Transcript BIO 100 S. Badran

BIO 107
Lab # 4
Cell membranes, Osmosis &
Diffusion
S. Badran
The plasma membrane
• Is the boundary that separates the living cell
from its nonliving surroundings
• Is the minimal requirement for the formation of
a cell
• Was the first cell part to form during cellular
evolution
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Selective permeability
• The plasma membrane allows some
substances to cross it but not others
Figure 7.1
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The fluid mosaic model
• States that the cell membrane is a fluid
structure with a “mosaic” of various proteins
embedded in it
• Fluidity results from the movement of
phospholipids & proteins.
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There are four primary types of membrane
proteins, each of which performs a different
function.
The Permeability of the Lipid Bilayer
• Small, hydrophobic molecules can pass through the
membrane rapidly
• Example: CO2, O2
• Polar molecules do not cross the membrane rapidly
• Example: H2O, sugars
• Charged ions DO NOT cross the lipid bi-layer on their own
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Facilitated Diffusion by Transport Proteins
• Allows passage of hydrophilic substances and
ions across the membrane and consist of:
– Channels with ionic amino acids for passage of
ions such as Na, K and Ca
– Carriers that are specific for sugar, water and
other hydrophilic molecules
• Example: glucose carrier and aquaporins
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Passive transport
• Cell does not expend any extra energy for
diffusion
• Driving force for diffusion:
– kinetic energy of particles diffusing
– Difference in their concentration (concentration
gradient)
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Diffusion
•
tendency for molecules of any substance to spread out evenly into the
available space
•
Activity: Diffusion
(a) Diffusion of one solute. The membrane
has pores large enough for molecules
of dye to pass through. Random
movement of dye molecules will cause
some to pass through the pores; this
will happen more often on the side
with more molecules. The dye diffuses
from where it is more concentrated
to where it is less concentrated
(called diffusing down a concentration
gradient). This leads to a dynamic
equilibrium: The solute molecules
continue to cross the membrane,
but at equal rates in both directions.
Molecules of dye
Membrane (cross section)
Net diffusion
Net diffusion
Figure 7.11 A
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Equilibrium
Diffusion
• Substances diffuse down their concentration
gradient, from high to low concentration
(b) Diffusion of two solutes. Solutions of
two different dyes are separated by a
membrane that is permeable to both.
Each dye diffuses down its own concentration gradient. There will be a net
diffusion of the purple dye toward the
left, even though the total solute
concentration was initially greater on
the left side.
Net diffusion
Net diffusion
Figure 7.11 B
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Net diffusion
Net diffusion
Equilibrium
Equilibrium
Effects of Osmosis on Water Balance
• Osmosis is the diffusion of water across a
semi-permeable membrane down its
concentration gradient
• Based on osmosis, solutions are classified as:
– Hypotonic (hypo = less)
– Isotonic (iso = same)
– Hypertonic (hyper = more)
• The prefix refers to the SOLUTE
concentration NOT the water concentration
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Tonicity of a solution
• The solution with higher concentration of
solutes is hypertonic.
• The solution with lower concentration of solutes
is hypotonic.
– These are comparative terms.
• Tap water is hypertonic compared to
distilled water but hypotonic when
compared to sea water.
• Solutions with equal solute concentrations are
isotonic.
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Osmosis
• Is affected by the concentration gradient of dissolved substances
• Activity: osmosis and water balance
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
Same concentration
of sugar
Selectively
permeable membrane: sugar molecules cannot pass
through pores, but
water molecules can
Water molecules
cluster around
sugar molecules
More free water
molecules (higher
concentration)
Fewer free water
molecules (lower
concentration)
Osmosis

Water moves from an area of higher
free water concentration to an area
of lower free water concentration
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Pressure due to osmosis
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Cells in isotonic solutions
• The concentration of solutes is the same as it
is inside the cell
• No net movement of water
• Dynamic equilibrium: water moves but at the
same rate in opposite directions
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Cells in hypertonic solutions
• The concentration of solutes is greater than it is
inside the cell
• The cell will lose water
– Animal cells shrivel
– Plasmolysis occurs in plant cells, when the
cytoplasm shrivels and detaches from cell wall
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Cells in hypotonic solutions
• The concentration of solutes is less than it is
inside the cell
• The cell will gain water
– Animal cells burst (or adapt by using
contractile vacuoles)
– Plant cells become turgid (preferred state)
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Water Balance of Cells with Walls
• Cell walls help maintain water balance
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3.9 Osmosis is the diffus the passive
diffusion of water
Animal adaptations
Paramecium, a protist, is hypertonic when compared to pond water
Adaptations:
• membrane is less permeable to water than usual
• specialized organelle (contractile vacuole) pumps excess water out
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Osmosis & Dialysis in today’s lab
• Dialysis tubing: an artificial semi-permeable
membrane
– Molecules can pass if they are small enough to
fit through the microscopic pores
– Dialysis: diffusion of solute (not osmosis of
water)
• You will observe:
– Osmosis and dialysis using dialysis tubing
– Osmosis leading to plasmolysis of Elodea cells
treated with high salt concentration
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Osmosis leading to plasmolysis of
Elodea cells treated with high salt concentration
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Elodea cells in isotonic solution
Question 1
•
A solution of 1 M glucose is separated by a selectively
permeable membrane from a solution of 0.2 M fructose
and 0.7 M sucrose. The membrane is not permeable to
the sugar molecules. Which of the following statements is
correct?
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–
Side A is hypotonic relative to
side B.
–
The net movement of water will
be from side B to side A.
–
The net movement of water will
be from side A to side B.
–
Side B is hypertonic relative to
side A.
–
There will be no net movement
of water.
For Question 2
An artificial cell consisting of an aqueous solution enclosed in
a selectively permeable membrane has just been immersed
in a beaker containing a different solution. The membrane is
permeable to water and to the simple sugars glucose and
fructose but completely impermeable to the disaccharide
sucrose.
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Question 3
• Which solute(s) will exhibit a net diffusion into
the cell?
–
sucrose
–
glucose
–
fructose
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Question 4
• Which solute(s) will exhibit a net diffusion out
of the cell?
–
sucrose
–
glucose
–
fructose
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Question 5
• Which solution is hypertonic to the other?
– the cell contents
– the environment
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