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Principles of cell and membrane function, imagen - Coggle Diagram
Principles of cell and membrane function
Overview of eukaryotic cells
: are distinguished from prokaryotic cells by the presence of a membrane-delimites nucleus.
Divided into 2 compartments: nucleus and cytoplasm (aqueous solution containin numeros organic molecules, ions, cytoskeleytal elements and a number of organelles.
Organelles: many of them are membane-enclosed.
Plasma membrane
: separates the intracelllar contents from de extracellular environment. It is involved in many cellular functions:
Selective transport of molecules.
Cell recognition
Cell comunication
Tissue organization
Membrane-dependent enzymatic activity
Determination of cell shape.
Structure and composition: 5-nm-thick with associated proteins. Some of them are integrated into the lipid bilayer, others are more loosely attached to the inner or otter surface of the membrane.
Membrane lipids: mayor lipids of the plasma membrane:
Phospholipids:amphipathic molecules thath has a charcher hidrophilic head (polar) and 2 hiprophobic fatty acyl chains or tails (non polar). This is important for the formation of the bilayer: the tails form the core of the bilayer and the head are exposed on the surface.
Phosphoglycerides: is the majority of the membrane phospholipids. It has a glycerol "backbone"to wich are attached the fatty acyl chains, and an alcohol is linked to glycerol via phosphate grup.
Sphingomyelin: has the amino alcohol sphingosine as "backbone".
Others: both amphipatic
Cholesterol: it represents 50% of the lipids of the bilayer. Servs to stabilize the membrane at normal body temperature (37°C)
Glicolipids
Lipid bilayer is not static structure: lipids and associated proteins can diffuse. The fluidity is determinated by temperature and its lipidcomposition.
Insaturated fatty acyl chains: the presence of a double bond introduces a "kink" in the molecule that prevents the molecule for asociating closely with sorrounding lipids.
Proteins: the movement of proteins in the membrane can be constrained or limited.
Lipid rafts: some regions on the membran contains lipids (sphingomyelin and cholesterol) tha aggregates.
Membrane proteins: 50% of the plasma membrane is composted of proteins. Are glycosylated, and with the glycolipids are colled the glycocalyx.
Integral membrane proteins: hydrophobic amino acid residues are associated with the hydophobic fatty acyl chains
Transmembrane proteins: the hydrophobic region spans the membrane and the hydrophilic amino acid residues are the exposed to he aqueus enviroment. May pass multiple times through the membrane.
Peripheral proteins: asossiated with the polar head of the membrane lipids.
Lipid anchors: the protein covalently attacher to a lipid (Glycosylphosphatidylinositol) molecule, then is embedded in one leaflet of the bilayer.
The protein can be attachd to the inner leaflet via their amino-terminus by fatty acids or their carboxyl-terminus by preny anchors.
Membrane transport
: the plas membrane is an effective barrier to he movement of virtually of important solutes (ions, glucose, amino acids) and restrictions of the movement of H2O across the membrane. So it has specific membrane transporters.
Membrane transpor proteins: they are classified:
Ion channels: impotant for the function of excitable cells. Classified by:
Selectivity: is defined as the nature of the ions that pass through the channel. Can be high or low selective.
Conductance: number of ions that pas through the channel and is tipically expressed in picosiemens. The conductance varies depending on the direction wich the ion is moving.
Chanel gating: ion channels fluctuate between an open state or a closed state.
Control factors: membrane voltage, extracellular agonist or antagonist, intracellular messengers, and mechanical stretch of the plasma membrane.
Solute carriers: divide according to their mode of transport:
Uniporters: or facilitated transporters. Transports a single molecule across the membrane.
Symporters: or cotransporter. Couples the movent of ≥2 molecules/ions across the membrane in the same direction.
Antiporters: or exchange transporters. Couples the movent of ≥2 molecules/ions across the membrane in oposite directions.
Water channels: or aquaporins. Main routes for H2O movement into an out fo the cell. There are different isoforms thath allows the passive movement of H2O acros the membrane. Some isoforms can provide a path way for other molecules (Glycero, urea, manitol, purines, pyrimidines, CO2 and NH3)
Adenosine triphosohate(ATP)-dependent transporters: use the ehergy in ATP to drive the movement of molecules/ions acros the membrane
ATPase ion transporters: P-type (phospholylated during the transpor cycle) and V-type
ATP-binding cassette transporters: large group of membrane transporters.
Vesicular transport
: in the endocytosis and the exocytosis the integity of the plasma membrane is maintained.
Endocytosis: a pice of the plasma membrane pinches off and is internalized into the cel interior. This occurs in 3 mechanisms:
Pinocytosis: nonspecific uptake of small molecules an water into the cell.
Phagocytosis: allows the cellular internalizacion of large particles. Oftes is a receptor-mediated process.
Receptor-mediated endocytosis: allows the uptake of specific molecules into the cell, where molecules bind tor receptors on the suface of the cell.
Transcytosis: endocytosis across one membrane of the cell is followed by exocytosis across the opposite membrane
Exocytosis: is the process whereby vesicless inside the cell fuse with the plasma membrane.
Regulated: the secretory product, after synthesis and processing, is stored in the cytoplasm in secretory granules until an apropiate signal of secretion is recived. The vesicle is fuses with the plasma membrane and releases its contents into the extracelular fluid. This is mediated by accessory proteins (SNARE) and usually triggered by an increase in the concentration of intracellular Ca++.
Constitutive.
Basic principles of solute and water transport
Difussion: process by wich molecules moves espontaneusly frhom an area of high concentration to one of low concentration. Is a random proces driven by thermal motion of the molecules.
Fick's 1st law of diffusion: quantifies the rate at wich a molecule diffuses from a point.
Diffusion coefficient: counts the thermal energy of hte molecule, its size, and the viscosity of the medium.
Strokes-Einstein equation: for spherical molecules.
Partition coefficient: to quantify the interaction of the molecule with the bilayer.
The rate of diffusion is linearly related to the concentrarion gradient:
The rate of diffusion will be faster for small molecules.
Diffusion rates are high at elevated temperatures, in the presence of large concentration gradientes, and when diffusion occurs in a low-viscosity medium.
The phospholipid portion of th plasma membrane represents an effective barrier to many biologically important molecules.
Diffusion through the lipid phase of the plasma membrane is not an efficient process for movement of these molecules across the membrane.
Electrochemical gradient
: or electrochemical potential difference. Used to quiantitate the driving force acting on a molecule to cause it to move across a membrane. A messire of the free energy available to carry out the useful work of transporting the molecule across the membrane. Components:
Chemical potential diference: the energy in the concentration gradient for a molecule across the membrane
Electrical potential difference: energy associated with moving charged molecules across the membane when a membrane potential exist
Nerst equation: used for the situation in wich a molecule is at equilibrium across the membrane
Nerst equilibrium potential: the equilibrium condition that quantitates the energy in a concentration gradients an express that energy in millivolts.
Passive transport
: or downhill transport or transport with the electrochemical gradient. Is when the net movement of a molecule across the membrane occurs in the direction predicted by the elctrochemical gradient.
Active transport
: or uphill transport or transport against the electochemical gradient. Is when the net movement of a molecule across the membrane is opposite to thath predicted by the electrochemical gradient, a process that requires the imput of energy.
1ry active transport: the transport is directly coupled to the hydolisis of ATP
2ry active transport: solutes carriest that couple of movements of ≥2 molecules often transport ≥1 molecules against the respetive electrochemical gradient through the use of the energy in the electrochemical gradient of the other mocelule or othres.
Osmosis
: movement of the water across cell membranes is passive, with the driving force for this movement being thre osmotic pressure diference across the cell membrane.
Osmotic pressure
: determinated by the number of solute molecules dissolved in the solution. Calculated by van't Hoff's law. To exert an osmoti pressure acoss a membrane, a molecule most not cross the membrane (effective osmole vrs. ineffective osmole).
Osmolarity vrs Osmolality
: the 1st is temperature dependent, the 2nd isn't.
Osmolarity: osmotic pressure generated by dissolved solute molecules in 1 L of solvent
Osmolality: is the number of molecules dissolved in 1 Kg of solvent
Is the preferred term of biologic systems.
Are expressed mOSm/Kg H2O
Tonicity
: is the effect of the solution on the volume of the cell
Isotonic: solution that do not change the volume of a cell
Hypotonic: solution that causes a cell to swell
Hypertonic: solution that causes a cell to shrink
