Transport Mechanisms Flashcards
Permeability Characteristics of Cell Membrane
Highly Permeable
-H2O
-Lipid-soluble substances
-Dissolved gasses (O2 CO2)
-Small uncharged molecules
Less permeable
-Larger molecules
-Charged particles
Impermeable
-Very large molecules
Fluid Mosaic Model
-Phospholipid bilayer
-Cholesterol
-Proteins
-Glycocalyx
Phospholipid bilayer
- 6-10nM thick
- Amphipathic: polar heads and non polar tails
~ 40-50 % of membrane by weight
Membrane Cholesterol
- Slightly amphipathic
-Maintains fluidity/rigidity
-Vesicle formation / fusion
-Lipid raft formation
-Lower temps: increases fluidity
-Higher temps: decreases fluidity
Membrane proteins
-Most diverse macromolecules
-25-75 % membrane by weight
-Integral: cross the membrane
-Peripheral: attached on cytoplasmic side
Membrane Glycocalyx
-glycans, glycoproteins and glycolipids forms fuzzy coating
-cell-cell recognition
-communication
-adhesion
-protection
-control vascular pemeability
Functions of plasma membrane proteins
-Selective transporters
-Enzymes
-Cell surface receptors
-Cell surface identity markers
-Cell adhesion
-Attachment to cytoskeleton
GLYCO = carbohydrate
Passive Cell Membrane Transport Mechanisms
(no energy)
1. Diffusion
2. Facilitated diffusion
3. Osmosis
Active Cell Membrane Transport Mechanisms
(energy dependent)
1. Active Transport (Primary and Secondary)
2. Pino/Phagocytosis
Diffusion
Movement of molecules from one location to another due to random thermal motion
-occurs in presence of membrane IF permeable
Diffusion Net Flux
particle crossing surface per unit time
Net flux is from high concentration to low concentration
Zero at equilibrium
Fick’s Law of Diffusion
J = PA (C0 – Ci)
J: flux (rate of diffusion)
P: permeability constant
A: surface area
C0 – Ci: concentration gradient
Diffusion Time
Time increases in proportion to the square of the distance travelled by solute
(10 m = 100 sec)
DIFFUSION ONLY EFFECTIVE OVER SHORT DISTANCES
Cell membrane diffusion
Related to the CONCENTRATION GRADIENT
- non-polar molecules and gases across the lipid bilayer
- ions through channels
Ion Channels
transmembrane proteins that show ion selectivity
-also affected by presence of electrical gradient
Electrochemical gradient
Concentration gradient + Electrical gradient
Usually for a particular ion through ion channel
Ion channel gating
Ion channels exist in open/closed states
1. Ligand gated
2. Voltage gated (Na+, K+, Ca+, Cl-)
3. Mechanically gated
Flow through channel depends on
1. Channel conductance
2. Channel open time
3. Frequency of channel opening
Mediated Transport
Movement of ions/molecules by integral proteins called transporters or carriers (slower than ion channel)
Can be active or passive
-Passive
1. Facilitated Diffusion
-Active
1. Primary Active Transport
2. Secondary Active Transport
Mediated Transport Characteristic
- Specificity: system only transports one type of molecule
- Saturation: max rate of transport (Tm)
- Competition: structurally similar substances complete for binding site on transporter
Transport Maximum (Tm)
CHARACTERISTIC TO MEDIATED TRANSPORT
rate of transport reaches a maximum when all binding sites on all transporters are occupied by substance
Factors that Determine Mediated Transport
- Solute concentration
- Affinity of transporter for solute
- Number of transporters
- Rate of transporter conformational change
Facilitated Diffusion
-“transporter” or “carrier” molecule undergoes conformational change
-PASSIVE
-net flux from high to low concentration
(Transporter affinity/number may be affected by hormones)
Transporters NOT channels
Active Transport
- Transporter-mediated
- Requires energy from ATP hydrolysis
- Susceptible to metabolic inhibitors
- Can be uphill against conc. gradient
Primary Active Transport
-hydrolysis of ATP by a transporter
-phosphorylation of the transporter changes the conformation of the transporter and its solute binding affinity (P from ATP -> ADP added to carrier)
Ex: Na+/K+-ATPase + other ATPase molecules
Na+/K+-ATPase
Primary Active Transport
- powered by ATP hydrolysis + phosphorylation and dephosphorylation
-Pumps 3 Na+ out of cell
-Pumps 2 K+ into cell
AGAINST CONC GRADIENT
FORMS ELECTROCHEMICAL GRADIENT
Secondary Active Transport
-Na+ down its concentration gradient coupled to solute uphill against its concentration gradient.
-uses the energy stored of the electrochemical gradient to move both (POWERED BY PRIMARY ACTIVE TRANSPORT)
-All Na+ pumps besides from Na+/K+ (usually amino acids, H+, Ca2+, HCO-3)
Directions of Secondary Active Transport
Symport/Cotransport: solute is transported in same direction as Na+
Antiport/Countertransport/Exchange: solute is transported in opposite direction as Na+
Endocytosis and Exocytosis
ACTIVE transport mechanisms (energy-dependent) involving participation of the cell membrane itself
Endocytosis
CELL TAKES IN: the cell membrane invaginates and pinches off to form a vesicle
- Pinocytosis (non-specific and constitutive): cells engulfs fluid and solutes
- Phagocytosis (specific and triggered): cells bind and take in matter
Exocytosis
CELLS SECRETES: an intracellular vesicle fuses with the cell membrane, and its contents are released into the ECF
- Constituitive Exocytosis (non-regulated)
-maintain membrane / remove substances - Regulated Exocytosis (triggers by extracellular signals and increases in cytosolic Ca2+)
-secrete hormones, enzymes, neurotransmitters
Phagocytosis
- Extensions of the membrane (pseudopodia) engulf particle
- Pseudopodia fuse to form large vesicles, called phagosomes
- Phagosomes migrate to and fuse with lysosomes
- Contents of the phagosome are degraded
Receptor-mediated Endocytosis
-ligands bind with high affinity to specific protein receptors on the plasma membrane
1. Clathrin-dependent receptor-mediated endocytosis
2. Potocytosis
Clathrin-dependent receptor-mediated endocytosis (LDL receptor)
- Ligand binds the receptor
- Conformational change
- Clathrin is recruited to the plasma membrane. 4. Adaptor proteins link the ligand-receptor to the clathrin
- Complex forms cagelike structure that leads to the aggregation of ligand bound receptors
- A clathrin coated pit is formed which then forms a “clathrin-coated vesicle”
- Vesicle pinches off it sheds the clathrin coat
- Vesicles can fuse with endosomes and lysosomes. or fuse with the membrane on another side of the cell (transcytosis)
Receptors and clathrin protein are recycled back to the cell membrane
Is ACTIVE process (needs ATP)
cholesterol transported as LDL into cell
Potocytosis
-molecules are sequestered and transported by tiny vesicles called caveolae
-clathrin-independent
Water Diffusion
freely across aquaporins in the cell membrane
Osmosis
net diffusion of H2O across a semipermeable membrane (permeable to solvent, but not to all solute)
from high water concentration to low water concentration
OR
from low solute concentration to high solute concentration
Osmotic Pressure
-pressure required to prevent the movement of water across a semi-permeable membrane
-equal to the difference in the hydrostatic pressures of the two solutions
P = nRT / V (ideally)
-osmotic pressure is proportional to the NUMBER of particles in solution/unit volume
NOT charge or size
R = gas constant
T = abs temp
V = volume
n = number of particles
Osmolarity (Osm)
-total solute concentration of a solution
-proportional to osmotic pressure
1 osmol = 1 mol of solute particle
1 mol glucose = 1 osmol of solute
1 mol NaCl = 1 mol Na+ + 1 mol Cl- = 2 osmol
1 mol MgCl2 = 1 mol Mg2+ + 2 mol Cl- = 3 osmol
Osm = osmol / L
1 mol NaCl/L = 2 osmol/L = 2 Osm
1 mol MgCl2/L = 3 osmol/L = 3 Osm
Osmotic Pressure Calculation
- Determine molarity
0.9% saline = 0.15 M NaCl solution - Determine osmolarity
0.15 M NaCl solution = 0.30 Osm - Calculate osmotic pressure
(Osm * atm/Osm)
Osmolarity of cells
Normal extracellular concentration = 300 mOsm
Isosmotic: solution = 300 Osm
Hypo-osmotic: solution < 300 Osm
Hyper-osmotic: solution > 300 Osm
Nonpenetrating particles
-effective in exerting a sustained osmotic pressure
-Extracellular Na+ behaves as nonpenetrating solute (Na+ into cell is pumped out by the Na-K ATPase)
Tonicity
Isotonic: solution has conc. of non-penetrating particles is 300 Osm
Hypotonic: solution has conc. of non-penetrating particles < 300 Osm
-water will enter the cell and swell
Hypertonic: solution has conc. of non-penetrating particles > 300 Osm
-water will leave cell and shrink
Capillaries
-Adult has ~40 km of capillaries
-Capillaries contain ~5% of total circulating blood
-Each capillary is ~1mm long, inner diameter ~8 μm
Capillary Wall
A single layer of flattened endothelial cells and a supporting basement membrane
-Very permeable as pores between endothelial cells
Transport across capillary wall
- Diffusion:
- Transcytosis: endocytosis on luminal side then exocytosis on interstitial side
- Bulk Flow: distributes ECF volume between the plasma and the ISF
-proportional to the hydrostatic pressure difference between the plasma and the ISF
-caplillary wall permits protein free plasma to move from caplillaries to the ISF
Osmotic Pressure drives reabsorbtion of fluid from ISF into capillaries
Penetrating solutes
-enter cell by diffusion and go down concentration gradient
-will affect Osm inside cell and water will enter to reach equilibrium
-Steroid hormones, CCP, tatins
