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Principles of cell and membrane function - Coggle Diagram
Principles of cell and membrane function
Overview of eukarotic cells
:
Distinguished from prokaryotic cells by the presence of a membrane-delimited nucleus.
Divided into 2 compartments: nucleus and cytoplasm.
Cytoplasm: aqueous solution containin numerous organic molecules ions, cytoskeletal elements, and a number of organelles.
Fig. 1.1 and table 1.1
The plasma membrane
:
fig 1.2
Separates the intracelullar contents from the extracelular enviroment. Is involved in:
Selective trasport of molecules into and out of the cell
Cell recognition and comunication
Tissue organization
Membrane-dependent enzymatic activity
Determination of cell shape
Structure and composition:
Consisits of a 5-nm-thick lipid bilayer with associated protein:
Integrated into the lipid bilayer
Attached to the inner or outer surfaces of the membrane
Membrane lipids: is not a static structure, it depends by temperature and by it's lipid composition (unsaturated chain increase fluidity). The mayor lipids are phospholipids and phosphoglycerids.
Phospolipids: are amphipathic (contain a charge hydrophylic head and 2 hidrophobic fatty acyl chains). It's nature form the bilayer (the head are exposed on the surface and the fatty acyl chains forme de core). The composition varies among different cell types.
Fig. 1.3 and table 1.2
Glycerol: backbone to which are attacher the chains. Except sphingomyelin, which it's backbone is amino alcohol.
Alcohol: is linked to glycerol (choline, ethanolamine, serine, inositol and glycerol).
Cholesterol: (sterol) servs to stabiliza the membrane at a normal body temperature (37°C). Represents 50% of the lipids.
Glycolipids: 2 fatty acyl chains linked to polar head groups that consist of carbohydrates.
Rafts: have association with specific proteins. Important function is to segregate signaling molecules
Membrane proteins: 50% of the plasma membrane is composed of proteins. Are classified as:
Integral: are imbedded in the lipid bilayer.
Transmembrane: have hydophobic (alfa helix) and hydrophilic regions. May pass through the membrane multiple times.
Lipid anchors.
Peripheral: associated with the polar head groups of the membrane lipids.
Membrane transport: approximately 10% of the human genes code for transporters.
The presence of specific membrane transporters in the membrane is responsible for the movement of the solutes and water across the membrane
Membrane transport proteins: 4 general groups:
Table 1.3
Water channels
Ion channels
Solute carriers
ATP-dependent transporters
Water channel: or aquaponis (AQP's), are the main routes for water movement into and out of the cell. Cells express different and multiple AQP's isoforms.
Some isoforms provide a pathway for other molecules: glycerol, urea, manitol, purines, pyrimidines, CO2, and NH3.
The regulation of the amount of H2O in the cell occurs by altering the number of AQP's in the memmbrane.
Ion Channels: this can be regulated by the change in number of channels o by gating of the chanels. Are classified by:
Selectivity: is defined as the nature of the ions thath pas through the channel.
Conductance: refers to the number of ions that pass through the channel, expressed in picosiemens (pS, 1-2 pS to >100 pS)
The conductance varies: inward rectifier or gating.
Mechanism of channel gating: factors that control it: membrane voltage, extracellular agonists o antagonists, intracellular messengers, and mecanical stretch of the plasma membrane.
Solute carriers: can be divided into 3 groups (mode of transport):
Uniporters: or facilitated transporters. Transport a single molecule across the membrane.
Symporters: or cotransporters.Couples the movement of 2 or more molecules/ions across the membrane in the same direction.
Antiporters: or exchange transporters. Couples the movement of 2 or more molecules/ions across the membrane in the opposite direction.
ATP-dependent transporters: use the energy in ATP to move molecules/ions across the membrane. There are 2 groups:
ATPase ion transporters: are subdivided intro:
P-type ATPase: are phosphorylated during the transport cycle.
V-type ATPase: or V-type H+-ATPase or H+-ATPase. Are found in the membranes of several intracellular organelles.
ATP-binding cassette (ABC) transporters: are found in both prokaryotic and eukaryotic cells, and they have aminoacid domains that bind ATP.
Vesicular transport
: the integrity of the plasma membrane is maintainer, and the vesicles allow for the transfer of the contents among cellular compartments.
Endocytosis: is the process where by a piece of the plasma membrane pinches off and is internalizad into the cell interior. Occurs in 3 mecanisms:
Pinocytosis: nonspecific uptake of small molecules and water into the cell.
Phagocytosis: allos for the cellular internalization of large paricles.
Receptor-madiated endocytosis.
Exocytosis: is the process whereby vesicles inside the cell fuse with the plasma membrane. Can be either constitutive or regulated.
Trancytosis: endocytosis across one membrane of the cell is followed by exocytosis across the opposite membrane.
Basic principles of solute and H2O transport
: the presence of a pathway is not sufficient for transport to occur; an approtiate driving force is also required.
Diffusion
: the process by which molecules move spontaneously from an area of high concentration to one of low concentration. Is a random process driven by the thermal motion.
Fick's 1rst law of diffusion: quantifies the rate at which a molecule diffuses.
Diffusion coefficient: takes into account the thermal energy of the molecule, its size, and the viscosity of the medium through which diffusion is taking place. Stokes-Einstein equations is for spherical molecules. "The rate of diffusion will be faster for small molecules than for large molecules an at elevated temperatures".
Partition coefficient: interaction of the molecule with the bilayer. "The more lipid soluble the molecule is, the large the partition coefficient is, and thus the diffusion coefficient is greater".
The vast majority of biologically molecules cross cell membranes via specific membrane transporters.
Permeability coefficient: reflects the properties of the pathway that the molecule use to cross the membrane.
Electrochemical gradient
: used to quantitate the driving force acting on a molecule to cause it to move across a membrane. When a molecule is at equilibrium across the membrane, we use the Nernst equation. It has 2 components:
Chemical potential difference: represents the energy in the concentration gradient across the membrane.
Electrical potential difference: represents the energy associated with moving charged molecules across the membrane when a membrane potetial exists.
Active and passive transport
:
Fig 1.6
Passive: or down hill transport or transpor with the electrochemical gradient. When the net movement of a molecule across a membrane occurs in the direction predicted by the electrochemical gradient
Active: uphill transport or transport against the electrochemical gradient. . When the net movement of a moelecule across the membrane is opposite to thath predicted by the electrochemical gradient.
Fig 1.7
Primary: the transport is coupled to the hydrolysis of ATP.
Secondary: the use of the energy in the electrochemical gradient of the other molecule or molecules.
Osmosis and osmotic pressure
:
Fig 1.8
Osmosis: the movement of the water across cell membranes.
Osmotic pressure: determined by the number of solute molecules dissolved in the solution, calculated by Van't Hoff's law.
Osmolarity vrs. Osmolality
:
Osmolarity: osmotic pressure generated by the dissolved solute molecules in 1 L of solvent. Is temperature dependent.
Osmolality: number of molecules dissolved in 1 Kg of solvent. Is temperature independent and is prefered for biologic systems (mOSm/KgH2O).
Tonicity
:related to the effect of the solution on the volume of a cell.
Isotonic: solutions that not change the volume of the cell.
Hypotonic: solutions that causes a cell to swell.
Hypertonic: solutions that causes a cell to shrink.