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