The cell membrane is the only structure that is found in all cells of all organisms. Biological membranes consist of an amphiphilic lipid bilayer with embedded, integral and peripheral proteins. The lipid bilayer provides a fluid matrix in which proteins laterally diffuse. The primary role of biological membranes is to separate aqueous compartments from their surroundings, maintaining separation of the cell from the environment, and, additionally in eukaryotic cells, separating functional areas within the cell from the rest of the cell. The permeability of this barrier to certain substances and the various modes by which membranes allow for the transport of certain substances are major topics. Additionally, biological membranes are involved in signal transduction through their role as the home of receptor molecules which function is to transmit messages in the form of chemical signals.
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Conceptual Vocabulary for Biological Membranes
The cell membrane is a semipermeable lipid bilayer found in all cells.
Phospholipids are a class of lipids, and a major component of all biological membranes, along with glycolipids, cholesterol and proteins.
Passive transport means moving biochemicals and other atomic or molecular substances across membranes in a process that does not require chemical energy.
Diffusion is the sponteneous net movement of particles from an area of high concentration to an area of low concentration.
Osmosis is the spontaneous net movement of water across a partially permeable membrane from a region of high solvent potential to an area of low solvent potential, up a solute concentration gradient.
Phagocytosis is the cellular process of engulfing solid particles by the cell membrane to form an internal phagosome, or food vacuole.
A semipermeable membrane is a membrane which will allow certain molecules or ions to pass through it by diffusion and occasionally specialized facilitated diffusion.
A lipid bilayer is a membrane or zone of a membrane composed of lipid molecules two molecules thick, a structure which is a critical component of all biological membranes.
A integral membrane protein is a protein molecule, or assembly of proteins, that is permanently attached to the biological membrane.
A transmembrane protein is a protein that spans the entire biological membrane.
Ion channels are pore-forming proteins that help to establish and control the small voltage gradient across the plasma membrane of all living cells.
Sodium channels are integral membrane proteins that conduct sodium ions through a cell's plasma membrane.
A receptor is a protein on the cell membrane, within the cytoplasm, or within the cell nucleus that binds to a specific ligand, such as a neurotransmitter, hormone, or other substance, and initiates a cellular response.
A hypotonic solution has the lower osmotic pressure of two fluids. The term also describes a cell environment with a lower concentration of solutes than the cytoplasm of the cell.
A hypertonic cell environment has a higher concentration of solutes than inside the animal or plant cell.
Osmotic pressure is the hydrostatic pressure produced by a solution in a space divided by a semipermeable membrane due to a differential in the concentrations of solute.
Active transport is the mediated transport of biochemicals, and other atomic/molecular substances, across a membrane which specifically requires the expenditure of cellular energy to move molecules against a gradient.
Endocytosis is a process whereby cells absorb material from the outside by engulfing it with their cell membrane.
A gap junction or nexus is a junction between certain animal cell-types that allows different molecules and ions, mostly small intracellular signaling molecules, to pass freely between cells.
A cell wall is a fairly rigid layer surrounding the cells of plants, bacteria, archaea, fungi, and algae, located external to the cell membrane, which provides the cell with structural support, protection, and acts as a filtering mechanism.
Sodium/Potassium-ATPase is an enzyme located in the plasma membrane of virtually every human cell and is common to all cellular life. It helps maintain cell potential and regulate cellular volume.
An amphiphile is a chemical compound possessing both hydrophilic and hydrophobic properties.
Phosphatidylcholine is a phospholipid which is a major constituent of cell membranes. It is such a major component of lecithin that in some contexts the terms are used as synonyms.
The secretory pathway is a series of steps a cell uses to move proteins out of the cell.
Membrane potential is the electrical voltage across a plasmalemma.
Primary active transport is directly coupled to ATP cleavage to transport molecules across a membrane.
In secondary active transport there is no direct coupling of ATP. Instead the energy derives from the electrochemical potential difference created by pumping ions out of the cell.
Pinocytosis is a form of endocytosis in which small particles are brought into the cell suspended within small vesicles which subsequently fuse with lysosomes.
Receptor-mediated endocytosis is a process by which cells internalize molecules into a cell by the inward budding of plasma membrane vesicles which contain proteins with receptor sites specific to the molecules being internalized.
A second messenger system is a method of cellular signalling where the signalling molecule does not enter the cell but instead utilizes a cascade of events to transduce the signal into a change inside the cell.
A desmosome, also known as macula adherens, is a cell structure specialized for cell-to-cell adhesion.
A cell junction is one of a variety of types of structures consisting of protein complexes that provide contact between neighbouring cells, between a cell and the extracellular matrix, or participate in building up the paracellular barrier of epithelia.
Adherens junctions are protein complexes that occur at cell-cell junctions in epithelial tissues. They are usually more basal than tight junctions.
Tight junctions, or zonula occludens, are the closely associated areas of two cells whose membranes join together forming a virtual impermeable barrier to fluid.
Exocytosis is the process by which a cell directs secretory vesicles to the cell membrane and releases their contents.
Potassium channels are the most common type of ion channel, forming pores selective for that ion spanning cell membranes.
Voltage-gated calcium channels are a group of ion channels found in excitable cells such as neurons, glial cells, muscle cells, etc.
Voltage-gated potassium channels are transmembrane channels specific for potassium and sensitive to changes in the cell's membrane potential.
Clathrin is a protein that is the major constituent of the 'coat' of the coated pits and coated vesicles formed during endocytosis of materials at the surface of cells.
Transmembrane receptors are integral membrane proteins that bind to a signalling molecule or sometimes to a pair of such molecules on one side of the membrane and initiate a response on the other side.
Peripheral membrane proteins are proteins that adhere only temporarily to the biological membrane with which they are associated.
Cellular adhesion is the binding of a cell to another cell or to a surface or matrix.
A symporter is an integral membrane protein that is involved in active transport of two or more different molecules or ions across a phospholipid membrane in the same direction.
Self-organization is a process in which the internal organization of a system, normally an open system, increases in complexity without being guided or managed by an outside source.
Sphingolipids are a class of lipids derived from the aliphatic amino alcohol sphingosine.
The outer membrane refers to the outside membranes of Gram-negative bacteria, the chloroplast, or the mitochondria.
ATP-binding cassette transporters are members of an ancient superfamily that function in the transport of a wide variety of substrates across biological membranes.
Calcium-activated potassium channels are divided into BK channels, IK channels, and SK channels based on their conductance (big, intermediate, and small conductance).
Inositol triphosphate receptor is a membrane glycoprotein complex acting as calcium ion channel activated by IP3.
G-protein-coupled receptors, also known as seven transmembrane receptors or heptahelical receptors, are a large protein family of transmembrane receptors that respond to signals outside the cell and activate signal transduction pathways and, ultimately, cellular responses.
Hemidesmosomes are very small stud- or rivet-like structures on the inner basal surface of keratinocytes in the epidermis of skin.
Paracellular transport refers to the transfer of substances between cells of an epithelium.
Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of eubacteria.
An antiporter is an integral membrane protein that is involved in active transport of two or more different molecules or ions in opposite directions across a plasma membrane.
A uniporter is an integral membrane protein that is involved in facilitated diffusion that works by binding to one molecule of solute at a time and transporting it with the solute gradient.
Transmembrane domain usually denotes a single transmembrane alpha helix of a transmembrane protein.
The membrane topology of an transmembrane protein describes which portions of the amino-acid sequence of the protein lie within the plane of the surrounding lipid bilayer and which portions protrude into the watery environment on either side.
The translocon (commonly known as a translocator) is the complex of proteins associated with the translocation of nascent polypeptides into the interior space of the endoplasmic reticulum from the cytosol.
Two-pore-domain potassium channels comprise a family of 15 members known as leak channels which possess Goldman-Hodgkin-Katz (open) rectification. These channels are regulated by several mechanisms including oxygen tension, pH, mechanical stretch, and G-proteins.
Porins are beta barrel proteins which cross a cellular membrane and act as a pore through which molecules can diffuse.
The primary role of SNARE proteins is to mediate fusion of cellular transport vesicles with the cell membrane or with a target compartment such as a lysosome.
Sphingosine is an 18-carbon amino alcohol with an unsaturated hydrocarbon chain, which forms a primary part of sphingolipids, a class of cell membrane lipids that include sphingomyelin, an important phospholipid.
The integral membrane protein, bacteriorhodopsin, is a photosynthetic pigment used by some archaea that acts as a proton pump. It captures light energy and uses it to move protons across the membrane out of the cell.
Transient receptor potential channels are a family of loosely related ion channels that are non-selectively permeable to cations, including calcium and magnesium.
A cyclic nucleotide-gated ion channels describe the category of ion channels opened by cyclic AMP or cyclic GMP.
Caveolae are small (50-100 nanometer) invaginations of the plasma membrane in many vertebrate cell types, especially in endothelial cells and adipocytes.
Plasmodesmata are microscopic channels of plants traversing the cell walls and middle lamella between pairs of plant cells and facilitating transport and communication between them.
Cadherins are a class of transmembrane proteins that play important roles in cell adhesion. They are dependent on calcium ions to function, hence their name.
A connexon is an assembly of 6 proteins called connexins that forms a bridge called a gap junction between the cytoplasm of two adjacent cells.
Glycocalyx is a general term referring to extracellular polymeric material produced by some bacteria, epithelia and other cells.
A lipid raft is a cholesterol-enriched microdomain in cell membranes.
Connexins, or gap junction proteins, are a family of structurally-related transmembrane proteins that assemble to form vertebrate gap junctions.
Pseudopeptidoglycan is a major cell wall component of some archaebacteria which is chemically different but morphologically, functionally, and structurally similar to eubacterial peptidoglycan.
Syntaxins are membrane integrated Q-SNARE proteins participating in exocytosis. They possess a single C-terminal transmembrane domain, a SNARE domain (known as H3), and an N-terminal regulatory domain
One of the SNARE proteins involved in formation of the SNARE complexes, synaptobrevins, are small integral membrane proteins of secretory vesicles that are part of the vesicle-associated membrane protein (VAMP) family.
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