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Cell Membrane/Plasma Membrane
• plasma membrane is the boundary that separates the living cell
from its surroundings
• functions:
• 1. integrity of the cell – size and shape
• 2. controls transport = “selectively permeable”
• 3. excludes unwanted materials from entering the cell
• forms a physical barrier with the external environment
• allows the cell to create different environments outside and inside
• 4. maintains the ionic concentration of the cell & osmotic pressure of
the cytosol
• allows for the creation of gradients – electrical and chemical
• 5. forms contacts with neighboring cells = tissue
• 6. sensitivity - first part of cell that is affected by changes in the
•
extracellular environment
Fibers of extracellular matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
Cell Membrane/Plasma Membrane
• comprised of a phospholipid bilayer
• phospholipids are the most abundant
lipid in the plasma membrane – about
75% of lipids
• phospholipid:
Hydrophilic
head
WATER
Hydrophobic
tail
WATER
– glycerol + 2 fatty acids
– addition of a phosphate group
• phospholipids are amphipathic
molecules
– 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 Function – Cell Integrity & Size
•
•
•
•
•
the amount of plasma membrane determines
the size of the cell
metabolic requirements of the cell set upper
limits on its size
the surface area to volume ratio of a cell is
critical
as the surface area increases by a factor of n2,
the volume increases by a factor of n3
small cells have a greater surface area
relative to volume
– i.e. 6 vs. 1.2
Total surface area
[sum of the surface areas
of all box sides  number of boxes]
SA = height x width
Surface area increases while
total volume remains constant
5
1
1
6 cm2
150 cm2
25x bigger
750 cm2
125x bigger
cm3
125 cm3
125x larger
125 cm3
125x larger
Total volume
[height  width  length
 number of boxes]
1
Surface-to-volume
(S-to-V) ratio
[surface area  volume]
6
1.2
6
Tissues
Plasma Membrane Models: Scientific Inquiry
• scientists began building models of plasma membranes before they
were seen with electron microscopes
• RBC membranes were chemically analyzed in 1915
– found to be made of proteins and lipids
• 10 yrs later – hypothesis that membranes were composed of
phospholipid bilayers
– with the hydrophobic tails facing inward away from the aqueous environment
• BUT – where were the proteins located in the membrane?
Plasma Membrane Models: Scientific Inquiry
• HYPOTHESIS: membrane is a phospholipid bilayer “sandwiched”
between two layers of proteins
• PROBLEM: membrane proteins are not very water soluble because
they are amphipathic (just like phospholipids)
– made up of hydrophilic and hydrophobic regions
• 1972 – Singer and Nicolson proposed the model seen below
Phospholipid
bilayer
Hydrophobic regions
of protein
Hydrophilic
regions of protein
• freeze-fracturing
• microtome splits the frozen
cell along the hydrophobic
interior
• SEM used to image the cell
layers
TECHNIQUE
Extracellular
layer
Knife
Plasma membrane
Proteins
Cytoplasmic layer
RESULTS
Inside of extracellular layer
Inside of cytoplasmic layer
Plasma Membrane Fluidity
• plasma membrane also contains embedded proteins
• many lipids and proteins are also modified through the addition of carbohydrates
– glycoproteins
– glycolipids
• these 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
–
–
–
–
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).
Scientific Inquiry: Do membrane proteins move?
• Frye and Eddin – fluorescently labelled membrane proteins of
mouse and human cells and fused the cells
• production of “hybrid” cells with a homogenous mixture of proteins
proved that proteins do move within the plasma membrane
RESULTS
Membrane proteins
Mouse cell
Mixed proteins
after 1 hour
Human cell
Hybrid cell
Evolution of Differences in Membrane
Lipid Composition
• Variations in lipid composition of cell membranes of
many species appear to be adaptations to specific
environmental conditions
• Ability to change the lipid compositions in response
to temperature changes has evolved in organisms
that live where temperatures vary
© 2011 Pearson Education, Inc.
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
Fibers of extracellular matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein CYTOPLASMIC SIDE
OF MEMBRANE
Membrane Proteins and Their Functions
• two kinds of membrane proteins:
• 1. Extrinsic or Peripheral: bound
to the surface of the membrane
EXTRACELLULAR
SIDE
N-terminus
– 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
•
6 major functions of integral membrane
proteins:
– 1. transport – channel proteins &
transporters
– 2. enzymatic activity
– 3. signal transduction – receptor
proteins
– 4. cell-cell recognition – cell identity
markers
– 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
matrix (ECM)
The Role of Membrane Carbohydrates in Cell-Cell
Recognition
• cell surface carbohydrates – found on the extracellular surface of the plasma
membrane
• these carbohydrates are covalently bonded to lipids (glycolipids) or to proteins
(glycoproteins)
• on the external side of the PM - carbohydrates vary among species, individuals,
and even cell types in an individual
• often used to identify a type of cell
• can be exploited by viruses – gain entry to a host cell
HIV
Receptor
(CD4)
Co-receptor
(CCR5)
HIV can infect a cell that
has CCR5 on its surface,
as in most people.
Receptor (CD4)
but no CCR5
HIV cannot infect a cell lacking
CCR5 on its surface, as in
resistant individuals.
Plasma
membrane
Synthesis and Sidedness of Membranes
• membranes have distinct inside and outside faces
• asymmetrical distribution of proteins, lipids, and
associated carbohydrates in the plasma membrane is
determined when the membrane is built by the ER and
Golgi apparatus
Transmembrane
glycoproteins
Secretory
protein
Golgi
apparatus
Vesicle
ER
ER lumen
Glycolipid
Plasma membrane:
Cytoplasmic face
Extracellular face
Transmembrane
glycoprotein
Secreted
protein
Membrane
glycolipid
Membrane structure results in selective
permeability
• a cell must exchange materials with its surroundings
– controlled by the plasma membrane
• plasma membranes are selectively permeable
• permeability = property that determines the effectiveness of the PM as a
barrier
• permeability varies depending on the organization and characterization of the
membrane lipids and proteins
• hydrophobic (nonpolar) molecules dissolve in the lipid bilayer and pass through
the membrane rapidly
• polar molecules do not cross the membrane easily
– require transport mechanisms
– provided by transport proteins
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
Membrane Permeability and Transport
•transport across the membrane may be: passive or
active
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
-uses a transporter protein
B. Osmosis = diffusion of water from [high] to [low]
OR movement of water from [low solute] to [high solute]
-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
-the solutes are surround by a hydration
shell of water molecules
-this decreases the amount of free water
molecules available to move
Lower
concentration
of solute (sugar)
Higher
concentration
of solute
Sugar
molecule
H2O
Selectively
permeable
membrane
-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]
-EXPERIMENT: U shaped tube divided by a membrane permeable to water only
-increase the solute concentration in the right half of the tube
-decrease free water concentration on the right side also
-water rises on the side of the solutes – due to osmosis
-counteract this rise in water by applying pressure = osmotic pressure (OP)
-therefore increasing solute concentration increases osmotic pressure
-water will move toward that area to decrease this OP
Water Balance of Cells Without Walls
•
•
•
How do cells behave in solutions?
must consider solute concentration and membrane permeability
the two things combine to create = TONICITY
•
Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water
– degree to which a the concentration of a specific solute surrounding a cell causes water to enter
or leave the cell
•
•
•
Isotonic solution: Solute concentration is the same as that inside the cell = no net water
movement across the plasma membrane
Hypertonic solution: Solute concentration is greater than that inside the cell = cell loses
water
Hypotonic solution: Solute concentration is less than that inside the cell = cell gains water
Water Balance of Cells Without Walls
(a) Animal cell
H2O
Lysed
(b) Plant cell
Isotonic
solution
Hypotonic
solution
H2O
Cell wall
Turgid (normal)
H2O
Hypertonic
solution
H2O
H2O
Normal
H2O
Flaccid
Shriveled
H2O
H2O
Plasmolyzed
Osmosis
• Isotonic solution: Solute concentration is the same as that inside the cell = no
net water movement across the plasma membrane
• Hypertonic solution: Solute concentration is greater than that inside the cell =
cell loses water
• Hypotonic solution: Solute concentration is less than that inside the cell = cell
gains water
• hypertonic or hypotonic environments create osmotic problems for
organisms
• Osmoregulation = control of solute concentrations and water balance
• the protist Paramecium- hypertonic to its pond water environment
– has a contractile vacuole that gets rid of the excess water it takes on due to
osmosis
• in medicine – hypertonic IVs are given to induce the movement of water out
of a tissue
– make the blood plasma hypertonic – increased OP results
– osmosis will drive water from the tissue into the plasma – low solute to high
solute
– used as a treatment for edema
Water Balance of Cells with Walls: Plants
• cell walls also help maintain water balance
• plant cell in a hypotonic solution swells until the wall opposes
uptake
– the cell is said to be turgid (firm)
• if a plant cell and its surroundings are isotonic, there is no net
movement of water into the cell
– the cell becomes flaccid (limp) - the plant may wilt
(a) Animal cell
H2O
Lysed
(b) Plant cell
Video: Turgid Elodea
Isotonic
solution
Hypotonic
solution
H2O
Cell wall
Turgid (normal)
H2O
Hypertonic
solution
H2O
H2O
Normal
H2O
Flaccid
Shriveled
H2O
H2O
Plasmolyzed
• in plant cells in a hypertonic environment – cells lose
water
• the membrane will pull away from the wall
• usually lethal effect called plasmolysis
Video: Plasmolysis
Facilitated transport = movement of a molecule aided by a protein
- movement of a solute from [high] to [low] either:
1. through a channel protein
e.g aquaporins- for facilitated diffusion of water
e.g. Ion channels that open or close in response to a
stimulus (gated channels)
2. through a carrier protein
EXTRACELLULAR
FLUID
(a) A channel
protein
Channel protein
CYTOPLASM
Solute
Carrier proteins and Facilitated Diffusion
-solute binds to the carrier protein located on the plasma membrane
-transported across the membrane by the carrier protein’s change in shape
-no energy required – still moving high to low concentration
-but there is a limit to the amount of facilitated diffusion the cells can undergo
- has to do with the # of carrier proteins on the PM
EXTRACELLULAR
FLUID
Carrier protein
CYTOPLASM
(b) A carrier protein
Solute
Active transport uses energy to move
solutes against their gradients
• some transport proteins can move solutes against their
concentration gradients
• Active transport moves substances against their
concentration gradients
– requires energy, usually in the form of ATP
– is performed by specific proteins embedded in the membranes
called pumps
• two kinds of active transport:
– Primary
– Secondary
A. Primary Active transport
•
•
requires a protein carrier called a pump – a pump that hydrolyzes ATP (ATPase)
e.g. sodium/potassium ATPase pump : maintains a specific concentration of Na+ and K+ in and
out of the cell
–
•
•
•
•
•
•
•
http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter6/a
nimations.html#
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
Selective Permeability: Membrane Gradients
• selective permeability of the plasma membrane PLUS the action
of pumps like the Na/K ATPase 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 and chemical composition
between the inside and outside produces a potential energy =
electrochemical gradient
• because it occurs across the PM – we call this potential energy =
membrane potential
• MEMBRANE GRADIENTS ARE THE UNDERLYING REASON
DRIVING SECONDARY ACTIVE TRANSPORT
B. Secondary Active transport = ATPase pumps are used to establish a concentration gradient
of one ion - which then drives the movement of another ion or ions
-i.e. the energy stored in a concentration gradient is used to drive the transport of other
materials
-the pump that creates this gradient = electrogenic pump
-S.A.T is usually is driven by the establishment of either a Na+ or proton gradient
-movement of the additional ions occurs through a membrane protein called a co-transporter
-two kinds: symporters and anti-porters
http://highered.mcgrawhill.com/sites/00724373
16/student_view0/chapt
er6/animations.html#
diffusion
diffusion
Na pump diffusion
diffusion
e.g. Na/Ca antiporter – opposite direction for Na and Ca movement
– the Na+ concentration gradient creates potential energy which is used by an
antiporter protein
- as Na+ leaks back in through this antiporter – the potential energy is converted
into kinetic energy which drives the movement of a Ca ion against its gradient
e.g. Na/glucose symporter
-Na+ diffusion coupled to movement of glucose in the same direction
ATP

H
H

H
Proton pump
H

H


H
H

H
Sucrose-H
cotransporter
Sucrose

Diffusion of H

Sucrose
Bulk transport across the plasma membrane
occurs by exocytosis and endocytosis
• small molecules and water enter or leave the cell through the
lipid bilayer or via transport proteins
• large molecules (e.g. polysaccharides and proteins) cross the
membrane in bulk via vesicles
– this bulk transport requires energy
• two kinds of bulk transport:
– 1. Exocytosis – known as secretion
– 2. Endocytosis – known as internalization
Exocytosis
http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter6/animations.html#
• exocytosis= migration of transport vesicles to the plasma
membrane – followed by fusion and release of contents
– transport vesicles bud from the Golgi apparatus
• many secretory cells use exocytosis to export their products
Endocytosis
• endocytosis = internalization following the formation of a vesicle
– vesicles form by invaginating from the plasma membrane
• reversal of exocytosis - involving different proteins
• three types of endocytosis:
– Phagocytosis (“cellular eating”)
– Pinocytosis (“cellular drinking”)
– Receptor-mediated endocytosis
1. pinocytosis = “cell drinking”
2. phagocytosis = “cell eating”
• molecules are taken up when extracellular • cell engulfs a particle into a vacuole
called a phagosome
fluid is “gulped” into tiny pinocytic
vesicles
• fusion with a lysosome to digest
the particle
• fusion with a lysosome and digestion into
component
3. receptor-mediated endocytosis = internalization of specific substances at specific
locations in the plasma membrane
-binding of a ligand with its receptor  internalization into the cell
-internalization requires a protein called clathrin
-receptor-ligand complexes internalized into clathrin-coated pits clathrincoated vesicle
-CC vesicle fuses with endosomes – eventually fuse to the lysosome for
processing
http://sumanasinc.com/webcontent/animations/co
ntent/endocytosis.html
Extracellular components and connections between cells
help coordinate cellular activities
• most cells synthesize and secrete materials that are external
to the plasma membrane
• these extracellular materials include:
– the extracellular matrix (ECM) of animal cells
– intercellular junctions
– cell walls of plants
Cell Junctions – Joining Plasma membranes
Tight junctions prevent
fluid from moving
across a layer of cells
Tight junction
TEM
0.5 m
Tight junction
Intermediate
filaments
Desmosome
TEM
1 m
Gap
junction
Ions or small
molecules
Space
between cells
Plasma membranes
of adjacent cells
Extracellular
matrix
TEM
• neighboring cells can interact, and
communicate through direct physical
contact
• one kind of contact is through the
formation of intercellular junctions
• several types of intercellular junctions
– Tight junctions - animals cells
– Desmosomes – animal cells
– Gap junctions – animal cells
– Plasmodesmata – plants cells
0.1 m
Tight Junctions, Desmosomes, and Gap Junctions –
Intercellular Junctions in Animal Cells
1. tight junctions: anchoring junction
•
PMs are bound together by interlocking membrane proteins
– forms a very tight union – can prevent passage of water and solutes between the
two cells and between the two cells
– also gives the tissue strength
-complex of numerous proteins:
-these protein complexes interact
with the actin of the cells
cytoskeleton
Tight Junctions, Desmosomes, and Gap Junctions –
Intercellular Junctions in Animal Cells
•
•
•
•
•
2. Desmosomes: anchoring junction
fasten cells together into strong sheets
comprised of cellular adhesion molecules/CAMs and proteoglycans
adhesion plaque links to components of the cytoskeleton (keratin)
desmosomes that link the cell to the underlying basement membrane = hemidesmosome
Tight Junctions, Desmosomes, and Gap Junctions –
Intercellular Junctions in Animal Cells
•
•
•
•
Gap junctions = communicating junction
provides cytoplasmic channels between adjacent cells
made of protein complexes called connexons
capable of opening and closing and connecting the two cell interiors
Cell Walls of Plants
• cell wall = extracellular structure that distinguishes plant cells from animal cells
– prokaryotes, fungi, and some protists have cell walls
• functions:
– 1. protects the plant cell
– 2. maintains its shape
– 3. prevents excessive uptake of water
• made of cellulose fibers embedded in a mixture of polysaccharides and protein
Cell Walls of Plants
• may have multiple layers:
– primary cell wall: thin and
flexible portion of the wall
• made of cellulose and a sugar
called pectin
– middle lamella: thin layer
between primary walls of
adjacent cells
– secondary cell wall –found in
some cells
• found between the plasma
membrane and the primary cell
wall
• impregnated with lignin for
strength
Secondary
cell wall
Primary
cell wall
Middle
lamella
1 m
Central vacuole
Cytosol
Plasma membrane
Plant cell walls
Plasmodesmata
Plasmodesmata in Plant Cells
• Plasmodesmata are channels that perforate plant cell walls
– like gap junctions between two plant cells
– plasmodesma (singular)
– clusters of plasmodesmata found in pit fields
• through plasmodesmata, water and small solutes (and sometimes
proteins and RNA) can pass from cell to cell
Cell walls
Interior
of cell
Interior
of cell
0.5 m
Plasmodesmata Plasma membranes