Membrane Structure and
Function
Ch 5
Gary Bates, PhD
Outline
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Fluid Mosaic Model
Structure and Function
Passive transport
Active transport
Fluid mosaic
• Fluid mosaic– Phospholipid bilayer with proteins partially or
completely embedded
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Structure
• Phospholipid bilayer
– Hydrophobic tails
– Hydrophillic heads
Proteins
• Embedded-integral
• Associated with the
membrane-peripheral
Protein functions
• Channel
– makes a pathway into cell
• Carrier
– actively carriers molecules
• Cell Recognition
– id substances
• Receptor
– binds with substance conformational change brings into cell
• Enzymatic
– Breaks down or joins together substances
• Cellular joining
– Allows cells to connect
• Signal transduction
– Starts signal pathway to lead to a response
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Crossing Plasma Membrane
Transport into and out of cell
• Transport
– active
• requires ATP
• Against gradient Lo -> Hi
– passive
• No ATP requirement
• With gradient Hi-> Lo
Passive transport
• Diffusion is movement of molecules from
Hi concentration to lo concentration until
equilibrium is reached
• Osmosis is diffusion of water across a
semi-permeable membrane
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Osmosis
• Creates osmotic pressure
• Greater osmotic pressure the greater chance
that water will diffuse from that direction
• Isotonic
= in and out causes equilibrium
• Hypotonic
lower in than out causes swelling to lysis
• Hypertonic
Lower out than in causes shrinkage to plasmolysis
Active transport
Carrier proteins bind with certain molecules which causes a
conformational change which brings the molecule into the cell.
Molecule can follow the gradient (passive) facilitative or be moved
against the gradient (active)
Membrane Assisted Transport
• Exocytosis- Vesicles fuse with plasma
membrane as secretion occurs
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Membrane Assisted Transport
– Endocytosis - Cells
take in substances by
vesicle formation.
• Phagocytosis - Large,
solid material.
• Pinocytosis - Liquid or
small, solid particles.
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