Transcript 7-3 Movement Through The Membranes

Cell Structure & Function
Structure & Movement
Through Membranes
(There and Back Again)
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The Cell Membrane
Key Concept:
The cell membrane regulates
what enters and leaves the
cell and also provides
protection and support
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The Cell Membrane
• The cell takes in nutrients and expels
wastes plus cell products through the
cell membrane
• The Basic Structure Of Nearly All Cell
Membranes Is The LIPID BILAYER
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LIPID BILAYERS
• Form Automatically When Certain
Lipids Are Dissolved In Water
• Tough Flexible Structure
• Strong Barrier Between Cell Contents
And Its Surroundings
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LIPID BILAYERS
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LIPID BILAYERS
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Fluid Mosaic Model
In Addition To The Bilayer,
The Cell Membrane Contains:
• Proteins That Form
– Channels & Pumps
• Proteins with Carbohydrates That Form
– Antigenic Sites
– Communication Sites
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Diffusion
All Living Cells Contain And Are
Surrounded By Liquid.
• THE CELL MEMBRANE REGULATES
THE MOVEMENT OF MOLECULES
FROM ONE SIDE OF THE MEMBRANE
TO THE OTHER
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Cytoplasm
Is A Solution
Of Many Different Substances
In Water
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Solution
A liquid mixture of 2 or more
substances evenly mixed.
• Solvent
– The liquid in which the substances are
mixed
• Solute
– Substances dissolved in the Solvent
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Concentration Of Solution
Concentration = Mass of the solute
in a given volume of solvent.
Mass/Volume e.g.
12g of NaCl dissolved in 3L H2O
equals
12g/3L
or
4.0 g/L
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In A Solution
Molecules Are Moving Constantly
They Collide and Bounce off each other
This causes them to spread out randomly
Resulting In:
Movement from:
an area of higher concentration to an
area of lower concentration
This process is called: Diffusion
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DIFFUSION
• Molecules move away
from an area of Higher Concentration
to area’s of Lower Concentration
• Equilibrium
– Concentration of a solute is the same
throughout the solution
• The concentration is the same EVERYWHERE
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Diffusion
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DIFFUSION
Key Concept:
Diffusion Causes Many Substances To
Move Across A Cell Membrane But:
Diffusion Does NOT
Require The Cell
To Use Energy
It just happens naturally!
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Diffusion & Cells
• Substances that can diffuse across cell
membranes will do so until equilibrium
is reached
Diffusion
continues after equilibrium
but the rates are equal
in ALL Directions
Atoms & Molecules Never Stop Moving
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OSMOSIS
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Osmosis Requires A Membrane
• Permeable Membrane
– Substance Can Easily Cross
• Impermeable Membrane
– Substance Can NOT Cross
• Selectively Permeable Membrane
aka = Semipermeable Membrane
– Some Substances Can Cross, Others Can Not
– Cell Membranes are Semipermeable
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OSMOSIS
Key Concept:
Osmosis =
The Diffusion Of Water Through A
Selectively Permeable Membrane
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The Tonics
• Hypotonic
– Less concentrated solution
• Isotonic
– Equally concentrated solution
• Hypertonic
– More Highly concentrated solution
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The Tonics
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The Tonics
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How Osmosis Works
• Overhead – Sugar & Water
• Membrane Impermeable To Sugar
• Water moves from
a higher concentration of WATER
to a
lower concentration of WATER
(NOT Sugar)
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Osmotic Pressure
• Pressure exerted on the hypertonic side of a
selectively permeable membrane
• May cause cells to shrink or burst
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Osmotic Pressure
Cell
Membrane
Contractile
Vacuoles
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Facilitated Diffusion
• Molecules that can easily diffuse
across the cell membrane:
–Alcohol
–Water
–Small Lipids
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Facilitated Diffusion
• Other molecules can not pass
through the cell membrane but still
get into the cell.
HOW?
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Facilitated Diffusion
• Thru Protein Channels in the Cell
Membrane that allow a substance to
cross.
–Allows Diffusion In Both Directions
–Each Channel Substance Specific
–DOES NOT REQUIRE ENERGY
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Facilitated Diffusion
Protein Channel Is Substrate Specific
Substrate Can Move Either Direction Depending On Concentration.
Always from area of Higher Concentration to area of Lower
Concentration!
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Active Transport
• Moves Material Across A Cell
Membrane
• Against A Concentration Gradient
(from area of Lower concentration into an area of
Higher concentration)
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Active Transport
Requires Energy
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Energy Molecule
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Active Transport
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Active Transport
• Endocytosis
– Used to bring in large molecules, food, and
whole cells
• Exocytosis
– Membrane surrounded material from inside
the cell fuses with the cell membrane,
releasing the contents.
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Foothill High School Science Department