Unit 2b: Cell Membrane Dynamics Flashcards
intracellular fluid (ICF
2/3 of fluid in the body
Extracellular fluids (ECF
1/3 of the fluid in the body
Interstitial fluid (ISF)
ECF
– surrounds the cells of a tissue; makes
up 75% of ECF volume
Intravascular fluid (IVF)
– includes blood plasma and lymph which make up 25% of ECF volume
All body fluids are
a solution consisting of a solvent (water) that contains solutes (ions, nutrients, gases, proteins, wastes
like urea, etc)
osmotic equilibrium
ECF and ICF are in this
meaning there is no
net movement of water because the two compartments have
the same concentration of solutes.
The concentration of
solutes is equivalent to a 0.9% NaCl solution, or 290 mOsm
How are the ECF and ICF in a chemical and electrical disequilibrium?
While the overall concentration is equal between
compartments, the composition and proportion of
different solutes is NOT the same
Which compartment has the highest concentration of protein? In which
compartment are proteins absent?
highest= blood plasma and ICF
ISF= lowest
concentration gradients
, some solutes are more concentrated in
certain compartments than others. This sets up
What does it mean for solutes to “ move down their concentration gradient”
from an area of high
concentration to an area of low concentration
What does it mean for solutes to “ move against their concentration”
from an area of
low concentration to an area of high concentration
Passive processes
move solute or solvent molecules
down their concentration gradient until equilibrium is
reached and do not require energy.
For example
moving Na+ from the ECF into the ICF; or movement of
water molecules
Describe what net movement means
molecules cross a
semipermeable membrane in both directions, but overall more are moving from the area on the left (high concentration) to the area on
the right (low concentration).
Active processe
move solute molecules against their
concentration gradients or move large molecules that
would otherwise be unable to cross the membrane
(vesicular transport).
All active transport processes require
the use of energy, which is usually obtained from ATP.
Ø For example moving Na+ from the ICF into the ECF.
Ø Na+ has a low concentration (15 mM) in the ICF, and a high
concentration (145 mM) in the ECF. In order to move Na+
against its concentration gradient, energy is required.
Describe the relative concentrations of key substances (ions, etc.) in the plasma, interstitial fluid, and cytoplasm inside of a cell
Blood plasma= high Na+ low K+ high Cl- high protein high HCO3-
ICF= low Na+ high K+ low Cl- high protein low HCO3-
ISF= high Na+ low K+ high Cl- low protein high HCO3-
List what is able to freely pass thru the membrane
hydrophobic lipid soluble molecules small polar molecules
ex; o2 co2 water urea
List what isn’t able to freely pass thru the membrane
large polar molecules; glc proteins amino acids
charged ions; Na+ K+ Cl-
How do large polar molecules and charged ion enter the cell?
require transport proteins
Name the types of transport proteins
a. channel proteins
b. carrier proteins
channel proteins
Ø is a water filled pore that can be open to both sides.
Ø each channel protein is specific for a particular solute (e.g. Na+
channel, K+ channel, .etc).
aquaporins
Channels specific for water
List the different types of channel proteins
- open channels
- gated channels
Open channels (pores)
always open (like a doorway with no
door)
Also called “leak channels”, as they allow the solute they
are specific for to continuously leak into/out of the cell
Gated channels
can open and close in response to a stimulus
(signal). Each gated channel type has a specific stimulus:
Describe the different types of gated channels
i. Chemically gated channels – open in response to a
chemical signal (e.g. a hormone or neurotransmitter)
ii. Voltage-gated channels – open in response to to a
change in the electrical state of the cell.
iii. Mechanically gated channels – respond to physical forces
(temperature or pressure
Carrier proteins
Never form an open channel between the ECF and ICF. Are open
to one side at time.
Ø Solute enters carrier protein and binds to it causing a
conformational change (change in protein shape) that cause the
protein to close on one side and open on the other. The solute is
then released
List and describe the different types of carrier proteins
a. Uniport – transport only one type of solute e.g. glucose transporters (GLUT) in red blood cells.
b. Symport – transports 2 or more types of solute in the same direction. E.g. Na+/glucose transporters (SGLT) in cells of the
small intestine that absorb glucose from the digestive tract.
c. Antiport – transports two or more types of solute in the
opposite directions. E.g. Na+/K+ - ATPase pump – carries 3 Na+ out of the cell and 2 K+ into the cell.
Osmosis
passive process
Movement of water molecules (solvent) down
their own concentration gradient due to kinetic
energy of water molecules.
Ø From an area of high [H2O] (lots of water molecules) to an an area of low [H2O] (few water molecules)
Adding solutes to pure water lowers the
concentration of water molecules. So the more
solute in a solution the less concentrated the water is
in that solution.
How does H20 get across the cell membrane
Movement of water molecules occurs directly
across the cell membrane OR through cell
membrane protein channels call aquaporins
osmotic pressure
Movement of water can cause pressure
The pressure/force required to stop the flow of water
from one solution/compartment to another
Osmolarity
(mOsm/ L)
The concentration of a solution based on the total number of solute particles
per liter (includes both penetrating and non-penetrating solutes)
What is a penetrating solute vs a non penetrating solute?
Penetrating solutes – are capable of freely crossing cell membranes
Ø Small, polar and non-polar
molecules
Ø E.g. urea, glycerol, ethanol
Non-penetrating solutes – cannot freely cross cell membranes
Ø Ions and larger polar (hydrophilic)
molecules that cannot cross the hydrophobic region of the
phospholipid bilayer
Ø E.g. Na+, glucose, amino acids
Isosmotic
have the same osmolarity as another solution
hyperosmotic
have a higher osmolarity than another solution
Hypoosmotic
have a lower osmolarity than another solution
crenation
Movement of water out of the cell causes it to shrink/shrivel
tonicity
how a solution will affect the volume of a cell
compares a solution to a cell’s intracellular
solution.
Specifically tells you whether or not a cell will swell or shrink
Depends ONLY on the concentration of non-
penetrating solutes
- Water will always go to high non penetrating solute
Hypertonic
causes cell to lose water and shrink
( higher concentration of water inside the cell than outside and lower concentration of non penetrating solutes)
Isotonic
does not change cell shape (no net osmosis)
Hypotonic
causes cell to swell and may burst.
(higher concentration of water outside the cell than inside and lower concentration of non penetrating solutes outside)
Hyposmotic solutions are always…..
hypotonic
What is diffusion?
Movement of solutes down a concentration gradient
(from an area of high concentration to an area of low concentration)
Ø Does not require energy.
Ø Occurs as a result of the kinetic energy (random motion) of
ions/molecules.
Ø Process continues until equilibrium is reached.
Ø Fast over short distances, slow over long distances.
Ø The time it takes to get from A to B is a “distance squared”
relationship. If the distance travelled doubles from 1 to 2, then the
time it takes for diffusion to occur quadruples from 1 to 4 (=2^2 )
Ø Rate of diffusion is faster at high temperatures (increases kinetic
energy/random motion of molecules)
Ø Rate of diffusion is slower across a membrane
List the different types of diffusion
simple diffusion
facilitated diffusion
simple diffusion
molecules pass directly through the phospholipid bilayer
- lipophilic substances can pass right thru
What does Fick’s Law of Diffusion say
the diffusion rate increases with increasing SA concentration gradient and mem. permeability
i
a. Surface area of membrane: ↑ surface area = ↑ rate of diffusion
(basically there is more space over which diffusion can occur)
b. Concentration gradient: ↑ gradient = ↑ rate of diffusion
Ø the larger the gradient the more molecules will move
into/out of the cell in an attempt to establish equilibrium
What does the permeability of the cell membrane to the molecule depend on?
i. Size (and shape) of molecules. ↑ molecular size = ↓ permeability
ii. Lipid solubility of the molecule. ↑ lipid solubility= ↑
permeability
iii. Composition of the membrane
Ø Relative proportions and types of phospholipids,
sphingolipids affect rate as does the amount of
cholesterol.
Ø For example ↑ cholesterol = ↓ permeability
(cholesterol gets in between fatty acid tails and
blocks movement of molecules through the
membrane)
Facilitated diffusion
protein mediated transport
Ø Movement of a molecule across the cell membrane via a channel protein or a
carrier protein.
The protein facilitates diffusion of the solute down its concentration gradient.
ØDoes not require ATP
ØThis process alone cannot accumulate a solute against a concentration gradient
channel-mediated facilitated diffusion
Ø uses a channel protein to move the solute down its concentration gradient.
Ø E.g. Na+ leak channels
Carrier-mediated facilitated diffusion
Ø uses a carrier protein to move the solute down its concentration gradient
.
Ø E.g. facilitated diffusion of glucose into skeletal muscle or liver cells via glucose
transporters (GLUT)
Ø Low concentration of glucose inside of cell are maintained because any glucose entering the cell is immediately converted to glucose-6-phosphate. Prevents equilibrium from being reached.
Primary Active Transport
Directly uses ATP (i.e. the transport protein that breaks
down ATP is the same protein that will transport the
solute)
b. Establishes concentration gradients.
c. Carrier proteins involved are sometimes called pump
Give an example of active transport
Na+/K+ ATPase is the most widely known example (found in all cells).
Ø Pumps 3 Na+ out of the cell and 2 K+ into the cell (antiport).
- CREATES THE NA+ AND K+ GRADIENT THAT EXISTS BETWEEN ECF AND ICF
Ø Carrier protein hydrolyzes ATP and undergoes several
conformational changes
Secondary Active Transport
Indirectly uses ATP
Ø the transport protein that breaks down ATP creates a concentration
gradient of one solute (solute A).
Ø The kinetic energy stored in this concentration gradient is then used to
move another solute (solute B) against its concentration gradient using a
separate carrier protein.
Give an ex of 2 active transport
Example: Na+-glucose secondary active transporter (SGLT-protein)
used to absorb glucose from lumen of intestine into intestinal cells.
i. Na+ binds to SGLT carrier protein (moving down its concentration gradient)
ii. Na+ binding creates a high-affinity site for glucose
iii. Glucose binding changes the conformation of the protein so that protein is
open to the inside of the cell.
iv. Na+ is released moving down its concentration gradient
v. Release causes decreases affinity of glucose binding site
vi. Glucose is released into the cytosl
Specificity
The transporter is specific for a particular
substrate/solute or a particular group of related substrates/solutes
ØE.g. GLUT proteins are specific for 6-carbon sugars, with a preference for
glucose (but can also move galactose and fructose
Competition
several substrates/solutes compete for the binding
sites on the carrier. This reduces the transport rate.
E.g. glucose and
galactose compete for the same binding sites, so when both are
present, the transport of glucose decreases.
Some competitors are
not transported across the membrane, but simply block the binding
site of the preferred solute (competitive inhibitor)
Saturation
if there are not enough carriers for the amount of
substrate/solute, the binding sites become saturated
At this point the
rate of transport cannot increase with any further increase in
substrate/solute concentration and so a transport maximum is
reached
ØCells can avoid reaching the transport maximum by building new protein
carriers and inserting them in the membrane.
ØE.g. Insulin acts on muscle cells to increase the number of GLUT4 protein
carriers in their membranes
List the different types of active transport
primary
secondary
vesicular
Vesicular transport
Used for large molecules that cannot be transported by membrane
channels or carriers
Phagocytosis
movement of a very large particle (e.g. bacterium) into
the cell in a large vesicle. ”Cell-eating”
Ø Cell uses cytoskeleton (microfilaments made of actin) and myosin motor
proteins to extend the cell membrane and wrap it around the particle.
Creates a membrane bound space within the cell called a phagosome
Ø E.g. a white blood cell surrounding a bacterium
Endocytosis
movement of large particles into the cell in small vesicles
Ø Cell membrane surface indents and forms vesicles
Pinocytosis
a on-selective form of endocytosis (cell takes in all particles – water and solutes – in the area where the vesicle forms). “Cell-drinking”.
Exocytosis
movement of large particles OUT of the cell in small
vesicles
Ø Intracellular vesicles move to membrane and merge with it (vescicle
membrane becomes part of cell membrane,; vesicle contents exit the cell).
Ø Transport of large lipophobic molecules (e.g. some proteins)
Ø Requires ATP
Ø Usually triggered by an increase in cytosolic Ca++ concentration
Absorption
transport from the outside of the body to the
inside.
Secretion
transport from the inside of the body to the outside
Apical surface
f aces the outside of the body (e.g. the
lumen of the intestine or the lumen of a kidney tubule
(nephron)).
Common location for SLGT proteins in intestinal
cells.
Basolateral surface/membrane
aces the ISF,. Common
location for Na+/K+ ATPase pumps
Paracellular transport
Through junctions between adjacent cells
Transcellular transport
Through cells themselves – substance being absorbed or secreted
must pass through two membranes
Ø Involves a combination of active and passive transport processes
Describe transcellular transport of glucose and Na+, including direction of movement, relevant membrane proteins, additional ions, and concentration gradients.
a. A Na+/Glucose symporter (SGLUT) in the apical membrane that
moves glucose into the cell using the concentration gradient for Na+
(secondary active transport).
b. GLUT transporter that transports glucose out of the cell into the ECF
(first into the ISF, then into the plasma) using carrier mediated
facilitated diffusion (passive).
c. Na+/K+ ATPase pump in the basolateral membrane that pumps Na+
out of the cell in exchange for K+, thereby keeping [Na+] low inside
the cell (allows for (a) to keep happening). – primary active transport
