physio lecture 2-4(Cellular transport mechanisms).

Report
Objectives
•Understand how proteins and lipids are assembled to form a
selectively permeable barrier known as the plasma membrane.
•Explain how the plasma membrane maintains an internal
environment that differs significantly from the extracellular fluid.
•Explain the importance and characteristics of carrier-mediated
transport systems.
•Understand how voltage-gated channels and ligand-gated
channels are opened.
•Explain, using specific examples, the difference between
primary and secondary active transport.
The Structure of the Plasma Membrane
Lipid bilayer – two sheets of lipids (phospholipids)-Polar
(water-soluble) heads face out and the non-polar fatty
acids hang inside.
Polar head
non-polar
tail
• Embedded with proteins and strengthened with cholesterol
molecules.
-- Integral proteins (or intrinsic proteins) are embedded in
the lipid bilayer, Include channels, pumps, carriers and
receptors.
-- Peripheral proteins (or extrinsic proteins) do not
penetrate the lipid bilayer. They are in contact with the outer
side of only one of the lipid layers either the layer facing the
cytoplasm or the layer facing the extracellular fluid
Integral-Membrane Proteins Can Serve as
Receptors
Integral-Membrane Proteins
Can Serve as Adhesion
Molecules, ex- integrins,
Cadherins.
Can Form a
Submembranous
Cytoskeleton,
Ex- spectrin,ankyrin
Hereditary Spherocytosis
•Hereditary spherocytosis (HS) is a
genetic disease that affects proteins
in the erythrocyte membrane, and the
result is a defective cytoskeleton.
•The most common defect is
deficiency of spectrin, and the result
is that regions of the membrane
break off because they are no longer
anchored to the cytoskeleton.
•cell eventually becomes small and
spherical.
•Hemolysis (cell bursting) is present
because the spherocytes are fragile
to osmotic stress.
Membrane Transport
Cell membrane is permeable to:
Non-polar molecules (02& C02).
Lipid soluble molecules (steroids).
H20 (small size, lack charge).
Cell membrane impermeable to:
Large polar molecules (glucose).
Charged inorganic ions (Na+ , K+
etc.,).
Types of Membrane Transport
• The movement of large molecules is carried out by endocytosis
and exocytosis, the transfer of substances into or out of the cell,
respectively, by vesicle formation and vesicle fusion with the plasma
membrane.
• Cells also have mechanisms for the rapid movement of ions
and solute molecules across the plasma membrane.
• These mechanisms are of two general types:
-- passive movement, which requires no direct expenditure of
metabolic energy, and substances move across the membrane
down their electrochemical gradient
-- and active movement, which uses metabolic energy to drive
solute transport against this gradient.
Passive transport Processes
Includes: A) Diffusion & B) Osmosis
Bulk Transport (Endocytosis and
Excytosis)
 Movement of many large molecules, that cannot be
transported by carriers.
 Exocytosis:
A process in which some large particles move from
inside to outside of the cell by a specialized function
of the cell membrane
 Endocytosis:
Exocytosis in reverse.
Specific molecules can be taken into the cell
because of the interaction of the molecule and
protein receptor.
11
 Exocytosis
 Vesicle containing the secretory protein fuses with
plasma membrane, to remove contents from cell.

12
Endocytosis
Material enters the cell through the plasma membrane
within vesicles.
13
Types of Endocytosis



Phagocytosis - (“cellular eating”) cell engulfs a
particle and packages it with a food vacuole.
Pinocytosis – (“cellular drinking”) cell gulps
droplets of fluid by forming tiny vesicles.
(unspecific)
Receptor-Mediated – binding of external
molecules to specific receptor proteins in the
plasma membrane. (specific)
14
Example of Receptor-Mediated Endocytosis
in human cells
15
Diffusion
• By which molecules move from areas of high
concentration to areas of low concentration and cations
move to anions.
• Its of 2 types. 1. Simple 2. Facilitated Diffusion
• Simple Diffusion: means molecules move through a
membrane without binding with carrier proteins.
• Facilitated: requires a carrier protein which aids in
passage of molecules through the membrane by binding
chemically and shuttling them through the membrane.
1. Simple Diffusion
 Molecules/ions are in constant state of random motion
due to their thermal energy.
 Simple diffusion occurs
 whenever there is a concentration difference across the
membrane
 the membrane is permeable to the diffusing substance.
17
Membrane Transport
particles
C1
●
● C1
●
●
●
●
●
●
●
C2
●
●
●
C2
●
●
●
●
time
●●
Another presentation of Fick’s law :
J = D A ΔC / ΔX
Where ,
D=Diffusion coefficient
factors that influence net flux are: (Fick`s law of diffusion)
1. Electrical gradient. If the molecule is charged then its net flux across a
membrane will be increased if the charge on the other side is opposite.
2. Temperature. Higher temperature ⇒ greater net flux
3. Surface area of membrane. Greater the surface area greater the net flux.
4. Molecule mass. Higher mass molecules move slower so the net flux would be
less
5. Membrane permeability. The greater the membrane permeability for the
molecule the greater the net flux
Diffusion Rate
Jx
↑Px
Flux
↓Px
[X]
Concentration
Simple Diffusion
• Most molecules partition poorly – i.e.
soluble in water but not lipid – therefore
cannot cross lipid bilayer
• Need – pores, channels and transporters
The major property of biological membranes
which makes them impermeable to most ions and
polar molecules is
A The presence of cholesterol
B The absence of all proteins from the
membrane
C The structure of the lipid bilayer
D The absence of charged groups on the
membrane surface
E The hydrophilic core of the membrane
Pores, Channels and Transporters
• Pore – transmembrane protein that is
open, ex-aquaporins
• Channel – transmembrane protein with a
pore that can open and close(gated)
• Transporter – transmembrane protein that
undergoes a conformational change and
facilitates the transport of a ‘packet’ of
substrate across the membrane
Ion Channels.
•The cell membrane has ion channels that increase the permeability of
the membrane for that ion species and allows the movement of those
ions down their electrochemical gradient, channels can be opened or
closed by gates
•These channels can show a high degree of specificity for a particular
ion species, e.g. the epithelial sodium channel is 30 times more
permeable to Na+ than K+.
Regulation of ion channels
Ion channels gates may be open or closed and the time and frequency
of opening may be regulated. There are three major factors that are
involved in the regulation of the frequency and duration of channel
opening.
1.Ligand-gated channels
2.Voltage-gated channels
3.Mechanosensitive channels
Ligand gated Ion Channel
• The channel is a channel/receptor
complex
• Upon ligand binding there is a
conformational change that opens the
channel
• Selectivity is conferred by charged amino
acids and size (selects for cations or
anions and then selects for size e.g. K+ ion
much larger than Na+ ion – hydrated form)
Ligand-Operated ACh
Channels
Ion channel runs
through receptor.
• Receptor has 5
polypeptide subunits
that enclose ion
channel.
• 2 subunits contain
ACh binding sites.
26
Voltage Gated Ion Channel
• change in membrane potential(Vm) moves
charged molecules within the channel
changing channel conformation either
opening or closing the channel.
• Charged amino acids inside the channel
- pore detect the electric field across the
membrane – and conformational change
can occur in response to a change in
electric field
Organ
of Corti
 When sound waves move the basilar membrane it moves the hair
cells that are connected to it,
 but the tips of the hair cells are connected to the tectorial
membrane
 the hair cell get bent .
 There are little mechanical gates on each hair cell that open when
they are bent.
 K+ goes into the cell and Depolarizes the hair cell. (concentration of
29
K+ in the endolymph is very high)
Facilitated Diffusion via carrier
• Ex- glucose, amino acid
transport.
– Down concentration
Gradient
– Chemical Specificity:
Carrier interact with specific
molecule only,
cysteinurea.
– Competitive inhibition:
 Molecules with similar
chemical structures
compete for carrier site.
– Saturation:
 Vmax (transport maximum):
 Carrier sites have
become saturated.
glucose transporter 4
(GLUT4) activated by insulin
Graph showing the
relationship between net
flux and concentration
gradient of a substance
moved across the
membrane via facilitated
diffusion. If the
concentration gradient (and
hence concentration)
increases enough the
transporters will become
saturated and the net flux
cannot be increased, this
net flux value is called the
transport maximum.
Active transport
When the cell membrane moves molecules
or ions uphill against a concentration
gradient
(or uphill against an electrical gradient),
the process is called active transport


Primary active transport
Secondary active transport:
Active transport
1 Primary active transport:
the energy used to cause the transport is
derived directly from the breakdown of ATP or
some other high-energy phosphate compound
2 Secondary active transport:
The energy is derived secondarily from energy
That has been stored in the form of ionic
concentration differences between the two
sides of the membrane
created by primarily active transport
Intracellular vs extracellular ion concentrations
Ion
Intracellular
Extracellular
Na+
K+
Mg2+
Ca2+
H+
5-15 mM
140 mM
0.5 mM
10-7 mM
10-7.2 M (pH 7.2)
145 mM
5 mM
1-2 mM
1-2 mM
10-7.4 M (pH 7.4)
Cl-
5-15 mM
110 mM
3.1 Primary Active Transport
Hydrolysis of ATP directly
required for the function of
the carriers.
Molecule or ion binds to
“recognition site” on one
side of carrier protein.
3.1 Primary Active Transport
 Binding stimulates
phosphorylation (breakdown
of ATP) of carrier protein.
 Carrier protein undergoes
conformational change.
Hinge-like motion releases
transported molecules to
opposite side of
membrane.
Mechanism of Acid Secretion
•The key player in acid secretion is a H+/K+ ATPase or
"proton pump" located in the parietal cell membrane.
•Hydrogen ion is pumped out of the cell, into the lumen, in
exchange for potassium through the action of the proton
pump.
Na+/K+ Pump
A Model of the Pumping Cycle of the Na+/K+ ATPase
39
Importance of the
+
+
Na -K
Pump
Control cell volume
Develop and Maintain Na+ and K+
concentration gradients across the membrane
Electrogenic action influences membrane
potential
Provides energy for secondary active
transport
2 Secondary Active Transport
 Energy needed for “uphill” movement obtained from
“downhill” transport of Na+.
 Hydrolysis of ATP by Na+/K+ pump required indirectly to
maintain [Na+] gradient.
41
Secondary active transport
co-transport
(symport)
out
in
Na+
glucose
Co-transporters will move one
moiety, e.g. glucose, in the same
direction as the Na+.
counter-transport
(antiport)
out
in
Na+
ca2+
Counter-transporters will move
one moiety, e.g.ca2+, in the
opposite direction to the Na+42.
Insulin Secretion and
Membrane Transport Processes
(b)
1
Beta cell secreting insulin. Closure of the KATP channel
depolarizes the cell, triggering exocytosis of insulin.
2
High glucose
levels in blood
3
4
KATP channels
Metabolism ATP
increases. increases. close.
5
Cell depolarizes and
calcium channels
open.
6
Ca2+ entry
acts as an
intracellular
signal.
Ca2+
Glucose
GLUT
transporter
Glycolysis
and citric
acid cycle
ATP
Ca2+
7
Ca2+ signal
triggers
exocytosis,
and insulin
is secreted.
Epithelial Transport
A 7-year-old boy is brought to the pediatrician because of
a chronic cough, fatty diarrhea, and failure to thrive.
Pseudomonas aeruginosa is cultured from his respiratory
tract. The physician informs the patient’s parents that
their son has a disease that is caused by a mutation in a
specific ion transporter. This patient has a mutation
in the ion transporter of which of the following
electrolytes?
(A) Bicarbonate
(B) Calcium
(C) Chloride
(D) Potassium
(E) Sodium
factors that influence net flux are: (Fick`s law of diffusion)
1. Electrical gradient. If the molecule is charged then its net flux across a
membrane will be increased if the charge on the other side is opposite.
2. Temperature. Higher temperature ⇒ greater net flux
3. Surface area of membrane. Greater the surface area greater the net flux.
4. Molecule mass. Higher mass molecules move slower so the net flux would be
less
5. Membrane permeability. The greater the membrane permeability for the
molecule the greater the net flux
The Movement of Water
Across the Plasma
Membrane
The Movement of Water Across the
Plasma Membrane
•Water can move in and out of cells.
•But the partition coefficient of water into lipids is low meaning
the permeability of the membrane lipid bilayer for water is low.
•Specific membrane proteins that function as water channels
explain the rapid movement of water across the plasma
membrane
•These water channels are small integral membrane proteins
known as aquaporins
Water Movement
NaCl 0 mOsm
NaCl 100 mOsm
[water] HIGH
[water] LOW
Aquaporin
If membrane impermeable to NaCl
CLINICAL CORRELATION•In the kidney, aquaporin-2 (AQP2) is abundant in the collecting duct and is the
target of the hormone vasopressin, also known as antidiuretic hormone. This
hormone increases water transport in the collecting duct by stimulating the
insertion of AQP2 proteins into the apical plasma membrane. Several studies
have shown that AQP2 has a critical role in inherited and acquired disorders of
water reabsorption by the kidney.
• For example, diabetes insipidus is a condition in which the kidney loses
its ability to reabsorb water properly, resulting in excessive loss of water and
excretion of a large volume of very dilute urine (polyuria). Although inherited
forms of diabetes insipidus are relatively rare, it can develop in patients
receiving chronic lithium therapy for psychiatric disorders, giving rise to the
term lithium-induced polyuria.
• Both of these conditions are associated with a decrease in the number of
AQP2 proteins in the collecting ducts of the kidney.
Osmosis
- Osmosis is the flow of water across a
semipermeable membrane from a solution with
low solute concentration to a solution with high
solute concentration.
The Movement of Water Across the Plasma Membrane Is Driven
by Differences in Osmotic Pressure
•Osmotic pressure of a solution is defined as the pressure necessary to
stop the net movement of water across a selectively permeable membrane
•When a membrane separates two solutions of different osmotic pressure,
water will move from –
the solution with low osmotic pressure (high water and low solute
concentrations) to
the solution of high osmotic pressure (low water and high solute
concentrations).
Osmolarity refers to osmotic pressure
generated by the dissolved solute molecules in 1L
of solvent.
It depends strictly on the number of particles in solution
(not the number of molecules, since some molecules (e.g.
NaCl) dissociate into ions when in solution).
Osmolarity is therefore, the number of particles per liter of
solution and is expressed in osmol/L or OsM or in the
case of dilute solutions as milliosmol/L.
Ex- A solution of 1 M CaC12 has a higher osmotic pressure
than a solution of 1 M KCl because the concentration of
particles is higher.
The higher the osmotic pressure of a solution, the greater the
water flow into it.
The osmotic
pressure of a solution can be calculated by -
Van't Hoffs law, which states that osmotic pressure depends on the
concentration of osmotically active particles. The concentration of
particles is converted to pressure according to the following equation:
where:
osmotic pressure (mm Hg or atm)
g = number of particles in solution(osm/mol)
7T =
R = gas constant (0.082 L-atm/mol-K)
σ = Reflection coefficient (varies from 0 to 1)
T = absolute temperature (K)
C = concentration (mol/L)
The Concept of Tonicity
•Tonicity describes a solution, and how that solution affects cell volume.
• The tonicity of a solution depends not just on the osmolarity of the solution
but also on whether the solutes (particles) in the solution are penetrating or
not.
•
Two solutions having the same effective osmotic
pressure are isotonic because no water flows across a
semipermeable membrane separating them.
•
If two solutions separated by a semipermeable
membrane have different effective osmotic pressures, the
solution with the higher effective osmotic pressure is
hypertonic and the solution with the lower effective
osmotic pressure is hypotonic.
• Water flows from the hypotonic to the hypertonic solution.
RBC hypotonic solution
Rules for predicting tonicity
If the cell has a higher concentration of non-penetrating solutes than the
solution, there will be net movement of water into the cell. The cell swells,
and by definition that solution is hypotonic.
SWELL
RBC isotonic solution
NO VOLUME CHANGE
RBC hypertonic solution
SHRINK
Tonicity
Tonicity describes the volume change of a cell
placed in a solution
SHIFTS OF WATER BETWEEN BODY FLUID COMPARTMENTS -
Types
Examples
Iso-osmotic volume contraction
Diarrhea,Burns
Hyperosmotic volume contraction
Severe dehydration (sweating,
fever, diabetes insipidus - ↓ ADH)
Hypo-osmotic volume contraction
Adrenal insufficiency (↓
aldosterone)
Iso-osmotic volume expansion
Infusion of isotonic saline
Hyperosmotic volume expansion
High NaCl intake
Hypo-osmotic volume expansion
Syndrome of inappropriate ADH
secretion (SIADH)
ISO-OSMOTIC VOLUME CONTRACTION
1. ECF Fluid change ?
2. ECF Osmolarity ?
3. ICF Osmolarity ?
4. Hematocrit ?
HYPEROSMOTIC VOLUME CONTRACTION
1. ECF Fluid change ?
2. ECF Osmolarity ?
3. ICF Osmolarity ?
4. Hematocrit ?
Hypo-osmotic volume contraction
1. ECF Fluid change ?
2. ECF Osmolarity ?
3. ICF Osmolarity ?
4. Hematocrit ?
ISO-OSMOTIC VOLUME EXPANSION
1. ECF Fluid change ?
2. ECF Osmolarity ?
3. ICF Osmolarity ?
4. Hematocrit ?
HYPEROSMOTIC VOLUME EXPANSION
1. ECF Fluid change ?
2. ECF Osmolarity ?
3. ICF Osmolarity ?
4. Hematocrit ?
HYPO-OSMOTIC VOLUME EXPANSION
1. ECF Fluid change ?
2. ECF Osmolarity ?
3. ICF Osmolarity ?
4. Hematocrit ?
A 23-year-old man is brought to the Emergency
Department after collapsing during basketball
practice. On admission he is lethargic and appears
confused. His coach reports that it was hot in the
gym and he was drinking a lot of water during
practice. An increase in which of the following is
the most likely cause of his symptoms?
a. Intracellular tonicity
b. Extracellular tonicity
c. Intracellular volume
d. Extracellular volume
e. Plasma volume
A 70kg man is given a treatment intravenously. The diagram shows the
intracellular(ICF) volume And extracellular fluid(ECF)volume before and
after treatment. Which of the following treatments was likely
administered to this man?
A. Hypertonic saline
B. Hypotonic saline.
C. Isotonic saline
D. Isotonic glucose.
7.A 14-year-old boy has a craniotomy performed under general endotracheal
anesthesia for removal of a craniopharyngioma. The anesthetic agent used is
halothane, and when he is fully awake in the recovery room, he is extubated and
sent to the floor. Five percent dextrose in one-third normal saline was dripping in his
intra-venous line at a rate of 125 mL/h. Four hours later, the nurses report that he
cannot be roused from a deep sleep. They also point out that his urinary output in
each of those 4 hours was 1059, 1100, 980, and 1250
mL, respectively. Laboratory studies show:
Sodium
156 mEq/L
Osmolarity 312 mOsm/L
pH
7.55
pco2
28 mm Hg
Bicarbonate
24 mEq/L
Which of the following best explains these findings?
(A) Brain edema
(B) Nephrogenic diabetes insipidus
(C) Respiratory depression induced by unmetabolized anesthetic
(D) Surgical trauma to the posterior pituitary
(E) Water retention
Oral Rehydration Therapy Is Driven by Solute
Transport
Oral administration of rehydration solutions has dramatically
reduced the mortality resulting from cholera and other diseases that
involve excessive losses of water and solutes from the
gastrointestinal tract. The main ingredients of rehydration solutions
are glucose, NaCl, and water. The glucose and Na+ ions are
reabsorbed by SGLT1 and other transporters in the epithelial cells
lining the lumen of the small intestine .
Deposition of these solutes on the basolateral side of the epithelial
cells increases the osmolarity in that region compared with the
intestinal lumen and drives the osmotic absorption of water.
Absorption of glucose, and the obligatory increases in absorption of
NaCl and water, helps to compensate for excessive diarrheal losses
of salt and water.
The Clinical Relevance of Understanding Tonicity
The importance of understanding this well is to make sure that you
understand the basis and rationale for intravenous fluid therapy.
Several IV fluids exist e.g.
0.9% saline(normal saline)
5% dextrose in normal saline
5% dextrose in water
half normal saline
5% dextrose in half normal saline. (Dextrose is glucose).
How does the clinician decide which fluid to use? Well, it depends
on what the objectives are – replacement of blood volume or
rehydration of cells in dehydrated individuals.
Discussion on IV solutions
First thing to do is to look at the relative osmolarity and
tonicity of the solution to the extracellular (and intracellular) fluid.
Then take into account what effect this will have on the volumes of
the two fluid compartments.
1. 0.9% saline. This has the same osmolarity as the intracellular
fluid. The saline is NaCl so the two particles Na and Cl are
considered to be non-penetrating. Therefore this solution is isoosmotic and isotonic. Because it is isotonic it will not change the
tonicity of the extracellular fluid and so the extracellular fluid will
remain isotonic to the intracellular fluid. Therefore NO FLUID
MOVEMENT INTO THE CELLS. This solution would be suitable for
replacing blood (extracellular fluid).
2.
5% Dextrose in normal saline. 5% dextrose is iso-osmotic to the intracellular
fluid, so is normal saline. Therefore you must take into account both of these
when working out the overall osmolarity. This solution has twice the
osmolarity of the intracellular fluid. Therefore it is HYPEROSMOTIC.
Dextrose is penetrating , so makes no contribution to the tonicity of the
solution. Saline is non-penetrating it does make a contribution. Therefore
the solution is ISOTONIC. Infusion of this solution into the veins would not
change the tonicity of the extracellular fluid so NO NET FLUID MOVEMENT
INTO THE CELLS. Notice the NET. Rapid infusion of this solution will lead
initially to some water movement out of the cells which will be reversed as
the dextrose moves into the cells. This solution would be suitable for
replacing blood.
3.
5% Dextrose in water. 5% dextrose is iso-osmotic to the intracellular fluid.
Water is, of course, hypo-osmotic (it has no particles). This solution
therefore is iso-osmotic to the intracellular fluid. Water has no tonicity and
dextrose is penetrating . This solution has no tonicity so is HYPOTONIC to the
intracellular fluid. Infusion of this solution will make the extracellular fluid
hypotonic to the intracellular fluid so some of the infused fluid will enter the
cell. THERE IS THEREFORE FLUID MOVEMENT INTO THE CELLS. This solution
would be suitable for rehydrating cells.
4. Half normal saline. This is 0.45% saline, so has half the number
of particles as normal saline so it is hypo-osmotic to the
intracellular fluid. The particles are non-penetrating but again
you have half the number of particles. This solution is therefore
HYPOTONIC to the intracellular fluid. Infusion of this solution
will make the extracellular fluid hypotonic to the intracellular
fluid so some of the infused fluid will enter the cell. THERE IS
THEREFORE FLUID MOVEMENT INTO THE CELLS. This solution
would be suitable for rehydrating cells.
5. 5% Dextrose in half normal saline. 5% dextrose is iso-osmotic
to the intracellular fluid, half normal saline is hypo-osmotic.
However if you have them together in a solution the result is
hyperosmotic to the intracellular fluid. Only the saline is nonpenetrating , therefore the solution is HYPOTONIC to the
intracellular fluid. Infusion of this fluid will decrease the tonicity
of the extracellular fluid. THERE IS THEREFORE FLUID MOVEMENT INTO
THE CELLS. This solution would be suitable for rehydrating cells.

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