(Dr. McLachlin) (Unit B) Topic Note 6 Flashcards
What are the main functions of biological membranes?
- Separate contents of a cell or organelle from the surrounding environment
- Control import and export of molecules into and out of the cell or organelle, using proteins that span the membrane
- Contain sensors or receptors that allow the cell to respond to external stimuli including communications from other cells
- They are involved in cell movement
What are lipids?
Biological molecules that have little or no solubility in water
* Soluble in organic solvents
Functions of lipids
- Structural components
- Energy storage molecules
- Enzyme cofactors
- Signalling molecules
- Pigments
Fatty acids
Hydrocarbon chains ending in a carboxylic acid group
What are general characteristics of fatty acid groups?
- Contain even number of carbon atoms
- Length from 4-24 carbons
What are saturated and unsaturated fatty acids?
- Saturated: No double bonds
- Monounsaturated: 1 double bond
- Polyunsaturated: More than 1 double bond
What conformation are double bonds in fatty acid chains?
Always cis
How are carbon atoms counted in a fatty acid chain?
- Chemists count from carboxyl end
- Nutrition literature counts from methyl end
What would an omega-6 fatty acid be like?
Double bond 6 carbons from the omega carbon (including the omega carbon)
What is the end terminal carbon known as? (Nutrition Literature)
Omega Carbon
Glycerol
3-carbon molecule with hydroxyl groups at each carbon
Triacylglycerols
Also known as triglycerides
Fatty acid chain attached to each hydroxyl of glycerol via an ester linkage
Do triacylgycerols contain different or same fatty acids?
Usually contain two or there different types of fatty acids
What are triacylglycerols used for?
Store fatty acids as energy reservoirs in adipocytes (fat cells)
Glycerophospholipids
Similar to triacylglycerols
* Fatty acid on one end of glycerol is replaced with a charged phosphate group
What usually happens to the charged phosphate group in glycerophospholipids?
Often conjugated to a polar alcohol like:
* Serine (makes phophatidylserine)
* Choline (makes phosphatidylcholine)
Sphingolipids
Based on molecule sphingosine
* Long hydrocarbon chain
* Second chain added by joining a fatty acid to sphingosine’s amin group to form a ceramide
What are modified ceramides called?
Sphingomyelins
* Modify the terminal hydroxyl group
* With phosphoethanolamine or phosphocholine
Where are sphingomyelins found?
Myelin sheath of nerve cells
Besides sphingomyelins, how else can ceramide be modified?
With one or more monosaccharide groups to make glycosphingolipids
Steroids
Based on a system of four fused rings
* Three with six carbons
* One with five carbons
Sterols
Steroids with a hydroxyl group at C3
Which lipids are amphipathic?
- Fatty acids
- Glycerophospholipids
- Sphingolipids
Amphipathic
One part of the molecule is hydrophobic and the other is hydrophilic
Micelle
Formed when hydrophilic groups have a larger diameter than the hydrophobic groups
* Lipid is triangular or wedge-shaped
* Water is excluded from the centre
Are micelles found in living systems?
Not common, often seen in soaps and detergents
Lipid bilayer
Formed when hydrophilic and hydrophobic groups of amphipathic lipids have approximately equal diameters
* Hydrophilic heads form two surfaces that contact water and protect the hydrophobic groups
Are lipid bilayers flexible?
Yes
Why are lipid bilayers flexible?
Prevents their hydrophobic edges from making contact with water by spontaneously closing to form spherical vesciles or liposomes that enclose an aqueous interior
What does it mean the lipid bilayers are “fluid”?
Individual lipid molecules can move and diffuse within the plane of the bilayer
What factors determine the fluidity of a lipid bilayer?
- Temperature
- Proportion of unsaturate fatty acyl chains whithin the phospholipids and glycolipids
- Length of fatty acyl chains present in the phospholipids anf glycolipids
- Amount of cholesterol in the bilayer
How does temperature affect fluidity of lipid bilayer?
Fluidity increases at high temperatures
Decreases as temperatures decrease
What happens to the lipid bilayer when the temperature gets below a certain phase transition temperature?
Lipid bilayer will exist in a gel state
* Lipid movement is reduced
How does unsaturated fatty acyl chains affect bilayer fluidity?
Cis double bonds impede tight packing, increasing fluidity
Are shorter or longer chains more mobile and flexible?
Shorter chains
How does cholesterol affect the lipid bilayer fluidity?
- Reduces fluidity at higher temperatures
- Increases fluidity at lower temperatures
Are biological lipid bilayers symmetrical?
No
What is the fluid mosaic model?
Describes that proteins are free to diffuse laterally wthin the bilayer, unless their movement is restricted by cellular components
Integral membrane proteins
Portion of the polypeptide chain contacts the hydrophobic interior of the membrane
Transmembrane proteins
Integral membrane proteins that completely crosses the membrane at least once
What secondary structure are transmembrane sections of proteins usually? Why?
Alpha helices
* Maximize hydrogen bonding
How do beta-sheets form transmembrane sections?
Form barrels
Peripheral membrane proteins
Not anchored to the lipid bilayer, associated with integral membrane proteins or with the hydrophilic head groups of membrane lipids
Lipid-linked proteins
One or more lipid molecules are covalently attached to the end of the protein or to particular side chains, usually cysteine
Where are lipids in a lipid-linked protein usually located?
Lipids are embedded in the membrane
What proteins cannot be removed from the membrane without disrupting the lipid bilayer?
- Integral membrane proteins
- Lipid-linked proteins
The use of detergents in purification may alter what protein structures?
Secondary, Tertiary, and Quaternary structure of proteins
What are membrane proteins often modified by?
Carbohydrates
* On asparagine, serine, or threonine side chains
* These proteins are called glycoproteins
Where do sugars never attach on proteins?
Cytosolic surface
What are the 4 main functions of membrane proteins?
- Transporters and channels
- Receptors
- Enzymes
- Anchors
What process do transport proteins and channels do?
Facilitated transport
Transporters vs Channels
Transporters undergo conformation changes to move ions or molecules across the membrane
Channels (when open) form continuous pores through the bilayer
Uniporters vs. Symporters vs. Antiporters
- Uniporters: Transporters that move a single species across the membrane
- Symporters: Transport two species across the membrane in the same direction
- Antiporters: Transport two species across the membrane in opposite directions
What forms across the membrane when the concentration of a given ion/molecule is larger on one side of the membrane than the other?
Electrochemical gradient
Passive transport
Pathway is provided via a transporter or channel, ions and molecules will spontaneously move across the membrane down their electrochemical gradient
Active transport
A source of energy is used to move ions of small molecules across the membrane
Receptor proteins
Sense chemical signals on the outside of the cell and carry the message to the inside of the cell, so the cell can react to its environment
Enzyme proteins
Catalyze chemical reactions; can be associated with membranes
Anchor proteins
Structural proteins that provide stability to the membrane and help control the shape of the cell and its position relative to other cells
* Binds to other macromolecules on one or both sides of the membrane
Give examples:
1. Transporters/Channels
2. Receptors
3. Enzymes
4. Anchors
- Potassium channels
- Insulin receptors
- Insulin receptors (adds phosphates)
- Integrins
