Cell Membrane Structure and Function: Fluid Mosaic Model & Transport Mechanisms

Plasma membrane

The boundary that separates the living cell from its surroundings.

Selective permeability

The property of the plasma membrane that allows some substances to cross it more easily than others.

Phospholipids

The most abundant lipid in the plasma membrane, which are amphipathic molecules containing hydrophobic and hydrophilic regions.

Fluid mosaic model

A model stating that a membrane is a fluid structure with a 'mosaic' of various proteins embedded in it.

Phospholipid bilayer

A structure that forms the basic framework of the plasma membrane, consisting of two layers of phospholipids.

Membrane fluidity

The ability of phospholipids in the plasma membrane to move within the bilayer, which is affected by the types of fatty acids present.

Unsaturated fatty acids

Fatty acids that increase membrane fluidity compared to saturated fatty acids.

Peripheral proteins

Proteins that are bound to the surface of the membrane.

Integral proteins

Proteins that penetrate the hydrophobic core of the membrane.

Transmembrane proteins

Integral proteins that span the membrane.

Functions of membrane proteins

Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, attachment to the cytoskeleton and extracellular matrix (ECM).

Membrane carbohydrates

Molecules that help cells recognize each other by binding to surface molecules on the extracellular surface of the plasma membrane.

Glycolipids

Membrane carbohydrates that are covalently bonded to lipids.

Glycoproteins

Membrane carbohydrates that are more commonly bonded to proteins.

Asymmetrical distribution

The distinct inside and outside faces of membranes, determined during the building process by the ER and Golgi apparatus.

Molecular traffic

The exchange of materials with the surroundings, controlled by the plasma membrane.

Hydrophobic molecules

Nonpolar molecules, such as hydrocarbons, that can dissolve in the lipid bilayer and pass through the membrane rapidly.

Polar molecules

Molecules, such as sugars, that do not cross the membrane easily.

Transport proteins

Proteins that allow passage of hydrophilic substances across the membrane.

Channel proteins

Transport proteins that have a hydrophilic channel allowing certain molecules or ions to pass through the membrane.

Aquaporins

Channel proteins that facilitate the passage of water across the membrane.

Carrier proteins

Transport proteins that bind to molecules and change shape to shuttle them across the membrane.

Passive transport

Diffusion of a substance across a membrane with no energy investment.

Diffusion

The tendency for molecules to spread out evenly into the available space.

Dynamic equilibrium

A state where as many molecules cross the membrane in one direction as in the other.

Concentration gradient

The region along which the density of a chemical substance increases or decreases.

Osmosis

The diffusion of water across a selectively permeable membrane.

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 the same as that inside the cell, resulting in no net water movement.

Hypertonic solution

A solution where the solute concentration is greater than that inside the cell, causing the cell to lose water.

Hypotonic solution

A solution where the solute concentration is less than that inside the cell, causing the cell to gain water.

Osmoregulation

The control of solute concentrations and water balance, necessary for life in varying environments.

Contractile vacuole

An organelle in the protist Paramecium that acts as a pump to manage water balance.

Facilitated diffusion

The process where transport proteins speed the passive movement of molecules across the plasma membrane.

Gated channels

Ion channels that open or close in response to a stimulus.

Turgid

The state of a plant cell when it swells in a hypotonic solution until the cell wall opposes further uptake.

Flaccid

The state of a plant cell when it is in an isotonic environment, resulting in no net movement of water.

Plasmolysis

The process where plant cells lose water in a hypertonic environment, causing the membrane to pull away from the wall.

Cystinuria

A kidney disease caused by malfunctions in specific transport systems.

Active Transport

The process of moving substances against their concentration gradients, requiring energy, usually in the form of ATP.

Sodium-Potassium Pump

A type of active transport system that maintains concentration gradients in animal cells.

Membrane Potential

The voltage difference across a membrane created by the distribution of positive and negative ions.

Electrochemical Gradient

The combined forces of a chemical force (the ion's concentration gradient) and an electrical force (the effect of the membrane potential on the ion's movement).

Electrogenic Pump

A transport protein that generates voltage across a membrane.

Proton Pump

The main electrogenic pump of plants, fungi, and bacteria.

Cotransport

A process where the active transport of a solute indirectly drives the transport of other solutes.

Exocytosis

The process where transport vesicles migrate to the membrane, fuse with it, and release their contents.

Endocytosis

The process where the cell takes in macromolecules by forming vesicles from the plasma membrane.

Phagocytosis

A type of endocytosis known as 'cellular eating' where a cell engulfs a particle in a vacuole.

Pinocytosis

A type of endocytosis known as 'cellular drinking' where molecules are taken up when extracellular fluid is 'gulped' into tiny vesicles.

Receptor-Mediated Endocytosis

A type of endocytosis where binding of ligands to receptors triggers vesicle formation.

Ligand

Any molecule that binds specifically to a receptor site of another molecule.

Vesicles

Small membrane-bound sacs that transport large molecules across the plasma membrane.

Lysosome

An organelle that digests particles engulfed by phagocytosis.