Membranes
Outline - Membranes
Membrane Phospholipids
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
Phosphorylated
alcohol
3. Transport Mechanisms
Passive Transport: Diffusion & Facilitated Diffusion
Active Transport: Molecular & Bulk
Polar
(hydrophilic) region
Fluid Mosaic Model of Cell Membrane
Glycoprotein
Phospholipid
Bilayer
Carbohydrate
1. Phospholipid bilayer
Glycolipid
Fatty acid
Fatty acid
Nonpolar (hydrophobic) region
Membrane Protein Functions
Composition of Cell Membranes
Outside Cell
G
L
Y
C
E
R
O
L
2. Proteins
Transmembrane
Peripheral - Interior
Outside
Plasma
membrane
Inside
Transporter
Enzyme
Cell surface
receptor
Cell surface identity
marker
Cell adhesion
Attachment to the
cytoskeleton
3. Carbohydrates
Attached to lipids Æ Glycolipids
Attached to proteins Æ Glycoproteins
Cholesterol
Transmembrane
proteins
Peripheral
protein Cytoplasm (inside cell)
4. Cholesterol
1
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Structure of Membrane Proteins
Anchoring
Proteins in the
Phospholipid Phospholipids
Bilayer
Nonpolar areas
of
protein
Moving Molecules into or out of Cells
PROTEINS have a key role in transport across membranes
I. Passive Transport
1. Always “down” a concentration gradient
1. Single-Pass… Anchors
Polar areas
of protein
2. Always involves proteins called
A. Channels
B. Carriers
C. Pores… “porins”
2. Multi-Pass
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
¾ Channels
¾ Pores
¾ Carriers
Moving Molecules into or out of Cells
Passive Transport
1. Multi-pass proteins create openings in the membrane
Moving Molecules into or out of Cells
- Passive Transport – Channels
Passive transport of
1) ions
2) Sugars
3) amino acids
K+ ion channel
Solute
molecule
Passive Transport
1. Channels
2. Carriers
3. Pores
Moving Molecules into or out of Cells
Passive Transport – Carriers
Selectivity filter
Facilitated Diffusion in Red Blood Cells
1) Cl- and bicarbonate ions
2) Glucose carrier
Outside cell
Outside cell
Side view
Multi-Pass
Protein
Transport
protein
Top view
Passive transport of
1) Water-soluble molecules
2) Ions
Inside cell
K+
Inside cell
ion
2
Moving Molecules into or out of Cells
Passive Transport – Pores
Aquaporins are Water Channels
Aquaporin-0
Porin Protein
Pleated folds
How do molecules move across membranes?
Major Sites of Expression Comments
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
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Example: Active Transport – Sodium-Potassium Pump
Moving Molecules into or out of Cells
Extracellular
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
Fig. 6.19 (TEArt)
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Active Transport - Cotransport
Outside cell
Active transport occurs against a concentration gradient.
Sugar
Na+
Active Transport proteins that move molecules = Pumps
Transport
protein
PP
P
ATP
PP ATP
P
A
Na+
P
A
Intracellular
1. Protein in membrane binds
intracellular sodium.
2. ATP phosphorylates protein
with bound sodium.
K+
Solute
1 Solute binding
ATP
P
ADP
2 Phosphorylation
Protein
changes shape
3 Transport
P
Phosphate
detaches
PP
ADP A
3. Phosphorylation causes
conformational change in
protein, allowing sodium to
leave.
Na/K
pump
Coupled
transport
protein
P
4 Protein reversion
P
PP
A
ADP
4. Extracellular potassium
binds to exposed sites.
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.
K+
Inside cell
Animation
3
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Bulk Transport Across Membranes
Bulk Tranport: Exocytosis
Bulk Transport: Endocytosis
• Exocytosis - discharge of material from vesicles at
the cell surface
• Endocytosis - enveloping food
Plasma
membrane
– phagocytosis - particulate material
– pinocytosis - liquid
– receptor-mediated - transport specific molecules
Cytoplasm
Animation
20
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Glucose Transport From Intestine to Blood
Examples of Transport
Carrier-Mediated Endocytosis
Coated pit
Clathrin
Receptor protein
Coated vesicle
4
Neurotransmitter Movement from Cell to Cell
END
Membranes &
Transport
Reuptake
transporter
5