Transcript Biol 178 Fall 2006 05

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
Membranes
Fig. 6.2 (TEArt)
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Phosphorylated
alcohol
G
L
Y
C
E
R
O
L
Fatty acid
Fatty acid
Polar
(hydrophilic) region
Nonpolar (hydrophobic) region
Fig. 6.3 (TEArt)
Polar
hydrophilic
heads
Nonpolar
hydrophobic
tails
Polar
hydrophilic
heads
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Fluid Mosaic Model of Cell Membrane
Outside Cell
Glycoprotein
Phospholipid
Bilayer
Carbohydrate
Cholesterol
Glycolipid
Transmembrane
proteins
Peripheral
protein Cytoplasm (inside cell)
Cell Membrane Structure
1. Phospholipid bilayer
2. Proteins
3. Carbohydrates
Attached to lipids  Glycolipids
Attached to proteins  Glycoproteins
4. Cholesterol
6
Membrane Protein Functions
Outside
Plasma
membrane
Inside
Transporter
Enzyme
Cell surface
receptor
Cell surface identity
marker
Cell adhesion
Attachment to the
cytoskeleton
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Anchoring
Proteins in the
Phospholipid Phospholipids
Bilayer
Nonpolar areas
of
protein
Polar areas
of protein
Structure of Membrane Proteins
1. Single-Pass… Anchors
2. Multi-Pass
 Channels & Pores
 Carriers
Examples of Single Pass Proteins
Receptors, anchors & enzymes
NO
Nitric oxide substrate
Anchors - Cell Surface Markers
Self-recognition
Protein receptor
Protein anchor
Unique
part of
protein
Protein enzyme
Guanylyl cyclase
Second messenger
cGMP
degradation by
phosphdiesterase
V
MHC Antigen
Smooth muscle relaxation
Fig. 6.12 (TEArt)
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Diffusion
Solute dissolves in a solvent.
Lump
ofSolutes
sugar move from a high to a low concentration.
Sugar
molecule
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Fig. 6.14 (TEArt)
Osmosis is Water Diffusion
Across a Semipermeable Membrane
Urea
molecule
Water
molecules
Direction of Water Diffusion
Hyperosmotic
solution
Shriveled cells
Isoosmotic
solution
Normal cells
Hypoosmotic
solution
Cells swell
and
eventually burst
Fig. 6.15c (TEArt)
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Water Diffusion in Plant Cells
Hyperosmotic
External
Solution
Plasmolysis
Cell body shrinks
from cell wall
Isoosmotic
External
Solution
Hypoosmotic
External
Solution
Normal turgid cell
Turgor Pressure
Maintaining Osmotic Balance
Life in a osmotic environment
1.
Extrusion
e.g. Contractile Vacuoles in Paramecium
2.
3.
Isoosmotic solutions
e.g. Blood Protein
Live with it
e.g. Turgor pressure
15
Moving Molecules into or out of Cells
- Overview of Passive & Active 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 “down” a concentration gradient
2. Small molecules transported through
A. Protein Pumps
3. Large molecules transported by vesicles
A. Endocytosis
B. Exocytosis
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
Multi-Pass
Protein
Transport
protein
Moving Molecules into or out of Cells
- Passive Transport – Example of a Channel
K+ ion channel
Selectivity filter
Outside cell
Side view
Top view
Passive transport of
1) Water-soluble molecules
2) Ions
Inside cell
K+ ion
Moving Molecules into or out of Cells
- Passive Transport – Carriers & Facilitated Diffusion
Passive transport of
1) ions
2) Sugars
3) amino acids
Outside cell
Inside cell
Examples of Facilitated Diffusion in
Red Blood Cells
1) Cl- and bicarbonate ions
2) Glucose carrier
Pores & Porin Proteins
Allow Water and Small Molecules into Cells
Pleated folds
Porins are transport channels
1.Allow movement of small molecules
Water
Ions
Organic Wastes
2003 Nobel Prize in Chemistry
Aquaporin Water Channels
Porin Protein
Aquaporins are Water Channels
Major Sites of Expression Comments
Aquaporin-0
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
How do molecules move across membranes?
1. Proteins allow transport
2. Mechanisms of movement through proteins
1. Passive Transport
 Channels, carriers & pores
 Simple Diffusion
 Facilitated Diffusion
2. Active Transport
 Molecular Transport
 Bulk Transport
Exocytosis
Endocytosis
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Active Transport – Sodium-Potassium Pump
Extracellular
PP ATP
P
A
Na+
PP
P
ATP A
Intracellular
1. Protein in membrane binds
intracellular sodium.
2. ATP phosphorylates protein
with bound sodium.
P
PP
ADP A
3. Phosphorylation causes
conformational change in
protein, allowing sodium to
leave.
K+
P
PP
A
ADP
4. Extracellular potassium
binds to exposed sites.
P
PP
ADP+Pi
A
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.
Fig. 6.19 (TEArt)
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Active Transport - Cotransport
Outside cell
Sugar
Na+
Na/K
pump
K+
Inside cell
Coupled
transport
protein
Example of Glucose Transport From Intestine to Blood
Bulk Transport Across Membranes
• Exocytosis - discharge of material from vesicles at
the cell surface
• Endocytosis - enveloping food
– phagocytosis - particulate material
– pinocytosis - liquid
– receptor-mediated - transport of specific molecules
Bulk Tranport: Exocytosis
27
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Bulk Transport: Endocytosis
Plasma
membrane
Cytoplasm
Fig. 6.16c (TEArt)
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Carrier-Mediated Endocytosis
Coated pit
Target molecule
Receptor protein
Coated
Vesicle
Example of Neurotransmitter Movement from Cell to Cell
Reuptake
transporter
END
Membranes &
Transport