Download Plasma membrane

Chapter 7.2 & 8.1
The Plasma Membrane
Review of the Plasma Membrane Structure and Function
• Function
• the plasma membrane is the flexible lipid
boundary of the cell and it helps the cell to
maintain homeostasis through selective
permeability
• homeostasis is the regulation of the
internal environment of the cell regardless
of the external environment
• remember, cells need certain chemicals
and molecules in certain amounts to
function correctly
• selective permeability is the act of
allowing certain molecules to move into
and out of the cell and not others
•
Structure
• Remember, the plasma membrane is a flexible
lipid bilayer
• It is composed of 3 types of molecules
– the main type of molecule that composes the
membrane is the phospholipids
• these phospholipids are composed of 2 parts;
the polar (they like water – hydrophilic) head
and the nonpolar (they don’t like water hydrophobic) fatty acid tails
• these phospholipids line up in 2 layers to
makeup the membrane; the polar heads on the
outside while the nonpolar fatty acid tails
point in because there is water found both
inside and outside of the cell (see diagram)
•
there are also proteins found in the bilayer
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•
there are several different types of proteins found in
the membrane
• some go only partway through the bilayer – these
serve as enzymes and chemical “markers”
• some go the entire way through the bilayer – these
allow larger molecules to pass through the
membrane
there is also cholesterol found in the bilayer
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•
the cholesterol serves 2 purposes; it gives the
membrane stability and helps to keep the fatty acid
tails separated
We call our model of the plasma membrane the
fluid mosaic model
–
it is made of many tiny parts that are free to move
around
•
Review of Diffusion
–
Diffusion is the random movement of particles from
an area of high concentration to low concentration
–
Diffusion allows particles in a solution to become
evenly spread out
• once the particles are evenly spread out, the
random movement of particles continues but there
is no overall change in the distribution of particles
• this condition is called dynamic equilibrium
• If nothing interferes with diffusion, it will continue
until dynamic equilibrium is reached and there is
no longer a difference in concentration
•
Diffusion depends on three things
• concentration gradient
– diffusion cannot occur unless there is a difference in
the concentration of a substance
– this difference in concentration in different areas
called a concentration gradient
– since particles randomly move from high
concentration to low concentration it is said that they
move with the gradient
– if something makes particles move from low
concentration to high concentration they are said to
move against the gradient
• size
– it is difficult for large molecules to pass through the
membrane by diffusion because there is not much
room between the phospholipids to diffuse
•
charge
– it is also difficult for charged particles to pass
through the memebrane by diffusion because
of the nonpolar fatty acid tails
– the nonpolar tails do not “like” polar
molecules and they will not allow them to
pass through the membrane by diffusion
•
there are only a few molecules that can pass
through the membrane by diffusion
– water, oxygen, nitrogen, carbon dioxide and a
few other small nonpolar molecules
•Osmosis: Diffusion of Water
- osmosis is the diffusion of water into and out of a cell
•
•
•
remember, diffusion occurs from an area of
high concentration to low concentration
this means within a cell osmosis occurs when
water moves into or out of a cell to even out the
concentration of water
concentration refers to how many molecules
are in one area relative to another area
•
•
•
for example, we can make salt water by adding sodium
chloride to water. If we do this in 2 separate beakers,
we can add more salt to one than to the other. This
gives each beaker a different concentration of dissolved
particles.
The concentration of dissolved particles can help us to
classify the solution, but we must look at the water
concentration to see in which direction the water
molecules will diffuse (see diagram)
Water concentration is opposite of the dissolved
substance concentration
What controls osmosis?
• Unequal distribution of particles, called a concentration gradient, is
one factor that controls osmosis.
Before
Osmosis
After
Osmosis
Water molecule
Selectively permeable
membrane
Sugar molecule
The concentration of dissolved substance is opposite of the
water concentration.
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Remember, water is one of the only
molecules that can diffuse across a
membrane
Water will move out of the cell if the
water concentration is greater inside the
cell than outside
Water will move into the cell if the water
concentration is greater outside of the
cell than inside
•
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Cells can be present or placed into three
types of environments that are classified
by the type of solution they are
these three types of solutions are
classified by the concentration of
dissolved substance outside of the cell
compared to the concentration of
dissolved substance inside the cell
these three types are called isotonic,
hypotonic and hypertonic
•
Isotonic solutions are solutions that have the same
concentration of dissolved substance that the
inside of the cell has. This means that their water
concentrations are equal as well.
– if there is no difference in water concentration,
then there will be no diffusion of water (osmosis)
• water molecules will still be moving in and out,
but since the concentrations are equal
dynamic equilibrium is occurring
•
cells in isotonic solutions maintain their “normal”
shape
•
most solutions that are injected into our bodies are
isotonic so the cell is not damaged by the loss or
gain of water
•
Hypotonic solutions are solutions in
which the dissolved substance
concentration is less outside the cell than
inside the cell.
– this means that the water concentration is
greater outside the cell than inside, so water
will move into the cell
– this will cause the cell to swell, because
water will move into it, the pressure inside
the cell will increase
•
in plant cells;
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this pressure is called turgor pressure.
Because plants have cell walls, they can resist turgor
pressure and not be damaged
This turgor pressure helps to maintain plant cell shape and
support
When turgor pressure is lost, plants wilt
In animal cell;
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animal cells have no cell walls, therefore, animal cells can
continue to swell until they burst (called lysis)
therefore, organisms that live in freshwater have to get rid
of excess water from their cells
fish in fresh water constantly urinate
some single-celled organisms have organelles called
contractile vacuoles that help them excrete excess water
•
Hypertonic solutions are solutions in
which the dissolved substance has a
higher concentration outside the cell than
inside the cell.
– this means that the water concentration is
less outside the cell than inside, so water
will move out of the cell.
– This will cause the cell to shrink, because
water is leaving the cell and the pressure
decreases
•
In plant cells;
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In animal cells;
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will lose water mainly from their large vacuole
this loss in pressure allows the cell membrane
to pull in and away from the cell wall which is
rigid and cannot move; this is called
plasmolysis
this results in wilting of the plant
will shrivel up, called crenation
animals that live in salt water must compensate
for the constant loss of water (ex. Salt water
fish drink the water and excrete the salt)
see page 197 for diagrams
• Summary
– Hypertonic – more solute, less water outside;
water will move out and cell will shrink
– Hypotonic – less solute, more water outside;
water will move into the cell and the cell will
swell
– Isotonic – same concentrations inside and
out; water will move in and out in equal rates,
so cell will remain “normal”
– Plant cells  “shrinkage” = plasmolysis,
“swelling” causes turgor pressure
– Animal cells  shrinkage = crenation,
swelling to bursting = lysis
•
Passive Transport
• passive transport is the movement of
materials into and out of the cell that does
not require the cell to use energy to
accomplish the movement
• there are 2 types of passive transport
• diffusion
– remember, diffusion occurs with the
concentration gradient and therefore it
happens randomly so the cell doesn’t
have to use energy to make it happen
– diffusion only occurs for a few small
molecules
•
facilitated diffusion
– this type of diffusion takes place through the
membrane transport proteins, but it is still
diffusion and therefore the particles are
moving with the gradient and the movement
does require the cell to expend energy
– there are 2 types of transport proteins
•
•
channel proteins – these proteins are like tubes
that go the entire way through the membrane and
allow larger molecules to flow right in (they are
like an open doorway)
carrier proteins – these proteins are like doors
that open and close (by changing shape) to allow
substances to pass through the membrane
channel proteins – these proteins are like tubes
that go the entire way through the membrane and
allow larger molecules to flow right in (they are like
an open doorway)
carrier proteins – these proteins are like doors that
open and close (by changing shape) to allow
substances to pass through the membrane
see figure 8.5 page 198
Passive Transport
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Active Transport
active transport is the movement of
materials through a membrane against a
concentration gradient
this means that the cell is moving
materials from an area of low
concentration to an area of high
concentration
this can be done, it just means the cell
has to use energy to move the materials
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active transport is accomplished by the use of
carrier proteins
remember, they are like doors that open and
close by changing shape
generally, each carrier protein has a specific
shape that fits a specific molecule or ion
when the proper molecule “binds” with the
protein, cellular energy is used to change the
shape of the protein allowing the material to be
released on the other side of the membrane
once the material is released, the proteins
original shape is restored
Active Transport
–
Transports of Large Molecules
• some cells can move very large molecules, groups
of molecules or even whole cells in and out
• when large particles are moved in, it is called
endocytosis
• when large particles are moved out it is called
exocytosis
• in these processes, the materials do not pass
directly through the membrane; instead they either
“engulfed” by the cell during endocytosis or put in
vacuoles and pushed out of the cell in exocytosis
• both of these processes require cellular energy
• see figure 8.7, page 200