Outline - Membranes
1. Fluid Mosaic Model of Membrane
Structure
2. Membrane Proteins
1. Kinds of membrane proteins
2. Membrane protein structure
3. Transport Mechanisms
Passive: Diffusion & Facilitated Diffusion
Active: Molecular & Bulk
Plasma Membrane Function
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Plasma Membrane Properties and Structure
Properties:
No Free Ends
Internal space
Fluid Mosaic Model of Membrane Structure
Fluid = Phospholipid bilayer
Mosaic = Embedded Proteins:
Transport proteins: channels and carriers
Receptor Proteins: gates, triggers
Recognition Proteins: ID Tags
Membranes
Gate Keeper
Regulation
Communication
Transport
Selectively permeable
Elastic
Protection
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Fluid Mosaic Model of Cell Membrane
Outside Cell
Glycoprotein
Polar
hydrophilic
heads
Phospholipid
Bilayer
Glycolipid
Carbohydrate
Nonpolar
hydrophobic
tails
Polar
hydrophilic
heads
Cholesterol
Transmembrane
proteins
Peripheral
protein Cytoplasm (inside cell)
Cell Membrane Structure
Summary
Six Functions of Membrane Proteins
Outside
1. Phospholipid bilayer
Plasma
membrane
2. Proteins
Transmembrane
Interior
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
4. Cholesterol
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Anchoring
Proteins in the
Phospholipid Phospholipids
Bilayer
Types of Transport
• Passive Transport
Follows concentration gradient
Does not require energy
Direct or via channels
Nonpolar areas
of
protein
Polar areas
of protein
Examples: Diffusion, Facilitated Diffusion and Osmosis
• Active Transport:
Against concentration gradient
Requires energy
Bulk Transport
• Exocytosis and Endocytosis
Fig. 6.12 (TEArt)
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”
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Diffusion
Solute dissolves in a solvent.
Lump
ofSolutes
sugar move from a high to a low concentration.
Sugar
molecule
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
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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
Isotonic
Hypertonic
Hypotonic
solution
solution
solution
Water diffuses out
Solute
molecule
Fig. 6.15c (TEArt)
Water
molecules
Shriveled cells
Water diffuses out
and in = equilibrium
Normal cells
Water diffuses in
Cells swell & burst
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Water Diffusion in Plant Cells
Hypertonic
External
Solution
Isotonic
External
Solution
Hypotonic
External
Solution
Maintaining Osmotic Balance
Life in a osmotic environment
1. Extrusion
e.g. Contractile Vacuoles in Paramecium
2. Isotonic solutions
e.g. Blood Protein
Plasmolysis
Cell body shrinks
from cell wall
Normal cell
Turgor Pressure
3. Live with it
e.g. Turgor pressure
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Moving Molecules into or out of Cells
Passive Transport
1. Multi-pass proteins create openings in the membrane
Selectivity filter
Outside cell
Side view
Multi-Pass
Protein
Transport
protein
Moving Molecules into or out of Cells
Passive Transport – Carriers
Outside cell
K+ ion channel
Solute
molecule
Passive Transport
1. Channels
2. Carriers
3. Pores
Passive transport of
1) ions
2) Sugars
3) amino acids
Moving Molecules into or out of Cells
- Passive Transport – Channels
Facilitated Diffusion in Red Blood Cells
1) Cl- and bicarbonate ions
2) Glucose carrier
Top view
Passive transport of
1) Water-soluble molecules
2) Ions
Inside cell
K+
ion
Moving Molecules into or out of Cells
Passive Transport – Pores
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
Inside cell
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Example: Active Transport – Sodium-Potassium Pump
Extracellular
Fig. 6.19 (TEArt)
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Active Transport - Cotransport
Outside cell
Sugar
Na+
PP
P
ATP A
PP ATP
P
A
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.
Na/K
pump
Coupled
transport
protein
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.
Bulk Transport Across Membranes
K+
Inside cell
Animation
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
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Bulk Transport: Endocytosis
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Carrier-Mediated Endocytosis
Coated pit
Clathrin
Receptor protein
Coated vesicle
Plasma
membrane
Cytoplasm
END
Membranes &
Transport
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