chapter 5- structure and function of plasma membranes

Active transport

Method of transporting material that requires energy.

Amphiphilic

Molecule possessing a polar or charged area and a nonpolar or uncharged area capable of interacting with both hydrophilic and hydrophobic environments.

Antiporter

Transporter that carries two ions or small molecules in different directions.

Aquaporin

Channel protein that allows water through the membrane at a very high rate.

Carrier protein

Membrane protein that moves a substance across the plasma membrane by changing its own shape.

Caveolin

Protein that coats the plasma membrane's cytoplasmic side and participates in the liquid uptake process by potocytosis.

Channel protein

Membrane protein that allows a substance to pass through its hollow core across the plasma membrane.

Clathrin

Protein that coats the plasma membrane's inward-facing surface and assists in forming specialized structures, like coated pits, for phagocytosis and receptor-mediated endocytosis.

Concentration gradient

Area of high concentration adjacent to an area of low concentration.

Diffusion

Passive transport process of low-molecular weight material according to its concentration gradient.

Electrochemical gradient

A combined electrical and chemical force that produces a gradient.

Electrogenic pump

Pump that creates a charge imbalance.

Endocytosis

Type of active transport that moves substances, including fluids and particles, into a cell.

Exocytosis

Process of passing bulk material out of a cell by fusing a membranous envelope or vesicle with the interior of the plasma membrane.

Facilitated transport

Process by which material moves down a concentration gradient (from high to low concentration) using integral membrane proteins.

Familial hypercholesterolemia

A human genetic disease in which LDL receptors are defective or missing entirely, preventing cells from clearing LDL particles and causing life-threatening levels of cholesterol in the blood.

Fluid mosaic model

Describes the plasma membrane's structure as a mosaic of components including phospholipids, cholesterol, proteins, glycoproteins, and glycolipids (sugar chains attached to proteins or lipids, respectively), resulting in a fluid character (fluidity).

Glycolipid

Combination of carbohydrates and lipids.

Glycoprotein

Combination of carbohydrates and proteins.

Hydrophilic

Molecule with the ability to bond with water; "water-loving".

Hydrophobic

Molecule that does not have the ability to bond with water; "water-hating".

Hypertonic

Situation in which extracellular fluid has a higher osmolarity than the fluid inside the cell, resulting in water moving out of the cell.

Hypotonic

Situation in which extracellular fluid has a lower osmolarity than the fluid inside the cell, resulting in water moving into the cell.

Integral protein

Protein integrated into the membrane structure that interacts extensively with the membrane lipids' hydrocarbon chains and often spans the membrane.

Isotonic

Situation in which the extracellular fluid has the same osmolarity as the fluid inside the cell, resulting in no net water movement into or out of the cell.

Osmolarity

Total amount of solutes dissolved in a specific amount of solution.

Osmosis

Transport of water through a semipermeable membrane according to the water's concentration gradient across the membrane that results from the presence of solute that cannot pass through the membrane.

Passive transport

Method of transporting material through a membrane that does not require energy.

Peripheral protein

Protein at the plasma membrane's surface either on its exterior or interior side.

Pinocytosis

A variation of endocytosis that imports macromolecules that the cell needs from the extracellular fluid by invaginating the plasma membrane, surrounding a small volume of fluid, and pinching off.

Plasmolysis

Detaching the cell membrane from the cell wall and constricting the cell membrane when a plant cell is in a hypertonic solution.

Potocytosis

Variation of pinocytosis that uses a different coating protein (caveolin) on the plasma membrane's cytoplasmic side to bring small molecules into the cell and transport them via caveolae.

Primary active transport

Active transport that moves ions or small molecules across a membrane and may create a difference in charge across that membrane.

Pump

Active transport mechanism that works against electrochemical gradients.

Receptor-mediated endocytosis

Variation of endocytosis that involves using specific binding proteins in the plasma membrane for specific molecules or particles, and clathrin-coated pits that become clathrin-coated vesicles.

Secondary active transport

Movement of material that results from primary active transport to the electrochemical gradient.

Selectively permeable

Membrane characteristic that allows some substances through (also known as semipermeable).

Solute

Substance dissolved in a liquid to form a solution.

Symporter

Transporter that carries two different ions or small molecules, both in the same direction.

Tonicity

Amount of solute in a solution.

Transcytosis

The process of bringing small molecules into the cell via potocytosis and transporting them through the cell for their release on the other side.

Transport protein

Membrane protein that facilitates a substance's passage across a membrane by binding it.

Transporter

Specific carrier proteins or pumps that facilitate movement.

Uniporter

Transporter that carries one specific ion or molecule.

Fluid Mosaic Model

The model proposed in 1972 by S.J. Singer and Garth L. Nicolson describing the plasma membrane structure as a fluid mosaic of components—including phospholipids, cholesterol, proteins, and carbohydrates—ranging from $5\text{ to }10\,\text{nm}$ in thickness.

Davson-Danielli Model

A 1935 model proposed by Hugh Davson and James Danielli theorizing that the plasma membrane resembles a sandwich with proteins as the bread and lipids as the filling, based on early electron micrographs showing a "railroad track" appearance.

Amphiphilic

A characteristic of molecules (such as phospholipids) that have a positively or negatively charged, polar ("water-loving") area and an uncharged, non-polar ("water-hating") area.

Phospholipid

The main fabric molecule of the plasma membrane, composed of a three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon.

Cholesterol

A lipid composed of four fused carbon rings embedded alongside phospholipids in animal cell membranes that dampens temperature effects, functioning as a fluidity buffer and organizing lipid rafts.

Integral Proteins

Membrane proteins that integrate completely into the membrane structure, possessing hydrophobic membrane-spanning regions (often $20\text{--}25$ amino acids in single-pass proteins) that interact with the phospholipid bilayer's hydrophobic core.

Peripheral Proteins

Proteins located on the exterior or interior surfaces of the membrane, bound either to integral proteins or to phospholipids, that serve as enzymes, structural attachments for cytoskeleton fibers, or cell recognition sites.

Glycocalyx

The highly hydrophilic "sugar coating" on the outer surface of a cell formed collectively by carbohydrate chains attached to glycoproteins and glycolipids, which aids in water attraction, cell identification, and tissue formation.

Unsaturated Fatty Acid Kinks

Bends of approximately $30^\circ$ introduced into phospholipid tails by carbon-carbon double bonds, which prevent tight compression at low temperatures and maintain membrane fluidity.

Selective Permeability

The property of a plasma membrane that allows certain substances to pass through freely while restricting or preventing the entry and exit of others.

Passive Transport

A naturally occurring form of membrane transport in which substances move down a concentration gradient from an area of higher concentration to an area of lower concentration without cellular energy expenditure.

Concentration Gradient

A physical space over which there is a difference or range in the concentration of a single substance, representing a form of potential energy.

Dynamic Equilibrium

The condition achieved when a substance diffuses completely through a space and eliminates its concentration gradient; molecules continue random movement, but there is no net movement.

Filtration

A variation of diffusion wherein material moves through a membrane according to its concentration gradient, accelerated or enhanced by pressure (such as blood pressure in the kidney's renal tubules).

Facilitated Transport

The passive movement of polar molecules or ions across the plasma membrane down their concentration gradient with the assistance of transmembrane channel or carrier proteins.

Aquaporins

Channel proteins that provide hydrated core openings allowing water molecules to cross the plasma membrane at extremely high rates.

Gated Channels

Channel proteins whose openings are controlled by mechanisms such as the binding of a specific ion or changes in electrical potential across the membrane.

Carrier Proteins

Transmembrane transport proteins that specifically bind a single substance, undergo a conformational shape change, and move the molecule across the membrane at rates of $10^3\text{ to }10^6$ molecules per second.

Glucose Transport Proteins (GLUTs)

A specific class of carrier proteins responsible for transporting glucose and other hexose sugars across plasma membranes in the body.

Osmosis

The diffusion of free water molecules through a semipermeable membrane down water's concentration gradient, moving inversely to the concentration of non-penetrating solutes.

Osmolarity

The total solute concentration of a solution, determined by the total number of dissolved solute particles per unit volume.

Tonicity

The capacity of an extracellular solution to alter a cell's volume by influencing the direction and extent of osmosis.

Hypotonic Solution

An extracellular solution having a lower osmolarity (and higher free water concentration) than the cytoplasm, causing water to enter the cell.

Hypertonic Solution

An extracellular solution having a higher osmolarity (and lower free water concentration) than the cytoplasm, causing water to flow out of the cell.

Isotonic Solution

An extracellular solution that shares the same osmolarity as the cell cytoplasm, resulting in dynamic equilibrium with zero net water movement.

Lysis

The bursting or destruction of a cell (such as an animal red blood cell) occurring when excessive water influx in a hypotonic medium expands the membrane past its structural limits.

Crenation

The shrinking and shriveling of an animal red blood cell that occurs when water leaves the cell in a hypertonic environment, concentrating intracellular solutes.

Turgor Pressure

The internal hydrostatic pressure generated in plant cells under hypotonic conditions as water enters and forces the plasma membrane outward against the rigid cell wall.

Plasmolysis

The detachment of the plant cell membrane from the rigid cell wall and subsequent constriction of the cytoplasm when the plant is exposed to a hypertonic environment.

Contractile Vacuoles

Specialized osmoregulatory vesicles found in protists lacking cell walls (such as paramecia and amoebas) that collect excess internal water and pump it out to prevent bursting.

Osmoreceptors

Specialized brain cells that monitor blood solute concentrations and stimulate hormone release to regulate kidney water retention and maintain osmotic balance.

Albumin

A large protein produced by the liver that remains in the bloodstream because it cannot cross plasma membranes, serving as a primary factor in regulating tissue osmotic pressure.

Electrochemical Gradient

The combined influence of an electrical gradient (membrane potential) and a chemical concentration gradient acting on an ion's movement across a plasma membrane.

Primary Active Transport

An energy-requiring transport mechanism that directly uses ATP hydrolysis to pump ions against their gradient, generating a charge differential across the membrane.

Secondary Active Transport

A transport process (co-transport) that does not directly hydrolyze ATP, but instead utilizes the kinetic energy of an electrochemical gradient (such as $Na^+$) created by primary active transport to move another molecule against its gradient.

Uniporter

A transport carrier protein that moves only one specific ion or molecule across the membrane.

Symporter

A transport carrier protein that moves two different ions or molecules simultaneously in the same direction across the membrane.

Antiporter

A transport carrier protein that carries two different ions or molecules across the membrane in opposite directions.

Sodium-Potassium Pump ($Na^+-K^+$ ATPase)

An electrogenic antiporter pump that utilizes ATP hydrolysis to export three $Na^+$ ions from the cell while importing two $K^+$ ions into the cell.

Electrogenic Pump

A transport pump that generates a net electrical charge imbalance across the plasma membrane, directly contributing to the membrane potential.

Endocytosis

A form of bulk active transport in which the cell membrane invaginates around external particles to form an intracellular vesicle, pulling large materials into the cell.

Phagocytosis

A form of endocytosis ("cell eating") where the plasma membrane encloses and engulfs large particles or whole microorganisms, such as when a neutrophil digests foreign invaders.

Clathrin

A protein that lines and stabilizes the inward-facing surface of the plasma membrane during the formation of endocytic vesicles in phagocytosis.

Pinocytosis

A bulk transport process ("cell drinking") discovered in 1929 by Warren Lewis in which small vesicles form to internalize extracellular fluid and dissolved molecules without fusing with lysosomes.