Chapter 7: Membrane Structure and Function

Vocabulary practice flashcards covering membrane structure, fluidity, transport mechanisms, osmosis, tonicity, and bulk vesicle transport from Chapter 7.

Selective Permeability

A fundamental property of the plasma membrane that allows some substances to cross it more easily than others, thereby regulating the cell's molecular traffic.

Amphipathic Molecule

A molecule that possesses both a hydrophilic ("water-loving") region and a hydrophobic ("water-fearing") region, such as a phospholipid.

Fluid Mosaic Model

The currently accepted model of cell membrane structure, depicting the membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

Lateral Movement (Membrane Lipids)

The rapid sideways movement of phospholipids within their layer of the lipid bilayer, occurring approximately $\sim 10^7$ times per second.

Flip-Flop (Transbilayer Movement)

The rare migration of a lipid molecule transversely across the membrane from one phospholipid layer to the other, occurring only about once per month.

Cholesterol (Membrane Function)

A steroid lipid in animal cell membranes that acts as a fluidity buffer: at warm temperatures (such as $37^\circ\text{C}$), it restrains phospholipid movement, while at cool temperatures, it prevents tight packing to maintain fluidity.

Phytosterols

Steroid lipids related to cholesterol used by plants to buffer membrane fluidity across varying temperatures.

Peripheral Proteins

Membrane proteins that are not embedded in the lipid bilayer but are loosely bound to the surface of the membrane, often to exposed parts of integral proteins.

Integral Proteins

Membrane proteins that penetrate the hydrophobic interior of the lipid bilayer; those that span the entire membrane are termed transmembrane proteins.

Transmembrane Protein

An integral protein that spans the entire membrane; its hydrophobic regions typically consist of one or more stretches of nonpolar amino acids, often coiled into $\alpha$ helices.

Glycolipids

Membrane lipids covalently bonded to short carbohydrates on the extracellular surface of the plasma membrane, participating in cell-cell recognition.

Glycoproteins

Membrane proteins covalently bonded to short carbohydrates on the extracellular side; they serve as specialized identification markers, distinguishing cells such as human blood types A, B, AB, and O.

CD4 and CCR5

Cell-surface proteins on human immune cells required for HIV entry; CD4 is the primary receptor and CCR5 functions as a co-receptor. Individuals lacking CCR5 are resistant to HIV infection.

Channel Proteins

Transport proteins that provide a hydrophilic corridor through the lipid bilayer, allowing specific ions or polar molecules to cross the membrane by diffusion.

Aquaporins

Channel proteins that facilitate the rapid passage of water molecules across cellular membranes, crucial in tissues like renal tubules where their absence would lead to losing $180\,\text{L}$ of water daily in urine.

Carrier Proteins

Transport proteins that bind to a specific substance and undergo a subtle conformational change in shape to shuttle the solute across the membrane.

Diffusion

The spontaneous tendency of a substance to spread down its concentration gradient into an available space due to thermal motion, moving from higher to lower concentration without energy input.

Concentration Gradient

The region along which the density or chemical concentration of a substance increases or decreases; moving down this gradient requires no metabolic work.

Passive Transport

The diffusion of a substance across a biological membrane down its concentration gradient without any expenditure of energy by the cell.

Osmosis

The diffusion of free water molecules across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration until equilibrium is reached.

Tonicity

The ability of an extracellular solution to cause a cell to gain or lose water, dependent on the concentration of non-penetrating solutes relative to the inside of the cell.

Isotonic

A surrounding solution having the same solute concentration as the interior of the cell, resulting in no net movement of water across the plasma membrane.

Hypertonic

A solution with a higher concentration of non-penetrating solutes than the cell interior, causing a cell lacking a rigid wall to lose water and shrivel.

Hypotonic

A solution with a lower concentration of non-penetrating solutes than the cell interior, causing water to enter the cell and leading a walled-less animal cell to swell and lyse (burst).

Osmoregulation

The active cellular control of solute concentrations and water balance to maintain homeostasis in hypotonic or hypertonic environments.

Contractile Vacuole

A specialized organelle found in freshwater unicellular organisms like Paramecium that collects and periodically expels excess water entering the hypertonic cell from its hypotonic environment.

Turgid

The very firm and healthy state of a plant cell resulting from osmotic water uptake when placed in a hypotonic solution, balanced by the back-pressure exerted by the rigid cell wall.

Flaccid

The limp condition of a plant cell that occurs when its surroundings are isotonic, leading to no net inward flow of water and insufficient turgor pressure.

Plasmolysis

The phenomenon in walled cells placed in a hypertonic environment where water exits the cell, shrinking the protoplast and pulling the plasma membrane away from the cell wall, causing wilting.

Facilitated Diffusion

The passive transport of molecules or ions across a biological membrane aided by specific transmembrane channel or carrier proteins, proceeding down a concentration gradient without cellular energy.

Gated Channels

Ion channels across the plasma membrane that open or close in response to specific chemical or physical stimuli, such as electrical voltage changes in nerve cells.

Active Transport

The energy-requiring movement of a solute against its electrochemical or concentration gradient across a membrane, mediated by specific carrier proteins using ATP hydrolysis.

Sodium-Potassium Pump

An electrogenic carrier protein energized by the transfer of a phosphate group from ATP hydrolysis that exports three $\text{Na}^+$ ions out of animal cells for every two $\text{K}^+$ ions imported.

Cotransport

The coupled transport of two solutes across a membrane by a single protein, where the downhill diffusion of one solute (e.g., $\text{H}^+$) powers the uphill transport of another (e.g., sucrose) against its concentration gradient.

Exocytosis

The cellular secretion of macromolecules by the fusion of transport vesicles with the plasma membrane, releasing vesicular contents into the extracellular fluid.

Endocytosis

The cellular uptake of biological macromolecules and particulate matter by localized invaginations of the plasma membrane, which pinch inward to pinch off into internal vesicles.

Phagocytosis

A form of endocytosis ("cellular eating") in which a cell wraps pseudopodia around a large particle or microorganism and packages it into a membranous food vacuole for lysosomal digestion.

Pinocytosis

A type of endocytosis ("cellular drinking") where droplets of extracellular fluid containing dissolved solutes are continuously taken up into tiny vesicles formed by invaginations of the plasma membrane.

Receptor-Mediated Endocytosis

A selective form of pinocytosis wherein extracellular solutes bind to specific cell-surface receptor proteins, triggering the clustering of receptors into coat-protein-lined pits that pinch off into coated vesicles.

Familial Hypercholesterolemia

An inherited human disease characterized by missing or defective LDL receptors on cell surfaces, preventing receptor-mediated endocytosis of cholesterol and resulting in dangerous cholesterol buildup in the blood.