Lec 5: Membranes (Exam 2) Flashcards
importance of cellular membranes (3):
- ) Define the physical limits (border)of the cell
- Plasmalemma - ) Partition the cell into different regions
- Endoplasmic reticulum (ER) - ) Define the limits of organelles
- Nucleus, mitochondria, Golgi, ER, lysosomes, etc.
cellular membrane functions (5):
- ) Define boundaries (act as selective permeability barriers)
- ) Serve as loci for specific functions
- ) Regulate movement of specific molecules
- ) Contain receptors for detection of external stimuli
- ) Provide for cell-cell communication
membrane main components (2):
& their proportions are…
lipids
proteins
…variable among different membrane types
Membrane Lipids (3): & examples
1.) Phospholipids
- Phosphoglycerides
(Glycerol + Phosphate + side) group + 2 fatty acid chains
- Sphingolipids
(Sphingosine + phosphate + side group + 1 fatty acid chain)
2.) Glycolipids
(Sphingosine + carbohydrate + fatty acid)
- Cerebrosides: neutral glycolipids – single uncharged sugar
- Gangliosides: have oligosaccharide & negative sialic acid
3.) Sterols
- Cholesterol + derivatives (in animal cells)
- Phytosterols (in plant cells. Plant cells also have cholesterol)
- Ergosterol in fungi
- Hopanoids in some prokaryotes
(FA) =
= Membrane Fatty Acids
(Membrane Fatty Acids (FA))
Chain length usually between…
- Mostly…
…12 – 20 carbons in length
…16 or 18
What is currently thought that to determine the membrane thickness? =
The longer the FA chains =
= the average chain length of the fatty acids in the membrane
= the thicker the membrane
2 Factors affecting Membrane Fatty Acids (FA)
Chain length
Double bonds/saturation
(Membrane Fatty Acids (FA))
Higher level of unsaturation (double bonds) =
Lower level of unsaturation =
= more fluid (less solid)
= less fluid (more solid)
(Membrane Structure)
Current Model =
& 2 basic components =
Amount of…
= Fluid Mosaic Model
= Proteins + Lipid Bilayer (Phospholipids, Glycolipids, Sterols)
…protein, sterols, and carbohydrate in membrane is variable
Fluid Mosaic Model focuses on
Phospholipid bilayer & associated proteins
(Fluid Mosaic Model)
The lipid bilayer would include (4):
Phosphoglycerides
Sphingolipids
Glycolipids
Embedded sterols
3 classes of Membrane Proteins are based on…
…interactions with the bilayer
3 classes of Membrane Proteins:
- ) Integral membrane proteins
- ) Peripheral membrane proteins
- ) Lipid Anchored proteins
3 classes of Membrane Proteins:
- ) Integral membrane proteins
- Embedded within…
- _____ _______ span the bilayer - ) Peripheral membrane proteins
- Located…
- weakly bound (noncovalent) to…
- or associated with… - ) Lipid Anchored proteins
- ________, reside on…
- Covalently bound to…
- ) Integral membrane proteins
- Embedded within the hydrophobic portion of the lipid bilayer
- Transmembrane segments span the bilayer - ) Peripheral membrane proteins
- Located at the surface of the membrane,
- weakly bound (noncovalent) to the phospholipids (electrostatic interactions)
- or associated with integral membrane proteins - ) Lipid Anchored proteins
- Hydrophilic, reside on the membrane surface
- Covalently bound to lipid molecules in the bilayer
Lipids and proteins in the membrane are, to some degree,…
& ex:
…ordered
(ex: electon transport chain in the inner mitochondrial membrane)
Proteins are not…
Although some have…
…floating around randomly
…a high degree of mobility
Characteristics of membranes (5):
- ) Membranes are asymmetric
- ) Lipid Bilayer is fluid
- ) Membranes only function in the fluid state
- ) Fatty acid composition will influence fluidity
- ) Sterols will also influence fluidity
- ) Membrane proteins have variable mobility
(Characteristics of membranes/ 1.) Membranes are asymmetric) - Each leaflet (monolayer) has... - Plasma membrane = (Outer leaflet = ) (Inner leaflet = ) (Inner leaflet has... ) - Asymmetry originates during... - Asymmetry is maintained because...
- …different lipid and protein composition
= plasmalemma
(Outer = highly glycosylated)
(Inner = Low glycosylation)
( …more phosphatidyl-inositol, -ethanolamine, -serine for cell signaling (generation of 2nd messengers) - …membrane biogenesis @ Golgi, ER, etc.
- …transverse diffusion of lipids is rare
(Characteristics of membranes/ 2.) Lipid Bilayer is fluid)
- Lipid components can…
- Lipid components can…
- Lipid components cannot…
- Membrane fluidity is important for…
- Measure fluidity via…
- …diffuse laterally
- …rotate
- …diffuse transversely very easily
- …membrane function
- …photobleaching (measures the rate of lateral diffusion)
(Phospholipid Mobility)
The more fluid-like (less solid) a membrane is…
…the higher the rate of return to baseline fluorescence
(Importance of membrane fluidity)
Membranes need to be…
What is dependent on membrane fluidity? =
Example:
…“pliable”.
= Membrane function (membrane protein conformational change)
Ex: Na+/K+-ATPase activity depends on membrane lipid fluidity
(Characteristics of membranes/ Membranes only function in the fluid state)
- Membrane _____ is dependent on _______
- Membrane ______ is dependent on _______
- If too cold =
- …which =
- fluidity, temperature
- function, temperature
fatty acids will transition to a
= more solid state… - …which = decreased ability for components to function
During the transition from solid to fluid, the membrane…
…absorbs heat.
(Characteristics of membranes/ 4.) Fatty acid composition will influence fluidity)
- Increase in the number of fatty acids with double bonds (unsaturated and polyunsaturated) will…
- Unsaturated can’t…
- Increase in the chain length of the fatty acids will…
- Longer carbon chain =
- …increase the fluidity
- …pack tightly
- …decrease the fluidity
- = thicker area of tight packing
(Membrane fluidity: Fatty Acids)
The temperature at which a membrane undergoes a phase transition from solid to liquid is related to…
The lower the value, the…
…its fluidity.
…more fluid.
(Membrane Fluidity: FA Chain Length)
The lower the transition temperature, the…
…more fluid the membrane is.
(Characteristics of membranes/ 5.) Sterols will also influence fluidity) - Most sterols are... - They tend to... Therefore... - high [Sterols] tends to decrease... - high [Sterols] tends to increase...
- …relatively rigid (hinder FA chain movements)
- …disrupt fatty acid chains from interacting
- …(mixed effect)
- …decrease fluidity at high temp (including ~body temp)
- …increase fluidity at low temp
(Sterols and membrane fluidity)
- As the ratio of cholesterol to phospholipid was increased…
- The membranes became…
- …fluidity decreased at body temperature!
- …more viscous (rigid) with increasing cholesterol.
Can changes in membrane fluidity have a physiological impact?
Example:
Yes
Main Conclusions:
1.) Blood plasma cholesterol is in equilibrium with red blood cell (RBC) membrane cholesterol content.
Implies that increased cholesterol in the blood will increase cholesterol in the RBC membrane.
2.) As seen from the previous slide, increased cholesterol in the RBC membrane makes it less fluid.
3.) The decreased fluidity of the RBC membrane reduces oxygen diffusion across the membrane.
4.) Ultimately, O2 uptake (at the lung) and O2 delivery (at the tissues) are impaired by high cholesterol in the plasma. Due to the effects of cholesterol on membrane fluidity.
(Characteristics of membranes/ 6.) Membrane proteins have variable mobility)
- Some show…
- Those that are…
- …a low degree of lateral diffusion, others have a high degree…
- …linked to either cytoskeletal components or extracellular components have restricted lateral diffusion
(Example of restricted mobility)
Membrane components of…
- Found at…
- Provide structural integrity to muscle fiber
- Provide a linkage from…
- May be associated…
Lack of any one of the components leads to…
- …the Dystrophin-glycoprotein complex (a-dystroglycan, b-dystroglycan, sarcoglycan subunits, sarcospan) in muscle
- …myotendinous junctions, costameres (rib like structures positioned periodically along the length of muscle fiber), and neuromuscular junction
- …cytoskeletal proteins to extracellular matrix (collagen…tendons)
- …with cell signaling (i.e., NOS)
- …muscular dystrophy (many types of muscular dystrophy)
(Lipid Rafts) =
- Characterized by high levels of…
- The rafts are…
- On the outer leaflet of the plasma membrane, …
- Allows for…
= Localized regions (microdomains) of membrane lipids that sequester proteins involved in cellular signaling.
- …cholesterol, and glycosphingolipids with long saturated fatty acids
- …thicker and less fluid than surrounding membrane
- …various receptor proteins with bound ligands are generally assembled into rafts that communicate with cell signaling proteins found in rafts on the inner membrane
- …enhanced transfer of signal from the outer leaflet to the inner leaflet
Draw Screenshot #1 (Lipid Raft)
plzzzzzz don’t your waannnaaa AAAA??
(Membrane proteins) (Integral Membrane Proteins) - Integral Membrane Proteins = - Hydrophobic regions within... - 2 Types of membrane proteins = - #1 Which... - #2 Which...
- 2 Types of #2 =
- Transmembrane segments typically have…
= Mostly amphipathic (hydrophobic and hydrophilic regions)
- …hydrophobic portion (interior) of bilayer
= Integral monotopic proteins & Transmembrane proteins
- …protrudes from only one side
- …protrudes from both sides
- 1.) Single pass – only one segment crosses the membrane
2. ) Multipass – multiple segments cross the membrane - …an a-helical structure or multiple b-sheets closed into a barrel (b-barrel)
(Membrane proteins)
(Peripheral membrane proteins)
- Do not…
- Generally bound to…
- …penetrate into the lipid barrier
- …the membrane surface via weak electrostatic interactions
(Membrane proteins) (Lipid Anchored Proteins) - Hydrophillic or Hydrophilic proteins? = - Covalently attached to... - Multiple classes: (3) - & found on the... - & an example
= Hydrophilic proteins
- …lipid molecules in the membrane
1.) Fatty Acid anchored proteins
- …inner surface linked to 14 or 16 carbon fatty acids
- Ex: Bid (important for apoptosis)
2.) Isoprenylated membrane proteins
- …inner surface linked to multiple isoprenyl groups (isoprenes).
(The isoprene groups insert into the membrane)
- Ex: Ras proteins
3.) GPI Anchored proteins (GPI = glycosylphosphatidylinositol)
- …outer surface
(Synthesized as single pass membrane proteins that then have the transmembrane domain removed and replaced via a covalent bound to GPI)
- Ex: Acetylcholinesterase
(Membrane proteins)
- Many membrane proteins are…
- Process occurs in…
- 2 types of links:
- …glycosylated
- …the ER and Golgi
1. ) N-linked: sugars are linked to the protein via an amino group on asparagine
2. ) O-linked: sugars are linked to the protein via an oxygen atom of a hydroxyl group on either serine, threonine, hydroxylysine, or hydroxyproline
(Protein Glycosylation)
- Sugars can be…
- Glycosylation plays an important role in…
…either straight chains or branched
…cell-cell recognition
