 minutes the labeled proteins mixed completely, demonstrating that membrane proteins move within the fluid phospholipid bilayer.

Freeze fracture

A research method used to visualize the interior structure of membranes.

Selective permeability

The property of biological membranes by which hydrophobic molecules move freely through the lipid bilayer, hydrophilic molecules move through slowly, and charged atoms/molecules are blocked by the hydrophobic core.

Passive transport

Transport that uses energy from the concentration gradient itself, without direct energy expenditure by the cell; includes simple diffusion, facilitated diffusion, and osmosis.

Active transport

Transport of substances across a membrane against a concentration gradient, requiring energy input; primary active transport uses ATP energy directly, secondary uses energy in forms other than ATP.

Simple diffusion

Passive transport through the lipid part of a biological membrane, depending solely on molecular size and lipid solubility; used by nonpolar gases (O2, N2, CO2) and other nonpolar organic molecules, driven by the concentration gradient.

Facilitated diffusion

Passive, specific diffusion of polar and charged molecules (such as water, amino acids, sugars, and ions) through transport proteins in the membrane.

Channel protein

An integral membrane protein that forms hydrophilic channels through which water and ions can pass.

Aquaporin

A channel protein specialized for transporting water; involved in urine formation, with mutations in the human gene resulting in an inability to make concentrated urine.

Ion channel

A channel protein that facilitates transport of ions (Na⁺, K⁺, Ca²⁺, Cl⁻); most are gated channels.

Gated channel

A channel protein that switches between open, closed, or intermediate states (e.g., a voltage-gated K⁺ channel).

Carrier protein

A transport protein that facilitates movement of ions/solutes by physically binding a molecule on one side of the membrane and releasing it on the other via a conformational change; specific, passive, and can become saturated.

Osmosis

The diffusion of water across a selectively permeable membrane in response to concentration gradients, moving from a region of higher water concentration (fewer solutes) to lower water concentration (more solutes).

Osmotic pressure

The force needed to stop osmotic flow; develops when a cell in a hypotonic solution gains water and swells.

Tonicity

A property of a solution relative to a particular membrane, describing the relative concentration of nonpenetrating solutes.

Hypotonic solution

A solution with a lower concentration of nonpenetrating solutes than inside the cell; water enters and the cell swells.

Hypertonic solution

A solution with a higher concentration of nonpenetrating solutes than inside the cell; water leaves and the cell shrinks.

Isotonic solution

A solution in which solute concentrations inside and outside the cell are balanced/equal.

Turgor pressure

Osmotic pressure in plant cells that pushes the cell membrane tightly against the cell wall, supporting soft tissues in a hypotonic environment.

Plasmolysis

The shrinking of plant cells in a hypertonic solution to the point that they retract from their cell walls.

Isosmotic regulation

The maintenance of osmotic balance by keeping cells isotonic with their environment.

Contractile vacuole

A structure used by some cells to eject excess water via extrusion, maintaining osmotic balance.

Membrane potential

An electrical charge difference (voltage) across the plasma membrane, generated by active transport of ions; used by neurons and muscle cells to respond rapidly to stimuli (e.g., nerve impulse transmission).

Primary active transport

Active transport in which the transport protein directly hydrolyzes ATP to power the movement of a substance.

Secondary active transport

Active transport indirectly driven by ATP hydrolysis, using a favorable ion concentration gradient (built by primary active transport) as the energy source to transport a different ion or molecule.

Sodium-potassium (Na⁺-K⁺) pump

A primary active transport mechanism that moves 3 Na⁺ out of the cell and 2 K⁺ into the cell using ATP energy to change the carrier protein's conformation; creates a membrane potential of about −50 to −200 mV.

Electrochemical gradient

The combination of ion concentration differences and electrical charge differences across a membrane, constituting a form of potential energy.

Calcium (Ca²⁺) pump

A primary active transport protein that moves Ca²⁺ from the cytoplasm to the cell exterior or into ER vesicles; regulates secretion, microtubule assembly, and muscle contraction.

Symport

A form of secondary active transport in which the transported solute moves through the membrane channel in the same direction as the driving ion.

Antiport

A form of secondary active transport in which the transported solute and driving ion move through the membrane channel in opposite directions.

Coupled transport

Secondary active transport that uses the energy released by diffusion of one molecule to power active transport of a different molecule (e.g., the glucose-Na⁺ symporter, which uses energy from Na⁺ diffusion to move glucose against its concentration gradient).

Endocytosis

The process by which proteins and other substances become trapped in pit-like inward depressions of the plasma membrane and are taken into the cell.

Pinocytosis

Non-specific (bulk) endocytosis in which a cell takes in only fluid.

Receptor-mediated endocytosis

Specific endocytosis in which target molecules bind to receptor proteins on the outer cell surface before being taken in.

Coated pit

A depression in the plasma membrane, reinforced on its cytoplasmic side by a network of clathrin proteins, where receptor-bound target molecules collect during receptor-mediated endocytosis.

Clathrin

A network of proteins that coats and reinforces the cytoplasmic side of a coated pit during receptor-mediated endocytosis.

Phagocytosis

The process by which phagocytes and protists (such as Amoeba) take in large particles or whole cells, using surface receptors and extending cytoplasmic lobes to form a large endocytic vesicle.