Biological Membranes Flashcards
Membranes
barrier surrounding cells and membranous organelles - but allow selective transport between compartments.
Receive extracellular signals which influence gene expression.
Anchorage points for structures, both inside the cell (cytoskeleton) and outside (extracellular matrix; other cells). (Lecture 6&7)
site for specialised enzyme function (e.g. mitochondrion or chloroplast).
Functions
Boundary and Permeability
Organisation and localisation
Transport processes
Nutrients
Signal
Cell-cell processing
Phospholipid bilayer
Permeable to hydrophobic molecules
Permeable to small, uncharged molecules
Very low permeability/not permeable to large, uncharged molecules
Not permeable to charged ions
Components
The lipid content is largely made up of phospholipids which are Amphipathic
This means molecules with a polar (hydrophilic) head and a non-polar (hydrophobic) tail
The chemical properties of water impact molecular interactions in cells
Hydrophilic head group
backbone (e.g. glycerol)
Hydrophobic carbon tail
Molecule is always amphipathic
Phospholipid backbone
Phospholipids constructed from a glycerol backbone are called phosphoglycerates
Examples: phosphatidylcholine (most common), phosphatidylserine, phosphatidylethanolamine….
The head group.
glycerol
Sphingosine
backbone in sphingomyelin – rich in myelin
Most phospholipids have the glycerol backbone
Sphingomyelin is the exception.
What happens if you mix water with uncharged (hydrophobic) molecules?
favorable interactions – a cage-like structure forms around it
Why do phospholipid bilayers form?
Bilayer formation is energetically favorable.
Hydrophilic head interact with water.
Hydrophobic tails are in contact with each other
Factors
Temperature
The nature of the phospholipids (Phosphatidylcholine, Phosphatidylinositol etc.)
The amounts of saturated and unsaturated fatty acids.
The presence of cholesterol or other sterols
Excitable Membranes
Excitable Membranes
The outer Carbohydrate layer
Glycoproteins - short oligosaccharide chain (3-10)
Proteoglycans – one/more polysaccharide chain (>10)
Cells have an outer sugar coating – ‘carbohydrate layer’
Protective – sugar attracts water – slimy layer that protects and helps motion
Cell-cell recognition
Sugars=Simple Carbs
Composed entirely of carbon, hydrogen and oxygen: CnH2nO
Form rings or linear strands
Monosaccharides,
disaccharides,
oligo-, poly-saccharides
Membranes also have carbohydrates on the outer surface that serve as recognition sites for other cells and molecules
Glycolipids—carbohydrate + lipid
Glycoproteins—carbohydrate + protein
Adding carbohydrate moieties (Glyco…
Membrane Asymmetry
The two leaflets of a membrane are not identical. Each may contain different types of phospholipid and a different arrangement of proteins and carbohydrates.
The two sides of the membrane may have different functions
Double bonds make a kink – influence fluidity
Double bond in unsaturated tail makes kinks in the tail
Gaps caused by kinks in plasma membrane filled with cholesterol
High cholesterol levels makes membranes more rigid.
Peripheral
Can be attached to an integral membrane protein
Attachments are temporarily:
Hydrophobic interaction
Electrostatic interaction
Non-covalent interactions
Easily removed from the cell membrane
Integrtal
Transmembrane proteins extend all the way through the phospholipid bilayer.
Permanently embedded – can only remove with detergent
They have one or more transmembrane domains:
Transmembrane domains are usually composed of a-helices
Hydrophilic amino acids are internalised
Hydrophobic amino acids are on the outside in contact with the lipid bilayer
Lipid anchored
covalently attached to lipid bilayer and are referred to as anchored membrane proteins
These lipid anchors include long-chain acyl or prenyl groups, glycosyl-phosphatidylinositol (GPI) and cholesterol.
The lipid association with proteins can be either reversible or irreversible and an individual protein may have more than one lipid anchor attached.
