Membranes Flashcards
Lipids
Nonpolar
Hydrophobic
Insoluble in water
Types of Lipids
Fatty acids
Triacylglycerides
Membrane lipids
Cholesterol
Fatty Acids
Long-chain hydrocarbon carboxylic acids
-general formula=CH3(CH2)nCOO-
Amphipathic (COO- is polar)
Saturated Fatty Acids
No double bonds
Unsaturated Fatty Acids
Has double bonds
Monounsaturated Fatty Acids
1 double bond
Polyunsaturated Fatty Acids
More than 1 double bond
Where are double bonds most common?
In cis conformation
How do double bonds affect the melting point?
Decreasing the melting point by introducing kinks
Shorthand Notation for Fatty Acids
(number of carbons):(number of double bonds)(delta)^where the bonds are
How does length affect the melting point
Longer=higher melting point
Shorter=lower melting point
How does saturation affect the melting point
Saturated=higher melting point
Unsaturated=lower melting point
Greater effect on melting point than length
Triacylglycerol
Way of storing fatty acids
Very hydrophobic
Triacylglycerol Structure
3 acyl chains attached to glycerol
Acyl chains from fatty acids (ester-linked)
What type of triacylglycerols is most common?
Mixed TAGs
the melting point will be lower for those containing unsaturated fatty acids or shorter chains
Glycerophospholipids
2 acyl groups and a large polar group attached to glycerol
amphipathic
Cholesterol
Rigid, nonpolar structure
Weakly amphipathic
Found in membrane lipids
Does not form membranes alone
-associates with polar headgroups of other lipids
-nonpolar portion is found in the membrane
Lipid Interactions in water
Fatty acids from micelles when mixed in water
Membrane lipids form a bilayer when mixed in water
Dimensions of a lipid bilayer
Variable
Lipid head groups have significantly different dimensions
Acyl chains vary in length
Cholesterol is almost entirely buried in the bilayer
The fluidity of membrane lipids
The melting temperature is the temperature of its transition from an ordered crystalline to a more fluid state
Depending on the acyl chain length and unsaturation
Why is the transition temperature not sharp?
Mixture of compounds
Membranes must operate above melting point but not be completely disordered
Lipids must adjust to maintain fluidity
How do membranes maintain fluidity?
Lower temperature = more unsaturated and shorter fatty acids are incorporated
Higher temperature = more saturated and longer fatty acids are incorporated
How does cholesterol help maintain membrane fluidity?
Lower temperature = prevents packing between acyl chains
Higher temperature = decreases motion/disorder of acyl chains and increases rigidity
Why do lipids only move laterally in the bilayer?
Provides membrane fluidity
-weakly hydrophobic interactions
Why don’t lipids undergo transverse diffusion?
The significant energy barrier for desolvating a polar head to move through the nonpolar bilayer
-flipases would be needed
Types of Membrane Proteins
Integral membrane protein -hydrophobic interactions Peripheral membrane protein -electrostatic/polar -easy to separate from lipid Lipid-linked protein -lipid prosthetic group -hydrophobic anchor
Integral Membrane Proteins and Hydrophobic Amino Acids
The portion of protein connected to acyl tails must have hydrophobic amino acids side chains
What can cross bilayer by diffusion
Small, nonpolar molecules
What does the rate of diffusion depend on?
Size of molecule -smaller moves faster Concentration gradient -larger gradient increases the rate Lipid solubility -increases the diffusion rate
Passive Transport
No ATP required
If G=negative, motion is spontaneous which is passive
Active Transport
ATP required
If G=positive, energy is required which is active
What does G need to be for transport to occur?
Negative
Porins
Relatively non-specific
Water-filled pore in the center of the barrel
Most are trimers (3 pores)
Passive
Ion Channels
Highly selective
Channel formed between subunits
Selectivity depends on the size of the pore and the properties of the side chains/functional groups found there
Can be passive or active
Transporter Proteins
Carrier proteins No membrane-spanning pores Conformational change alternates openings Selective Passive or active
Uniport
Transports one type of substrate
Symport
Trasport of two molecules in the same direction
Antiport
Transports two molecules in opposite directions
Secondary Active Transport
Primary vs Secondary Active Transport
Primary = uses ATP as a source of free energy Secondary = uses ion gradient as a source of free energy
