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Cell Membranes
Chapter 4
The Cell:
The Fundamental Unit of Life
•
Materials must constantly cross the cell
membrane
•
Food energy molecules in
•
Waste molecules out
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Some things can cross membranes freely; others
cannot
•
Cells have developed a number of ways around
this
Lecture 4: Membrane Permeability
Cell Membranes
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Cell membrane is semisemi-permeable
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Some things can cross, others can not
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Primary influences:
Diffusion
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Movement of atoms, ions, molecules from an area
of high concentration to an area of low
concentration (along a diffusion gradient)
Size
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Will equalize concentrations on both sides
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Polarity
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Costs no cellular energy = passive transport
•
Charge
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Diffusion
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If more than one solute:
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Each solute diffuses independently
Diffusion
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Several types of diffusion
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Simple diffusion
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Substance flows
freely across membrane
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Examples: O2, CO2
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Diffusion
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Facilitated diffusion
Diffusion
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Facilitated diffusion
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Membrane protein facilitates movement
across membrane
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Highly specific
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May be a channel
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Based on Shape
Based on size,
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Example: glucose
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DOES THIS REQUIRE ENERGY??
•
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May be carrier
charge
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Example: ions
Facilitated Diffusion
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Both channels and carriers can be regulated
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Turned on and off
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Number on membrane changed
Diffusion Rate
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Influenced by many factors
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Temperature
•
•
Size
•
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Higher temp = faster diffusion
Smaller solute = faster diffusion
Concentration gradient
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Greater gradient = faster diffusion
Diffusion Rate
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Membrane surface area
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Membrane permeability characteristics
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More SA = faster diffusion
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Makeup of individual membrane
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Carriers/channels present
Osmosis
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Diffusion of water molecules only
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Occurs when…
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Concentration gradient
exists
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Membrane is not
permeable to solute(s)
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Membrane is permeable
to water
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Selectively permeable
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Osmosis
Non-diffusable solutes = polar
NonHydration spheres will form
• Reduces amount of water free to diffuse
• Indicate which solution has the most free water
•
Osmosis
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Use arrows to indicate the direction that water
will move across the membrane
•
Remember: the goal is to reach the same
concentration on both sides
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2M
4M
2M
5mM
22mM
5mM
5%
1.3%
5%
1.3%
2.4M/kg
1.5M/kg
350mM
0.35M
2.4M/kg
1.5M/kg
350mM
0.35M
Osmosis
•
Tonicity
Osmosis: Movement of water from an area of low
•
solute concentration to an area of high solute
concentration
A way to describe solutions surrounding a living
cell, and the osmotic effect on the cell
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Three possibilities:
•
Diffusible/penetrating solutes do not cause
osmosis
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Only nonnon-diffusible/nondiffusible/non-penetrating solutes
•
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Hypertonic
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Solution has higher osmotic concentration than cell
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Hypotonic
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Isotonic
If you can’t move the solute, move the solvent
•
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Solution has lower osmotic concentration than cell
Solution has equal osmotic concentration to cell
Tonicity
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4M
22mM
Isotonic
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Solution does not cause osmotic flow of water
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No change in cell shape
Tonicity
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Hypotonic
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Solution with lower osmotic concentration
than the cell
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Cell will swell
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Tonicity
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Hypertonic
Practice Activities
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Solution with higher osmotic concentration
than the cell
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Cell will shrink
Describe the movement of water by osmosis
between the two solutions below.
0.1 M glucose
Practice Activities
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Describe the movement of water by osmosis
between the two solutions below.
Practice Activities
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Describe the movement of water by osmosis
between the two solutions below.
0.1 M NaCl
0.25M NaCl
0.1 M NaCl
(electrolyte)
(electrolyte)
(electrolyte)
Practice Activities
•
Describe the movement of water by osmosis
between the two solutions below.
10 M albumin
10M glucose
0.1 M sucrose
0.15M glucose
(non(non-electrolyte)
electrolyte)
Practice Activities
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Describe the movement of water by osmosis
between the two solutions below.
2M NaCl
1M glucose
(electrolyte)
2M sucrose
1M KCl (electrolyte)
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Practice Questions
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Which of the following would increase the rate
of diffusion? (choose all that apply)
Practice Questions
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a. Increase the concentration of a molecule
a. 0.20 mM
b. Increase the temperature of the
b. 0.8 mM
environment
c. 12 mM
c. Decrease the molecular size
d. 15 mM
d. Increase the molecular size
e. 8 mM
Practice Questions
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Which of the following solutions would have the
least amount of free water?
The term osmosis refers to the diffusion of
what molecule?
Practice Questions
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When the extracellular solution has a greater
solute concentration than the intracellular fluid,
the extracellular solution is described as being
a. Isotonic
b. Hypertonic
c. Hypotonic
d. Ultrasonic
Practice Questions
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How would adding salt to the soil affect a plant?
(The Romans did this after defeating Carthage
in 149 BC)
Practice Questions
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A 0.9mM glucose solution is separated from a
0.9mM sucrose solution by a membrane that is
impermeable to both. What best describes the
resulting osmosis?
a. Water moves equally in both directions
b. Water moves from the glucose solution to
the sucrose solution
c. Water moves from the sucrose solution to
the glucose solution
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Practice Questions
•
A 10mM NaCl (an electrolyte) solution is
separated from a 10mM sucrose (a nonnonelectrolyte) solution. What best describes the
resulting osmosis?
a. Water moves equally in both directions
b. Water moves from the sucrose solution to
the NaCl solution
c. Water moves from the NaCl solution to the
sucrose solution
Practice Questions
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The ______ portion of the plasma membrane is
responsible for isolating the cytoplasm from the
extracellular environment, while the ______
portion regulates the exchange of solutes and
makes it possible for the cell to sense the
environment
a. Lipid; protein
b. Cholesterol; lipid
c. Protein; cholesterol
d. Carbohydrate; lipid
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