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Chapter 5
Fluid Mosaic Model
• Cell Membrane Structure and
Function
• Membrane is a mosaic of
– A. Phospholipids
– B. Proteins
– C. Glycoproteins and Glycolipids
– Who regulates
exchanges of
substances
between the living
and nonliving
world?
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– D. Steroids
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PHOSPHOLIPIDS
CH2
CH2
CH3
CH2
CH2
CH2
O
CH2
H3C N CH2 CH2O P O CH2 O
CH
CH3
O HC O C CH2CH2CH2CH2CH2CH2CH2 CH
A. PHOSPHOLIPID BILAYER
CH2
CH3
O
H2C O C CH2CH2CH2 CH2CH2CH2 CH2CH2 CH2CH2 CH2 CH2CH2 CH2CH2 CH2 CH3
tails
(hydrophobic)
head
(hydrophilic)
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phospholipid
extracellular fluid
(watery environment)
hydrophilic
heads
B. Membrane Proteins
a. Surface protein
b. Transmembrane protein
hydrophobic
tails
bilayer
adhesion
protein
receptor
protein
recognition
protein
hydrophilic
heads
communication
protein
passive
transporter
active transporters
cytoplasm
(watery environment)
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How Is the Structure of a
Membrane Related to Its
Function?
• 1. Phospholipids:
• 3. Glycoproteins
– Insulation. Isolates the Cell While Allowing
Communication with Its Surroundings
– Flexibility.
– Fluidity
– Cell identity and recognition
• 2. Proteins: embedded and moving within
layers of lipids
– Transport (i.e.: channel proteins, carrier proteins)
– Receptor (i.e. N.T. brain receptors)
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• 4. Steroids
– Makes membrane stronger
– Less permeable to water soluble substances
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extracellular fluid (outside)
glycoprotein
binding site
Selective Permeability
phospholipid bilayer
carbohydrate
O2, CO2,
glucose and other large, polar,
and other small, nonpolar water-soluble molecules; ions,
molecules; some water
water molecules
molecules
cholesterol
phospholipid
receptor protein
transport protein
protein filaments
recognition protein
cytoplasm (inside)
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How Do Substances Move
Across Membranes?
Membrane Crossing
Mechanisms
PASSIVE TRANPORT
a.
b.
Simple Diffusion
Facilitated diffusion
c.
Osmosis
ENERGY REQUIRING TRANSPORT
a. Active transport
b. Endocytosis
c. Exocytosis
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Simple diffusion
drop of dye
water molecule
Simple diffusion
Facilitated diffusion through a channel
lipid-soluble molecules
(O2, CO2, H2O)
(extracellular fluid)
(cytoplasm)
Some Molecules Move Across Membranes by Simple Diffusion
ions
channel
protein
Plasma Membranes Are Selectively Permeable to Diffusion of Molecules
H2O
selectively
permeable
membrane
Facilitated diffusion through a carrier
amino acids,
sugars,
small proteins
(extracellular fluid)
sugar
carrier
protein
pore
(cytoplasm)
–Other Molecules Cross the Membrane by Facilitated Diffusion,
–with the Help of Membrane Transport Proteins
–Osmosis Is the Diffusion of Water Across a
semipermeable Membrane
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selectively
permeable
membrane
sugar
molecule
10 micrometers
Isotonic solution
Hypertonic solution
Hypotonic solution
water
molecule
–Osmosis Across the Plasma Membrane Plays
–an Important Role in the Lives of Cells
ACTIVE TRANSPORT
(extracellular fluid)
1 The transport protein
binds both ATP and Ca2+.
Active Transport. It Uses Energy to
Move Molecules Against Their
Concentration Gradients
recognition
site
ATP
2 Energy from ATP changes
the shape of the transport
protein and moves the ion
across the membrane.
3 The protein releases
the ion and the remnants
of ATP (ADP and P) and
closes.
ADP
ATP
binding
site
Ca2+
P
(cytoplasm)
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Pinocytosis Pinocytosis Moves Liquids into the Cell
(extracellular fluid)
Cells Engulf Particles or Fluids by
Endocytosis
Three types of endocytosis:
1
(cytoplasm)
1 A dimple forms in the plasma membrane, which
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Pinocytosis
Pinocytosis
(extracellular fluid)
(extracellular fluid)
3
2
vesicle containing
extracellular
fluid
(cytoplasm)
(cytoplasm)
2 deepens and surrounds the extracellular fluid.
Receptor-mediated endocytosis
–Moves Specific Molecules into the Cell
nutrients
receptors
3 The membrane encloses the extracellular fluid,
forming a vesicle.
Receptor-mediated endocytosis
(extracellular fluid)
nutrients
receptors
(extracellular fluid)
1
coated pit
coated pit
(cytoplasm)
2
(cytoplasm)
1 Receptor proteins for specific molecules or complexes of
molecules are localized at coated pit sites.
2 The receptors bind the molecules and the membrane
dimples inward.
Receptor-mediated endocytosis
Receptor-mediated endocytosis
nutrients
receptors
coated vesicle
(extracellular fluid)
nutrients
receptors
(extracellular fluid)
coated pit
coated pit
3
(cytoplasm)
4
coated vesicle
3 The coated pit region of the membrane encloses the
receptor-bound molecules.
(cytoplasm)
coated vesicle
4 A vesicle ("coated vesicle") containing the bound
molecules is released into the cytoplasm.
5
(c) Phagocytosis
–Moves Large Particles into the Cell
extracellular particles
bound to receptors
coated vesicle
(extracellular fluid)
(extracellular fluid)
food particle
pseudopods
(cytoplasm)
protein coated pit
coating
plasma membrane
0.1 micrometer
1
food
vacuole
(cytoplasm)
1 The plasma membrane extends pseudopods toward an
extracellular particle (for example, food).
(c) Phagocytosis
(c) Phagocytosis
food particle
(extracellular fluid)
food particle
pseudopods
(extracellular fluid)
pseudopods
2
(cytoplasm)
food
vacuole
(cytoplasm)
2 The ends of the pseudopods fuse, encircling the particle.
3
food
vacuole
3
A vesicle called a food vacuole is formed containing the
engulfed particle.
• Exocytosis Moves Material Out of the Cell
(extracellular fluid)
plasma membrane
secreted
material
plasma membrane
Specialized Junctions
• Functions:
vesicle
– Cell attachment
• Desmosomes
• Tight junctions (leak-proof cells)
– Cell Communication
(cytoplasm)
0.2 micrometer
• Gap junctions (liver)
• Plasmodesmata (plants)
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Desmosome
Tight junction
urinary bladder
small intestine
Gap junctions
Plasmodesmata
liver
root
cells lining small
intestine
desmosome
cells lining
bladder
liver cells
plasma
membranes
(edge view)
root
cells
plasma membrane
cell wall
protein
filaments in cytoplasm
primary
cell wall
plasma
membrane
secondary
cell wall
middle
lamella
cytoplasm
–Some Cells Are Supported by Cell Walls
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