Unit 3: Membranes Flashcards
Functions of biological membranes
- Compartmentalization
- Scaffolding for biochemical activities
- Selectively permeable barrier
- Transporting solutes
- Responding to external signals
- Intercellular interaction
- Energy transduction
Specific function of the plasma membrane
- Import/export of molecules
- Receiving information
- Capacity for mvmnt and expansion
What determines the function of biological membranes?
The proteins embedded!
Cell membranes composed of
Lipids and proteins
Lipid bilayer is approximately ____ thick
~ 5nm
Each row of lipids called a
Leaflet
When talking about organelles within a cell, the outer faces the _____ and the inner faces the _____
Cytosol,
Lumen.
The plasma membrane outer leaflet is facing ______ and the inner leaflet is facing _____
Outside cell,
Inside cell (cytosol)
Inner of golgi faces the ______, outer of golgi faces _____
Lumen,
cytosol
____ portion of leaflets face eachother
Hydrophobic
Molecules with hydro(philic and phobic portions) called
Amphipathic
Proteins that can cross membrane
Must be longer than 5nm and hydrophobic
Fatty acid composed of
Carboxyl group makes it acidic
Long hydrocarbon chain (saturated or unsaturated)
____ fatty acids can pack more tightly, whereas _______ fatty acids with kinks creates more space and hence fluidity.
Saturated,
Unsaturated
Three main types of membrane lipids (most to least abundant)
- Phosphoglycerides
- Sphingolipids
- Cholesterol
Phosphoglycerides
Are diacylglycerides with small functional head groups linked to the glycerol backbone by phosphate ester bonds.
Sphingolipids
are ceramides formed by the attachment of sphingosine to fatty acids.
Most abundant phosphoglyceride
Phosphotidylcholine: choline + phosphate + glycerol+ 2x Fatty acid tails.
Most abundant phospholipid in most cell membranes is:
Phosphatidylcholine
Least abundant phospholipid
Phosphatidylinositol: important for cellular signaling because of ability to cleave between glycerol and phosphate. Releases phosphoinositol
Release of phosphoinositol pathway
IP3 due to phosphorylation (regulating proteins)
Sphingolipids (chain of command)
- Sphingosine
- Add second fatty acid to sphingosine via amine group ~ Ceramide
- Add phosphorylcholine to ceramide ~ Sphingomyelin
- Add galactose to ceramide ~ cerebroside
- Add complex carbohydrate to ceramide ~ ganglioside
Sphingolipids abundant in
Nervous tissue
Sugar substituted lipids known as
Glycolipids
All cell membranes (aside from phosphoglycerides, sphingolipids and glycolipids) contain
Cholesterol
Cholesterol is
Amphipathic
What dictates hydrophilic behaviour in bilayer?
Hydrophilic molecules in water behave by dissolving due to polar charges and form H-bonds with water molecules (it is more energy favourable state)
Hydrophobics behaviour in bilayers:
Can’t form bonds with water molecules. Water makes a cage-like structure around it. This disturbs energy favourable state with non-energy favourable state because H-bond is disripted.
Why membranes form a bilayer?
Cage structure or water molecules around the hydrophobic requires energy. System wants to be in lowest energy state.
Energy cost minimized if hydrophobic molecules cluster together
When hydrophobics cluster together, two things can happen… what happens depends on this:
- Micelle
- Lipid bilayer
Depends on the shape of the fatty acid tail.
Ends of bilayers result in
Self-sealing properties: exposed edges rearrange spontaneously to be energetically more favourable state not exposed to water.
Why do microsomes and vesicles (made of plasma membrane) form a ball and not a sheet?
Because edges of sheet are hydrophobic, exposed to water. For most energy favourable state, those lipids will spont. form a sphere and fold so edges associate one another (inside sphere is liquid).
Pure phospholipids in water will spontenously form
Liposomes
Lipid bilayer behaves as 2D fluid, why is this necessary?
For many proteins, for them to do their function they need to move around a bit. They need that environment to be a bit fluid-like.
Three types of lipid mobility account for fluidity:
- Lateral diffusion
- Rotation
- Flip-flop
Lipid movement 1: Lateral diffusion
Lipids rapidly exchange places with their neighbors on same leaflet
Lipid movement 2: Rotation
Rotate on own axis. Kinks in unsaturateds help do this
Lipid movement 3: Flip-flop
From one leaflet to the other, by route of flippases (enzyme) to move it.
Cholesterol can flip rapidly on its own.
How did they determine fluid nature of membranes?
Using FRAP: Fluorescence Recovers After Photobleaching
laser beam bleaches area with fluorescent dye. then dye fills it in eventually by diffusing in.
Why don’t bilayers fuse spontaneously to the closest…
Because the cage-structure (hydration shell) of the polar head acts as insulator and isn’t easily displaced. Membrane fusion requires energy.
Two main properties of lipids contribute to fluidity
- Length of hydrocarbon tail
- Degree of saturation.
Shorter tail length ~
increases fluidity because of less hydrophobic interactions.
Cholesterol makes bilayer less fluid at _____, but keeps it fluid at
High temperature,
Low temperature
Unless acted on by flippases, the orientation of lipids on leaflet is the same.
_____ and _____ are only in the outside (extracellular) leaflet
Glycololipids,
Sphingomyelin
Cytosol cannot have specificity for
sphingomyelin and glycolipids
Ceramide is the precursor for
Sphingomyelin and glycolipids
Ceramide sitting on the ______ and can thus be converted to sphingomyelin
lumenal leaflet
Phosphatidylcholine is mostly found in the
Outer leaflet
Asymmetry of phospholipids is due to
flippases
Phosphatidylinositol found mostly on
Cytosolic side (inner leaflet)
Phospholipids synthesized in the
Smooth ER.
Membrane orientation does not change during transfer between compartments, thus:
Lipids facing the cytosol remain cytosolic even at the plasma membrane. Those facing the lumen of a compartment will face the extracellular space.
