Lecture 2: Membranes - Biophysical Properties & Membrane Bilayer Flashcards
What are the functions of biological membranes?
- Continuous, highly selective permeability barrier
- Control of the enclosed chemical environment
- Communication
- Recognition - signalling molecules (adhesion proteins and immune surveillance)
- Signal generation in response to stimuli (electrical, chemical)
What is the composition of biological membranes?
- By dry weight, 40% lipid, 60% protein and 1-10% carbohydrate
- 20% of total weight is water
What are the lipids involved in a biological membrane?
- Phospholipids
- Glycolipids
- Cholestrol
What is a phospholipid?
- Glycerol backbone with 2 fatty acid chains and a phosphate head group
What is a glycolipid?
Sugar containing lipids
What is cholestrol?
Lipid with a polar head group, rigid planar steroid ring structure and a non-polar hydrocarbon tail
What is meant by a fluid membrane?
- Dynamic environment
- Lipids move around (flexion, rotation, lateral diffusion, rare flip flop)
How does cholestrol contribute to membrane stability?
- Small polar -OH head forms hydrogen bonds with acrylic C=O group of lipid, limiting the temperature dependent movement by acting as an additional mass = making it more stable
- 4 rigid sterol plate rings interferes with phospholipid packing = increased fluidity = can allow lipid movement
- Overall extends the range of temperature over which both membrane fluidity and stability can be maintained
What is the fluid mosaic model of the cell membrane?
- describes the structure of the plasma membrane as a mosaic of components - phospholipids, cholesterol, proteins and carbohydrates - that gives the membrane a fluid character
What is the ‘head and tails’ structure of membrane lipids?
Head - C=O bond has strongly polar qualities
Tails - acyl tails with saturated or unsaturated carbon bonds
How does the ‘head and tails’ structure of membrane lipids result in their amphipathic properties?
Phospholipid Head - C=O group is polar = hydrophilic
Phospholipid tails - non-polar so = hydrophobic
How do amphipathic molecules behave in aqueous environments?
Form either micelles or bilayers as the hydrophilic heads interact with the aqueous environment and the hydrophobic tails interact with each other
How are lipid bilayers formed in aqueous environments?
- Bilayer is favored structure as it is thermodynamically driven
- Extensive hydrophobic interactions between the tails
- Charged lipid heads extend over extra and intracellular surfaces of lipid bilayer
- Stabilized by electrostatic and hydrogen bonds between hydrophilic heads and the environment
How do unsaturated fatty acids influence membrane fluidity?
- The cis C=C bond sticking out interferes the structure significantly
- Allows more movement
- Extends temperature range downwards in which the bilayer will remain fluid-like
- But at increasing temperature, membrane will be too fluid
How does cholesterol influence membrane fluidity?
- Extend membrane stability at lower temperatures
- Extend membrane stability by reducing excess fluidity at higher temperatures
- Polar OH head associates with the acyl C=O group, limiting the temperature dependent movement
- 4 rigid sterol plate rings interferes with bilayer structure, allowing more movement
- Stable but highly dynamic structure
