Schematic three dimensional cross section of a cell membrane.
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
Biological Membranes
Each list begins with basic conceptual vocabulary you need to know for MCAT questions and proceeds to advanced terms that might appear in context in MCAT passages. The terms are links to Wikipedia articles.
Cell membrane
The cell membrane is a semipermeable lipid bilayer found in all cells.
The cell membrane is a semipermeable lipid bilayer found in all cells.
Phospholipid
Phospholipids are a class of lipids, and a major component of all biological membranes, along with glycolipids, cholesterol and proteins.
Phospholipids are a class of lipids, and a major component of all biological membranes, along with glycolipids, cholesterol and proteins.
Passive transport
Passive transport means moving biochemicals and other atomic or molecular substances across membranes in a process that does not require chemical energy.
Passive transport means moving biochemicals and other atomic or molecular substances across membranes in a process that does not require chemical energy.
Diffusion
Diffusion is the sponteneous net movement of particles from an area of high concentration to an area of low concentration.
Diffusion is the sponteneous net movement of particles from an area of high concentration to an area of low concentration.
Osmosis
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.
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
Phagocytosis is the cellular process of engulfing solid particles by the cell membrane to form an internal phagosome, or food vacuole.
Phagocytosis is the cellular process of engulfing solid particles by the cell membrane to form an internal phagosome, or food vacuole.
Semipermeable membrane
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 semipermeable membrane is a membrane which will allow certain molecules or ions to pass through it by diffusion and occasionally specialized facilitated diffusion.
Lipid bilayer
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 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.
Integral membrane protein
A integral membrane protein is a protein molecule, or assembly of proteins, that is permanently attached to the biological membrane.
A integral membrane protein is a protein molecule, or assembly of proteins, that is permanently attached to the biological membrane.
Transmembrane protein
A transmembrane protein is a protein that spans the entire biological membrane.
A transmembrane protein is a protein that spans the entire biological membrane.
Ion channel
Ion channels are pore-forming proteins that help to establish and control the small voltage gradient across the plasma membrane of all living cells.
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 channel
Sodium channels are integral membrane proteins that conduct sodium ions through a cell's plasma membrane.
Sodium channels are integral membrane proteins that conduct sodium ions through a cell's plasma membrane.
Receptor
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 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.
Hypotonic
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 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.
Hypertonic
A hypertonic cell environment has a higher concentration of solutes than inside the animal or plant cell.
A hypertonic cell environment has a higher concentration of solutes than inside the animal or plant cell.
Osmotic pressure
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.
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
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.
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
Endocytosis is a process whereby cells absorb material from the outside by engulfing it with their cell membrane.
Endocytosis is a process whereby cells absorb material from the outside by engulfing it with their cell membrane.
Gap junction
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 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.
Cell wall
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.
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
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.
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.
Amphiphile
An amphiphile is a chemical compound possessing both hydrophilic and hydrophobic properties.
An amphiphile is a chemical compound possessing both hydrophilic and hydrophobic properties.
Phosphatidylcholine
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.
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.
Secretory pathway
The secretory pathway is a series of steps a cell uses to move proteins out of the cell.
The secretory pathway is a series of steps a cell uses to move proteins out of the cell.
Membrane potential
Membrane potential is the electrical voltage across a plasmalemma.
Membrane potential is the electrical voltage across a plasmalemma.
Primary active transport
Primary active transport is directly coupled to ATP cleavage to transport molecules across a membrane.
Primary active transport is directly coupled to ATP cleavage to transport molecules across a membrane.
Secondary active transport
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.
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
Pinocytosis is a form of endocytosis in which small particles are brought into the cell suspended within small vesicles which subsequently fuse with lysosomes.
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
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.
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.
Second messenger system
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 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.
Desmosome
A desmosome, also known as macula adherens, is a cell structure specialized for cell-to-cell adhesion.
A desmosome, also known as macula adherens, is a cell structure specialized for cell-to-cell adhesion.
Cell junction
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.
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 junction
Adherens junctions are protein complexes that occur at cell-cell junctions in epithelial tissues. They are usually more basal than tight junctions.
Adherens junctions are protein complexes that occur at cell-cell junctions in epithelial tissues. They are usually more basal than tight junctions.
Tight junction
Tight junctions, or zonula occludens, are the closely associated areas of two cells whose membranes join together forming a virtual impermeable barrier to fluid.
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
Exocytosis is the process by which a cell directs secretory vesicles to the cell membrane and releases their contents.
Exocytosis is the process by which a cell directs secretory vesicles to the cell membrane and releases their contents.
Potassium channel
Potassium channels are the most common type of ion channel, forming pores selective for that ion spanning cell membranes.
Potassium channels are the most common type of ion channel, forming pores selective for that ion spanning cell membranes.
Voltage-gated calcium channel
Voltage-gated calcium channels are a group of ion channels found in excitable cells such as neurons, glial cells, muscle cells, etc.
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 channel
Voltage-gated potassium channels are transmembrane channels specific for potassium and sensitive to changes in the cell's membrane potential.
Voltage-gated potassium channels are transmembrane channels specific for potassium and sensitive to changes in the cell's membrane potential.
Clathrin
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.
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 receptor
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.
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 protein
Peripheral membrane proteins are proteins that adhere only temporarily to the biological membrane with which they are associated.
Peripheral membrane proteins are proteins that adhere only temporarily to the biological membrane with which they are associated.
Cell adhesion
Cellular adhesion is the binding of a cell to another cell or to a surface or matrix.
Cellular adhesion is the binding of a cell to another cell or to a surface or matrix.
Symporter
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.
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
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.
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.
Sphingolipid
Sphingolipids are a class of lipids derived from the aliphatic amino alcohol sphingosine.
Sphingolipids are a class of lipids derived from the aliphatic amino alcohol sphingosine.
Outer membrane
The outer membrane refers to the outside membranes of Gram-negative bacteria, the chloroplast, or the mitochondria.
The outer membrane refers to the outside membranes of Gram-negative bacteria, the chloroplast, or the mitochondria.
ATP-binding cassette transporter genes
ATP-binding cassette transporters are members of an ancient superfamily that function in the transport of a wide variety of substrates across biological membranes.
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 channel
Calcium-activated potassium channels are divided into BK channels, IK channels, and SK channels based on their conductance (big, intermediate, and small conductance).
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
Inositol triphosphate receptor is a membrane glycoprotein complex acting as calcium ion channel activated by IP3.
Inositol triphosphate receptor is a membrane glycoprotein complex acting as calcium ion channel activated by IP3.
G-protein-coupled receptor
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.
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.
Hemidesmosome
Hemidesmosomes are very small stud- or rivet-like structures on the inner basal surface of keratinocytes in the epidermis of skin.
Hemidesmosomes are very small stud- or rivet-like structures on the inner basal surface of keratinocytes in the epidermis of skin.
Paracellular transport
Paracellular transport refers to the transfer of substances between cells of an epithelium.
Paracellular transport refers to the transfer of substances between cells of an epithelium.
Peptidoglycan
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.
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.
Antiporter
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.
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.
Uniporter
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.
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
Transmembrane domain usually denotes a single transmembrane alpha helix of a transmembrane protein.
Transmembrane domain usually denotes a single transmembrane alpha helix of a transmembrane protein.
Membrane topology
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 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.
Translocon
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.
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.
Tandem pore domain potassium channel
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.
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.
Porin
Porins are beta barrel proteins which cross a cellular membrane and act as a pore through which molecules can diffuse.
Porins are beta barrel proteins which cross a cellular membrane and act as a pore through which molecules can diffuse.
SNARE
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.
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
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.
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.
Bacteriorhodopsin
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.
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
Transient receptor potential channels are a family of loosely related ion channels that are non-selectively permeable to cations, including calcium and magnesium.
Transient receptor potential channels are a family of loosely related ion channels that are non-selectively permeable to cations, including calcium and magnesium.
Cyclic nucleotide-gated ion channel
A cyclic nucleotide-gated ion channels describe the category of ion channels opened by cyclic AMP or cyclic GMP.
A cyclic nucleotide-gated ion channels describe the category of ion channels opened by cyclic AMP or cyclic GMP.
Caveolae
Caveolae are small (50-100 nanometer) invaginations of the plasma membrane in many vertebrate cell types, especially in endothelial cells and adipocytes.
Caveolae are small (50-100 nanometer) invaginations of the plasma membrane in many vertebrate cell types, especially in endothelial cells and adipocytes.
Plasmodesmata
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.
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.
Cadherin
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.
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.
Connexon
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.
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
Glycocalyx is a general term referring to extracellular polymeric material produced by some bacteria, epithelia and other cells.
Glycocalyx is a general term referring to extracellular polymeric material produced by some bacteria, epithelia and other cells.
Lipid raft
A lipid raft is a cholesterol-enriched microdomain in cell membranes.
A lipid raft is a cholesterol-enriched microdomain in cell membranes.
Connexin
Connexins, or gap junction proteins, are a family of structurally-related transmembrane proteins that assemble to form vertebrate gap junctions.
Connexins, or gap junction proteins, are a family of structurally-related transmembrane proteins that assemble to form vertebrate gap junctions.
Pseudopeptidoglycan
Pseudopeptidoglycan is a major cell wall component of some archaebacteria which is chemically different but morphologically, functionally, and structurally similar to eubacterial peptidoglycan.
Pseudopeptidoglycan is a major cell wall component of some archaebacteria which is chemically different but morphologically, functionally, and structurally similar to eubacterial peptidoglycan.
Syntaxin
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
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
Synaptobrevin
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.
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.