Biology 200

2.2 The Lipid Bilayer

Features of Biological Membranes● The membrane is a bilayer; contains phospholipids; asymmetric● localizes reactions (digestion in lysosomes); regulates cell communication (receptors embedded in membranes, signals cross the barrier)● Serve as barrier to outside environment; forms cellular compartments (ionically distinct)● organized, but fluid● Control transport processes (pumps and pores in membrane)● has different permeability for different types of molecules● asymmetric
Lipids in Bilayer● Broad category of molecules that can have different structures; high hydrophobic content● Characterized by chemical properties:1) Phospholipid (phosphate group, 2 tails, forms bilayer or liposomes)2) Sterols (ring structures, OH group can be found somewhere, monolayer on water surface) Rigid Ring Structure3) Fatty acids ( 1 tail, monomer, micelles)4) Triacylglycerols ( 3 tails, emulsion with water, droplets)

Hydrophobic Effect >:(

● Water likes to form hydrogen bonds with other water molecules; energetically favourable● H2O is a polar molecule; So by putting in a hydrophobic molecule, water thus decreases in entropy→ A hydrophobic molecule forces water to reorganize into a 'cage' around any hydrophobic molecules. This would in turn, affect the water's ability/efficacy to form hydrogen bonds● when a micelle is formed, the entropy of hydrophobic molecules decrease, but the entropy of water increases
NOTE: The OVERALL system increases in entropy

Ways That Lipids Interact with Water Determines the Structure Formed

● Amphipathicity; Shape
Triacylglycerols (fat):● Forms droplets, not bilayer● triacylglycerol - 3 fatty acids esterified to a glycerol through a condensation reaction● storage form of fatty acids
Amphipathicity:● Bilayer formation requires amphipathicity→ Having both hydrophobic/hydrophilic properties● Must be able to associate with water● Must form relatively stable sheets in water
Right Shape:● Fatty acids form micelles, phospholipids form bilayers● Fatty acid are usually cone shaped; phospholipids are usually cylinder shaped
Thermodynamics of Hydrophobic Effect● minimum energy conformation (most stable) is achieved by minimizing exposure of hydrophobic groups to water● Conflicting forces stabilize the bilayer - free energy to maintain the system is minimized if the hydrophobic regions cluster together to minimize contact with water, increasing its (the water's) movement.
motion of the ocean XD

The Effects of Temperature on Lipids

● At cold temperatures, membrane bilayers freeze and turn into a more ordered crystalline-like state with low(er) fluidity, and is very fragile● As temperatures decrease, molecular motion slows down and molecules are trapped by London dispersion forces
How can a cell change its lipid composition to maintain appropriate fluidity? :● plants and fungi are more likely to do this because they cannot regulate their own body temperatures (#Unstable #GottaChangeToLive #Real #SoNotFair #WishIWereAnAnimal)● Degree of unsaturation in lipids (If unsaturated fatty acids are compressed, the “kinks” in their tails push adjacent phospholipid molecules away, which helps maintain fluidity in the membrane).● Fatty acid tail length (A shorter chain length reduces the tendency of the hydrocarbon tails to interact with one another, and cis-double bonds produce kinks in the hydrocarbon chains that make them more difficult to pack together, so that the membrane remains fluid at lower temperatures)● Amount of Sterol in the Membrane (As sterols reduce membrane fluidity, by decreasing the amount of sterols in the membrane, the plant is able to increase membrane fluidity at lower temperatures)

Fatty Acid Saturation

Higher Saturated Lipids:● More tightly packed● more Van der Waals interactions● Less fluid
Higher Unsaturated Lipids● more kinks in the fatty acid tails due to double bonded structures (see image __ ) ● more fluid
Fatty Acid Tail Length● Fatty acid tail length = 14-24 Carbon atoms ( 18-20 Carbons is more common though)● Shorter tails (reducing the tendency of the hydrocarbon tails to interact with each other) are more fluid● Phospholipids with shorter fatty acid chains have less surface area and thus have fewer Van der Waals interactions
Sterol Content● Amount of sterol in the membrane (see image __)● Cholesterol (animal) and Phytosterol (plant)● Sterols are short, rigid lipids (very stubby hehe) (see image __)● At high temperatures, the ring structure of cholesterol stiffens the cell membranes● At low temperatures, cholesterol can increase fluidity by preventing tight packing of fatty acids
NOTE: Organisms like their cell membranes to have just the right amount of fluidity (not too fluid, not too stiff or rigid) . We tend to think of fluid membranes as being good for the cell but the membranes that are too fluid can be detrimental as well 0_0.
CELL TYPE 1 VS CELL TYPE 2 DIAGRAM (see image__)

Membrane Organization

Lipid Rafts:● Micro-domains in the plasma membrane are RICH in specific types of lipids (sphingomyelin and cholesterol)● More ordered than surrounding membrane (more neat - kinda like Squilliam Fancyson vs Squidward)● Lipid rafts are thicker than other regions of the cell membrane● Less fluid than rest of cell membrane due to higher sterol content● Plasma membrane is reinforced inside the cell by association of membrane proteins with the cell cortex (see image __)● Some transmembrane proteins are anchored in place via spectral dimers (red), actin (light green), and attachment proteins (yellow and blue) network forming the cell cortex
Adhesion to ECM (extracellular matrix)● (e.g.) Integrin proteins can connect to the extracellular matrix.
Adhesion to neighbouring cells● Cell to cell adhesion molecules (cadherins) linking the plasma membranes to neuronal cells

Diffusion Barriers

Tight Junctions● tight junctions are adhesions between neighbouring epithelial cells that form "kissing points" between the two cells so nothing leaks in between the cells● Segregates tops and sides/bottoms of cells in to distinct membrane proteins● ESPECIALLY important for cells like the intestine epithelium