Membranes
Outline - Membranes
1. Fluid Mosaic Model of Membrane Structure
2. Membrane Proteins
1. Kinds of membrane proteins
2. Membrane protein structure
Single pass
Multi-pass: Channels, Pores & Carriers
3. Transport Mechanisms
Passive: Diffusion & Facilitated Diffusion
Active: Molecular & Bulk
Fig. 6.2 (TEArt)
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Phosphorylated
alcohol
G
L
Y
C
E
R
O
L
Fig. 6.3 (TEArt)
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Fatty acid
Fatty acid
Polar
hydrophilic
heads
Nonpolar
hydrophobic
tails
Polar
(hydrophilic) region
Nonpolar (hydrophobic) region
Polar
hydrophilic
heads
1
Fluid Mosaic Model of Cell Membrane
Cell Membrane Structure
Outside Cell
Glycoprotein
Phospholipid
Bilayer
Glycolipid
Carbohydrate
Cholesterol
Transmembrane
proteins
1. Phospholipid bilayer
2. Proteins
3. Carbohydrates
Attached to lipids Æ Glycolipids
Attached to proteins Æ Glycoproteins
4. Cholesterol
Peripheral
protein Cytoplasm (inside cell)
6
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Membrane Protein Functions
Outside
Plasma
membrane
Anchoring
Proteins in the
Phospholipid Phospholipids
Bilayer
Inside
Transporter
Cell surface identity
marker
Enzyme
Cell adhesion
Cell surface
receptor
Nonpolar areas
of
protein
Polar areas
of protein
Attachment to the
cytoskeleton
2
Fig. 6.12 (TEArt)
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Structure of Membrane Proteins
Diffusion
Solute dissolves in a solvent.
Lump
ofSolutes
sugar move from a high to a low concentration.
Sugar
molecule
1. Single-Pass… Anchors
2. Multi-Pass
¾ Channels & Pores
¾ Carriers
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Fig. 6.14 (TEArt)
Osmosis is Water Diffusion
Across a Semipermeable Membrane
Direction of Water Diffusion
External environment of a cell can vary
Isoosmotic
Hyperosmotic
Hypoosmotic
solution
solution
solution
Water diffuses out
Solute
molecule
Water
molecules
Shriveled cells
Water diffuses out
and in = equilibrium
Normal cells
Water diffuses in
Cells swell & burst
3
Fig. 6.15c (TEArt)
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Water Diffusion in Plant Cells
Hyperosmotic
External
Solution
Isoosmotic
External
Solution
Hypoosmotic
External
Solution
Maintaining Osmotic Balance
Life in a osmotic environment
1. Extrusion
e.g. Contractile Vacuoles in Paramecium
2. Isoosmotic solutions
e.g. Blood Protein
Plasmolysis
Cell body shrinks
from cell wall
Normal cell
Turgor Pressure
Moving Molecules into or out of Cells
- Overview of Types of Transport
I. Passive Transport
1. Always “down” a concentration gradient
2. Always involves proteins called
A. Channels
B. Carriers
C. Pores… “porins”
II. Active Transport
1. Always “up” a concentration gradient
2. Small molecules transported through
A. Protein Pumps
3. Large molecules transported by vesicles
A. Endocytosis
B. Exocytosis
3. Live with it
e.g. Turgor pressure
Moving Molecules into or out of Cells
- Passive Transport
1. Multi-pass proteins create openings in the membrane
Solute
molecule
Passive Transport
1. Channels
2. Carriers
3. Pores
Multi-Pass
Protein
Transport
protein
4
Moving Molecules into or out of Cells
- Passive Transport –Channels
Moving Molecules into or out of Cells
- Passive Transport – Carriers
Passive transport of
1) ions
2) Sugars
3) amino acids
K+ ion channel
Selectivity filter
Facilitated Diffusion in Red Blood Cells
1) Cl- and bicarbonate ions
2) Glucose carrier
Outside cell
Outside cell
Side view
Top view
Inside cell
Passive transport of
1) Water-soluble molecules
2) Ions
K+
Inside cell
ion
Moving Molecules into or out of Cells
- Passive Transport – Pores
Aquaporins are Water Channels
Major Sites of Expression Comments
Aquaporin-0
Porin Protein
Pleated folds
Porins are transport channels
1.Allow movement of small molecules
Water
Ions
Organic Wastes
2003 Nobel Prize in Chemistry
Aquaporin Water Channels
Aquaporin-1
Aquaporin-2
Aquaporin-3 *
Aquaporin-4
Aquaporin-5
Eye: lens fiber cells
Fluid balance within the lens
Red blood cells
Osmotic protection
Kidney: proximal tubule
Concentration of urine
Eye: ciliary epithelium
Production of aqueous humor
Brain: choriod plexus
Production of cerebrospinal fluid
Lung: alveolar epithelial cells
Alveolar hydration state
Kidney: collecting ducts
Mediates antidiuretic hormone activity
Kidney: collecting ducts
Reabsorbtion of water into blood
Trachea: epithelial cells
Secretion of water into trachea
Kidney: collecting ducts
Reabsorbtion of water
Brain: ependymal cells
CSF fluid balance
Brain: hypothalamus
Osmosensing function?
Lung: bronchial epithelium
Bronchial fluid secretion
Salivary glands
Production of saliva
Lacrimal glands
Production of tears
5
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Example: Active Transport – Sodium-Potassium Pump
How do molecules move across membranes?
1. Proteins allow transport
2. Mechanisms of movement through proteins
1. Passive Transport – “down” concentration gradient
¾ Channels, carriers & pores
¾ Diffusion
Simple
Facilitated
2. Active Transport – “up” concentration gradient
¾ Molecular Transport
¾ Bulk Transport
Exocytosis
Endocytosis
Extracellular
Na+
Intracellular
1. Protein in membrane binds
intracellular sodium.
2. ATP phosphorylates protein
with bound sodium.
K+
P
PP
ADP A
3. Phosphorylation causes
conformational change in
protein, allowing sodium to
leave.
P
PP
A
ADP
4. Extracellular potassium
binds to exposed sites.
Fig. 6.19 (TEArt)
PP
P
ATP A
PP ATP
P
A
Animation
P
PP
A
ADP+Pi
5. Binding of potassium causes
dephosphorylation of protein.
PP ATP
P
A
6. Dephosphorylation of
protein triggers change back
to original conformation,
potassium moves into cell,
and the cycle repeats.
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Active Transport - Cotransport
Example of Glucose Transport From Intestine to Blood
Outside cell
Sugar
Na+
Na/K
pump
K+
Inside cell
Coupled
transport
protein
Animation
6
Bulk Transport Across Membranes
Bulk Tranport: Exocytosis
• Exocytosis - discharge of material from vesicles at
the cell surface
• Endocytosis - enveloping food
– phagocytosis - particulate material
– pinocytosis - liquid
– receptor-mediated - transport specific molecules
Animation
26
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Bulk Transport: Endocytosis
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Carrier-Mediated Endocytosis
Coated pit
Clathrin
Receptor protein
Coated vesicle
Plasma
membrane
Cytoplasm
7
Example of Neurotransmitter Movement from Cell to Cell
END
Membranes &
Transport
Reuptake
transporter
8