Transport Mechanisms Flashcards
Two fonctions of cell membrane
- Support distinct compositions of ICF and ISF
- Be selectively permeable
Membrane is permeable to:
H20
Lipid-soluble substances
Dissolved gases
Small uncharged molecules
Membrane is a little permeable to:
Larger molecules
Charged particles
Membrane is impermeable to:
very large molecules
Membrane is amphipathic. What does that mean?
Polar and non-polar ends
Phospholipid bilayer has hydrophilic …
heads
Phospholipid bilayer has hydrophobic …
tails
Cholesterol in the membrane provides :
firmness and fluidity
Integral membrane proteins are :
transmembrane
Peripheral membrane are on the … side
inner (cytoplasmic)
Carbohydrates and glycoproteins constitue the
glycocalyx
Name of the current model for cell membrane
Fluid Mosaic Model
Functions of plasma membrane proteins (6)
Selective transport Enzyme Cell surface receptor Cell surface identity marker (glycoproteins) Cell-cell adhesion Attachment to cytoskeleton
Transmembrane transport pathways
- Via phospholipid bilayer
- Via interaction with protein cluster (channel or carrier/transporter)
What are the factors affecting movement across the cell membrane
- Lipid solubility
- Particle size
- Electrical charge
- Availability and number of carriers and ion channels
Passive (energy independant) and active (energy dependant) movements across the cell
Passive: 1. Diffusion
- Facilitated diffusion (carrier-mediated)
- Osmosis
Active: 1. Carrier-mediated active transport
- Primary
- Secondary
2. Pino/Phagocytosis
What is diffusion?
Movement of solute particles resulting from random thermal molecular motion
As they collide, they become dispersed
What is Net flux and equilibium?
Net flux : from a region of high concentration to one of lower concentration
At equilibrium, net flux = 0 (Diffusion fluxes in opposite directions are equal)
What are the factors that affect diffusion?
- Temperature
- Mass of the molecule
- Medium through which the molecule is diffusing
- Surface area between the two region
What is the relationship between the diffusion time and the distance?
Diffusion time increases in proportion to the square of the mean distance travelled by the solute
molecules = diffusion is only effective over short distances
What are the factors that affect diffusion across the cell membrane?
- Mass of the molecule
- Concentration gradient across the cell membrane
- Lipid solubility
- Electrical charge
- Availability of selective ion channels or membrane carriers
How the solute particule could penetrate the cell ?
- Dissolving in the lipid component (non-polar molecules —> O2, CO2, fatty acids)
- Diffusing through channels (ions)
Movement of the particule depends of what?
Movement depends of the concentration gradient
+ electrical gradient for ions (electrochemichal gradient)
What are ion channels?
Ion channels: clusters of proteins or single protein (sometimes)
Show selectivity by their diameter and distribution of charges
Exist in open or closed (gated) states (conformational changes)
How ion channels can be gated?
Can be gated in 3 ways :
- Ligand-gated (chemical messengers)
- Voltage-gated (membrane potential)
- Mechanically-gated (stretching of the membrane)
What are the Voltage-gated ion channels?
- Na+ channels
- K+ channels
- Ca+ channels
- Cl- channels
Total number of ions that flow through these channels, generating ionic current, depend upon what?
- The channel conductance
- How often the channel opens
- How long the channel stays open
On which cell membranes water is diffused and what is facilitating this diffusion?
Water diffuses freely across most cell membranes
Facilitated by aquaporines (proteins) that form water permeable channels
What is osmosis?
Spontaneous net movement of H2O across a semipermeable membrane (permanent to solvant, not to solute)
From the lower concentration in solute to the higher concentration in solute
What is the osmotic pressure?
Pressure required to prevent the mvt of water = difference in the hydrostatic pressures of the two solution
What is the equation defining the osmotic pressure in an ideal solution?
In an ideal solution, osmotic pressure —> P = nRT/V
What is osmolarity (Osm)?
Osmolarity(Osm) is the total solute concentration of a solution
Define the value of one osmol and the value of osmolarity (Osm). If you double the value of Osm, how is the osmotic pressure affected?
1 osmol = 1 mol of solute particles 1 mol glucose = 1 osmol of solute 1 mol NaCl = 1 mol Na+ + 1 mol Cl-= 2 osmol of solute
Osm = osmol/liter
1 mol glucose/L = 1 osmol/L = 1 Osm 1 mol NaCl/L = 2 osmol/L = 2 Osm
Osmotic pressure is proportional to osmolarity
Give an exemple of osmotic pressure of physiological saline.
1.Determine Molarity
0.9% saline = 9 g NaCl/L H2O (liter (l) ~1000g), Moles = 9g NaCl x 1 mole /58.5 g NaCl = 0.15 moles NaCl Molarity = moles/L = 0.15 moles NaCl/L = 0.15 M NaCl solution
2. Determine Osmolarity
0.15 M NaCl solution = 0.15 moles Na+ + 0.15 moles Cl= 0.15 osmol + 0.15 osmol = 0.30 osmol = 0.30 osmol/L H2O = 0.30 Osm = 300 mOsm
3. Calculate Osmotic Pressure
Osmotic pressure = 0.30 Osm x 22.4 atm/Osm = 6.7 atm = 6.7 atm x 760 mmHg/atm = 5092 mm Hg
When ISF is 3 mOsm> ICF osmotic pressure gradient is 50.92 mm Hg
What are isosmotic, hyposmotic and hyperosmotic solutions?
Isosmotic: solution with same number of osmotically active particles as normal extracellular solutions
Hyposmotic: solution with a lower number of solute particles
Hyperosmotic: solution with a higher number of solute particles
What is needed for particles to sustain the osmotic pressure? How does extracellular Na acting?
For particles to be effective in exerting a sustained osmotic pressure, they must be nonpenetrating (not be able to cross the membrane which separates the two solutions)
Extracellular Na acts as a nonpenetrating solute because Na that has moved in is pumped out
What are the characteristics of Carrier-Mediated Transport Systems?
Characteristics:
- Facilitated diffusion
- Active Transport
- Primary
- Secondary
What does specificity means for Carrier-Mediated Transport Systems?
Transport of one particular type of molecule only
What does saturation means for Carrier-Mediated Transport Systems?
Rate of transport reaches maximum when all binding siteson all transportersare occupied
Limit for a given substance across a given membrane : transport maximum (Tm)
What does competition means for Carrier-Mediated Transport Systems?
When structurally similar substances compete for the same binding site on a membrane carrier
What determines the Flux magnitude of mediated-transport systems?
- Solute concentration
- Affinity of transporter for the solute
- Number of transporters
- Rate of transporter conformational change
What is involved in Facilitated diffusion?
Involves the presence of a ‘‘transporter’’ or ‘‘carrier’’ molecule —> enables a solute to penetrate more readily than it would be expected to by simple diffusion
a) Solute binds to transporter
b) Transporter changes configuration
c) Solute is delivered to other side of membre (??? membrane???)
d) Transporter resumes original configuration
Passive (no energy) —-> net flux from high to low concentration
Hormones may increase the number/affinity of transporters in some membranes
What is involved in active transport?
- Transporter-mediated
- Requires supply of chemical energy (usually derived from enzymatic hydrolysis of ATP)
- Susceptible to metabolic inhibitors
- Can transport solute against its concentration gradient
Primary active transport:
- Involves hydrolysis of ATP by a transporter
- Phosphorylation of the transporter changes the conformation of the transporter and its solute binding affinity
What is the Na+/K+-ATPase Mechanism?
The transporter involved is the enzyme ATPase, which splits ATP on the cytoplasmic side of the membrane, and thus releases the necessary energy
(i) The carrier molecule binds Na+ on the cytoplasmic surface of the membrane. This results in the activation of ATPase and the consequent release of energy
(ii) Through a change in configuration, the carrier “delivers” the Na+ to the outer surface, as it permits the exposure of a binding site for K+ on the outer surface, as well
(iii) The attachment of K+ to its binding site is accompanied by the return of the carrier to its original configuration, andthe ultimate release of K+ to the inside of the cell, while the Na+ binding site becomes available, again
What are the active transport mechanisms (energy dependant) involving participation of the cell membrane itself?
Active transport mechanisms (energy-dependant) involving participation of the cell membrane itself
Endocytosis: invagination of the cell membrane forming a channel —> pinches to form vesicle
Exocytosis: intracellular vesicle fuses with the cell membrane and releases its content into the ECF
Name three types of Endocytosis and describe them.
Pinocytosis: ingestion of dissolved materials by endocytosis -small droplets of fluid then transferred to cytosol
Phagocytosis: ingestion of solid particles by endocytosis -phagocytic vacuole (phagosome) then fuses with lysosomes to degrade particles
Receptor-mediated endocytosis: invagination happens when ligand bounds to a receptor on the surface (then recycled)
- Clathrin-dependant: vesicles are coated with clathrin –> fuses with endosomes and content is sorted
- Clathrin-independant: vesicles called ‘‘caveolae’’ , deliver content to cytosolvia channels or carriers
What are the kinds of body fluids?
There are two kinds of body fluids, namely, the intracellular (ICF) and extracellular (ECF) which is further categorized into plasma and interstitial fluid (ISF).
Osmotic pressure of ECF?
300 mOsm
