Cell Membrane Structure and Transport Mechanisms

Vocabulary practice flashcards covering cell membrane composition, transport proteins, passive and facilitated diffusion, osmosis, active transport mechanisms, and vesicular transport.

Cell (Plasma) Membrane

A thin, elastic, semi-permeable membrane with a thickness of $7.5\,\text{nm}$ that separates the intracellular fluid (ICF) from the extracellular fluid (ECF), maintains the internal environment, regulates transport, contains receptors, and generates transmembrane potentials.

Membrane Chemical Composition

The structural proportion of macromolecular components in the plasma membrane: $55\%$ proteins, $42\%$ lipids, and $3\%$ carbohydrates.

Phospholipid Bilayer

The basic framework of the cell membrane, consisting of two layers of phospholipids; each molecule features a hydrophilic phosphate head facing the ECF and ICF, and hydrophobic lipid tails facing each other in the membrane's interior.

Integral (Intrinsic) Proteins

Membrane proteins bound directly to the hydrophobic core of the lipid bilayer; they include transmembrane proteins spanning the entire bilayer (functioning as channels, carriers, pumps, and receptors) or proteins situated on one side functioning as enzymes.

Peripheral (Extrinsic) Proteins

Membrane proteins attached to the hydrophilic polar heads of lipids or to intrinsic proteins; those on the intracellular surface contribute to the cytoskeleton, while those on the extracellular surface contribute to the glycocalyx.

Passive Diffusion

The movement of substances across the cell membrane down their electrochemical gradient without the requirement of external energy.

Simple Diffusion

The movement of substances across the cell membrane down an electrochemical gradient without carrier proteins; characterized by being passive, not rate-limited (linear with concentration gradients), non-saturable, and lacking competition or stereospecificity.

Lipid-Soluble Substance Diffusion

Transport across the membrane occurring when a substance dissolves directly into the lipid bilayer, as seen with $O_2$, nitrogen, $CO_2$, and alcohol.

Water Diffusion

Rapid diffusion through the cell membrane at a high velocity (described as like a bullet) attributed to water's small size and high kinetic energy.

Protein Channels

Watery pathways through the cell membrane providing selective permeability based on pore diameter, shape, and internal surface charge ($+ve$ channels allow $-ve$ particles to pass, while $-ve$ channels allow $+ve$ particles to pass).

Channel Gating

The mechanism by which protein channels open or close via gates, such as $Na^+$ channels possessing two gates (an outer $m$ gate and an inner $h$ gate) and $K^+$ channels possessing one extracellular gate (an $n$ gate).

Ligand-Gated Ion Channels

Nonselective ion channels associated with membrane receptors that undergo a conformational change to open upon chemical ligand binding; an example is the cholinergic receptor at the motor end plate (MEP) of skeletal muscle.

Voltage-Gated Ion Channels

Selective ion channels opened by changes in membrane potential reaching a specific threshold level; essential for action potential generation (e.g., voltage-gated $Na^+$, $K^+$, and $Ca^{2+}$ channels in nerves).

Facilitated Diffusion

Passive, carrier-mediated transport of substances down an electrochemical gradient; characterized by rate-limitation, saturation, competition, and stereospecificity (e.g., glucose transport).

Fick's Law of Diffusion

A physical law stating that net diffusion flux is directly proportional to solute concentration gradient, diffusion coefficient ($D$), and surface area ($A$), and inversely proportional to membrane thickness ($X$), modeled as $\text{Flux} = -\frac{D \times A}{X} (C_{\text{in}} - C_{\text{out}})$.

Osmosis

The passive flow of water across a semi-permeable membrane down the water concentration gradient (from high water/low solute concentration to low water/high solute concentration).

Osmolarity

A property of a solution expressed in $\text{osmol/L}$ that depends strictly on the total number of dissolved particles, completely independent of their mass, electrical charge, or chemical composition.

Iso-osmotic Solution

A solution that possesses the same osmolarity as normal blood plasma, which is $290\text{--}300\,\text{mosm/L}$.

Tonicity

The ability of solute particles in a solution to cause changes in cell volume based on osmolarity relative to plasma; categorized into isotonic (no volume change), hypotonic (cell swelling), and hypertonic (cell shrinkage).

Active Transport

Carrier-mediated movement of substances across cell membranes against an electrochemical gradient (uphill) that consumes energy and demonstrates specificity, saturation, and competition.

Primary Active Transport

Active transport powered directly by the enzymatic hydrolysis of ATP by specialized transport ATPases.

Sodium-Potassium Pump ($Na^+\text{-}K^+$ ATPase)

A primary active transport enzyme complex comprising $2\alpha$ and $2\beta$ subunits that extrudes $3\,Na^+$ from the cell and imports $2\,K^+$ per hydrolyzed ATP molecule; inhibited pharmacologically by Digitalis (Digoxin).

Two-Step Cycle of $Na^+\text{-}K^+$ ATPase

The operational cycle consisting of a phosphorylation step (where $3\,Na^+$ and ATP bind the $\alpha$ subunit, causing ATP cleavage and phosphorylation to expel $Na^+$) and a de-phosphorylation step (where $2\,K^+$ bind, triggering phosphate hydrolysis and transporting $K^+$ inward).

Calcium Pump ($Ca^{2+}\text{-ATPase}$)

A primary active transporter found in cell membranes and the sarcoplasmic reticulum responsible for maintaining very low resting intracellular $Ca^{2+}$ concentrations.

Proton Pump ($K^+\text{-}H^+\text{-ATPase}$)

A primary active transport ATPase located in gastric parietal cells that pumps $H^+$ ions into the stomach lumen; selectively inhibited by proton pump inhibitors such as omeprazole.

Secondary Active Transport

A form of active transport that does not directly hydrolyze ATP, but instead utilizes the potential energy stored within the transmembrane $Na^+$ electrochemical gradient.

Symport (Cotransport)

Secondary active transport wherein two solutes move in the identical direction across the membrane, such as the $Na^+\text{-glucose}$ cotransporter located in the luminal border of intestinal and renal proximal tubule cells.

Antiport (Counter-transport)

Secondary active transport where driven solutes cross the membrane in opposing directions, such as $Na^+\text{-}Ca^{2+}$ exchange (in ventricular cardiac myocytes) and $Na^+\text{-}H^+$ exchange.

Endocytosis

A form of vesicular transport in which extracellular substances are internalized into vesicles formed by membrane invagination; includes phagocytosis (bacteria/dead tissues), pinocytosis (fluids/proteins), and receptor-mediated endocytosis (iron/cholesterol).

Exocytosis

A vesicular transport mechanism (referred to as cell vomiting) wherein intracellular vesicles fuse with the plasma membrane to discharge their contents into the extracellular fluid.