Types of adherent junctions ctin filament(microfilament) attachments Cell-to-cell: adherens junctions: cadherins Cell-to-extracellular matrix: focal adhesions: integrins Intermediate filament attachment sites Cell-to-cell: desmosomes(spot and belt): cadherins Cell-to-extracellular matrix: hemi-desmosomes: integrins Cell-cell Endocrine: inter-glandular or inter-structure, i.e. hormones produced by an endocrine gland act on target cells at a distant body site Paracrine: local intercellular, i.e. act on neighbouring target cells Autocrine: intracellular, i. e act on the cell responsible for production Cell adhesion molecules Surface molecules involved in cell-cell interactions(cell adhesion molecules are integral membrane proteins with extracellular, transmembrane and cyto plasmic domains. They mediate cell adhesion by forming non-covalent bond with corresponding surface molecules of neighbouring cells Adhesion molecules can be classified as being involved in: e Cell body to cell body adhesion: Calcium-dependent adhesion molecules: cadherins(classic cadherins; mosomal cadherins. Cadherins are involved in homophilic cell-to-cell sion in the presence of calcium ions. They are cell surface adhesion molecules that interact with the intracellular actin cytoskeleton via plakoglo- bulin and catenin molecules. Neural(N)-cadherins, placental(P)-cadherins and epithelial(E)-cadherins are recognised Calcium-independent adhesion molecules, which belong to the immuno- globulin superfamily including intercellular adhesion molecules (ICAMs) nd neural cell adhesion molecules(N-CAMs) Cell to cell surface carbohydrate ligand-binding proteins: selectins, which are divalent cation-dependent glycoproteins Cell to extracellular matrix adhesion: the integrins. Integrins are a family f transmembrane proteins that act as receptors for extracellular matrix molecules, integrating the matrix and the cytoskeleton functionally and structurally. They are non-covalently attached heterodimeric glycoproteins, composed of alpha and beta subunits
Cell–cell signalling mechanisms may be: Endocrine: inter-glandular or inter-structure, i.e. hormones produced by an endocrine gland act on target cells at a distant body site. Paracrine: local intercellular, i.e. act on neighbouring target cells. Autocrine: intracellular, i.e. act on the cell responsible for production. Cell adhesion molecules Surface molecules involved in cell–cell interactions (cell adhesion molecules) are integral membrane proteins with extracellular, transmembrane and cytoplasmic domains. They mediate cell adhesion by forming non-covalent bonds with corresponding surface molecules of neighbouring cells. Adhesion molecules can be classified as being involved in: * Cell body to cell body adhesion: Calcium-dependent adhesion molecules: cadherins (classic cadherins; desmosomal cadherins). Cadherins are involved in homophilic cell-to-cell adhesion in the presence of calcium ions. They are cell surface adhesion molecules that interact with the intracellular actin cytoskeleton via plakoglobulin and catenin molecules. Neural (N)-cadherins, placental (P)-cadherins, and epithelial (E)-cadherins are recognised. Calcium-independent adhesion molecules, which belong to the immunoglobulin superfamily including intercellular adhesion molecules (ICAMs) and neural cell adhesion molecules(N-CAMs). Cell to cell surface carbohydrate ligand-binding proteins: selectins, which are divalent cation-dependent glycoproteins. * Cell to extracellular matrix adhesion: the integrins. Integrins are a family of transmembrane proteins that act as receptors for extracellular matrix molecules, integrating the matrix and the cytoskeleton functionally and structurally. They are non-covalently attached heterodimeric glycoproteins, composed of alpha and beta subunits. Types of adherent junctions Actin filament (microfilament) attachments Cell-to-cell: adherens junctions: cadherins Cell-to-extracellular matrix: focal adhesions: integrins Intermediate filament attachment sites Cell-to-cell: desmosomes (spot and belt): cadherins Cell-to-extracellular matrix: hemi-desmosomes: integrins Intercellular junctions 5
The role of cell adhesion molecules Cell-cell recognition Cell signalling Cell growth Cell migratio Information transfer from the extracellular matrix to the cell Establishment of the blood-brain barrier Cancer metastasis Cell membrane transport classification of transport mechanisms Membrane transport mechanisms can be classified as Passive diffusion: along an electrochemical gradient. Diffusion refers to the random movement of particles in solution from an area of higher concentra- tion to one of lower. This may involve either dissolution and diffusion in ne lipid, or passage through ion channels. Ion channels may be permanently open(non-gated, passive, or leakage) or be gated, i.e. can be opened or closed: e.g., voltage-gated; extracellular or intracellular ligand gated; mechanically gated (mechanical deformation); ion-gated; or gap junc- tion activation Voltage-gated channels are found in neurons and muscle cells Mechanical gating is exemplified by the mechanical deformation of cilia of the hair cells of the inner ear brought about by sound waves Facilitated transport: aided by membrane transporters(carrier proteins)in the direction of the electrochemical gradient. The process is more rapid than simple diffusion. Carriers must be able to recognise the substance trans ported, to permit translocation, followed by release of the substance with recovery of the carrier Active transport: an energy requiring process operating against an electro chemical gradient. The process can be mediated by Primary ATPases: Na*/K+-ATPase; H*-ATPase; K*/H*-ATPase: Ca2+- ATPase: these transporters are known as pumps Adenosine 5-triphosphate(ATP)-binding cassette proteins, which bind ATP and use the free energy from ATP hydrolysis to selectively transport materi als, e.g. the cystic fibrosis transmembrane conductance regulator. These
& Cell membrane transport Classification of transport mechanisms Membrane transport mechanisms can be classified as: * Passive diffusion: along an electrochemical gradient. Diffusion refers to the random movement of particles in solution from an area of higher concentration to one of lower. This may involve either dissolution and diffusion in membrane lipid, or passage through ion channels. Ion channels may be permanently open (non-gated, passive, or leakage) or be gated, i.e. can be opened or closed: e.g., voltage-gated; extracellular or intracellular ligandgated; mechanically gated (mechanical deformation); ion-gated; or gap junction activation. Voltage-gated channels are found in neurons and muscle cells. Mechanical gating is exemplified by the mechanical deformation of cilia of the hair cells of the inner ear brought about by sound waves. * Facilitated transport: aided by membrane transporters (carrier proteins) in the direction of the electrochemical gradient. The process is more rapid than simple diffusion. Carriers must be able to recognise the substance transported, to permit translocation, followed by release of the substance with recovery of the carrier. * Active transport: an energy requiring process operating against an electrochemical gradient. The process can be mediated by: Primary ATPases: Naþ/Kþ-ATPase; Hþ-ATPase; Kþ/Hþ-ATPase; Ca2 þ- ATPase: these transporters are known as pumps. Adenosine 50 -triphosphate (ATP)-binding cassette proteins, which bind ATP and use the free energy from ATP hydrolysis to selectively transport materials, e.g. the cystic fibrosis transmembrane conductance regulator. These The role of cell adhesion molecules Cell–cell recognition Cell signalling Cell growth Cell migration Embryogenesis Information transfer from the extracellular matrix to the cell Establishment of the blood–brain barrier Cancer metastasis Cell physiology 6
steps in receptor-mediated endocytosis Specific binding of ligand to high-affinity receptor, which is clustered in pits coated with clathria Internalisation of the receptor in its coated pit, forming a coated vesicle. Coated vesicles lose their clathrin coats after endocytosis, and fuse with other vesicles to form early endosomes The receptors are recycled to the surface in vesicles that fuse with the cell membrane The process is used in cellular uptake of cholesterol (via the low density lipoprotein LDL) receptor) and of iron, among other substances proteins comprise a ligand-binding domain at one surface and an ATP-binding domain at the other. Secondary mechanisms, being coupled to Na or H transport. The mechan- ism can be either a co-transport(symport) or a counter-transport(anti port system): K+/H+-ATPase or proton pump Osmosis: the passage of water from a region where its concentration is high, through a semi-permeable membrane, into a region where its concentration is Vesicular transport, which can be classified as Pinocytosis: the plasma membrane forms vesicles that trap extracellular fluid; Phagocytosis Receptor-mediated endocytosis allowing their contents to be released into the extracellular space brane Exocytosis: fusion of membrane-bound vesicles with the plasma membr Diffusion across a membrane This depends on: The concentration gradient of the solute across the membrane The permeability of the membrane to the solut The transmembrane voltage gradient The molecular weight of the solute The membrane surface area The distance over which diffusion occurs The rate of diffusion is proportional to the cross-sectional area and to the change in concentration per unit distance, i. e the concentration gradient across the membrane(Fick's law). Fick's law may be stated as
proteins comprise a ligand-binding domain at one surface and an ATP-binding domain at the other. Secondary mechanisms, being coupled to Naþ or Hþ transport. The mechanism can be either a co-transport (symport) or a counter-transport (antiport system): Kþ/Hþ-ATPase or proton pump. * Osmosis: the passage of water from a region where its concentration is high, through a semi-permeable membrane, into a region where its concentration is lower. * Vesicular transport, which can be classified as: Endocytosis: Pinocytosis: the plasma membrane forms vesicles that trap extracellular fluid; Phagocytosis; Receptor-mediated endocytosis. Exocytosis: fusion of membrane-bound vesicles with the plasma membrane, allowing their contents to be released into the extracellular space. Diffusion across a membrane This depends on: The concentration gradient of the solute across the membrane; The permeability of the membrane to the solute; The transmembrane voltage gradient; The molecular weight of the solute; The membrane surface area; The distance over which diffusion occurs. The rate of diffusion is proportional to the cross-sectional area and to the change in concentration per unit distance, i.e. the concentration gradient across the membrane (Fick’s law). Fick’s law may be stated as: Steps in receptor-mediated endocytosis Specific binding of ligand to high-affinity receptor, which is clustered in pits coated with clathrin. Internalisation of the receptor in its coated pit, forming a coated vesicle. Coated vesicles lose their clathrin coats after endocytosis, and fuse with other vesicles to form early endosomes. The receptors are recycled to the surface in vesicles that fuse with the cell membrane. The process is used in cellular uptake of cholesterol (via the low density lipoprotein (LDL) receptor) and of iron, among other substances. Cell membrane transport 7
Q Where Q= the rate of flow of solute at right angles to the interface between two solutions (mg/s) dc/dx= the concentration gradient(mg/mD)across the interface A= the area of the interface(cm) D= the diffusion coefficient(sq cm/s) The permeability constant P= D/d, where D is the diffusion coefficient and d is the width of the membrane Facilitated transport Facilitated transport demonstrates the following characteristics Specificity for the transported solute Movement along an electrochemical gradient; Saturation kinetics: saturation at high substrate concentrations owing to the limited number of binding sites on the carrier Inhibition by structurally similar substrates; No energy expenditure Active transport Active transport demonstrates the following characteristics: Specificity for the transported solute. Movement against an electrochemical gradient. Saturation kinetics: saturation at high substrate concentrations. metabolic energy requirement. Energy dependence leads to active transport being substrate and oxygen dependent Inhibition by metabolic poisons such as cyanide and dinitrophenol may occur. Profound inhibition may result from lowering of ambient temperature Competition for uptake by similar substrates Kinetic characteristics shared by facilitated diffusion and active transport processes Stereochemical specificity. Thus amino acid transport systems of cell mem- branes are much more active with L-amino acids than the d isomers Saturation, i.e. the transport system can become saturated with the substance being transported. Plots of the rate of transport against substrate concentra tion usually show a hyperbolic curve approaching a maximum at which the rate is zero order with respect to substrate concentration
Q = AD (dc/dx) Where Q = the rate of flow of solute at right angles to the interface between two solutions (mg/s) dc/dx = the concentration gradient (mg/ml) across the interface A = the area of the interface (cm) D = the diffusion coefficient (sq cm/s) The permeability constant P = D/d, where D is the diffusion coefficient and d is the width of the membrane Facilitated transport Facilitated transport demonstrates the following characteristics: Specificity for the transported solute; Movement along an electrochemical gradient; Saturation kinetics: saturation at high substrate concentrations owing to the limited number of binding sites on the carrier; Inhibition by structurally similar substrates; No energy expenditure. Active transport Active transport demonstrates the following characteristics: Specificity for the transported solute. Movement against an electrochemical gradient. Saturation kinetics: saturation at high substrate concentrations. Metabolic energy requirement. Energy dependence leads to active transport being substrate and oxygen dependent. Inhibition by metabolic poisons such as cyanide and dinitrophenol may occur. Profound inhibition may result from lowering of ambient temperature. Competition for uptake by similar substrates. Kinetic characteristics shared by facilitated diffusion and active transport processes These include: * Stereochemical specificity. Thus amino acid transport systems of cell membranes are much more active with L-amino acids than the D isomers. * Saturation, i.e. the transport system can become saturated with the substance being transported. Plots of the rate of transport against substrate concentration usually show a hyperbolic curve approaching a maximum at which the rate is zero order with respect to substrate concentration. Cell physiology 8
Competitive inhibition by other transported species (structurally related Non-competitive inhibition by carrier poisons, which can block or alter lon channels Ion channels are integral membrane proteins that form aqueous (water-filled) macromolecular membrane-spanning pores in the plasma membrane. They are involved in the generation and propagation of nerve impulses, synaptic trans- mission, muscle contraction salt balance and hormone release The advantages of ion channel transport High selectivity for specific ion species(substrate specificity). The ability to be gated. The gating mechanism is a regulatory system con trolling the opening and closing of gates in ion channels. Gating is a process of transition through open(conducting), closed and inactive states accompa nied by conformational changes in the ion channels. Forward and backward rate constants for the transitions determine the likelihood of the various channel states The ability to allow very large ion fluxes in short time periods, i.e. a very high catalytic power to substantially increase the flow rate of ions over the free diffusion rate in water Ion channel flow rates The rate of ion flow through an open ion channel depends on The concentration gradient across the plasma membrane. The voltage gradient across the plasma membrane The conductance of the ion channels, which is expressed in units of charge/ second per volt. a high conductance channel allows more ionic flow for a iven driving voltage than a low conductance channel. Opening may lead to either inward current generation, leading to depolarise tion;outward current generation, leading to hyperpolarisation; or increased conductance, leading to stabilisation of membrane potential. Closure may lead to either switching off of the inward current, leading to hyperpolarisation; switching off of the outward current, leading to depolarisa- tion;or reduced conductance, leading to increased sensitivity of the cell to other components
* Competitive inhibition by other transported species (structurally related compounds). * Non-competitive inhibition by carrier poisons, which can block or alter specific functional groups of proteins. Ion channels Ion channels are integral membrane proteins that form aqueous (water-filled) macromolecular membrane-spanning pores in the plasma membrane. They are involved in the generation and propagation of nerve impulses, synaptic transmission, muscle contraction, salt balance and hormone release. The advantages of ion channel transport * High selectivity for specific ion species (substrate specificity). * The ability to be gated. The gating mechanism is a regulatory system controlling the opening and closing of gates in ion channels. Gating is a process of transition through open (conducting), closed and inactive states accompanied by conformational changes in the ion channels. Forward and backward rate constants for the transitions determine the likelihood of the various channel states. * The ability to allow very large ion fluxes in short time periods, i.e. a very high catalytic power to substantially increase the flow rate of ions over the free diffusion rate in water. Ion channel flow rates The rate of ion flow through an open ion channel depends on: The concentration gradient across the plasma membrane. The voltage gradient across the plasma membrane. The conductance of the ion channels, which is expressed in units of charge/ second per volt. A high conductance channel allows more ionic flow for a given driving voltage than a low conductance channel. Opening may lead to either inward current generation, leading to depolarisation; outward current generation, leading to hyperpolarisation; or increased conductance, leading to stabilisation of membrane potential. Closure may lead to either switching off of the inward current, leading to hyperpolarisation; switching off of the outward current, leading to depolarisation; or reduced conductance, leading to increased sensitivity of the cell to other components. Cell membrane transport 9