Transport mechanisms Flashcards
How do we ensure to preserve the constancy of the Milieu Intérieur and the homeostasis?
by exchanging nutrients, salts, gases, and waste in and out of the body
What is the plasma membrane/ cell membrane?
What are its characteristics?
It is the membrane that covers every cell (membrane between ICF and ISF)
Varies depending on the cell
Bimolecular Phospholipid Layer (phospholipid bilayer)
polar/hydrophilic head and non-polar/hydrophobic tail
Phospholipids = 40-50% of plasma membrane by weight
What organs make the link between the plasma and the interstitial fluid?
Lungs and the kidneys
What is the cell membrane permeable/ less permeable and impermeable to?
Highly permeable to :
H2O
Lipid-soluble substances
Dissolved gases(CO2, O2)
Small uncharged molecules
Less permeable to:
Larger molecules
Charged particules
Impermeable to:
Very large molecules (macro-molecules)
What role does cholesterol play in the plasma membrane?
inserted into phospholipid bilayer to reduce packing of fatty acid tails –> increase fluidity of membrane and avoid aggregation
In high temperature, role changes: adds firmness, stabilize cell membrane
What are the different types of protein we can find in the cell membrane and their importance?
25-75% of membrane by weight
Integral proteins:
closely associated with phospholipids, cross membrane (Trans-membrane, amphipathic –> both hydrophilic and hydrophobic parts)
Peripheral proteins:
more loosely associated, mostly on the cytoplasmic side
What is Glycocalyx?
layer of carbohydrates formed by a chain of monosaccharides that extend extracellular surface on cell membrane bound ot proteins
Provides protection from infections
Enable cells to indentify each other and interact
What are the different functions of plasma membrane proteins?
Selective transport channel
Enzyme that catalyse membrane associated ractions (amino acid transport/Na-K pump)
Cell surface receptors for chemical signal of cell environment
Cell surface identity marker
Cell adhesion
Attachement to cytoskeleton
Which are the 2 possibilities for transmembrane transport pathways?
- via phosphilpid bilayer
- via interaction with protein cluster (channel or carrier/trnasporter)
What are the 3 passive transport mechanisms across the cell membrane ?
Energy independent
1. Diffusion (most important)
- Carrier-mediated Facilitated Diffusion (form of diffusion)
3.Osmosis (mvt of water across semi-permeable membrane)
What are the 3 active transport mechanisms across the cell membrane ?
Energy dependent
1.Carrier-mediated Active Transport:
- primary
-secondary
- Pino/phagocytosis
What is simple diffusion
mvt of molecules from on location to another as result of RANDOM THERMAL MOTION
Flux = amount particules crossing surface/ unit of time
Net Flux = high concentration to lower concentration
Equilibrium = when diffusion fluxes are equal and net flux = 0
*Membrane has to be permeable to particles
What happens with diffusion and concentrations when cell is exposed to solute in large extracellular solution?
extracellulare cellular concentration = constante C0
intracellular concentration = Ci
Ci will eventually = C0
because when Ci diffuses, doesn’t change C0 concentration but when C0 diffuses, changes Ci concentration
What is Fick’s Law of Diffusion?
J = PA(C0 - Ci)
J = Net flux: rate of diffusion
P = Permeability/diffusion coefficient
A = surface area of membrane
C0 - Ci = concentration gradient
*Diffusion time increases proportionnally to the square of distance travelled by solute molecule –> effective over short distances –> why many tiny capillaries close to cells
What factors affect diffusion across the cell membrane?
- Mass of the molecule
- Concentration gradient across cell membrane
- Lipid Solubility (if diffusion trough bilayer)
- Electrical charge
- Availability of selective ion channel or membrane carriers (to move olecules through celle membrane)
Which are the 2 ways DIFFUSION particules may penetrate into cell?
- Dissolving in the lipid component (for non-polar molecules) ex: O2, CO2, fatty acids, etc.
- Diffusing through channels (for ions –> polar) (mediated transport?)
Both case, mvt depends on existence on concentration gradient
What is an ion channel?
What are the 3 ways a channel can be gated?
A single protein or clusters of proteins (which cross the membrane multiple times)
Shows selectivity based on their diameter (why small particles enter more easily) and on the distribution of charges
Eache type of selectivity channels has specific ions which go through –> form energy barriers
3 possible ways to channel is gated:
- Ligand-gated
- Voltage-gated (based on resting potential?)
- Mechanically-gated (stress around channels –> pain ex)
What is the electrical gradient?
Mouvement of ions also affected by presence of an ELECTRICAL GRADIENT because inside of cell = negative charge
simultanous existence of concentration + electrical gradient for particular ion = electrochemical gradient
How do voltage gated ion channel work?
4 classes of ion channels:
Na+ channels
K+ channels
Ca+ channels
Cl- channels
total # ions that flow through these channels generate ionic current depending on:
1. channel conductance
2. how often channel opens
3. how long channel says open
What is Mediated Transport Systems?
And its characteristics
mouvement of ions and other molecules (glucose, amino acids) by integral membrane proteins called transporters
Ion mvt across membrane via transporters = much slower that trough ion channels
- Specificity - system transports 1 specific type of molecule only
- Saturation - rate of transport = max when all binding sites on all transporters occupied.
Tm (transport max) for given substance across given membrane - Competition - when (structurally) similar substances compete for same binding site on membrane carrier
*Compared to simple diffusion which increase linearly, mediated-transport curves at asymptote Tm
2 Types:
1. Facilitated diffusion (passive)
- Active transport (active)
- Primary active transport
- Secondary active transport
Which are the factors that determine flux magnitude of mediated-transport systems?
- solute concentration
- Affinity of transporter for the solute
- # transporters
- Rate of transporter conformational change
What is involved in facilitated diffusion?
(mediated-transport)
presence of a transporter/carrier molecule which enable solute to penetrate nore readily than by simple diffusion
- Passive (no energy)
- net flux from high to low concentration (thats why no energy?)
- Hormones may increase # or affinity of transporters
- Solute binds transporter
- Transporter changes configuration
- Solute is delivered to other side membrane
- Transporter resumes original conformation
What is involved in active mediated-transport?
- Transport-mediated
- Requires supply of chemical energy (usually hydrolysis of ATP by carrier)
- Susceptible to metabolic inhibitors
- Can transport solute against concentration gradient (“uphill”)
Primary: ATP
Phosphorylation of transporter changes conformation of transporter and its solute binding affinity
Necessary to maintain concentration gradient between inside and outside of cell (ICF = more K+ than Na+ and ECF = more Na+ so have to go uphill to keep gradient bc naturally goes downhill)
Secondary: use downhill for uphill, electrochemical gradient
- Na+ binds to transporter outside cell (where Na+ concentration = high so downhill) allowing glucose or amino acid to bind to the same carrier
- Through change of configuration, transporter delivers both into cell
- Transporter then reverts to original configuration and Na+ is extruted from cell by Na+/K+ -ATPase (primary)
*Uses energy stored of electrochemical gradient to move Na+ and transport solute.
Creation + maintenance of electrochemical gradient depends on primary active transport
What is Na+/K+ -ATPase?
It is the active mediated-transport of Na+ ouside the cell and K+ inside the cell to keep concentration gradient (uphill) to compensate for simple diffusion goind downhill
Changes in the binding site affinity for a transported solute are produced by phosphorylation and dephosphorylation of the Na+/K+-ATPase.
Name 4 active transporters
Na+/K+ -ATPase : concentration gradient in/out cell (K+ inside, Na+ outside)
Ca2+-ATPase : maintain low intracellular Ca2+ level, plasma membrane move Calcium in storage sites in muscles
H+-ATPase : maintain low lysosomal pH
H+/K+ -ATPase : Acidity of the stomach
What is cotransport ?
What is countertransport?
In secondary activated mediated-transport
Cotransport = when solute X transported in same direction Na+.
Also called Symport (ex: Na+ - Amino Acids)
Countertransport = when solute X transported in opposite direction to Na+
Also called Antiport
(Na+ - Ca2+ –> Na goes in, Ca2+ taken out of cell)
What molecule is affected by simple diffusion?
Review p.54-55
Glucose goes into cell to be used in cellular respiration
What is endocytosis?
ACTIVE transport mechanisms (energy dependent) involving participation of cell membrane itself
Cell membrane invaginates and piches off to form vesicle
- Pinocytosis (fluid endocytosis)
- All endocytotic vesicles that engulf the ECF including whatever solutes present
- Non-specific + constitutive
- Vesicle travels into cytoplasm and fuses with other vesicles (endosomes or lysosomes)
- Cell drinking!!! - Phagocytosis
- cell internalize particulate matter (>0.75um) –> small dust particles, cell debris, microorganiisms
- Specific + triggered
- Pseudopodia (extension of cell membrane) folds around and fully engulf particle
- Pseudopodia fuses to form vesicles called phagosomes that pinch off membrane
- Phagosomes fuse with lysosomes where content = degraded
- Protects against infection, scavenge senescent, dead cells
- Cell eating!!
What is exocytosis?
ACTIVE transport mechanisms (energy dependent) involving participation of cell membrane itself
Intracellular vesicle fusses with cell membrane and content released into ECF
Types:
1. Constitutive Exocytosis: replenishment of cell membrane –> Nutrients
- Non-regulated
- To replace plasma membrane, deliver membrane proteins to the cell membrane and get rid of substances from cell
- Regulated Exocytosis:
- Triggered by extracellular signals and the increase of cytosolic Ca2+
- Responsible for secretion of hormones, digestive enzymes, neurotransmitters
Which are the 2 processes by which Receptor-memdiated endocytosis can occur?
Clathrin-dependent receptor-mediated endocytosis:
a. Ligand (chemical messenger) binds –> receptor
b. Conformational change of receptor + clathrin is recruited to the plasma membrane
c. complex = cagelike structure –> leads to aggregation of ligand bound receptors
d.Clathrin pit formed –> invaginates –> clathrin coated vesicle
e. Vesicle pinched off –> sheds clathrin coat
f. can fuse with cell organells (endosomes, lysosomes) or fuse with membrane on other side of cell (transcytosis)
g. Receptors and clathrin protein are recycled back to the cell membrane
2.Potocytosis:
a. Molecules sequestred and trasported by Caveolae (tiny vesicles) (clathrin-independent)
b. Caveolae can deliver directly to cytoplasm!! (main difference), endoplasmic reticulum, organelles or do transcytosis
*Implicated in uptake of low MW molecules such as vitamins.
What are aquaporins?
Group of protein which facilitate diffusion of water across cell membranes –> form water permeable channels
What is osmosis?
What is osmotic pressure?
Which way goes the pressure induced?
NET diffusion of H2O across semi-permeable membrane (permeable to solvent but not to all solute)
Osmotic pressure (mm Hg)
Pressure requieres to prevent mvt of water (across semipermeable membrane)
= difference in hydrostatic pressures both sides
pressure to push down collumn for both to be equal (higher is the difference of height, higher is pressure) in U tube
Pressure enduced goes opposite way than net flow
What is Van’t Hoff Equation?
In ideal solution, osmotic pressure related to temperature in same way as pressure of a gas:
PV = nRT –> P = nRT/V
n = # particules, R = gas constant, T = abs. temp., V = volume
osmotic pressure = proportional to # particles, not to size, configuration or charge
What is osmolarity?
Osm = total solute concentration of a solution
1 osm = 1 mol of solute particle (if ions, when dissolved)
Osm = osmol/L
molarity = non dissolved particles (NaCl = 1)
osmolarity = # of mol so if given in grams, g*mol dissolved particles (Na+, Cl- = 2)
Osmotic pressure is proportional to osmolarity (Osm)
When think osmolarity, think # particles dissolved
*0.9g% = 0.9g/100mL = 9g/L
**22.4 atm/Osm
1 atm = 760 mm Hg
What is an Isosmotic solution?
same osmolarity as NORMAL extracellular (or intracellular) solutions
–> 300 mOsm
What is a Hypoosmotic solution?
What is a Hyperosmotic solution?
Hypoosmotic = osmolarity < 300 mOsm
Hyperosmotic = osmolarity > 300 mOsm
*ISF and ICF normal osmolarity = 300 mOsm
What is essential to be effective in exerting a sustained osmotic?
Does Na+ respond to that characteristic?
Review p.73-74
Molecules have to be nonpenetrating (not able to cross membrane)
Na+ is considered as being nonpenetrating because Na+ that moves ingets into the cell by being pumped by
ALL ABOUT CONCENTRATION
What would happen with ECF Osm and ECF volume if:
- Excessive H2O intake
- iv infusion of 0.9% NaCl
- Hemorrhage
- Drinking sea water
- Severe sweating
- Excessive H2O intake = ↓ ECF Osm, ↑ECF vol
- iv infusion of 0.9% NaCl = = ECF Osm, ↑ ECF vol
- Hemorrhage = =, ↓ ECF vol
- Drinking sea water = ↑ ECF Osm, ↑ EFC vol
- Severe sweating↓↑ = ↑, ↓ ECF volume
*If CONCENTRATION is important, not volume
Where do the main exchanges between ECF and ICF take place?
At the levels of the capillaries:
- 5% of total circulating blood
- Each capillariy = 1mm long
- Inner diameter = 8um
How do the capillary walls function?
Structure and permeability
They are single layer of flattened endothelial cells + supporting basement membrane
Permeability:
1. DIFFUSION –> Small water-soluble substances pass through water pores (Defenestration through water filled channels)
2. DIFFUSION –> Lipid-soluble substance –> through endothelial cells (most common is diffusion)
3. Trancytosis –> Exchangeable protein move across by vesicular transport (not so important)
4. BULK FLOW –> Distribute ECF between plasma and IFS , Magnitude proportional to hydrostatic pressure difference. Proteins are nonprenetrating so protein free plasma move from capillaries to ISF
