IV
Cellular Membranes
A. Membrane Structure
1. Overview of Membrane Structure
A phospholipid bilayer with associated proteins
2. Structure of the Phospholipid bilayer
Phospholipids are amphipathic, having both hydrophobic regions and hydrophilic “polar head groups”
Glycerol-based Phospholipids
• Glycerol Molecule
• Two Fatty Acid Chains
• Polar Head Group, attached via phosphate
• The fatty acid chains may be saturated or unsaturated
• Double bonds in unsaturated fatty acids may be cis or trans
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Other Lipids that contribute to the lipid bilayer are sphingosine-based lipids and cholesterol
Liposomes: Models for Studying Bilayer Structure.
• These are artificial bilayers, generally composed of pure phospholipid and water.
• In aqueous suspension, amphipathic lipids will spontaneously arrange a bilayer structure, forming the boundary of a sphere that has water on both its inside and outside.
3. Membrane Proteins
Peripheral Membrane Proteins
Integral Membrane Proteins
4. Properties of Membranes
a. Fluidity
The lipid molecules and some proteins have the ability to move laterally (side-to-side) within the membrane. They do not have the ability to randomly “flip-flop” (transverse mobility)
The degree of fluidity is influenced by the polar head group sizes, the fatty acid composition (especially by the length of the chains and the amount of unsaturation) and by the presence of cholesterol.
b. Compositional Asymmetry
The inner leaflet of a naturally occurring biological membrane will have a different composition than the outer leaflet.
Both the lipid composition and the protein composition of the outer and inner membranes are different.
Example:
Erythrocyte Membrane
(numbers are % of total lipid)
Outside Inside
Sphingomyelin 20% 5%
P. choline 25% 5%
P. ethanolamine 5% 25%
P. serine 0% 5%
c. Semipermeability
Biological membranes are semipermeable, permitting some substances to pass but not others.
B. Transport Processes across Membranes
1. Diffusion and Osmosis
Diffusion: The movement of molecules due to kinetic energy
Net Diffusion: The diffusion of a substance from a region of high concentration to a region of low concentration until an equilibrium is reached
Osmosis: The diffusion of water molecules across a semipermeable membrane
2. Simple Diffusion across a Membrane
• Movement of substances directly across a phospholipid bilayer or through an open protein channel
• Movement from high to low concentration
• No energy expenditure (e.g. ATP) from cell
3. Facilitated Diffusion
• Movement of substances across a membrane with the assistance of a carrier protein
• Movement from high > low concentration
• No energy expenditure (e.g. ATP) from cell
4. Active Transport
• Movement of substances across a membrane with the assistance of a transport protein
• Movement from low > high concentration
• Energy expenditure (e.g. ATP or ion gradients) from cell
• Active transport pumps are usually carrier proteins
5. Endocytosis
• The ingestion of large particles or macromolecules by the plasma membrane of a cell through the formation of an endocytotic vesicle
• The endocytotic vesicle is a membrane-enclosed packet coated by a protein called clathrin
• Typically mediated by cell-surface receptors that recognize the target being engulfed
• “Phagocytosis” refers to the ingestion of relatively large particles (for example, bacteria by a red blood cell)
• “Pinocytosis” refers to the ingestion of dissolved substances and fluid
6. Exocytosis
• The process by which secretory vesicles (from the Golgi apparatus) fuse with the plasma membrane to secrete large particles or macromolecules
• Secretory vesicles are also coated with clathrin
• Exocytosis and Endocytosis provide for a dynamic recycling of membrane components
C. Functions of Membranes
• Defining the outside of the cell and the organelle compartments (a barrier)
• Regulating the contents of the cell and the organelles
• Processing information: Signal Transduction
• Processing Energy
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