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BiologySylvia S. Mader
Michael WindelspechtChapter 5
Membrane Structure and Function
Lecture Outline
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See separate FlexArt PowerPoint slides for all figures and tables pre-inserted into
PowerPoint without notes.
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Outline• 5.1 Plasma Membrane Structure and
Function
• 5.2 Passive Transport Across a Membrane
• 5.3 Active Transport Across a Membrane
• 5.4 Modification of Cell Surfaces
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5.1 Plasma Membrane Structure and Function
• The plasma membrane is common to all cells• Separates:
Internal cytoplasm from the external environment of the cell
• Phospholipid bilayer: External surface lined with hydrophilic polar heads Cytoplasmic surface lined with hydrophilic polar heads Nonpolar, hydrophobic, fatty-acid tails sandwiched in
between
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Plasma Membrane Structure and Function
• Components of the Plasma Membrane Three components:
• Lipid component referred to as phospholipid bilayer• Protein molecules
– Float around like icebergs on a sea– Membrane proteins may be peripheral or integral
» Peripheral proteins are found on the inner membrane surface
» Integral proteins are partially or wholly embedded (transmembrane) in the membrane
• Cholesterol affects the fluidity of the membrane
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Membrane Proteins
5
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hydrophobicregion
phospholipidWater outside cell
integralprotein
hydrophilicregions
Water inside cell
peripheralproteins
cholesterol
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Plasma Membrane Structure and Function
• Carbohydrate Chains Glycoproteins
• Proteins with attached carbohydrate chains Glycolipids
• Lipids with attached carbohydrate chains These carbohydrate chains exist only on the
outside of the membrane• Makes the membrane asymmetrical
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Plasma Membrane of an Animal Cell
7
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Plasma membrane
filaments of cytoskeleton Inside cell
integral protein
cholesterol
peripheral protein
glycolipid
carbohydratechain Outside cell
hydrophilicheads
phospholipidglycoprotein
extracellularMatrix (ECM)
phospholipidbilayer
hydrophobictails
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Plasma Membrane Structure and Function
• Functions of Membrane Proteins Channel Proteins:
• Allow passage of molecules through membrane via a channel in the protein
Carrier Proteins:• Combine with the substance to be transported• Assist passage of molecules through membrane
Cell Recognition Proteins:• Glycoproteins• Help the body recognize foreign substances
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Plasma Membrane Structure and Function
• Functions of Membrane Proteins (continued) Receptor Proteins:
• Bind with specific molecules• Allow a cell to respond to signals from other cells
Enzymatic Proteins:• Carry out metabolic reactions directly
Junction Proteins:• Attach adjacent cells
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Channel Protein:Allows a particularmolecule or ion tocross the plasmamembrane freely.Cystic fibrosis, aninherited disorder,is caused by afaulty chloride (Cl–)channel; a thickmucus collects inairways and inpancreatic and liver ducts.
a.
Membrane Protein Diversity
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b.
Carrier Protein:Selectively interactswith a specificmolecule or ion sothat it can cross theplasma membrane.The inability of somepersons to useenergy for sodium-potassium (Na+–K+)transport has beensuggested as thecause of their obesity.
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Cell RecognitionProtein:The MHC (majorhistocompatibilitycomplex) glycoproteinsare different for eachperson, so organtransplants are difficultto achieve. Cells withforeign MHCglycoproteins areattacked by white bloodcells responsible forimmunity.
c.
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Receptor Protein:Is shaped in such away that a specificmolecule can bind toit. Pygmies are short,not because they donot produce enoughgrowth hormone, butbecause their plasmamembrane growthhormone receptorsare faulty and cannotinteract with growthhormone.
d.
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Enzymatic Protein:Catalyzes a specificreaction. The membraneprotein, adenylatecyclase, is involved inATP metabolism. Cholerabacteria release a toxinthat interferes with theproper functioning ofadenylate cyclase;sodium (Na+) and waterleave intestinal cells, andthe individual may diefrom severe diarrhea.
e.
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Junction Proteins:Tight junctions joincells so that a tissuecan fulfill a function, aswhen a tissue pinchesoff the neural tubeduring development.Without thiscooperation betweencells, an animalembryo would have nonervous system.
f.
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Channel Protein:Allows a particularmolecule or ion tocross the plasmamembrane freely.Cystic fibrosis, aninherited disorder,is caused by afaulty chloride (Cl–)channel; a thickmucus collects inairways and inpancreatic andliver ducts.
Carrier Protein:Selectively interactswith a specificmolecule or ion sothat it can cross theplasma membrane.The inability of somepersons to useenergy for sodium-potassium (Na+–K+)transport has beensuggested as thecause of their obesity.
Receptor Protein:Is shaped in such away that a specificmolecule can bind toit. Pygmies are short,not because they donot produce enoughgrowth hormone, butbecause their plasmamembrane growthhormone receptorsare faulty and cannotinteract with growthhormone.
Enzymatic Protein:Catalyzes a specificreaction. The membraneprotein, adenylatecyclase, is involved inATP metabolism. Cholerabacteria release a toxinthat interferes with theproper functioning ofadenylate cyclase;sodium (Na+) and waterleave intestinal cells, andthe individual may diefrom severe diarrhea.
Junction Proteins:Tight junctions joincells so that a tissuecan fulfill a function, aswhen a tissue pinchesoff the neural tubeduring development.Without thiscooperation betweencells, an animalembryo would have nonervous system.
Cell RecognitionProtein:The MHC (majorhistocompatibilitycomplex) glycoproteinsare different for eachperson, so organtransplants are difficultto achieve. Cells withforeign MHCglycoproteins areattacked by white bloodcells responsible forimmunity.
a. b.
d. e.
c.
f.
Membrane Protein Diversity
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How Cells Talk to One Another
• Signaling molecules serve as chemical messengers allowing cells to communicate with one another
Cell receptors bind to specific signaling molecules Once the signaling molecule and the cell receptor
bind a cascade of events occurs that elicits a cellular response
• Signal transduction pathway
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Cell Signaling
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Cellularresponse:
Altered shapeor movementof cell
Alteredmetabolismor cellularfunction
Altered geneexpressionand the typesand amountof proteinsproduced
generegulatory
protein
Nucleus
b.
Cytoplasm
unactivatedreceptorprotein
Nuclearenvelope
enzyme
structuralproteinreceptor
activation
signalingmolecule
Targetedprotein:plasma
membrane
newborna. egg embryo
Left: © Anatomical Travelogue/Photo Researchers, Inc.; Middle: © Neil Harding/Stone/Getty Images; Right: © Photodisc Collection/Getty RF
2.Transduction pathway: Series of relay proteins that ends when a protein is activated.
3. Response: Targeted protein(s) bring about a cellular response.
1. Receptor: Binds to a signaling molecule, becomes activated and initiates a transduction pathway.
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Plasma Membrane Structure and Function
• Permeability of the Plasma Membrane The plasma membrane is selectively permeable
• Allows some substances to move across the membrane• Inhibits passage of other molecules
Small, non-charged molecules (CO2, O2, glycerol, alcohol) freely cross the membrane by passing through the phospholipid bilayer
• These molecules follow their concentration gradient– Move from an area of high concentration to an area of
low concentration.
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Plasma Membrane Structure and Function
• Permeability of the Plasma Membrane Water moves across the plasma membrane
• Specialized proteins termed aquaporins speed up water transport across the membrane
The movement of ions and polar molecules across the membrane is often assisted by carrier proteins
Some molecules must move against their concentration gradient with the expenditure of energy
• Active transport Large particles enter or exit the cell via bulk transport
• Exocytosis• Endocytosis
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How Molecules Cross the Plasma Membrane
21
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water inside cell phospholipidmolecule
protein
water outside cell
nonpolar,hydrophobic core
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How Molecules Cross the Plasma Membrane
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+
–+
water inside cell phospholipidmolecule
protein
water outside cellcharged moleculesand ions–
nonpolar,hydrophobic core
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How Molecules Cross the Plasma Membrane
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+
–+
water inside cell phospholipidmolecule
protein
charged moleculesand ions
H2O
–
nonpolar,hydrophobic core
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How Molecules Cross the Plasma Membrane
24
+
–+
water inside cell phospholipidmolecule
protein
nonchargedmolecules
water outside cellcharged moleculesand ions
H2O
–
nonpolar,hydrophobic core
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How Molecules Cross the Plasma Membrane
25
+
–+
water inside cell phospholipidmolecule
protein
macromolecule
nonchargedmolecules
water outside cellcharged moleculesand ions
H2O
–
nonpolar,hydrophobic core
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Passage of Molecules Into and out of the Cell
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5.2 Passive Transport Across a Membrane
• A solution consists of: A solvent (liquid), and A solute (dissolved solid)
• Diffusion Net movement of molecules down a concentration
gradient Molecules move both ways along gradient, but net
movement is from high to low concentration Equilibrium:
• When NET movement stops• Solute concentration is uniform – no gradient
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Process of Diffusion
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crystaldye
a. Crystal of dye is placed in water
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Process of Diffusion
29
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time
crystaldye
a. Crystal of dye is placed in water b. Diffusion of water and dye molecules
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Process of Diffusion
30
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time time
crystaldye
a. Crystal of dye is placed in water b. Diffusion of water and dye molecules c. Equal distribution of molecules results
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Gas Exchange in Lungs
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O2
oxygen
O2
O2
O2
O2
O2
O2O2
O2 O2
O2
O2O2
O2
highO2
concentrationlowO2
concentration
bronchiole
capillaryalveolus
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Passive Transport Across a Membrane
• Osmosis: Special case of diffusion Focuses on solvent (water) movement rather than
solute Diffusion of water across a selectively permeable
membrane• Solute concentration on one side is high, but water
concentration is low• Solute concentration on other side is low, but water
concentration is high Water can diffuse both ways across membrane but
the solute cannot Net movement of water is toward low water (high
solute) concentration• Osmotic pressure is the pressure that develops
due to osmosis
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Osmosis Demonstration
33
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a.
less water (higherpercentage of solute)
more water (lowerpercentage of solute)
10%
5%
<10%
>5%
solute
differentiallypermeablemembrane
water
b.
c.
less water (higherpercentage of solute)
more water (lowerpercentage of solute)
beaker
thistletube
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Passive Transport Across a Membrane
• Isotonic Solutions Solute and water concentrations are equal on
both sides of membrane No net gain or loss of water by the cell
• Hypotonic Solutions Concentration of solute in the solution is lower
than inside the cell Cells placed in a hypotonic solution will swell
• Causes turgor pressure in plants• May cause animal cells to lyse (rupture)
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Passive Transport Across a Membrane
• Hypertonic Solutions Concentration of solute is higher in the
solution than inside the cell Cells placed in a hypertonic solution will
shrink • Crenation in animal cells• Plasmolysis in plant cells
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Osmosis in Animal and Plant Cells
36
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In a hypertonic solution, watermainly leaves the cell, whichshrivels (crenation).
In a hypertonic solution, vacuoleslose water, the cytoplasm shrinks(plasmolysis), and chloroplastsare seen in the center of the cell.
In a hypotonic solution, vacuolesfill with water, turgor pressuredevelops, and chloroplasts areseen next to the cell wall.
In an isotonic solution, there is nonet movement of water.
In an isotonic solution, there is no netmovement of water.
In a hypotonic solution, watermainly enters the cell, which mayburst (lysis).
plasmamembrane
Animalcells
nucleus
Plantcells
centralvacuole
chloroplast
nucleus
cellwall
plasmamembrane
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Passive Transport Across a Membrane
Facilitated Transport• Movement of molecules that cannot pass directly
through the membrane lipids• These molecules must combine with carrier
proteins to move across the membrane• Follow concentration gradient, moving from high
concentration to low concentration
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solute
Outside
Inside
plasma membrane
carrier protein
Facilitated Transport
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solute
Outside
Inside
plasmamembrane
carrier protein
Facilitated Transport
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solute
Outside
Inside
plasma membrane
carrier protein
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Facilitated Transport
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solute
Outside
Inside
plasma membrane
carrier protein
Facilitated Transport
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5.3 Active Transport Across a Membrane
Active Transport• The movement of molecules against their
concentration gradient– Movement from low to high concentration
• Movement is facilitated by carrier proteins • Requires the expenditure of energy in the form of
ATP• Ex: sodium-potassium pump
– Uses ATP to move sodium ions out of the cells and potassium ions into the cell against their concentration gradients.
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The Sodium-Potassium Pump
43
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carrierprotein
1. Carrier has a shape that allowsit to take up 3 Na+
Outside
Inside
K+
K+
Na+
K+
K+
Na+
Na+ Na+
Na+
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The Sodium-Potassium Pump
44
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carrier
protein
1. Carrier has a shape that allowsit to take up 3 Na+.
2. ATP is split, and phosphategroup attaches to carrier
Outside
Inside
ATP
K+
P
Na+
Na+
K+
K+
K+K+
K+
Na+Na+ Na+
Na+
K+
K+
Na+
Na+
Na+
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The Sodium-Potassium Pump
45
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carrierprotein
1. Carrier has a shape that allowsit to take up 3 Na+.
3. Change in shape results andcauses carrier to release 3 Na+
outside the cell.
Outside
Inside
ATP
K+
K+
K+
P
P
Na+
Na+
Na+
Na+
K+
K+
K+
Na+
Na+ Na +
K+ K+
K+
Na+
Na+
Na+
Na+
2. ATP is split, and phosphategroup attaches to carrier
K+
Na+Na+
K+
K+
Na +
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The Sodium-Potassium Pump
46
carrierprotein
1. Carrier has a shape that allowsit to take up 3 Na+.
4. Carrier has a shape thatallows it to take up 2K+.
3. Change in shape results andcauses carrier to release 3 Na+
outside the cell.
Outside
Inside
ATP
K+
K+
K+
K+
K+
K+K+
K+
P
P
P
Na+Na+
Na+
Na+
Na+
Na+
Na+
K+
K+
K+
Na+
Na+ Na +
Na+
Na+
Na+
Na+
Na +
Na+
K+K+
K+
2. ATP is split, and phosphategroup attaches to carrier.
K+Na+Na+
K+
Na+
Na +
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The Sodium-Potassium Pump
47
carrierprotein
1. Carrier has a shape that allowsit to take up 3 Na+.
4. Carrier has a shape thatallows it to take up 2 K+.
2. ATP is split, and phosphategroup attaches to carrier.
3. Change in shape results andcauses carrier to release 3 Na+
outside the cell.
5. Phosphate group is releasedfrom carrier.
Outside
Inside
ATP
K+
K+
K+
K+P
P
P
P
Na+
Na+
Na+
Na+ Na+
Na +
Na+
Na+
Na +
Na+
Na+
K+
K+K+
Na+
Na+ Na +
Na+
K+K+
K+
Na+
Na+
Na+
Na+
K+K+
K+
Na+
K+
K+
K+
Na+ Na+
K+
K+
Na+Na+
K+
K+
Na+
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The Sodium-Potassium Pump
48
carrierprotein
1. Carrier has a shape that allowsit to take up 3 Na+.
4. Carrier has a shape thatallows it to take up 2 K+.
2. ATP is split, and phosphategroup attaches to carrier.
3. Change in shape results andcauses carrier to release 3 Na+
outside the cell.
5. Phosphate group is releasedfrom carrier.
Outside
Inside
ATP
K+
K+
K+
K+P
P
P
P
Na+
Na+
Na+
Na+ Na+
Na +
Na+
Na+
Na +
Na+
Na+
K+
K+K+
Na+
Na+ Na +
Na+
K+K+
K+
Na+
Na+
Na+
Na+
K+K+
K+
Na+
K+
K+
K+
Na+ Na+
K+
K+
Na+Na+
K+
K+
Na +
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
6. Change in shape results andcauses carrier to release 2K+
inside the cell.
K+
K
K+Na+
Na+
Na +
Na+ Na+
K+
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Active Transport Across a Membrane
• Macromolecules are transported into or out of the cell inside vesicles via bulk transport Exocytosis – Vesicles fuse with plasma membrane and
secrete contents Endocytosis – Cells engulf substances into a pouch
which becomes a vesicle• Phagocytosis – Large, solid material is taken in by endocytosis• Pinocytosis – Vesicles form around a liquid or very small
particles• Receptor-Mediated Endocytosis– Specific form of pinocytosis
using receptor proteins and a coated pit
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Exocytosis
50
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OutsidePlasma membrane
Insidesecretoryvesicle
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Three Methods of Endocytosis
51
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pseudopod
paramecium
vacuoleforming
vesiclesforming
coatedpit
coatedvesicle
solute
solute
a. Phagocytosis
b. Pinocytosis
vacuole
coated vesicle
plasma membrane
receptor protein
coated pit
c. Receptor-mediated endocytosis
vesicle
0.5 μm
399.9 μm
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5.4 Modifications of Cell Surfaces
• Cell Surfaces in Animals Extracellular Matrix (ECM)
• Meshwork of proteins and polysaccharides in close connection with the cell that produced them
– Collagen – resists stretching
– Elastin – provides resilience to the ECM
– Integrin – play role in cell signaling
– Proteoglycans – regulate passage of material through the ECM to the plasma membrane
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Animal Cell Extracellular Matrix
53
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collagenproteoglycan
actin filament
fibronectin
elastin
integrin
Outside (extracellular matrix)
Inside (cytoplasm)
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Modifications of Cell Surfaces
• Cell Surfaces in Animals Junctions Between Cells
• Adhesion Junctions - Intercellular filaments between cells
– Desmosomes – internal cytoplasmic plaques
– Tight Junctions – form impermeable barriers
• Gap Junctions– Plasma membrane channels are joined (allows
communication)
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Junctions Between Cells of the Intestinal Wall
55
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plasmamembranescytoplasmic
plaque
Filamentsofcytoskeleton
adhesionproteins
intercellularspace
a. Adhesion junction
b. Tight junction
c. Gap junction
plasmamembranes
light junctionproteins
intercellularspace
plasmamembranes
intercellularspace
membranechannels
a: From Douglas E. Kelly, J. Cell Biol. 28 (1966): 51. Reproduced by copyright permission of The Rockefeller University Press; b: © David M. Phillips/Visuals Unlimited;c: Courtesy Camillo Peracchia, M.D.
20 nm
50 nm
100 nm
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Modifications of Cell Surfaces • Plant Cell Walls
Plants have a freely permeable cell wall, with cellulose as the main component
• Plasmodesmata penetrate the cell wall• Each contains a strand of cytoplasm• Allow passage of material between cells
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Plasmodesmata
57
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cell wall
plasmodesmata
cell wall
Cell 1 Cell 2
plasmamembrane
cell wall cell wall
cytoplasm
plasmamembrane
cytoplasm
middle lamella
plasmodesmata
0.3mm
© E.H. Newcomb/Biological Photo Service