Cell Membrane Structure and
Function
Prof. Dr. Turgut Ulutin
Why do we need the plasma
membrane?
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“Keeping the goods concentrated”
Keeping harmful materials out
Transports substances in and out of the
cell
Communication with other cells – chemical
signals
Cell membrane functions include:
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Physical isolation
•Regulation of exchange with the environment
Changes in ECF, pH, receptor
recognition
•Structural support
The Cell Membrane
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The cell membrane is a phospholipid bilayer
with proteins, lipids and carbohydrates.
Membrane Structure
Primarily made up of lipids
With lipids
With carbohydrates
1. Lipids are the most abundant
Membrane called phospholipid bilayer
Outermost portions - hydrophilic
Innermost layers - hydrophobic
Four Components of the Plasma
Membrane
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Phospholipid Bilayer
Cholesterol
Proteins
Glycocalyx
Membranes are “Fluid Mosaics”
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Phospholipids are the fluid part of the
membrane
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Changes shape without breaking apart
The proteins move through the
phospholipids
Think of it as icebergs moving through the
ocean
Phospholipid Bilayer
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Phospholipids–two fatty-acid chains and a polar
phosphate group attached to glycerol
Phospholipids in water -two layers, heads
pointed out, and tails pointed in
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Outside of cell mostly water and inside of cell mostly
water so polar heads point towards the watery
extracellular fluid and watery cytoplasm inside the cell
Passage through bilayer–lipid center is a barrier
to passage of large hydrophilic molecules, but it
allows nonpolar, hydrophobic molecules to pass.
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Small molecules like water and uncharged, lipid
soluble moleucles can pass freely
Cholesterol
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Lipid material
Two Functions
Acts as patching substance on the bilayer
that prevents passage of some molecules
(such as ions or simple sugars)
Helps keep membrane stronger and
flexible
Proteins
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Two types with many functions
Integral - span entire membrane, popping out on both
sides or extending partway into the membrane; bound to
the hydrophobic interior
Peripheral - lie on either side of the membrane, not
bound to hydrophobic interior
Function
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Structural support–attach to cytoskeleton
Recognition– binding sites on some proteins identify the cell to
other cells
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help immune system determine self from foreign cells
Communication–signals transferred through receptor proteins
Transport–molecules can pass
Glycocalyx – “sugar coat”
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Sugar components protruding from lipids
and proteins
Binding sites for proteins in
communication and recognition
Lubricate cells
Stick cells down – sticky adhesion layer
Membranes serve a wide variety of functions
1. Barriers to separate cells from environment
(plasma membrane)
and one compartment from another (organelle membranes)
2. Transport – Passive, active, and vesicular transport
3. Sites for enzyme systems, e.g.
a) ATP synthase
b) Cytochrome p450s
4. Conduct electrical signals
5. Recognize and respond to molecules or surfaces
a) Signal mol. Receptors
b) Immune response to antigens
Moving Materials In and Out:
Diffusion and Gradients
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A. Random Movement and Diffusion
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Concentration - # of molecules in a given unit of
volume
Concentration gradient = difference between the
highest and lowest concentration of a solute
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Example – bike coasting down a hill
Diffusion = movement of molecules from region of
higher to lower concentration
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The greater the concentration gradient the faster the rate
Diffusion will continue until concentration gradient is
eliminated (as long as no other processes interfere)
Structure and Function of Organelles
cell (plasma) membrane
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semi-permeable membrane which
regulates the passage of substances into
and out of the cell
composition: protein molecules dispersed
throughout a double layer of lipid (fluid
mosaic)
Permeability
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The ease with which substances can cross the cell membrane
•Nothing passes through an impermeable barrier
•Anything can pass through a freely permeable
barrier
•Cell membranes are selectively permeable
Selective permeability is based on size, electrical charges, molecular
shape, and lipid solubility.
Transport of substances across the membrane can be Passive or Active
Active transport requires energy to occur
Passive transport does not require energy
Diffusion, Osmosis and Active Transport are different
types of movement
Factors affecting permeability of
cell membrane to a particular
substance:
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1. molecular size of the substance
2. electrical charge on the atom or
molecule of the substance
3. solubility of the substance in water
4. solubility of the substance in lipid
What determines the direction of the
movement of molecules (diffusion)
across the cell membrane?
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concentration of substances on each side
of the membrane
Diffusion is a form of passive transport.
(no energy required)
Osmosis
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diffusion of water through a semipermeable membrane from an area where
the water molecules are more
concentrated to an area where the water
molecules are less concentrated
Diffusion through Membranes
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Permeability verses selectively permeable
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Permeable
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Selectively permeable
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Membrane permeable to both water and solute
solute diffuses down its concentration gradient
diffuses through the membrane becoming evenly distributed on both sides
water freely moves, but solute cannot
more solute on one side of the membrane
water moves both ways, but more flows to the greater concentration of
solutes
result – more solution on one side of the membrane than the other
Osmosis - net movement of water across a selectively
permeable membrane from an area of lower solute concentration
to higher solute concentration
Importance of osmosis to membrane function in animals and plants
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Cells shrinking or bursting in animals
Cells expanding and shrinking in plants (wilting and standing up)
Two Main Types of Transport
across Cell Membranes
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Passive Transport – no energy expended
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Simple diffusion - membrane is permeable to water, gases, and fat-soluble
molecules, so can enter without energy output by cell
Facilitated diffusion - membrane is impermeable to larger polar molecules, so to
travel down the concentration gradient, need help from a transport protein
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Channel protein
Carrier protein
Osmosis
Active Transport– molecules passing across the membrane up their
concentration gradient
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cannot use diffusion - must expend energy (ATP)
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Transport protein binds with ATP and the molecule being transported. The protein
changes shape to move the ion across the membrane
Ex. Sodium (Na) – Potassium (K) pump
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Maintain high K inside the cell and high Na outside the cell
Think of it as pumping out seawater from a leaking boat
Active transport requires energy
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A transport protein actively pumps a
specific solute across a membrane against
the solute’s concentration gradient; i.e.,
away from where the solute is less
concentrated. Membrane proteins use
ATP as their energy source for active
transport.