Lipids in Cell Membranes Flashcards
What is a lipid ?
Have low solubility in water
- Phospholipids
- Fats
- Sterols e.g. cholesterol
- (Some) Vitamins
What do lipids do ?
Sources of energy
Form membranes
Participate in cell signalling
Function of fatty acids
Principal store of energy
Where are fatty acids found ?
Rarely free in the body, found more frequently in :
- Part of a lipid molecule
- Complexed to a carrier protein e.g. human serum albumin
Which fatty acids can be absorbed into the bloodstream ?
Short and medium chain fatty acids
Which fatty acids cannot be absorbed into the bloodstream ?
Long chain fatty acids
So, they are synthesised
Why are trans fats difficult to break down ?
They have one or more double bonds which are stronger and more difficult for the body to break down.
They are UNsaturated fatty acids
Ratio of single to double bonds in saturated fatty acids
16:0
Ratio of single to double bonds in unsaturated fatty acids
18:1
Ratio of single to double bonds in polyunsaturated fatty acids
20:4
How are double bonds numbered in fatty acids ?
Delta
Describe a feature of polyunsaturated fatty acids
The double bonds are never conjugated and are separated.
Stearic acid
Saturated
Oelic acid
Unsaturated - cis
Eladic acid
Unsaturated - trans
Omega fatty acids
Unsaturated
Sterols
Bile acids
Steroid hormones
Vitamins
Cell membranes - e.g. cholesterol
Name some inherited disorders in lipid pathways
Gaucher’s
Neimann Pick
Tay-Sachs
Fabry disease
Describe inherited disorders in lipid pathways
Defects in enzymes which metabolise lipids leading to lipid accumulation.
Largely affect neurological system, liver, spleen and bone marrow.
Failure to thrive.
Name some membrane lipids
Phospholipids
Glycolipids
Sterols
Name the types of phospholipids
Glycerophospholipids
- Choline
- Ethanolamine
- Serine
- Inositol
Sphingolipids
Name a glycolipid
Sphingolipids
Name a sterol
Cholesterol
Describe a phospholipid
Polar head group
Phosphate
Glycerol backbone
Fatty acid chains
Describe a feature of glycolipids
They have a sphingosine core
Describe the phospholipid bilayer
Hydrophilic heads - face the water - energetically favourable
Hydrophobic tails - are shielded from the water and lie next to each other
CONFLICTING FORCES
Why are membranes important ?
Compartmentalisation
Organelles as well as plasma membrane
Highly selective barriers
Have sensors to respond to internal and external conditions.
Name the 2 classes of proteins
Intergal proteins - (intrinsic)
Peripheral proteins - (extrinsic)
Describe the movement of lipids and proteins ?
Rotational and lateral movement
Briefly describe membrane fluidity with unsaturated hydrocarbon tails
FLUID
Lipids containing unsaturated fatty acids increase membrane fluidity.
The presence of double bonds introduces kinks in the fatty acid chains, making them more difficult to pack together.
Briefly describe membrane fluidity with saturated hydrocarbon tails
VISCOUS
In their saturated form, the fatty acids in phospholipid tails are saturated with bound hydrogen atoms; there are no double bonds between adjacent carbon atoms.
This results in tails that are relatively straight.
Thus, if saturated fatty acids, with their straight tails, are compressed by decreasing temperatures, they press in on each other, making a DENSE AND FAIRLY RIGID membrane.
What determines the fluidity in the membrane at cold temperatures ?
The ratio of saturated and unsaturated fatty acids determines the fluidity in the membrane at cold temperatures.
FRAP
Fluorescence recovery after photobleaching
What is FRAP ?
A standard technique used to study the diffusion properties of biomolecules in artificial or cell membranes.
Why is composition of cell membrane important ?
Important for cell function.
Not all membranes have the same composition.
Where is sphingomyelin mainly found ?
Outer leaflet
Where is phosphatidylcholine mainly found ?
Outer leaflet
Where is phosphatidylethanolamine mainly found ?
Inner leaflet
Where is phosphatidylserine mainly found ?
Inner leaflet only
How is asymmetry generated ?
New phospholipid is synthesised by enzymes at the ER facing the cytosol.
They use fatty acids available in the cytosol.
They release new phospholipid into the outer leaflet of the ER bilayer.
Lipids are then transferred to other side by flippases.
Some flippases are selective for particular phospholipids so different types may be concentrated in each monolayer.
Flippase mechanisms
Pore model
“Slip-pop” model
Describe membrane synthesis
Membrane synthesis occurs in the ER.
New membrane is then transported to other parts of the cell.
Occurs through a series of vesicle budding and fusion.
Bits of membrane pinch off from ER to form vesicles and then fuse with other membranes (this is called vesicle trafficking).
Exocytosis
Delivery of lipids (and membrane proteins!) to cell membrane.
Allows movement of other molecules out of cell.
Endocytosis
Movement of materials into cells via membrane bound vesicles.
Especially important in immune response.
May be mediated by specific receptors.
Describe lipid rafts
Organising centre :
- Vesicle trafficking
- Cell signalling
Function of membrane proteins
Many molecules cannot diffuse across a lipid membrane.
Require specific transport (active/passive)
Transmembrane signalling.
Name the 4 types of membrane proteins
- Transmembrane
- Membrane associated
- Lipid-linked
- Protein-attached
What are transmembrane proteins ?
Alpha helixes
What can proteins do through membranes ?
They can create channels and pores through membranes.
Membrane associated proteins
Usually contain domains that selectively bind to phospholipids.
Compartment-specific association
For example, pleckstrin homology domains (PH)
~100 amino acid region found in many proteins that binds to certain types of phospholipid
What does removal of the PH domain do ?
It alters a protein’s membrane binding properties.
e.g. Phospholipase C-n2
GPI
Glycosylphosphatidylinositol
How are lipid-linked proteins linked ?
Through GPI-anchors
Protein-attached proteins
Especially important in signal transduction
Beta2 adrenergic receptor
Plasma membrane
G protein signalling complex
