Transport Mechanisms Flashcards
Permeability of Cell Membrane
Highly permeable to:
H2O,
lipid-soluble substances, dissolved gasses (O2, CO2),
small uncharged molecules
Less permeable to:
Large molecules
charged particles
Impermeable to:
Very large molecules
Plasma membrane composition
Phospholipid bilayer (40-50% of plasma membrane)
Membrane is 6-10 nM thick
Amphipathic: polar (hydrophilic heads) and nonpolar ends (Hydrophobic tails)
Role of Cholesterol
Inside bilayer, slightly amphipathic
Acts as buffer and ensures fluidity
temp is low = keeps fluidity
temp is high = prevents to much fluidity
involved in forming vesicles and lipid rafts
Types of proteins in plasma membrane
most diverse macromolecule 25-75% of membrane weight
Integral: mostly cross membrane (trans-membrane), associated with phospholipids
Peripheral: mostly on cytoplasmic side and next to polar heads of phospholipids
What is Glycocalyx
Coating surrounding cell membrane formed of glycans, glycoproteins and glycolipids.
helps in cell-cell recognition
communication
adhesion
protection
permeability
Fluid mosaic model
Things are not completely stuck in place in the membrane.
Fluidity can be changed with cholesterol
some proteins are created and others are removed
proteins move or stay attached to cytoskeleton
Functions of plasma membrane proteins (6)
- Selective transport:
channels and transporters - Enzyme Catalyst
- Cell surface receptor
- Cell surface identity marker:
CD4 T lymphocytes, CD proteins are important for cell recognition during immune response,
markers can help detect cancerous cells - Cell adhesion: CAMs, integrins, cadherins
- Attachment to cytoskeleton: actin, microtubules, septins
Types of passive transport
(energy independent)
- diffusion channels
- facilitated diffusion (carriers, transporters)
- osmosis
Types of active transport
Energy dependent
- Active transport
primary
secondary - Endocytosis Pino/Phagocytosis, receptor-mediated endocytosis
- Exocytosis
Diffusion
Thermal motion makes molecules move from a location to another until equilibrium is reached
(net flux is 0)
net flux:
high => low concentration
down concentration gradient
Why is diffusion effective on very short distances
diffusion time increases in proportion to the square of the distance travelled by solute
1um = 1msec
10 um = 100 msec
100 um = 10 000 msec
Very slow as distance is increased
(explain why we have a lot of capillaries across body, to have diffusion everywhere)
Ion channels distinctions
Show ion selectivity
movement of ion is affected by electrical gradient in addition to concentration gradient
Combination = electrochemical gradient
ions go where they are smaller in concentration, but at the same time some are sent back because the charge is different where they come from
Types of gating (3)
some ion channels can be closed by conformational changes
Ligand-gated
voltage-gated
mechanically-gated
Voltage gated ion channels depend on
channel conductance
channel open time
frequency of opening
What is mediated transport
Movement of ions and molecules by integral membrane proteins (transporters or carriers)
Types of mediated transport
Facilitated diffusion (passive)
Active transport
1. primary
2. secondary
Specificity of Mediated transport
system usually transports one particular type of molecule
Saturation of mediated transport
rate of transport can increase but reaches a maximum velocity once each binding sites are occupied
Transport maximum (Tm) is the limit of the rate at which substance cross the membrane
Increasing concentration of solute does not increase rate of transport
Competition in mediated transport
similarly structured molecules can compete for the same binding site of a carrier.
can slow rate of transport of desired molecule
Factors that determine mediated transport efficiency
solute concentration
affinity of transporter to solute
numbers of transporters
rate of transporter conformational change
What is Facilitated diffusion
presence of transporter or carriers enable solute to move across membrane from high to low concentration without use of energy
down concentration gradient
solutes binds
binding changes configuration
solute is delivered on other side
transporter resumes original config.
Characteristics of active transport
Needs energy supply, ATP
susceptible to metabolic inhibitors
Transport solute against concentration gradient
low to high
Primary active transport
Involves hydrolysis of ATP by carrier (becomes ADP + P)
P binds to carrier = phosphorylation, which changes conformation and binding affinity to solute
against concentration gradient
Na+/K+-ATPase, sodium potassium pump
outside high [Na+], low [K+]
inside, ATP binds to carrier, 3 Na+ ions bind to carrier
Phosphorylation occurs P ions attached to carrier and ADP leaves = conformation change
3 Na+ ions leave cell and 2 K+ ions attach to carrier
P ion detaches from carrier to form ATP molecule => conformation change
2 K+ ions are released in cell where [K+] is high, [Na+] is low
Active transports to remember
Ca2+-ATPase
maintain low intracellular Ca2+ levels
H+-ATPase
maintain low lysosomal pH
H+/K+-ATPase
acidification of the stomach
What is secondary active transport
Primary active transport creates an electrochemical gradient.
Secondary transport uses this gradient to send Na+ down its concentration gradient while being coupled with another solute molecule (ion, glucose, amino acid) on the carrier
conformation change delivers Na+ and other into cell
Transporter reverts to original configuration and Na+ leaves cell with Na+/K+ATPase
What are some kinds of transport (directions)
Symport, cotransport
two solutes transported in same direction
Antiport, Counter transport
Two solutes are transported in opposite directions
What is Exocytosis
Intracellular vesicle fuses with cell membrane to release content into ECF
What is endocytosis
Cell membrane invaginates and pinches off to form a vesicle (inside)
2 Types of Exocytosis
Constituitive exo.
non-regulated, replace plasma membrane, deliver membrane proteins to cell membrane and get rid of substances
Regulated exo.
Triggered by extracellular signals (increase of cystolic Ca2+)
Does secretion of
hormones, digestive enzymes, neurotransmitters
Pinocytosis
(cell drinks)
vesicle engulfs ECF and everything inside, nonspecific
vesicles fuse with other vesicle like endosomes or lysosomes
Phagocytosis
(Cell eats)
Pseudopodia (extensions of cell membrane) engulfs solid matter, small-sized particles, microorganisms.
Pseudopodia fuse into phagosomes which fuse with lysosomes where contents are degraded
Receptor-mediated Endocytosis
Molecules in ECF (ligands) bind with protein receptors on plasma membrane
when binding occurs, conformational change happens in receptors. Clathrin proteins form a cagelike structure with the receptors
Clathrin coated pit (Cage) fuses with lysosome to process substance inside
receptors and clathrin are recycled in cell membrane
What is potocytosis
molecules are transported by clathrin-independent vesicles (caveolae)
Caveolae deliver contents in cytoplasm or endoplasmic reticulum
Uptake of vitamins
How water diffuses
Aquaporins are water permeable channels across the cell membrane
what is osmosis
net diffusion of H2O across semipermeable membrane
(solvent can go trough and some but not all solutes)
water goes from High to low concentration of water
What is osmotic pressure
Pressure to prevent movement of water across semi-permeable membrane
Equal to difference in hydrostatic pressures of two solutions
Pressure (P) is proportional to number (n) of particles in solution, not their size
P = nRT/V
What is Osmolarity (Osm)
Total solute concentration of a solution. (number of solute particles per unit of solvent)
osmol = mol of solute
1 mol glucose = 1 osmol solute
Osm = osmol / unit of solvent
2 mol glucose/L = 2 osmol/L
= 2 Osm
Isomotic solutions
Solutions with the same osmolarity as normal extracellular or intracellular solutions (which are at 300 mOsm)
Hypoosmotic solutions
Osmolarity is lower than 300 mOsm
Hyperosmotic
Osmolarity is greater than 300 mOSm
Importance of nonpenetrating particles in osmotic pressure
Particles must not be able to cross the membrane freely all the time to allow a difference in solute concentration between each side of the cell membrane
Na+ is non penetrating because what goes in is pumped out by the Na-K ATPase
Isotonic solution
Solution with concentration of 300 mOsm = no net shift of water
Hypotonic solution
Solution concentration is less than 300 mOsm, there is more solute inside the cell
water enters cell and it swells
Hypertonic Solution
Solution concentration of nonpenetrating solute is greater than 300 mOsm, solute concentration is higher outside cell
Water leaves cell and it shrinks
Capillaries details
40 km in an adult
5% of total circulating blood
each: 1mm long
diameter 8 micrometer
Capillary wall
Single layer of flattened endothelial cells
supporting basement membrane
Diffusion across capillary wall
Diffusion across the membrane is most important
Also diffusion through water filled channels
Transcytosis in capillary wall
Endocytosis on luminal side followed migration of vesicle across cell and exocytosis on interstitial side
Bulk flow in capillary wall
Distribution of ECF volume between plasma and ISF
Bulk flow: Proportional to hydrostatic pressure difference between plasma and ISF
protein free plasma is filtered from capillaries to ISF
