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Structure and Function of Plasma Membranes > Passive Transport
Passive Transport
• Introduction
• Selective Permeability
• Diffusion
• Facilitated transport
• Osmosis
• Tonicity
• Tonicity in Living Systems
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Structure and Function of Plasma Membranes > Passive Transport
Introduction
• Plasma membranes are selectively permeable; if they were to lose this selectivity,
the cell would no longer be able to sustain itself.
• In passive transport, substances simply move from an area of higher
concentration to an area of lower concentration, which does not require the input
of energy.
• Concentration gradient, size of the particles that are diffusing, and temperature of
the system affect the rate of diffusion.
• Some materials diffuse readily through the membrane, but others require
specialized proteins, such as channels and transporters, to carry them into or out
Passive Transport
of the cell.
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Structure and Function of Plasma Membranes > Passive Transport
Selective Permeability
• The interior and exterior surfaces of the plasma membrane are not identical,
which adds to the selective permeability of the membrane.
• Fat soluble substances are able to pass easily to the hydrophobic interior of the
plasma membrane and diffuse into the cell.
• Polar molecules and charged molecules do not diffuse easily through the lipid
core of the plasma membrane and must be transported across by proteins,
sugars, or amino acids.
Asymmetry in Plasma Membranes
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Structure and Function of Plasma Membranes > Passive Transport
Diffusion
• Substances diffuse according to their concentration gradient; within a system,
different substances in the medium will each diffuse at different rates according to
their individual gradients.
• After a substance has diffused completely through a space, removing its
concentration gradient, molecules will still move around in the space, but there will
be no net movement of the number of molecules from one area to another, a
state known as dynamic equilibrium.
• Several factors affect the rate of diffusion of a solute including the mass of the
solute, the temperature of the environment, the solvent density, and the distance
traveled.
Diffusion
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Structure and Function of Plasma Membranes > Passive Transport
Facilitated transport
• A concentration gradient exists that would allow ions and polar molecules to
diffuse into the cell, but these materials are repelled by the hydrophobic parts of
the cell membrane.
• Facilitated diffusion uses integral membrane proteins to move polar or charged
substances across the hydrophobic regions of the membrane.
• Channel proteins can aid in the facilitated diffusion of substances by forming a
hydrophilic passage through the plasma membrane through which polar and
charged substances can pass.
• Channel proteins can be open at all times, constantly allowing a particular
substance into or out of the cell, depending on the concentration gradient; or they
Channel Proteins in Facilitated Transport
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can be gated and can only be opened by a particular biological signal.
• Carrier proteins aid in facilitated diffusion by binding a particular substance, then
altering their shape to bring that substance into or out of the cell.
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Structure and Function of Plasma Membranes > Passive Transport
Osmosis
• Osmosis occurs according to the concentration gradient of water across the
membrane, which is inversely proportional to the concentration of solutes.
• Osmosis occurs until the concentration gradient of water goes to zero or until the
hydrostatic pressure of the water balances the osmotic pressure.
• Osmosis occurs when there is a concentration gradient of a solute within a
solution, but the membrane does not allow diffusion of the solute.
Osmosis
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Structure and Function of Plasma Membranes > Passive Transport
Tonicity
• Osmolarity describes the total solute concentration of a solution; solutions with a
low solute concentration have a low osmolarity, while those with a high osmolarity
have a high solute concentration.
• Water moves from the side of the membrane with lower osmolarity (and more
water) to the side with higher osmolarity (and less water).
• In a hypotonic solution, the extracellular fluid has a lower osmolarity than the fluid
inside the cell; water enters the cell.
• In a hypertonic solution, the extracellular fluid has a higher osmolarity than the
fluid inside the cell; water leaves the cell.
• In an isotonic solution, the extracellular fluid has the same osmolarity as the cell;
Changes in Cell Shape Due to Dissolved Solutes
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there will be no net movement of water into or out of the cell.
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Structure and Function of Plasma Membranes > Passive Transport
Tonicity in Living Systems
• In a hypotonic environment, water enters a cell, causing it to swell and burst
(lyse).
• In a hypertonic environment, water leaves a cell, causing it to shrink and die.
• In plants, in a hypotonic environment, the inflow of water produces turgor
pressure, which stiffens the cell walls of the plant, while in a hypertonic
environment, turgor pressure is lost, and the plant wilts.
• Some protists that lack cell walls have contractile vacuoles, which collect excess
water from the cell and pump it out, keeping the cell from lysing.
• Freshwater fish living in a hypotonic environment take in salt through their gills
and excrete diluted urine to rid themselves of excess water; saltwater fish living in
Turgor Pressure and Tonicity in a Plant Cell
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a hypertonic environment secrete salt through their gills and excrete highly
concentrated urine.
• In vertebrates, osmoreceptors monitor the concentration of solutes in the blood,
and albumin controls osmotic pressure in tissues.
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Appendix
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Structure and Function of Plasma Membranes
Key terms
• amphiphilic Having one surface consisting of hydrophilic amino acids and the opposite surface consisting of hydrophobic (or
lipophilic) ones.
• concentration gradient A concentration gradient is present when a membrane separates two different concentrations of
molecules.
• concentration gradient A concentration gradient is present when a membrane separates two different concentrations of
molecules.
• diffusion The passive movement of a solute across a permeable membrane
• facilitated diffusion The spontaneous passage of molecules or ions across a biological membrane passing through specific
transmembrane integral proteins.
• hypertonic having a greater osmotic pressure than another
• hypertonic having a greater osmotic pressure than another
• hypotonic Having a lower osmotic pressure than another; a cell in this environment causes water to enter the cell, causing it to
swell.
• hypotonic Having a lower osmotic pressure than another; a cell in this environment causes water to enter the cell, causing it to
swell.
• isotonic having the same osmotic pressure
• membrane protein Proteins that are attached to, or associated with the membrane of a cell or an organelle.
• osmolarity The osmotic concentration of a solution, normally expressed as osmoles of solute per litre of solution.
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Structure and Function of Plasma Membranes
• osmoregulation the homeostatic regulation of osmotic pressure in the body in order to maintain a constant water content
• osmosis The net movement of solvent molecules from a region of high solvent potential to a region of lower solvent potential
through a partially permeable membrane
• passive transport A movement of biochemicals and other atomic or molecular substances across membranes that does not
require an input of chemical energy.
• permeable Of or relating to substance, substrate, membrane or material that absorbs or allows the passage of fluids.
• polar a separation of electric charge leading to a molecule or its chemical groups having an electric dipole
• semipermeable membrane A type of biological membrane that will allow certain molecules or ions to pass through it by diffusion
and occasionally by specialized facilitated diffusion
• solute Any substance that is dissolved in a liquid solvent to create a solution
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Structure and Function of Plasma Membranes
Passive Transport
Diffusion is a type of passive transport.Diffusion through a permeable membrane moves a substance from an area of high concentration (extracellular
fluid, in this case) down its concentration gradient (into the cytoplasm).
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Structure and Function of Plasma Membranes
Asymmetry in Plasma Membranes
The exterior surface of the plasma membrane is not identical to the interior surface of the same membrane.
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Structure and Function of Plasma Membranes
Diffusion
Diffusion through a permeable membrane moves a substance from an area of high concentration (extracellular fluid, in this case) down its concentration
gradient (into the cytoplasm).
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Structure and Function of Plasma Membranes
Carrier Proteins
Some substances are able to move down their concentration gradient across the plasma membrane with the aid of carrier proteins.Carrier proteins
change shape as they move molecules across the membrane.
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Structure and Function of Plasma Membranes
Channel Proteins in Facilitated Transport
Facilitated transport moves substances down their concentration gradients.They may cross the plasma membrane with the aid of channel proteins.
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Structure and Function of Plasma Membranes
Osmosis
In osmosis, water always moves from an area of higher water concentration to one of lower concentration.In the diagram shown, the solute cannot pass
through the selectively permeable membrane, but the water can.
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Structure and Function of Plasma Membranes
Changes in Cell Shape Due to Dissolved Solutes
Osmotic pressure changes the shape of red blood cells in hypertonic, isotonic, and hypotonic solutions.
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Structure and Function of Plasma Membranes
Contractile Vacuoles
A paramecium's contractile vacuole, here visualized using bright field light microscopy at 480x magnification, continuously pumps water out of the
organism's body to keep it from bursting in a hypotonic medium.
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Structure and Function of Plasma Membranes
Turgor Pressure and Plasmolysis
Without adequate water, the plant on the left has lost turgor pressure, visible in its wilting; the turgor pressure is restored by watering it (right).
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Structure and Function of Plasma Membranes
Turgor Pressure and Tonicity in a Plant Cell
The turgor pressure within a plant cell depends on the tonicity of the solution in which it is bathed.
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Structure and Function of Plasma Membranes
Which of the following statements about passive transport is true?
A) Substances move from areas of high concentration to areas of low
concentration.
B) Membrane proteins are always needed in order for it to take place.
C) Substances move from areas of low concentration to areas of high
concentration.
D) The input of ATP is required in order to facilitate transportation.
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Structure and Function of Plasma Membranes
Which of the following statements about passive transport is true?
A) Substances move from areas of high concentration to areas of low
concentration.
B) Membrane proteins are always needed in order for it to take place.
C) Substances move from areas of low concentration to areas of high
concentration.
D) The input of ATP is required in order to facilitate transportation.
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Structure and Function of Plasma Membranes
Which of the following substances will NOT diffuse easily across
the plasma membrane?
A) Vitamin K
B) Water
C) Sodium ions
D) Oxygen
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Structure and Function of Plasma Membranes
Which of the following substances will NOT diffuse easily across
the plasma membrane?
A) Vitamin K
B) Water
C) Sodium ions
D) Oxygen
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Structure and Function of Plasma Membranes
Which of the following factors does NOT affect the rate of diffusion
across a membrane?
A) the temperature of a solution
B) the size of the diffusing particles
C) the number of different types of particles in a solution
D) the concentration gradient across the cell membrane
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Structure and Function of Plasma Membranes
Which of the following factors does NOT affect the rate of diffusion
across a membrane?
A) the temperature of a solution
B) the size of the diffusing particles
C) the number of different types of particles in a solution
D) the concentration gradient across the cell membrane
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Structure and Function of Plasma Membranes
Which of the following characteristics distinguishes facilitated
transport from simple diffusion? Passive transport
______________.
A) uses membrane proteins and channels to move molecules down
concentration gradients.
B) requires the input of cellular energy to transport biomolecules.
C) describes the movement of biomolecules such as proteins,
carbohydrates, and fatty acids.
D) explains the movement of molecules or ions from high to low
concentration.
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Structure and Function of Plasma Membranes
Which of the following characteristics distinguishes facilitated
transport from simple diffusion? Passive transport
______________.
A) uses membrane proteins and channels to move molecules down
concentration gradients.
B) requires the input of cellular energy to transport biomolecules.
C) describes the movement of biomolecules such as proteins,
carbohydrates, and fatty acids.
D) explains the movement of molecules or ions from high to low
concentration.
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Structure and Function of Plasma Membranes
Which of the following statements about channel proteins is true?
A) They change shape as they move molecules across a membrane.
B) Polar compounds pass through them to avoid nonpolar regions.
C) All of them are open at all times to allow passage of materials.
D) all of these answers
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Structure and Function of Plasma Membranes
Which of the following statements about channel proteins is true?
A) They change shape as they move molecules across a membrane.
B) Polar compounds pass through them to avoid nonpolar regions.
C) All of them are open at all times to allow passage of materials.
D) all of these answers
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Structure and Function of Plasma Membranes
In a beaker separated by a membrane permeable only to water,
one area has a high solute concentration and the other has a low
solute concentration. What do you expect will happen in the
beaker?
A) Water will move to the area of low solute concentration.
B) Water will move to the area of high water concentration.
C) Water will move to the area of high solute concentration.
D) The solute will move to the area of low solute concentration.
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Structure and Function of Plasma Membranes
In a beaker separated by a membrane permeable only to water,
one area has a high solute concentration and the other has a low
solute concentration. What do you expect will happen in the
beaker?
A) Water will move to the area of low solute concentration.
B) Water will move to the area of high water concentration.
C) Water will move to the area of high solute concentration.
D) The solute will move to the area of low solute concentration.
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Structure and Function of Plasma Membranes
You take a sample of blood cells and place them in a solution.
Fifteen minutes later, you observe that they appear shriveled.
What is the tonicity of the solution they were placed in?
A) Hypotonic
B) Hypertonic
C) Isotonic
D) Equitonic
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Structure and Function of Plasma Membranes
You take a sample of blood cells and place them in a solution.
Fifteen minutes later, you observe that they appear shriveled.
What is the tonicity of the solution they were placed in?
A) Hypotonic
B) Hypertonic
C) Isotonic
D) Equitonic
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Structure and Function of Plasma Membranes
Which of the following is used as a method of osmoregulation in
humans?
A) plasmolysis
B) albumin
C) contractile vacuoles
D) turgor pressure
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Structure and Function of Plasma Membranes
Which of the following is used as a method of osmoregulation in
humans?
A) plasmolysis
B) albumin
C) contractile vacuoles
D) turgor pressure
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Structure and Function of Plasma Membranes
Attribution
• Connexions. "Passive Transport." CC BY 3.0 http://cnx.org/content/m44417/latest/?collection=col11448/latest
• Wikipedia. "concentration gradient." CC BY-SA 3.0 http://en.wikipedia.org/wiki/concentration%20gradient
• Wikipedia. "passive transport." CC BY-SA 3.0 http://en.wikipedia.org/wiki/passive%20transport
• Wiktionary. "permeable." CC BY-SA 3.0 http://en.wiktionary.org/wiki/permeable
• Connexions. "Passive Transport." CC BY 3.0 http://cnx.org/content/m44417/latest/?collection=col11448/latest
• Wiktionary. "polar." CC BY-SA 3.0 http://en.wiktionary.org/wiki/polar
• Wiktionary. "amphiphilic." CC BY-SA 3.0 http://en.wiktionary.org/wiki/amphiphilic
• Connexions. "Passive Transport." CC BY 3.0 http://cnx.org/content/m44417/latest/?collection=col11448/latest
• Wikipedia. "concentration gradient." CC BY-SA 3.0 http://en.wikipedia.org/wiki/concentration%20gradient
• Boundless Learning. "Boundless." CC BY-SA 3.0 http://www.boundless.com//biology/definition/diffusion
• Connexions. "Passive Transport." CC BY 3.0 http://cnx.org/content/m44417/latest/?collection=col11448/latest
• Wikipedia. "membrane protein." CC BY-SA 3.0 http://en.wikipedia.org/wiki/membrane%20protein
• Wikipedia. "facilitated diffusion." CC BY-SA 3.0 http://en.wikipedia.org/wiki/facilitated%20diffusion
• Connexions. "Passive Transport." CC BY 3.0 http://cnx.org/content/m44417/latest/?collection=col11448/latest
• Wiktionary. "solute." CC BY-SA 3.0 http://en.wiktionary.org/wiki/solute
• Wiktionary. "osmosis." CC BY-SA 3.0 http://en.wiktionary.org/wiki/osmosis
• Connexions. "Passive Transport." CC BY 3.0 http://cnx.org/content/m44417/latest/?collection=col11448/latest
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Structure and Function of Plasma Membranes
• Wiktionary. "isotonic." CC BY-SA 3.0 http://en.wiktionary.org/wiki/isotonic
• Wiktionary. "hypertonic." CC BY-SA 3.0 http://en.wiktionary.org/wiki/hypertonic
• Wiktionary. "hypotonic." CC BY-SA 3.0 http://en.wiktionary.org/wiki/hypotonic
• Wiktionary. "osmolarity." CC BY-SA 3.0 http://en.wiktionary.org/wiki/osmolarity
• Connexions. "Passive Transport." CC BY 3.0 http://cnx.org/content/m44417/latest/?collection=col11448/latest
• Wiktionary. "osmoregulation." CC BY-SA 3.0 http://en.wiktionary.org/wiki/osmoregulation
• Wiktionary. "hypertonic." CC BY-SA 3.0 http://en.wiktionary.org/wiki/hypertonic
• Wiktionary. "hypotonic." CC BY-SA 3.0 http://en.wiktionary.org/wiki/hypotonic
• Wiktionary. "tonicity." CC BY-SA http://en.wiktionary.org/wiki/tonicity
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