Cell Membrane & Transport Flashcards

A comprehensive vocabulary review of cell membrane architecture, fluidity, integral and peripheral proteins, transport mechanisms (diffusion, osmosis, active transport), and bulk vesicular transport.

Amphipathic

The property of having both a hydrophilic (polar phosphate) head and hydrophobic (non-polar fatty acid) tails, causing phospholipids to self-assemble into bilayers in water.

Bilayer Orientation

The structural arrangement in plasma membranes where hydrophilic phosphate heads face the cytoplasm and extracellular fluid, while hydrophobic fatty acid tails remain tucked into the middle, shielded from water.

Micelle vs. Liposome

A micelle is a single-layer sphere with heads facing outward and tails inward without a watery interior; a liposome is a lipid bilayer sphere containing a water-filled center.

Phospholipid Lateral Movement

The normal sideways movement of phospholipids within their own monolayer; flip-flop between layers is rare because the polar phosphate head would have to traverse the hydrophobic core.

Membrane Fluidity Factors

Structural and environmental conditions that increase fluidity, specifically a higher proportion of unsaturated (kinked) fatty acid tails preventing tight packing, as well as higher temperatures.

Cholesterol Membrane Roles

A steroid that reduces membrane fluidity when warm, prevents crystallization when cold, and reduces permeability to ions; it does not function as an enzyme that makes phospholipids.

Integral Protein

A protein permanently embedded in the phospholipid bilayer, often spanning it (polytopic when spanning multiple times, or monotopic when penetrating one surface); contrasts with temporary, surface-bound peripheral proteins.

TRACIE

An acronym outlining the six functions of membrane proteins: Transport, Receptors, Anchorage, Cell recognition, Intercellular joining, and Enzymes.

Simple Diffusion Across Bilayer

The direct, unassisted crossing of small, non-polar molecules like oxygen ($O_2$) across the phospholipid bilayer; charged ions like $Na^+$ and polar molecules like glucose and amino acids require transport proteins.

Diffusion Distance

The length of the path molecules must travel during diffusion; a greater diffusion distance slows the rate of diffusion, whereas a steeper gradient, larger surface area, and higher temperature accelerate it.

Channels vs. Carriers

Channels form an open pore, are purely passive, and transport solutes very rapidly ($\sim 10^7\text{ ions/sec}$); carriers bind solute, undergo conformational changes, can be passive or active, operate more slowly ($\sim 10^2\text{--}10^4\text{/sec}$), and can saturate.

Voltage-Gated Potassium Channel (Axon)

An axon membrane channel that opens when the inside of the axon becomes relatively positive, allowing $K^+$ to exit via facilitated diffusion, and closes rapidly upon voltage change.

Osmosis Direction

The net passive movement of water molecules toward the region of higher solute concentration (lower water potential); water follows the solutes.

Cytolysis

The swelling and bursting of an animal cell (such as a red blood cell) placed in a hypotonic solution due to osmotic water influx through a membrane lacking a cell wall.

Plasmolysis

The phenomenon occurring in plant cells in a hypertonic solution where water leaves the cell, causing the plasma membrane to pull away from the rigid cell wall.

$Na^+/K^+$ Pump Net Result

The stoichiometric outcome of one cycle of the sodium-potassium pump: $3\,Na^+$ pumped out, $2\,K^+$ pumped in, and $1\,\text{ATP}$ hydrolyzed.

Secondary Active Transport

Active transport driven by the stored potential energy of an existing electrochemical gradient (such as a $Na^+$ gradient previously established by primary active transport) rather than direct ATP hydrolysis.

Symport

A cotransporter mechanism that translocates two different substances across a membrane simultaneously in the same direction.

Phagocytosis vs. Pinocytosis

Phagocytosis ('cell eating') is the engulfment of large solid particles or cells (e.g., a macrophage engulfing a bacterium); pinocytosis ('cell drinking') is the non-specific uptake of extracellular fluid and dissolved solutes.

Secretory Pathway

The intracellular transport sequence for exported proteins: Rough Endoplasmic Reticulum (RER) ribosome $\rightarrow$ transport vesicle $\rightarrow$ Golgi apparatus $\rightarrow$ secretory vesicle $\rightarrow$ plasma membrane (exocytosis).

Phospholipid Bilayer Assembly

The spontaneous formation of a bilayer in cells because water exists on both sides of the membrane and two-tailed phospholipids are too bulky to form a micelle, keeping every polar head in water and shielding every tail without energy input.

Emergent Property (Membranes)

A macro-level characteristic (such as the self-assembly of phospholipids into a bilayer) that arises from the collective interactions of multiple molecules via the hydrophobic effect, rather than from any individual molecule.

Membrane Self-Sealing

The spontaneous closure of tears in a lipid bilayer driven by the thermodynamic instability of exposed hydrophobic tails in water, which also permits vesicles to bud and fuse.

Singer–Nicolson Fluid Mosaic Model

The membrane model describing proteins embedded in and floating within a fluid phospholipid bilayer, which replaced the Davson–Danielli protein–lipid–protein sandwich model.

Cholesterol Structural Class

A lipid molecule classified structurally as a steroid, defined by a core of four interconnected carbon rings (distinct from fats or oils).

Cholesterol Orientation in Bilayer

The precise alignment of cholesterol within the bilayer, placing its polar hydroxyl ($-OH$) group near the hydrophilic heads while its hydrophobic steroid rings and tail nestle among the fatty acid tails.

Fluidity Buffer

A function of cholesterol that dampens temperature effects by restricting phospholipid movement when warm and disrupting regular fatty acid tail packing to prevent solidifying when cold.

Membrane Fluidity Regulation

The homeostatic maintenance of membrane viscosity; excessive fluidity compromises the selective permeability barrier, whereas excessive rigidity impedes molecular motion and vesicle budding or fusion.

Polytopic Protein

An integral membrane protein that spans completely through the phospholipid bilayer multiple times.

Glycophorin A

An integral membrane protein found in red blood cells that carries oligosaccharide chains; distinct from peripheral proteins like beta-secretase 1 and muscle fatty acid binding protein.

Glycoprotein

A membrane protein covalently linked to an oligosaccharide (short sugar chain) facing the extracellular fluid, functioning in cell recognition and as hormone receptors.

Cell Recognition Proteins

Surface membrane markers, such as Major Histocompatibility Complex (MHC) proteins and antigens, that allow the immune system to distinguish self from non-self.

Intercellular Joining

A membrane protein role dedicated to establishing physical connections and communication between adjacent cells, such as tight junctions in animals and plasmodesmata in plants.

Membrane Receptor (Insulin)

A transmembrane protein that binds non-permeable extracellular ligands like peptide hormones (e.g., insulin) to initiate intracellular signal transduction cascades.

Ribosome (vs. Membrane Proteins)

The ribonucleoprotein complex that synthesizes new polypeptide chains, distinguishing its role from membrane protein functions like transport, signal reception, adhesion, and catalysis.

Bilayer Permeability to Ions

The inability of charged particles (such as $Cl^-$) to penetrate the membrane unaided due to electrostatic repulsion by the hydrophobic fatty acid core; contrasts with $CO_2$, $O_2$, and steroid hormones.

Diffusive (Dynamic) Equilibrium

A state in which solute molecules continue to move across a boundary at equal rates in both directions, resulting in equal concentrations and zero net movement.

Surface Area Adaptations for Diffusion

Specialized folded structures like alveoli, intestinal villi, mitochondrial cristae, and plant root hairs that increase surface area relative to volume, thereby accelerating the rate of diffusion.

Pulmonary Blood Flow in Gas Exchange

The continuous perfusion of blood through alveolar capillaries that rapidly whisks away oxygen ($O_2$), preserving a steep concentration gradient that sustains rapid diffusion.

Cell Membrane Thickness

The physical dimension of the lipid bilayer, measuring approximately $7\text{--}10\text{ nm}$, which minimizes diffusion distance across the cellular boundary.

Facilitated Diffusion Driving Force

The movement of solute molecules down their concentration gradient through a transport protein without expenditure of ATP, relying entirely on the kinetic energy of the gradient.

Carrier Protein Saturation

The plateau in transport rate that occurs when all binding sites on carrier proteins are occupied by solute; a kinetic property that rarely applies to open channel proteins.

Aquaporin

An integral channel protein that facilitates the rapid diffusion of water (osmosis) across a membrane down its water potential gradient without consuming ATP.

Potassium Channel Specificity

The selective filtration mechanism where only $K^+$ correctly fits and coordinates with amino acid residues in the narrowest part of the pore; other ions are either too large or too small to bond effectively.

Electrochemical Gradient

The dual driving force governing net ion movement across a membrane, comprising both the chemical concentration gradient and the electrical potential (charge) gradient.

Maximum Water Potential

The thermodynamic baseline of pure water, which has the highest possible water potential; dissolving any solute lowers the water potential.

Hypotonic Solution

An external solution that has a lower solute concentration (higher water potential) relative to the cytoplasm of the cell being compared.

Turgor Pressure

The internal hydrostatic pressure exerted by the cytoplasm and vacuole against the rigid plant cell wall in a hypotonic environment, maintaining structural firmness (turgid state).

Crenation

The shriveling and scalloping of an animal red blood cell resulting from net osmotic water loss when placed into a hypertonic solution.

Isotonic Saline

A salt solution formulated to match the physiological osmolarity of cytoplasm, preventing net water flow, cytolysis, or crenation during wound irrigation and organ storage.

De-icing Salt Dehydration

The phenomenon where road salts make soil water hypertonic relative to root cells, drawing water out of the roots down its water potential gradient.

Zero-Crossing (Osmosis Graph)

The point of zero percentage mass change on an osmosis graph of tissue mass change vs. external solution concentration, indicating the solute concentration isotonic to the cytoplasm.

ATP Hydrolysis

The enzymatic breaking of the terminal high-energy phosphate bond of ATP, producing $ADP + P_i$ and releasing free energy, which is subsequently regenerated by respiration.

$Na^+/K^+$ Pump Phosphorylation (Step 2)

The transfer of a phosphate group from ATP to the $Na^+/K^+$ pump following the binding of $3\,Na^+$ internally, triggering a conformational change that closes the pump to the inside.

$Na^+/K^+$ Pump Dephosphorylation

The release of the bound inorganic phosphate ($P_i$) triggered by the extracellular binding of $2\,K^+$, causing the pump to revert to its original inward-facing conformation.

Sodium-Glucose Cotransporter (SGLT)

A secondary active symporter that harnesses the downhill electrochemical gradient of $Na^+$ (built by the primary active $Na^+/K^+$ pump) to drive glucose uphill into the cell.

Antiport

A transport protein that cotransports two different substances across a membrane simultaneously in opposite directions (e.g., the $Na^+/K^+$ pump).

Transport Rate Hierarchy

The relative transport speeds of membrane transport mechanisms from fastest to slowest: open channels ($\sim 10^7\text{ ions/sec}$), carriers ($\sim 10^2\text{--}10^4\text{/sec}$), and active pumps ($\sim 10^1\text{--}10^3\text{/sec}$).

Constitutive vs. Regulated Secretion

Constitutive secretion is an ongoing, continuous exocytic release of substances (such as mucus); regulated secretion occurs strictly in response to an extracellular chemical or electrical signal (such as neurotransmitter release).

Bulk Transport Energy Requirement

The active mechanism underlying endocytosis and exocytosis, which requires ATP for cytoskeletal transport, membrane remodeling, and vesicle fusion, all enabled by membrane fluidity.