How Does Phospholipid Solute Across Membranes Build A Cell Membrane
MOVEMENT ACROSS MEMBRANES
Intracellular fluid Extracellular fluid Pond water Blood plasma
K+ A-
ClNa+
Cl+
K+ Na+ Cl-
Na+
Na+
Gill epithelial cell Intestinal epithelial cells
Cl-
Cell membrane Anionic proteins (a) Ion concentration inside a single animal cell
(b) Ion concentration across gill epithelium of a freshwater fish Blood capillary
Intestinal lumen
Glucose from meal
Cross section of small intestine
(c) Glucose transport across intestinal epithelium into the blood system
MEMBRANE STRUCTURE
Extracellular fluid Glycoproteins and -lipids
Phospholipid bilayer Cytoplasm
Integral and peripheral proteins
Cholesterol Cytoskeleton
How do phospholipids build a membrane?
1
Phospholipid …show more content…
structure
Choline+ PhosphateGlycerol
Bends symbolize double bonds – less densely packed lipids. Many positively charged groups can occupy this position.
Polar head
Non-polar fatty acid chains
Nonpolar tails Hydrophilic Hydrophobic
Phospholipid “behavior” in water
Water Micelle Hydrophillic heads
Hydrophobic tails
Phospholipid bilayer
2
Fluidity of membranes
(a)
Lateral Movement (frequent) Flip-flops (rare) Frequent lateral movement of lipids; infrequent flip-flops across membrane leaflets.
Fluid (b)
Unsaturated fatty acid tails with kinks
Viscous
Saturated fatty acid tails
Degree of unsaturated/saturated fatty acid tails (hydrocarbon) determines membrane fluidity.
Phospholipid composition and habitat temperature
% of hydrocarbon tails unsaturated
Phosphatidyl-ethanolamines
Phosphatidyl-cholines
Adaptation temperature (°C)
Fatty acid chains vary in chemical unsaturation = double bonds that produce a bend in the chain. Unsaturated phospholipids are less densely packed and increase membrane fluidity. To maintain membrane fluidity at low temperatures, cold adapted species incorporate more unsaturated lipids into their membrane.
after Logue et al. (2000), J Exp Biol 203, 2105-2115
3
Membrane proteins
PASSIVE AND ACTIVE TRANSPORT
What are equilibrium conditions? A system moves towards its “equilibrium” when there is no further input of energy or matter – has minimal potential for more work.
Passive transport mechanisms work only in the direction of equilibrium.
Active transport mechanisms are capable of working in a direction against the equilibrium.
4
PASSIVE: Diffusion as passive solute transport
High Low glucose concentration
Membrane Diffusion of glucose toward equilibrium
Due to molecular movement glucose passes through the membrane passively. Because there is more glucose on the left side, on average and by chance more molecules pass from left to right. This leads to equal concentrations on both sides – the equilibrium for this system.
PASSIVE: Solute diffusion
High C1 Low C2 solute concentration
J = D A C1 – C 2
X
Fick’s diffusion equation (Adolf Fick, 1829-1901)
X= l en gth
J ~ C1 – C2: Diffusion rate is proportional to concentration gradient JY ~ C solute Y : Diffusion rate for solute Y depends on concentration gradient of the solute Y J ~ 1/X: Diffusion rate is greater when diffusion distance is shorter J ~ A: Diffusion rate is greater when diffusion area (surface) is greater
J=net rate of diffusion
J ~ D: Diffusion rate is proportional to the diffusion coefficient D is a measure of the ease with which a solute moves through a medium separating the two concentrations; temperature dependent; defines permeability.
How do membranes affect diffusion?
5
Boundary layers and diffusion
Outward diffusion from the animal or cell increases the concentration in the environmental solution next to the outer surface. The boundary layer thus created may be very thin yet still have a substantial impact on the rate of diffusion.
6
PASSIVE: Permeability of a membrane for a solute
High Low glucose concentration
J =
P A
C1 – C 2 X
P = DmK
The permeability of a membrane to a solute is the ease with which it can move through the membrane by simple diffusion. Membrane permeability is based on Dm, the diffusion coefficient: rate of diffusion of a substance through a membrane K, the partition coefficient: an indicator for the ratio between dissolved and undissolved solute in the membrane
Membrane Diffusion of glucose toward equilibrium
PASSIVE: Membranes and the diffusion of solutes
• The diffusion of a solute across membranes depends on the hydrophobic or hydrophilic nature of the solute.
1. Lipid solutes (steroids and fatty acids): enter the interior of the cell membrane because of their hydrophobic nature. 2. Molecular oxygen (O2): enter interior of membrane because of their small and nonpolar nature. 3. Inorganic ions: low permeability across membrane, but can diffuse passively through cell membranes at rapid rates due to ion channels. Ion channels allow only passive transport across membranes. They do not bind the ions that pass through them. They are selective in determining which ion can pass. Types include: voltage-gated, stretch-gated, phosphorylation-gated and ligand-gated …show more content…
channels.
7
Types of gated ion channels
Types of gated ion channels
8
PASSIVE: Selectively permeable membranes can create electrical gradients
Donnan equilibrium
K+ A1-
0 0 0
Na+ 0 0 0 A2-
K+
A1Membrane is only permeable to K+.
-
+ + +
Na+
K+
A2-
At the beginning there is zero charge across the membrane.
Cross membrane diffusion of K+ creates positive charges on the right side, preventing K+ from further diffusing.
PASSIVE: Selectively permeable membranes can create electrical gradients: Nernst equation
R: Gas constant T: absolute temp in Kelvin (18°C) F: Faraday constant z: charge on the permeant ion electrical gradient = chemical gradient
Ex = RT ln Cout zF Cin
Nernst equation
Substituting the constants results in:
Ex = 0.058 log Cout z Cin
Consequences: Unequal concentrations of ions across a membrane can create a membrane potential = resting potential. A membrane potential can affect the concentration gradient across a membrane.
9
PASSIVE: Diffusion of ions is affected by a chemical and an electrical gradient
High Na+ concentration Low High Na+ concentration Low
Electrical gradient moves ions in same direction as conc. gradient = fast diffusion.
+ + + Membrane Concentration gradient
Electrical gradient opposes conc. gradient = slowing diffusion or reversing direction.
- + - + - +
Membrane Concentration gradient
+
-
-
+
Electrical gradient
Electrical gradient
Diffusion (fast)
Diffusion (slow)
PASSIVE: An electrochemical view of a single cell
+ +
Cl- is near electrochemical equilibrium because the concentration gradient and electrical effect oppose each other. K+: The conc. effect is greater than the electrical effect, but not as much as for Na+.
+ - + Cl- + ClK+ K+ - + Na+ A- - + Na+ + + Cell membrane + Anionic + proteins (a) Ion concentration inside a single animal cell
Intracellularfluid
-
+
Extracellular fluid
Na+ is far from electrochemical equilibrium, because the conc.
gradient and electrical effect move it into the cell.
10
PASSIVE: Facilitated diffusion
Polar organic solutes (glucose, amino acids) are hydrophilic: diffuse through membrane with the help of carrier proteins: • In direction of electrochemical gradient • Facilitated because faster than simple diffusion • Reversible and non-covalent binding
PASSIVE SOLUTE TRANSPORT - SUMMARY
• Passive transport moves towards the electrochemical equilibrium. • The permeability of a membrane for a lipid solute depends on how readily the the solute enters into and moves across the membrane lipid bilayer – simple diffusion. For inorganic ions, the permeability depends on the number of ion channels. • Polar organic solutes (glucose, amino acids) move across membranes with the help of transporter proteins in the direction of the electrochemical equilibrium (facilitated diffusion).
• Simple diffusion depends on the concentration gradient for an uncharged solute. In case of a charged solute, conc. gradients and electrical effects contribute to diffusion.
11
Primary and secondary ACTIVE
TRANSPORT
• Na+-K+-ATPase is an example of a primary active transport mechanism: it draws energy directly from ATP. An ATPase is a transporter and an enzyme. • Other important ATPases: Ca2+-ATPase in the sarcoplasmic reticulum of muscle cells. H+-K+-ATPase is the proton-pump that acidifies stomach content. vesicular or v-type H+-ATPase in gills and kidneys.
• Secondary active transport draws energy from an electrochemical gradient of a solute, and therefore indirectly from ATP.
ACTIVE TRANSPORT
Extracellular + fluid Intracellular- + fluid - + Cl-
+
+
Primary active-transport mechanisms.
Pond water
Blood plasma
Na+
Na+ Cl-
- + K+ - + Na+ A- - + Na+ + + Cell membrane + Anionic +
K+
Cl-
Cl-
proteins
Primary and secondary activetransport mechanisms.
Intestinal epithelial cells
Gill epithelial cell (b) Ion concentration across gill epithelium of a freshwater fish
(a) Ion concentration inside a single animal cell
Intestinal lumen
Glucose from meal
Blood capillary
Secondary activetransport mechanisms.
(c) Glucose transport across intestinal epithelium into the blood system
Cross section of small intestine
12
ACTIVE: Primary active transport mechanisms
Active transport Diffusion Cl- is close to electrochemical equilibrium.
Intracellular -fluid Cl ATP
-
+
+
+
+
Cl-
Extracellular fluid
2K+
+
Anionic proteins
+ +
K+
Electrochemical gradients favor steady diffusion of Na+ and K+ through resting channels in the cell membranes.
A
-
-
ADP
-
Na+
3Na+
+
+
The Na+-K+ pump maintains Na+ and K+ ions out of equilibrium by using ATP-bond energy.
(a) Ion concentration inside a single animal cell
Anionic proteins are trapped inside the cell.
ACTIVE: Secondary active transport mechanisms
Intestinal epithelial cells Blood capillary
Intestinal lumen
Glucose from meal
Cross section of small intestine
(c) Glucose transport across intestinal epithelium into the blood system
A transporter carries Basolateral Apical Na+ towards its EC membrane equilibrium, and thereby forces the cotransport + Na + - Na+ 2K+ of glucose across the ATP + - Glu membrane.
Na+
Cotransporter
Glu
+-
Glu
2 Na+
ADP
The Na+-K+-ATPase in the basolateral membrane pumps Na+ outside the cell, and thereby maintains the Na+ gradient across the apical membrane.
3Na+
Transport against and in direction of electrochem. gradient
13
ACTIVE: Ion movements in fresh water fish gills
Apical Pond water Blood plasma Basolateral membrane Na+ ATP ADP ADP ATP
Na+
Na+ Cl-
+ H+ + Na+
2K+
+ + -
3Na+
A H+-ATPase generates a negative charge inside – Na+ follows towards its EC equilibrium.
Cl-
Cl-
Gill epithelial cell (b) Ion concentration across gill epithelium of a freshwater fish Channel Countertransporter Transport against and in direction of electrochem. gradient
HCO3-
HCO3-+H+
CO2
CO2 – metabolic waste
A gradient favors the outward diffusion of bicarbonate ions (HCO3-), which drives the influx of Clagainst the Cl- EC gradient.
ACTIVE SOLUTE TRANSPORT - SUMMARY
• Solute transport is active if it can move solutes away from electrochemical equilibrium. • Active transport is primary if the transporter is an ATPase (energy comes directly from ATP), secondary if the energy comes from a solute electrochemical gradient. Organic solutes are pumped by secondary-active transport mechanism. • The transport of ions can create voltage differences (electrogenic) or not (electroneutral).
14
MEMBRANE TRANSPORT - SUMMARY
• Equilibrium is the state of a system without input of energy or matter from outside. • Passive transport moves towards, active transport can move against the electrochemical equilibrium of a system. • Uncharged molecules equalize concentrations by simple diffusion, charged molecules are also moved by electrical forces. • The permeability of a membrane for a solute depends how readily it dissolves into the membrane and the number of channels (inorganic ions). • Active transport uses metabolic energy (ATP) to transport a solute against the electochemical gradient. • Organic solutes can be moved across membranes by transporters, but only towards the electrochemical equilibrium (facilitated diffusion). • Active transport is primary when it directly uses ATP (ATPases), secondary if the energy for transport comes from a concentration gradient.
OSMOSIS - The transport of water
Osmotic water uptake across the gill epithelium of a freshwater fish Pond water
Na+
Blood plasma
Higher osmotic pressure – lower concentration of H20 – solution will gain water. The transport of water depends on the number not the nature of dissolved solutes per unit of volume – colligative properties (freezing point and water vapor pressure.
Na+
H20
Cl-
H20
Cl300 mOsm
Rate of water transport = K Π 1 – Π2
X K depends on temperature and permeability of the membrane to osmotic water transport (water channels – aquaporins).
5 mOsm
hypoosmotic
hyperosmotic
• Osmotic pressure is measured in osmolarity: 1 Osm is equivalent to a solvent with 1 Mole of dissolved solutes per liter.
15
OSMOSIS – An example
Isotonic solution: empirical evaluation if cell loses or gains water
Transport across membranes – summary II
• The transport of water depends on the colligative properties of a solvent. • Water moves from solution of lower osmotic pressure (more abundant water) to the solution of higher osmotic pressure (less abundant water). • If two solutions have the same osmotic pressure they are called iso-osmotic. A solution with higher osmotic pressure is hyper-osmotic relative to one with lower pressure (hypo-osmotic).
16
Intracellular fluid Extracellular fluid Pond water Blood plasma
K+ A-
ClNa+
Cl+
K+ Na+ Cl-
Na+
Na+
Gill epithelial cell Intestinal epithelial cells
Cl-
Cell membrane Anionic proteins (a) Ion concentration inside a single animal cell
(b) Ion concentration across gill epithelium of a freshwater fish Blood capillary
Intestinal lumen
Glucose from meal
Cross section of small intestine
(c) Glucose transport across intestinal epithelium into the blood system
MEMBRANE STRUCTURE
Extracellular fluid Glycoproteins and -lipids
Phospholipid bilayer Cytoplasm
Integral and peripheral proteins
Cholesterol Cytoskeleton
How do phospholipids build a membrane?
1
Phospholipid …show more content…
structure
Choline+ PhosphateGlycerol
Bends symbolize double bonds – less densely packed lipids. Many positively charged groups can occupy this position.
Polar head
Non-polar fatty acid chains
Nonpolar tails Hydrophilic Hydrophobic
Phospholipid “behavior” in water
Water Micelle Hydrophillic heads
Hydrophobic tails
Phospholipid bilayer
2
Fluidity of membranes
(a)
Lateral Movement (frequent) Flip-flops (rare) Frequent lateral movement of lipids; infrequent flip-flops across membrane leaflets.
Fluid (b)
Unsaturated fatty acid tails with kinks
Viscous
Saturated fatty acid tails
Degree of unsaturated/saturated fatty acid tails (hydrocarbon) determines membrane fluidity.
Phospholipid composition and habitat temperature
% of hydrocarbon tails unsaturated
Phosphatidyl-ethanolamines
Phosphatidyl-cholines
Adaptation temperature (°C)
Fatty acid chains vary in chemical unsaturation = double bonds that produce a bend in the chain. Unsaturated phospholipids are less densely packed and increase membrane fluidity. To maintain membrane fluidity at low temperatures, cold adapted species incorporate more unsaturated lipids into their membrane.
after Logue et al. (2000), J Exp Biol 203, 2105-2115
3
Membrane proteins
PASSIVE AND ACTIVE TRANSPORT
What are equilibrium conditions? A system moves towards its “equilibrium” when there is no further input of energy or matter – has minimal potential for more work.
Passive transport mechanisms work only in the direction of equilibrium.
Active transport mechanisms are capable of working in a direction against the equilibrium.
4
PASSIVE: Diffusion as passive solute transport
High Low glucose concentration
Membrane Diffusion of glucose toward equilibrium
Due to molecular movement glucose passes through the membrane passively. Because there is more glucose on the left side, on average and by chance more molecules pass from left to right. This leads to equal concentrations on both sides – the equilibrium for this system.
PASSIVE: Solute diffusion
High C1 Low C2 solute concentration
J = D A C1 – C 2
X
Fick’s diffusion equation (Adolf Fick, 1829-1901)
X= l en gth
J ~ C1 – C2: Diffusion rate is proportional to concentration gradient JY ~ C solute Y : Diffusion rate for solute Y depends on concentration gradient of the solute Y J ~ 1/X: Diffusion rate is greater when diffusion distance is shorter J ~ A: Diffusion rate is greater when diffusion area (surface) is greater
J=net rate of diffusion
J ~ D: Diffusion rate is proportional to the diffusion coefficient D is a measure of the ease with which a solute moves through a medium separating the two concentrations; temperature dependent; defines permeability.
How do membranes affect diffusion?
5
Boundary layers and diffusion
Outward diffusion from the animal or cell increases the concentration in the environmental solution next to the outer surface. The boundary layer thus created may be very thin yet still have a substantial impact on the rate of diffusion.
6
PASSIVE: Permeability of a membrane for a solute
High Low glucose concentration
J =
P A
C1 – C 2 X
P = DmK
The permeability of a membrane to a solute is the ease with which it can move through the membrane by simple diffusion. Membrane permeability is based on Dm, the diffusion coefficient: rate of diffusion of a substance through a membrane K, the partition coefficient: an indicator for the ratio between dissolved and undissolved solute in the membrane
Membrane Diffusion of glucose toward equilibrium
PASSIVE: Membranes and the diffusion of solutes
• The diffusion of a solute across membranes depends on the hydrophobic or hydrophilic nature of the solute.
1. Lipid solutes (steroids and fatty acids): enter the interior of the cell membrane because of their hydrophobic nature. 2. Molecular oxygen (O2): enter interior of membrane because of their small and nonpolar nature. 3. Inorganic ions: low permeability across membrane, but can diffuse passively through cell membranes at rapid rates due to ion channels. Ion channels allow only passive transport across membranes. They do not bind the ions that pass through them. They are selective in determining which ion can pass. Types include: voltage-gated, stretch-gated, phosphorylation-gated and ligand-gated …show more content…
channels.
7
Types of gated ion channels
Types of gated ion channels
8
PASSIVE: Selectively permeable membranes can create electrical gradients
Donnan equilibrium
K+ A1-
0 0 0
Na+ 0 0 0 A2-
K+
A1Membrane is only permeable to K+.
-
+ + +
Na+
K+
A2-
At the beginning there is zero charge across the membrane.
Cross membrane diffusion of K+ creates positive charges on the right side, preventing K+ from further diffusing.
PASSIVE: Selectively permeable membranes can create electrical gradients: Nernst equation
R: Gas constant T: absolute temp in Kelvin (18°C) F: Faraday constant z: charge on the permeant ion electrical gradient = chemical gradient
Ex = RT ln Cout zF Cin
Nernst equation
Substituting the constants results in:
Ex = 0.058 log Cout z Cin
Consequences: Unequal concentrations of ions across a membrane can create a membrane potential = resting potential. A membrane potential can affect the concentration gradient across a membrane.
9
PASSIVE: Diffusion of ions is affected by a chemical and an electrical gradient
High Na+ concentration Low High Na+ concentration Low
Electrical gradient moves ions in same direction as conc. gradient = fast diffusion.
+ + + Membrane Concentration gradient
Electrical gradient opposes conc. gradient = slowing diffusion or reversing direction.
- + - + - +
Membrane Concentration gradient
+
-
-
+
Electrical gradient
Electrical gradient
Diffusion (fast)
Diffusion (slow)
PASSIVE: An electrochemical view of a single cell
+ +
Cl- is near electrochemical equilibrium because the concentration gradient and electrical effect oppose each other. K+: The conc. effect is greater than the electrical effect, but not as much as for Na+.
+ - + Cl- + ClK+ K+ - + Na+ A- - + Na+ + + Cell membrane + Anionic + proteins (a) Ion concentration inside a single animal cell
Intracellularfluid
-
+
Extracellular fluid
Na+ is far from electrochemical equilibrium, because the conc.
gradient and electrical effect move it into the cell.
10
PASSIVE: Facilitated diffusion
Polar organic solutes (glucose, amino acids) are hydrophilic: diffuse through membrane with the help of carrier proteins: • In direction of electrochemical gradient • Facilitated because faster than simple diffusion • Reversible and non-covalent binding
PASSIVE SOLUTE TRANSPORT - SUMMARY
• Passive transport moves towards the electrochemical equilibrium. • The permeability of a membrane for a lipid solute depends on how readily the the solute enters into and moves across the membrane lipid bilayer – simple diffusion. For inorganic ions, the permeability depends on the number of ion channels. • Polar organic solutes (glucose, amino acids) move across membranes with the help of transporter proteins in the direction of the electrochemical equilibrium (facilitated diffusion).
• Simple diffusion depends on the concentration gradient for an uncharged solute. In case of a charged solute, conc. gradients and electrical effects contribute to diffusion.
11
Primary and secondary ACTIVE
TRANSPORT
• Na+-K+-ATPase is an example of a primary active transport mechanism: it draws energy directly from ATP. An ATPase is a transporter and an enzyme. • Other important ATPases: Ca2+-ATPase in the sarcoplasmic reticulum of muscle cells. H+-K+-ATPase is the proton-pump that acidifies stomach content. vesicular or v-type H+-ATPase in gills and kidneys.
• Secondary active transport draws energy from an electrochemical gradient of a solute, and therefore indirectly from ATP.
ACTIVE TRANSPORT
Extracellular + fluid Intracellular- + fluid - + Cl-
+
+
Primary active-transport mechanisms.
Pond water
Blood plasma
Na+
Na+ Cl-
- + K+ - + Na+ A- - + Na+ + + Cell membrane + Anionic +
K+
Cl-
Cl-
proteins
Primary and secondary activetransport mechanisms.
Intestinal epithelial cells
Gill epithelial cell (b) Ion concentration across gill epithelium of a freshwater fish
(a) Ion concentration inside a single animal cell
Intestinal lumen
Glucose from meal
Blood capillary
Secondary activetransport mechanisms.
(c) Glucose transport across intestinal epithelium into the blood system
Cross section of small intestine
12
ACTIVE: Primary active transport mechanisms
Active transport Diffusion Cl- is close to electrochemical equilibrium.
Intracellular -fluid Cl ATP
-
+
+
+
+
Cl-
Extracellular fluid
2K+
+
Anionic proteins
+ +
K+
Electrochemical gradients favor steady diffusion of Na+ and K+ through resting channels in the cell membranes.
A
-
-
ADP
-
Na+
3Na+
+
+
The Na+-K+ pump maintains Na+ and K+ ions out of equilibrium by using ATP-bond energy.
(a) Ion concentration inside a single animal cell
Anionic proteins are trapped inside the cell.
ACTIVE: Secondary active transport mechanisms
Intestinal epithelial cells Blood capillary
Intestinal lumen
Glucose from meal
Cross section of small intestine
(c) Glucose transport across intestinal epithelium into the blood system
A transporter carries Basolateral Apical Na+ towards its EC membrane equilibrium, and thereby forces the cotransport + Na + - Na+ 2K+ of glucose across the ATP + - Glu membrane.
Na+
Cotransporter
Glu
+-
Glu
2 Na+
ADP
The Na+-K+-ATPase in the basolateral membrane pumps Na+ outside the cell, and thereby maintains the Na+ gradient across the apical membrane.
3Na+
Transport against and in direction of electrochem. gradient
13
ACTIVE: Ion movements in fresh water fish gills
Apical Pond water Blood plasma Basolateral membrane Na+ ATP ADP ADP ATP
Na+
Na+ Cl-
+ H+ + Na+
2K+
+ + -
3Na+
A H+-ATPase generates a negative charge inside – Na+ follows towards its EC equilibrium.
Cl-
Cl-
Gill epithelial cell (b) Ion concentration across gill epithelium of a freshwater fish Channel Countertransporter Transport against and in direction of electrochem. gradient
HCO3-
HCO3-+H+
CO2
CO2 – metabolic waste
A gradient favors the outward diffusion of bicarbonate ions (HCO3-), which drives the influx of Clagainst the Cl- EC gradient.
ACTIVE SOLUTE TRANSPORT - SUMMARY
• Solute transport is active if it can move solutes away from electrochemical equilibrium. • Active transport is primary if the transporter is an ATPase (energy comes directly from ATP), secondary if the energy comes from a solute electrochemical gradient. Organic solutes are pumped by secondary-active transport mechanism. • The transport of ions can create voltage differences (electrogenic) or not (electroneutral).
14
MEMBRANE TRANSPORT - SUMMARY
• Equilibrium is the state of a system without input of energy or matter from outside. • Passive transport moves towards, active transport can move against the electrochemical equilibrium of a system. • Uncharged molecules equalize concentrations by simple diffusion, charged molecules are also moved by electrical forces. • The permeability of a membrane for a solute depends how readily it dissolves into the membrane and the number of channels (inorganic ions). • Active transport uses metabolic energy (ATP) to transport a solute against the electochemical gradient. • Organic solutes can be moved across membranes by transporters, but only towards the electrochemical equilibrium (facilitated diffusion). • Active transport is primary when it directly uses ATP (ATPases), secondary if the energy for transport comes from a concentration gradient.
OSMOSIS - The transport of water
Osmotic water uptake across the gill epithelium of a freshwater fish Pond water
Na+
Blood plasma
Higher osmotic pressure – lower concentration of H20 – solution will gain water. The transport of water depends on the number not the nature of dissolved solutes per unit of volume – colligative properties (freezing point and water vapor pressure.
Na+
H20
Cl-
H20
Cl300 mOsm
Rate of water transport = K Π 1 – Π2
X K depends on temperature and permeability of the membrane to osmotic water transport (water channels – aquaporins).
5 mOsm
hypoosmotic
hyperosmotic
• Osmotic pressure is measured in osmolarity: 1 Osm is equivalent to a solvent with 1 Mole of dissolved solutes per liter.
15
OSMOSIS – An example
Isotonic solution: empirical evaluation if cell loses or gains water
Transport across membranes – summary II
• The transport of water depends on the colligative properties of a solvent. • Water moves from solution of lower osmotic pressure (more abundant water) to the solution of higher osmotic pressure (less abundant water). • If two solutions have the same osmotic pressure they are called iso-osmotic. A solution with higher osmotic pressure is hyper-osmotic relative to one with lower pressure (hypo-osmotic).
16