Membrane Structure and Function

Vocabulary flashcards covering cellular membrane structure, fluidity, transport mechanisms, water potential, and bulk transport process terms from Chapter 7.

Phospholipid

The most abundant lipid in the plasma membrane, composed of amphipathic molecules containing both hydrophobic and hydrophilic regions.

Amphipathic molecule

A molecule that contains both hydrophobic (water-fearing) and hydrophilic (water-loving) regions.

Fluid mosaic model

A model stating that a membrane is a fluid structure with a mosaic of various proteins individually inserted into and dispersed throughout the phospholipid bilayer, as proposed by Singer and Nicolson in 1972.

Freeze-fracture

A specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer, providing visual support for the fluid mosaic model.

Membrane fluidity

The property of cellular membranes to remain fluid; membranes rich in unsaturated fatty acid tails with kinks remain more fluid at lower temperatures than those rich in packed saturated fatty acid tails.

Cholesterol (in animal membranes)

A steroid that restrains the movement of phospholipids at warm temperatures ($37^\circ\text{C}$) and maintains fluidity at cool temperatures by preventing tight lipid packing.

Peripheral proteins

Membrane proteins that are not embedded in the lipid bilayer and are bound loosely to the surface of the membrane.

Integral proteins

Membrane proteins that penetrate the hydrophobic core of the phospholipid bilayer.

Transmembrane proteins

Integral proteins that span completely across the hydrophobic interior of the phospholipid bilayer.

Glycolipids

Molecules formed by carbohydrates covalently bonded to lipids on the outer face of the plasma membrane, serving in cell-cell recognition.

Glycoproteins

Molecules formed by carbohydrates covalently bonded to membrane proteins that assist in cell-cell recognition.

Selective permeability

The property of plasma membranes that regulates cellular traffic by allowing nonpolar hydrophobic molecules to cross rapidly while restricting polar molecules.

Aquaporins

Specific channel proteins that facilitate the rapid passage of water molecules across the membrane.

Passive transport

The movement of a substance across a biological membrane down its concentration gradient that requires no energy investment from the cell.

Diffusion

The tendency for molecules to spread out evenly into available space down their concentration gradient.

Dynamic equilibrium

The state reached across a membrane when equal numbers of molecules cross in both directions per unit time.

Osmosis

The diffusion of water across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration.

Tonicity

The ability of a surrounding solution to cause a cell to gain or lose water.

Isotonic solution

A solution where the solute concentration is equal to that inside the cell, resulting in no net movement of water.

Hypertonic solution

A solution with a higher solute concentration than the inside of a cell, causing the cell to lose water.

Hypotonic solution

A solution with a lower solute concentration than the inside of a cell, causing the cell to gain water and swell.

Osmoregulation

The control of water balance and solute concentrations in organisms lacking rigid cell walls.

Plasmolysis

A lethal condition in plant cells exposed to a hypertonic environment, where water loss causes the plasma membrane to pull away from the cell wall.

Solute potential formula

Represented as $\Psi_s = -iCRT$, where $i$ is the ionization constant, $C$ is molar concentration, $R$ is the pressure constant ($0.0831\,\text{liter bar/mole K}$), and $T$ is temperature in Kelvin ($273 + ^\circ\text{C}$).

Facilitated diffusion

Passive transport aided by transport proteins (channel or carrier proteins) that speed up the movement of molecules down their concentration gradient.

Active transport

The movement of solutes across a membrane against their concentration gradient, requiring metabolic energy in the form of ATP.

Sodium-potassium pump

A specific active transport system that expels $\text{Na}^+$ from the cytoplasm and imports $\text{K}^+$ into the cell against their respective concentration gradients.

Membrane potential

The voltage difference across a plasma membrane created by an unequal distribution of cations and anions.

Electrochemical gradient

The combination of a chemical force (concentration gradient) and an electrical force (membrane potential) that drives the movement of ions across a membrane.

Proton pump

The main electrogenic pump in plants, fungi, and bacteria that actively stores energy by pumping hydrogen ions ($\text{H}^+$) out of the cell.

Cotransport

A mechanism in which the active transport of one solute indirectly drives the transport of another solute across the membrane.

Exocytosis

A form of bulk transport where secretory vesicles migrate to the membrane, fuse with it, and release their contents to the extracellular space.

Endocytosis

A form of bulk transport where the cell takes in macromolecules by forming new vesicles derived from the plasma membrane.

Phagocytosis

A form of endocytosis ('cellular eating') in which a cell engulfs a particle by extending pseudopodia around it to enclose it within a membrane-bound vacuole.

Pinocytosis

A form of endocytosis ('cellular drinking') in which droplets of extracellular fluid are gulped into tiny vesicles.

Receptor-mediated endocytosis

A specialized type of endocytosis where the binding of specific ligands to surface receptors triggers the formation of a coated vesicle.