Download Diffusion, Osmosis, and The Cell Membrane

DIFFUSION, OSMOSIS, AND
THE CELL MEMBRANE
Structure and Function of Cell Membrane
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Structure – phospholipid bilayer embedded with protein
pumps
cells want to maintain equilibrium
selectively regulates passage of substances in and out of
cell
How do things pass through the cell membrane?
Through passive transport: no additional
energy required
 Three types
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 small
molecules such easily pass through via
simple diffusion
 large molecules (starch) require use of protein
pumps for facilitated diffusion
 Water passes through the cell membrane via
osmosis
Simple Diffusion
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Simple diffusion is process in which molecules intermingle as a
result of kinetic energy
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all molecules are in constant motion and continually spreading apart
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Simple diffusion occurs because of a concentration gradient
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A concentration gradient exists when one area is more or less
concentrated than another.
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molecules move from an area of high to low concentration, aka
simple diffusion
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Example:
 Tea Bag or sugar cube in water
Let's
 intracellular circulation –
movement of materials
within a cell occurs through diffusion
see diffusion in action
(I am a link!)
Facilitated Diffusion
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Transport of substances across the cell membrane
from higher concentrations to areas of lower
concentration through means protein pump
No energy required
Let’s see facilitated diffusion in action (I am a link!)
OSMOSIS
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Diffusion of water is known as osmosis
water moves via kinetic energy from a less
salty/sugary solution (lower solute concentration) to
a more salty/sugary solution (higher solute
concentration) in order to dilute out the salt/sugar
if too much water builds up inside a membrane,
osmotic pressure results
if osmotic pressure is too high, the cell can
burst/lyse
Let’s see osmosis in action (I am a link!)
Osmosis and Cells
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Whether there is net movement of water into or out
of the cell and which direction it moves depends on
whether the cell’s environment is isotonic,
hypotonic, or hypertonic.
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Tonicity: is a measure of the osmotic (water)
pressure gradient of two solutions separated by a
semipermeable membrane.
 Hypertonic refers to a higher concentration
 Hypotonic refers to a lower concentration
 Isotonic refers to equal concentrations
Tonicity and cells: Cells in
an Isotonic Solution
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When two environments are
isotonic, the concentration of
dissolved solutes is the same in
both of them.
When cells are in isotonic
solution, movement of water
out of the cell is exactly
balanced by movement of
water into the cell.
Tonicity and cells: Cells in a
Hypertonic Solution
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Greek "hyper," = over, and "tonos," = stretching.
In a hypertonic solution the concentration of
solutes is greater than that of the concentration
in a cell.
If concentrations of solutes are greater outside
the cell, the concentration of water outside is
correspondingly lower.
As a result, water inside the cell will flow
outwards to attain equilibrium, causing the cell
to shrink.
As cells lose water, they lose the ability to
function or divide.
Tonicity and cells: Cells in a
Hypotonic Solution
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Greek "hypo,"=under, and "tonos,"=stretching.
In a hypotonic solution the concentration of
solutes is less than that of a cell.
When concentrations are less outside the cell
than inside, the concentration of water outside
is correspondingly greater outside the cell.
When a cell is in hypotonic conditions, there is
net water movement into the cell.
Cells without walls will swell and may burst
(lyse) if excess water is not removed from the
cell. Cells with walls often benefit from the
turgor pressure that develops in hypotonic
environments.
Active Transport

movement of particles with the use of
energy other than kinetic

sometimes active transport can move
molecules from an area of low to high
concentration (against the gradient)
Two Types of Active Transport
First Type:
Primary Active Transport
 Often
uses ATP (the
cell’s energy)
 involves protein
molecules in the cell
membrane that act as
carriers aiding in
transportation
Two Types of Active Transport
Second type:
ENDOCYTOSIS
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Phagocytosis (cell eating) – white blood
cells that keep the body healthy by engulfing
foreign pathogens
Membrane folds in around large molecules
so they can be taken in by the cell resulting
in a vacuole containing the molecule
Pinocytosis (cell drinking) – fluids
containing large molecules such as DNA or
proteins are taken into the cell in the same
manner
Putting it all together!