Cell Membrane:
Structure and Function 6.2-6.3
The cell
membrane
is the
gateway
into the cell.
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Key words you should know:
Phospholipids
Polar
Hydrophilic
Hydrophobic
Solution
Solute
Solvent
Partially permeable
Fluid mosaic model
Glycoproteins
Plasmolysis
Cholesterol
Proteins
Transport proteins
Enzymes
Receptor molecules
Diffusion
Concentration gradient
Facilitated diffusion
Osmosis
Surface area
Turgid
Passive transport
Plasmolysed
Selectively permeable
Active transport
Carrier protein
Bulk transport
Endocytosis
Phagocytosis
Phagocytes
Crenated
Pinocytosis
Exocytosis
Phospholipid bilayer
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Cell Membrane Structure
 All living cells are surrounded by a membrane.
 A cell membrane is also known as plasma membrane.
 To understand the function of anything in biology, you
must study the structure first!
Nerve cell
Cell membrane
Gap between cells
{
}
Cell Membranes
from Opposing
Neurons
(TEM x436,740).
cell membrane
Nerve cell
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Functions of the Plasma Membrane
•
Protective barrier
• Regulate transport in & out of cell
• Allow cell-cell recognition
• Provide anchoring sites for filaments of cytoskeleton
•Provide a binding site for enzymes and hormones
• Interlocking surfaces bind cells together (junctions)
•Contains the cytoplasm (fluid in cell)
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Structure of Cell Membrane
Phospholipids
Cholesterol
Proteins
(peripheral and integral)
Carbohydrates
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PHOSPHOLIPID REVIEW
Phospholipids are
important structural
components of cell
membranes. Phospholipids
are modified so that a
phosphate group (PO4-)
replaces one of the three
fatty acids normally found
on a lipid.
The addition of this group
makes a polar "head" and
two nonpolar "tails".
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HYDROPHILIC HEAD
At one end of the phospholipid is a
phosphate group and several double
bonded oxygens. The electrons at
this end of the molecule are not
shared equally. This end of the
molecule has a charge and is
attracted to water. It is POLAR
HYDROPHOBIC TAILS
The two long chains coming off of
the bottom of this molecule are
made up of carbon and hydrogen.
Because both of these elements
share their electrons evenly these
chains have no charge. They are
NON POLAR.
Molecules with no charge are not
attracted to water; as a result
water molecules tend to push them
out of the way as they are
attracted to each other. This
causes molecules with no charge not
to dissolve in water.
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Phospholipids can
form:
BILAYERS
-2 layers of phospholipids with
hydrophobic tails protected inside by the
hydrophilic heads.
The PHOSPHOLIPID BILAYER is the
basic structure of membranes.
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Polar heads are hydrophilic “water loving”
Nonpolar tails are hydrophobic “water fearing”
Phospholipids make membranes
Selectively Permeable- able to control what crosses
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Polar,
Hydrophilic
“head”
Nonpolar,
Hydrophobic
Fatty acid “tails”
Polar,
Hydrophilic
“head”
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FLUID MOSAIC MODEL
Cell membranes also contain proteins within the phospholipid bilayer.
This ‘model’ for the structure of the membrane is called the:
FLUID MOSAIC MODEL
FLUID- because individual phospholipids and proteins can move around freely within
the layer, like it’s a liquid.
MOSAIC- because of the pattern produced by the scattered protein molecules when the
membrane is viewed from above.
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Proteins Are Critical to
Membrane Function
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Proteins can float in the membrane or be fixed and also
have hydrophobic and hydrophilic portions. Proteins may
be embedded in the outer layer or in the inner layer and
some span the two layers.
Hydrophobic and Hydrophilic parts of the protein
molecules sit next to the Hydrophobic and Hydrophilic
portions of the phospholids of the membrane. This ensures
the proteins stay in the membrane.
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Some of the proteins
have carbohydrates
attached to them –
GLYCOPROTEINS.
Glycoproteins act as
chemical id tags.
Blood types are the
result different
glycoproteins.
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The membrane also contains molecules of
CHOLESTEROL
Cholesterol
•regulates the fluidity of the membrane
•gives mechanical stability
•helps to prevent ions from passing through the
membrane.
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The fluid mosaic model
was developed using
Freeze Fracture Studies.
The fracture occurs
between the two
phospholipid layers.
You can clearly see the
exposed proteins sticking
out of the two layers.
Individual phospholipids
are too small to see.
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Cell Membrane Permeability
Hydrophobic pass easily
Hydrophilic DO NOT
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Semipermeable Membrane
Small molecules and larger non-polar molecules move
through easily.
e.g. O2, CO2, H2O
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Semipermeable Membrane
Ions
Hydrophilic molecules larger than water
Large molecules such as proteins
do not move through the membrane on their own.
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Types of Transport
Across
Cell Membranes
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Two Forms of Transport Across the Membrane
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Passive Transport
Simple Diffusion
Doesn’t require cell energy
Molecules move from high to low
concentration
Example: Oxygen or water
diffusing into a cell and carbon
dioxide diffusing out.
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Molecules move
from area of
HIGH to LOW
concentration,
down their
concentration
gradient.
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Diffusion is a
PASSIVE process
which means no
cell energy is used
to make the
molecules move,
they have a natural
KINETIC ENERGY
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Diffusion of Liquids
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Diffusion through a Membrane
Cell membrane
Solute moves DOWN the concentration gradient
from HIGH to LOW concentration
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Osmosis
Diffusion of water across a
membrane
Moves from HIGH water
potential (low solute) to LOW
water potential (high solute)
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Osmosis-Diffusion of H2O Across
A Membrane
Low solute concentration
High solute concentration
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10% NaCL
90% H2O
ENVIRONMENT
CELL
10% NaCL
90% H2O
NO NET
MOVEMENT
What is the direction of water movement?
equilibrium
The cell s at _______________.
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10% NaCL
90% H2O
CELL
20% NaCL
80% H2O
What is the direction of water movement?
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15% NaCL
85% H2O
ENVIRONMENT
CELL
5% NaCL
95% H2O
What is the direction of water movement?
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Cells in Solutions
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Isotonic Solution
Hypotonic
Solution
Hypertonic
Solution
NO NET
MOVEMENT OF
H2O
CYTOLYSIS
CRENATION
(equal amounts
entering & leaving)
(water diffuses
into the cell)
(water diffuses
out of the cell)
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Cytolysis & Crenation
Cytolysis
Crenation
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Osmosis in Red Blood Cells
In Isotonic
Solution
In Hypotonic
Solution
In Hypertonic
Solution
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Indicate the diffusion gradient of water with an arrow.
hypotonic
hypotonic
hypertonic
hypertonic
isotonic
(Crenated cell)
hypertonic
(Plasmolyzed cell)
isotonic
hypotonic
(Turgid cell)
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Transport Proteins
Channel proteins are embedded in the cell
membrane & have a pore for materials to cross
Carrier proteins can change shape to move
material from one side of the membrane to the
other
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Passive Transport
Facilitated Diffusion
Doesn’t require energy
Uses transport proteins to move
high to low concentration
Moves materials with the
concentration gradient.
Examples: Glucose or amino
acids moving from blood into a cell.
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Facilitated Diffusion
Molecules will randomly move through
the pores in Channel Proteins.
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Facilitated Diffusion
 Some Carrier proteins
do not extend through
the membrane.
 They bond and drag
molecules through the
lipid bilayer and
release them on the
opposite side.
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Carrier Proteins
 Other carrier
proteins change
shape to move
materials across
the cell
membrane
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Active Transport
Requires energy or ATP
Moves materials from
LOW to HIGH concentration
AGAINST concentration
gradient
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Active Transport
Examples:
 Pumping Na+ (sodium
ions) out and K+
(potassium ions) in
against strong
concentration
gradients.
 Called Na+-K+ Pump
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Sodium-Potassium Pump
(+)
(-)
3 Na+ pumped out for every 2 K+
pumped in; creates a membrane potential
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Moving the “Big Stuff” Out
Exocytosis
- moving
things
out.
Molecules are moved out of the cell by vesicles that fuse with
the plasma membrane.
This is how many hormones are secreted and how nerve cells
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communicate with one another.
Exocytosis
Large molecules that are manufactured in the
cell are released through the cell membrane.
Inside Cell
Cell environment
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Exocytosis
Exocytic
vesicle
immediately
after fusion
with plasma
membrane.
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Moving the “Big Stuff” In
Large molecules move materials into the cell by
one of three forms of endocytosis.
1. Pinnocytosis
2. Receptor-mediated EndoCytosis
3. Phagocytosis
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1. Pinocytosis
Most common form of endocytosis.
Takes in dissolved molecules as a vesicle.
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Pinocytosis
 Cell forms an
invagination
 Materials dissolve
in water to be
brought into cell
 Called “Cell
Drinking”
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Example of Pinocytosis
pinocytic vesicles forming
mature transport vesicle
Transport across a capillary cell (blue).
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2. Receptor-Mediated Endocytosis
Some integral proteins have receptors
on their surface to recognize & take in
hormones, cholesterol, etc.
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Receptor-Mediated Endocytosis
Coated pit
Vesicle
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3. Phagocytosis
Used to engulf large particles such as
food, bacteria, etc. into vesicles
Called “Cell Eating”
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