Transport mechanisms Flashcards
What is the cell membrane highly permeable to? (4)
-H2O
-lipid-soluble substances
-dissolved gases (O2, CO2)
-small uncharged molecules.
What is the cell membrane less permeable to? (2)
-larger molecules
-charged particles
What is the cell membrane impermeable to? (1)
Very large molecules
What is the structure of the cell membrane?
The bimolecular phospholipid layer
Describe the polarity of the cell membrane.
Amphipathic
Polar and Nonpolar ends
Why is cholesterol inserted in the phospholipid bilayer? (3)
-reduces packing of fatty acid tails
-tends to increase membrane fluidity keeping fatty acid tails apart
-May also be involved in the formation of vesicles that pinch off the plasma membrane and in lipid rafts.
Describe the polarity of the cholesterol
Slightly amphipatic
How does cholesterol function at high and low temperatures?
At high temperatures, cholesterol acts to stabilize the cell membrane and adds firmness;
while at low temperatures, it inserts into phospholipids and prevents them from interfering
with each other to avoid aggregation
What is the difference between integral and peripheral proteins in the cell membrane?
Integral proteins are closely associated with phospholipids and mostly cross the membrane,
while peripheral proteins are more loosely associated and mostly found on the cytoplasmic
side.
What is the glycocalyx in the cell membrane?
The glycocalyx is a layer of carbohydrate formed by a chain of monosaccharides that extend from the extracellular surface of the cell membrane.
What is the function of the glycocalyx?
-provides protection from infection
-enables cells to identify each other and interact.
What are the functions of plasma membrane proteins ? (6)
- Transport and diffusion of specific molecules into and out of the cell.
- Act as enzymes that catalyze membrane associated reactions.
- Serve as receptors for receiving and transducing chemical signals from the
cell environment. - Cell surface identity markers
- Cell – cell adhesion
- Attachment to cytoskeleton
What are the two types of transmembrane transport
pathways?
-Via phospholipid bilayer
-via interaction with protein cluster (channel or carrier).
What are the two types of transport mechanisms
across the cell membrane?
Passive (energy independent) and active (energy
dependent).
What are the passive transport mechanisms? (3)
- Diffusion
- Carrier-mediated Facilitated Diffusion
- Osmosis
What are the active transport mechanisms?
- Carrier-mediated Active Transport
a) Primary
b) Secondary - Exocytosis
- Endocytosis
What is simple diffusion?
Simple diffusion is the movement molecules from one
location to another as a result of random thermal motion.
What is flux?
The amount of particles crossing a surface per unit
time.
What is the direction of net flux in diffusion?
From high concentration to lower concentration.
What is the net flux in diffusion at equilibrium?
zero
What is the condition for diffusion to occur in the
presence of a mechanical partition?
The partition must be permeable to diffusing
particles.
What is the equation for Fick’s Law of Diffusion?
J = PA (C0 - Ci).
What does J represent in Fick’s Law of Diffusion?
Net flux, or the rate of diffusion.
What is the relationship between diffusion time and
the distance traveled by solute molecules?
Diffusion time increases in proportion to the square
of the distance traveled by the solute molecules.
What are the factors that affect diffusion across the
cell membrane? (5)
-Mass of the molecule
-concentration gradient across the cell membrane
-lipid solubility
-electrical charge
-availability of selective ion channels or membrane carriers.
How do non-polar molecules penetrate the cell
membrane in diffusion?
They dissolve in the lipid component.
How do ions penetrate the cell membrane in
diffusion?
They diffuse through channels.
Diffusion depends upon the existence of a ___________ gradient.
concentration
What is the basis of selectivity in ion channels? (2)
based on
-their diameter
-on the distribution of charges lining the channel
What is an electrochemical gradient?
An electrochemical gradient is the simultaneous
existence of an electrical and a concentration
gradient for a particular ion.
What is gating in ion channels?
Gating in ion channels is the process of channels
existing in open or closed states as they undergo
conformational changes.
What are the three ways in which channels may be
gated?
-ligand-gated,
-voltage-gated
-mechanically-gated.
What are the four types of voltage-gated ion
channels?
-Na+ channels
-K+ channels
-Ca++ channels
-Cl- channels
What are the factors that determine the total
number of ions that flow through ion channels? (3)
-the channel conductance
-how often the channel opens
-how long the channel stays open.
What is the definition of mediated transport?
The movement of ions and other molecules by
integral membrane proteins called transporters
What is the main difference between ion channels
and mediated transport systems for ion movement?
Ion movement across membranes via transporters
is much slower than through ion channels.
What are the types of mediated transport systems? (3)
- Facilitated Diffusion
- Active Transport
Primary Active Transport
Secondary Active Transport
What is specificity in mediated transport systems?
The system usually transports one particular type of
molecule only.
What is competition in mediated transport systems?
It occurs when structurally similar substances
compete for the same binding site on a membrane
carrier.
What is saturation in mediated transport systems?
The rate of transport reaches a maximum when all binding sites on all transporters are occupied.
What are the factors that determine the flux magnitude of mediated transport systems? (4)
- Solute concentration
- Affinity of transporter for the solute
- Numbers of transporters
- Rate of transporter conformational change
What is facilitated diffusion?
Facilitated diffusion involves the presence of a “transporter” or “carrier” molecule, which enables a solute to penetrate more readily than it would be
expected to by simple diffusion.
What is the mechanism of facilitated diffusion?
a) Solute binds transporter
b) Transporter changes
configuration
c) Solute is delivered to
other side of membrane
d) Transporter resumes
original configuration
What is the net flux of facilitated diffusion?
Facilitated diffusion is a passive (no-energy) process that results in a net flux from high to low
concentration.
What is the role of hormones in facilitated diffusion?
Hormones may increase the number and/or affinity
of transporters in some membranes.
What is active transport?
Active transport is a transporter-mediated process
that requires a supply of chemical energy (usually
derived from enzymatic hydrolysis of ATP).
What is the unique feature of active transport?
Active transport can transport solute against its
concentration gradient
Active transport is susceptible to ___________.
metabolic inhibitors
What are the two types of carrier mediated active transport?
-primary active transport
-secondary active transport.
What is primary active transport?
Active transport involves the hydrolysis of ATP by a
transporter (carrier) and phosphorylation of the
transporter changes the conformation of the
transporter and its solute binding affinity
How are the changes in the binding site affinity for a transported solute produced?
Changes in the binding site affinity for a transported solute
are produced by phosphorylation and dephosphorylation of the Na+/K+-ATPase.
What is the function of the H+/K+-ATPase?
responsible for acidification of the stomach.
What is the function of the Ca2+-ATPase?
maintaining low intracellular Ca2+ levels.
What is the function of the H+-ATPase?
maintaining low lysosomal pH.
What is secondary active transport?
In secondary active transport, the movement of Na+
down its concentration gradient is coupled to the
transport of another solute molecule (ion, glucose,
amino acid) uphill against its concentration gradient.
How does secondary active transport move both
Na+ and the transported solute?
Secondary active transport uses the energy stored
in the electrochemical gradient to move both the
Na+ and the transported solute.
What is the mechanism of the secondary active transport?
- Na+ binds to a transporter outside the cell (where the
Na+ concentration is high) allowing glucose or amino acid
to bind to the same carrier. - Through a change in configuration, the transporter
“delivers” both Na+ and glucose or amino acid into the cell. - The transporter then reverts to its original
configuration, and the Na+ is extruded from the cell by the
Na+/K+ -ATPase.
What is cotransport?
It is when solute X is transported in the same
direction as Na+. (symport)
What is countertransport?
It is when solute X is transported in the opposite
direction to Na+.
What are the secondary active transport
mechanisms? (5)
-Na+/H+ exchanger (antiport)
-Na+/Ca2+ exchanger (antiport)
-Na+/HCO3− symporter
-Na+-amino acid symporter
-Na+-glucose symporter
(Sodium-glucose linked transporter SGLT)
What is endocytosis?
Endocytosis is an active transport mechanism
where the cell membrane invaginates and pinches
off to form a vesicle
What is exocytosis?
Exocytosis is an active transport mechanism where
an intracellular vesicle fuses with the cellmembrane, and its contents are released into the
ECF.
What is constitutive exocytosis?
Constitutive exocytosis is a non-regulated type of exocytosis that functions to replace plasma
membrane, deliver membrane proteins to the cell membrane, and to get rid of substances from the cell.
What is regulated exocytosis?
Regulated exocytosis is a type of exocytosis that is triggered by extracellular signals and the increase of cytosolic Ca2+. Responsible for the secretion of hormones, digestive
enzymes, and neurotransmitters.
What is phagocytosis?
Phagocytosis is the process by which cells bind and
internalize particulate matter (>0.75 µm) such as
small-sized dust particles, cell debris, and
microorganisms. It is specific and triggered.
How does phagocytosis occur?
Extensions of the cell membrane called pseudopodia fold around
the particle and fully engulf it. The pseudopodia fuse to form large vesicles, called phagosomes, that pinch off the membrane.
Phagosomes migrate to and fuse with lysosomes where the
contents of the phagosome are degraded. Defend against
infection and scavenge senescent and dead cells.
What is the difference between phagocytosis and
pinocytosis?
Phagocytosis involves the engulfment of large
particles (specific), while pinocytosis involves the engulfment
of extracellular fluid and solutes (nonspecific).
What is the function of phagocytosis?
Phagocytosis defends against infection and scavenges senescent and dead cells.
What is pinocytosis?
anendocytotic vesicle that engulfs the extracellular fluid includingwhatever solutes are present. It is nonspecific and constituitive. The vesicles travel into the cytoplasm and fuse with other vesicles such as endosomes or lysosomes.
What is the mechanism of clathrin-dependent receptor-mediated endocytosis
-When the ligand binds the receptor undergoes conformational change and clathrin is recruited to the plasma membrane.
-Adaptor proteins link the ligand-receptor to the clathrin.
-The complex forms a cagelike structure that leads to the aggregation of ligand bound receptors.
-A clathrin coated pit is formed which then invaginates and forms a “clathrin-coated
vesicle.
-Once the vesicle pinches off it sheds the clathrin coat and vesicles can fuse with the membrane of cellular organelles such as endosomes and lysosomes. Or, they can sometimes fuse with the membrane on another side of the cell (transcytosis) .
-Receptors and clathrin protein are recycled back to the cell membrane.
How is cholesterol transported in the blood?
As lipid-protein particles known as low density lipoproteins (LDL). The lipoprotein is recognized by PM LDL receptors and endocytosis follows.
What is the process by which molecules are
sequestered and transported by tiny vesicles called
caveolae?
Potocytosis
What are caveolae?
They are tiny vesicles called caveolae.
These vesicles are clathrin-independent. Caveolae can deliver their contents directly into the cell cytoplasm as well as to the endoplasmic reticulum (ER) or other organelles and to the plasma membrane on the opposite side of the cell
(transcytosis).
What is the function of aquaporins?
They form water permeable channels that facilitate
the diffusion of water across most cell membranes.
What is osmosis?
Osmosis is the net diffusion of H2O across a
semipermeable membrane (i.e., permeable to
solvent, but not to all solute).
What is the mechanism by which caveolae can
deliver their contents directly into the cell cytoplasm
as well as to the endoplasmic reticulum (ER) or
other organelles and to the plasma membrane on
the opposite side of the cell?
Transcytosis.
What is the osmotic pressure?
The pressure required to prevent the movement of
water across a semi-permeable membrane is
referred to as the osmotic pressure.
How is the osmotic pressure related to
temperature?
In an ideal solution, the osmotic pressure is related
to temperature in the same way as the pressure of a
gas.
What is the Van’t Hoff Equation for osmotic
pressure?
P =nRT/V
, where n is the number of particles, R is the
gas constant, T is the absolute temperature, and V
is the volume
What is osmolarity?
Osmolarity (Osm) is the total solute concentration of
a solution. (osmol/L).
What is the relationship between osmotic pressure
and osmolarity?
Osmotic pressure is proportional to osmolarity
(Osm).
What is an osmol?
An osmol is a unit of measurement for the number
of particles in solution/unit volume.
What is the osmolality of 1 mol glucose/L?
The osmolality of 1 mol glucose/L is 1 osmol/L = 1 Osm.
What is the osmolality of 1 mol NaCl/L?
The osmolality of 1 mol NaCl/L is 2 osmol/L = 2 Osm.
What is the osmolarity of a 0.15 M NaCl solution?
The osmolarity of a 0.15 M NaCl solution is 300
mOsm.
What is the definition of an isosmotic solution?
Solutions which have the same osmolarity as
normal extracellular or intracellular solution (300
mOsm) are called isosmotic.
What is the definition of a hypoosmotic solution?
Solutions which have an osmolarity lower than 300
mOsm are called hypoosmotic.
What is the definition of a hyperosmotic solution?
Solutions which have an osmolarity greater than
300 mOsm are called hyperosmotic.
What is the definition of nonpenetrating particles?
Nonpenetrating particles are particles that cannot
cross the membrane and are effective in exerting a
sustained osmotic pressure.
Does extracellular Na+ behave as a nonpenetrating
particle?
Yes, extracellular Na+ behaves as a nonpenetrating
particle.
What happens to a cell placed in a hypertonic
solution?
Water will leave the cell, and the cell will shrink.
What happens to a cell placed in a hypotonic
solution?
Water will enter the cell, and the cell will swell.
What is the normal osmolarity of the extracellular
fluid (ECF)?
300 mOsm.
What is the main exchange site for substances
between the plasma and interstitial fluid (ISF)?
Capillaries
What is bulk flow?
Bulk flow distributes the extracellular fluid volume
between the plasma and ISF.
What is the magnitude of bulk flow proportional to?
The magnitude of bulk flow is proportional to the
hydrostatic pressure difference between the plasma
and the ISF.
What is the function of the capillary wall in bulk
flow?
The capillary wall acts as a filter that permits
protein-free plasma to move from capillaries to the
ISF.
What are the 4 types of transports across the capillary wall?
- Diffusion through across cell membrane
- Diffusion through water filled channels
- Transcytosis
- Bulk flow
