Transcript Slide 1
Cell Membrane/Plasma Membrane
• plasma membrane is the boundary that separates the living cell from
its surroundings
• functions:
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1. integrity of the cell – size and shape
2. controls transport = “selectively permeable”
3. excludes unwanted materials from entering the cell
4. maintains the ionic concentration of the cell & osmotic pressure of the
cytosol
• 5. forms contacts with neighboring cells = tissue
• 6. sensitivity - first part of cell that is affected by changes in the extracellular
environment
Cell Membrane/Plasma Membrane
• comprised of a phospholipid bilayer
• phospholipids are the most abundant lipid in the
plasma membrane – about 75% of lipids
• phospholipid:
– glycerol + 2 fatty acids
– addition of a phosphate group
Hydrophilic
head
WATER
Hydrophobic
tail
WATER
• phospholipids are amphipathic molecules
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containing hydrophobic and hydrophilic regions
hydrophobic fatty acid “tails”
hydrophilic phosphate “head group”
makes the phospholipid both non-polar and polar
http://www.bio.davidson.edu/people/macampbell/111/memb-swf/membranes.swf
Plasma Membrane Composition:
polar heads out
Fibers of extracellular matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
non-polar tails in
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
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the polar and non-polar attributes of the lipids results in a bilayer arrangement
inserted into the membrane will also be cholesterol - which is also polar (OH group and non-polar steroid
rings)
-so the OH group faces out and the steroid rings face inward
-gives the membrane fluidity
also associated with membrane proteins and sugars
Plasma Membrane Fluidity & Membrane proteins
• plasma membrane also contains embedded proteins
• many lipids and proteins are also modified through the addition of carbohydrates
– glycoproteins
– glycolipids
• membrane proteins must be able to change shape to function
• some of them even laterally diffuse through the PM
• so the membrane must be fluid
Plasma Membrane Fluidity
• fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of
various proteins embedded in it
• membrane fluidity is due to several factors
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temperature - lipids move around more with increased temp
lipid packing – lipids with shorter fatty acid tails are less stiff
saturation of fatty acids – more C=C bonds (more unsaturated) increase fluidity
cholesterol – decreases fluidity at warmer temps; increases fluidity at lower temps
Lateral movement occurs
107 times per second.
Flip-flopping across the membrane
is rare ( once per month).
Membrane Proteins and Their Functions
• membrane proteins determine most of the membrane’s specific
functions
• proteins link on the extracellular side to an extracellular matrix of
proteins – support the cells within a tissue
• proteins link on the cytoplasmic side to the cytoskeleton
- via adaptor proteins
Membrane Proteins and Their Functions
EXTRACELLULAR
SIDE
two kinds of membrane proteins:
N-terminus
1. Extrinsic or Peripheral: bound to the surface of
the membrane
◦ often are linked to integral membrane proteins
◦ function mainly as enzymes
2. Intrinsic or Integral: penetrate the hydrophobic
core
◦ those that span the membrane are called
transmembrane proteins
helix
C-terminus
CYTOPLASMIC
SIDE
Membrane Proteins and Their Functions
6 major functions of integral membrane
proteins:
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1. transport – channel proteins &
transporters
2. enzymatic activity
3. signal transduction – receptor
proteins
4. cell-cell recognition – cell identity
markers
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e.g. ABO antigens
5. intercellular joining - linkers
6. attachment- to the cytoskeleton
and extracellular matrix (ECM)
Enzymes
Glycoprotein
Enzymatic activity
Cell-cell recognition
Intercellular joining
Attachment to
the cytoskeleton
and extracellular
Membrane structure results in selective permeability
a cell must exchange materials with its surroundings
◦ controlled by the plasma membrane
plasma membranes are selectively permeable
◦ regulating the cell’s molecular traffic
permeability = property that determines the effectiveness of the PM as a
barrier
hydrophobic (nonpolar) molecules dissolve in the phospholipid bilayer and pass
through the membrane rapidly
◦ must be small enough to diffuse across the PM
ionic and polar molecules do not cross the membrane easily
◦ require transport mechanisms
◦ provided by transport proteins
Membrane Gradients
selective permeability of the PM allows the cells to
control the concentration of ions within the cell (in the
ICF) and outside the cell (in the ECF)
this results in a distinct distribution of positive and
negative ions inside and outside the cell
◦ typically the inside of the cell is more negatively charged
this difference in electrical charge between inside and
outside = electrical gradient
because it occurs across the PM – we call this difference in
charge = membrane potential
can be measured with tiny glass electrodes
varies from cell to cell
very important in the functioning of neurons and muscle
cells
Transport Proteins
transport proteins allow passage of hydrophilic substances across the membrane
◦ specific for the substance it moves
numerous types:
1. channel proteins – have a hydrophilic channel that certain molecules or ions can
use as a tunnel to move across the membrane
2. carrier proteins - bind to molecules and change shape to shuttle them across the
membrane
3. pumps – carrier proteins that require the hydrolysis of ATP (or GTP) to move
substances
What determines the direction of transport??
two basic things
1. concentration of what is being moved
2. available energy
passive transport
diffusion
osmosis
facilitated
active transport
primary AT
secondary AT
endocytosis
exocytosis
http://programs.northlandcollege.edu/biology/Biology1111/animations/transport1.html
A. Diffusion = movement of materials from [high] to [low]
-random movement, no energy needs to be synthesized
-the movement is driven by the inherent kinetic energy of the particles moving
down their concentration gradient
-three ways to diffuse:
1. through the lipid bilayer: lipid soluble (non-polar), alcohol,
gases, ammonia, fat-soluble vitamins
2. through a channel: charged ions or small polar molecules
-some channels are “gated” – open and close
3. facilitated diffusion: larger, polar molecules too big for channels
B. Osmosis = diffusion of water from [high] to [low]
OR movement of water from [low solute] to [high solute]
Lower
concentration
of solute (sugar)
in osmosis – the membrane is permeable
to water and NOT to the solutes
BUT it is the concentration of solutes
that causes the water to move
Higher
concentration
of solute
Sugar
molecule
H2O
the solutes are surrounded by a
hydration shell of water molecules
Selectively
permeable
membrane
this decreases the amount of free water
molecules available to move
so increased solute concentration
decreases the concentration of free
water molecules
water movement is affected by this drop in
free water molecule concentration
Osmosis
Same concentration
of solute
B. Osmosis = diffusion of water from [high] to [low]
OR movement of water from [low solute] to [high solute]
in osmosis – it is the concentration of solutes that causes the water to move
-known as tonicity
experiment – U shaped tube divided by a membrane permeable to water only
-increase the solute concentration in the right half of the tube
-this increases the pressure caused by the increase in solutes = osmotic
pressure (OP)
-therefore increasing solute concentration increases osmotic pressure
-water will move in to decrease this OP
OP is important in determining how much fluid remains in your blood and how
much leaves to surround the cells in your tissues
Osmosis is controlled by tonicity = degree to which a the concentration of a
specific solute surrounding a cell causes water to enter or leave the cell
hypertonic
e.g. isotonic = [S]in = [S]out hypotonic = [S]in > [S]out hypertonic = [S]in < [S]out
water enters cell
water exits cell
no water movement
C. Facilitated transport = molecules move by a carrier protein from [high] to [low]
-binds to a carrier protein on the plasma membrane
-transported by the carrier protein
-no energy required – transported down the concentration gradient by the
carrier protein
-but there is a limit to the amount of facilitated diffusion cells can undergo
and it has to do with the number of carrier proteins on the PM
-molecules that are insoluble, too polar or
too large
e.g. glucose
amino acids
Active transport = transport requires the expenditure of energy
-usually provided through the hydrolysis of ATP ADP+Pi
-cell is moving a substance against its concentration gradient
-cell is forming transport vesicles
-cell is internalizing something
Several kinds:
1. primary active transport = molecules are moved against its concentration gradient
i.e. from [low] to [high]
-directly uses the energy of ATP hydrolysis for this
2. secondary active transport = molecules are moved against its concentration gradient
i.e. from [low] to [high]
-movement is dependent upon another ion’s concentration gradient
3. Exocytosis – cell secretion
4. Endocytosis – cell internalization
-pinocytosis
Exocytosis and Endocytosis
-phagocytosis
are referred to as
-receptor-mediated endocytosis
Bulk Transport
(Active process)
A. Primary Active Transport =
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requires a protein carrier called a pump - pump that hydrolyzes ATP (ATPase)
e.g. sodium/potassium ATPse pump : maintains a specific concentration of Na+ and K+ in and out
of the cell
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three Na+ are pumped out of a cell and 2 K+ are pumped into the cell
3 Na+ ions bind to the pump
ATP binds to the pump and is hydrolyzed
a phosphate group remains bound to the pump = phosphorylation
phosphorylation changes the activity of the pump by altering its shape
Na+ is expelled out of the cell – against its concentration gradient
2 K+ ions then bind the pump and causes the release of the P group
another shape change by the pump - releases the K+ into the cell
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html#
Animation: Active Transport
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.
2. secondary active transport:
-the energy stored in a concentration gradient is used to drive the transport of other materials
– primary active transport establishes high [Na] outside the cell - creates a Na gradient
-diffusion of Na back into the cell allows the movement of a second ion – either in the same
direction as the Na+ (symporter) or in the opposite direction (antiporter)
e.g. Na+/Ca2+ antiporter – opposite direction for Na+ and Ca2+ movement
– the Na+ concentration gradient creates potential energy which is used by an antiporter pump
- as Na leaks back in through this antiporter – the potential energy is converted into kinetic energy
which drives the movement of a Ca2+ ion against its gradient
-most of our cells use the energy created by the Na+ gradients to power the movement of other ions in and
out of our cells
diffusion
diffusion
Na pump
diffusion
diffusion
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html#
B. Exocytosis = secretion of a substance outside the cell
-products made within the cell, packaged by the Golgi into transport vesicles fusion
with the plasma membrane and release outside the cell
e.g. nerve cells - neurotransmitter release
http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter6/animations.html#
C. Endocytosis = reverse of exocytosis, internalization of substances
-3 forms: 1. pinocytosis = “cell drinking”
2. phagocytosis = “cell eating”
3. receptor-mediated endocytosis (RME)= internalization of specific substances
-binding of a ligand with its receptor internalization into the cell
-occurs at specific sites within the PM called clathrin-coated pits
-proteins accumulate at these clathrin-coated pits
-internalization clathrin-coated vesicle
-CC vesicle fuses with endosomes – eventually fuse to the lysosome for
processing
http://sumanasinc.com/webcontent/animations/content/endocytosis.html