Midterm 2 Flashcards
plasma membrane
the outer boundary of the cell separating the cellular content from the outside world
What holds the lipid bilayer together?
The van der whal interactions between the acyl tails of phospholipids
Functions of the plasma membrane
1)Compartmentalization
2)Scaffolding for biological activities
3)Selective permeable barrier
4)Allowing transport of solute
5)Responds to stimuli
6)Cell to cell communication
7)Energy transduction
Lipid Bilayer
the structural backbone and barrier to prevent random movements of materials into and out of the cell
What type of interaction holds lipids and proteins together in the bilayer
noncovalent bonds
Three lipid types found in mammalian membranes
1)Phosphoglycerides
2)Sphingolipids
3)Cholesterol
Phosphoglycerides
Contain 2 fatty acids, a glycerol, and phosphate group usually linked to a small polar group
Sphingolipids
Lipids derivatived from ceramide that can add groups to their terminal alcohol
Glycosphingolipid
Sphingolipid with an additional carbohydrate found exclusively on the exoplasmic face of the plasma membrane
cholesterol
A polar ampipathic molecule that stabilizes and adds fluidity to membranes composed of a polar head group, steroid ring and hydrocarbon tail
Why is lipid asymmetry between the different leaflets of the bi layer important?
1) Negatively charged cytosolic face can help proteins bind via its charge
2)Signalling can be done via movement of material from the inner layer to the outer layer
3)Glycolipids only are found on the extra cellular side acting as a ligand receptor
glycosylation
Post transcriptional modification which adds carbohydrates to protiens
Three sub classes of membrane proteins
1)Integral membrane proteins- fully integrated into the bilayer
2)Peripheral Membrane proteins- anchored to the outside of the bilayer
3)Lipid anchored proteins- Covaletly linked to the membrane by a membrane protein
Functions of Integral Membrane proteins
1)Transporters: moving ions and solutes across the membrane
2)Anchors: binding intra- or extra- cellular components to the membrane
3)Receptors: binding ligands to initiate signal transduction pathways
4)Electron Transporters: transfer electrons during photosynthesis and respiration
Structure of Integral Membrane proteins
Composed of Alpha Helices and Beta Barrels
Why would the cytosolic side of a integral membrane protein be positively charged?
To anchor to the negatively charged cytosolic side of the membrane
Hydropathy plot
measures the hydrophobicity of amino acids where hydrophobic = +ve, hydrophilic = -ve
Peripheral Membrane proteins
Protiens which do not interact with the core of the lipid bilayer and are anchored via non covalent bonding between the head groups, they are easily removed via changes in Ph or ionization strength
Three types of Lipid anchored membrane proteins
1)Fatty acid- anchored membrane proteins
2)Isoprenylated membrane proteins
3)GPI-anchored membrane proteins
Fatty acid-anchored membrane proteins
Synthesized within the cytosol Attached to a saturated fatty acid, usually myristic acid(14C) or palmitic acid(16C), that is embedded in the membrane
Isoprenylated membrane proteins
Attached to multiple isoprenyl groups(5C), usually farnesyl(15C) or geranygeranyl(20C) groups,then embedded in the membrane, Synthesized in the Cytosol
GPI-anchored membrane proteins
synthesized in the ER, post-translationally attached to the C-terminus and can be released from the membrane via cleavage by phospholipase C
Fluid Mosaic Model
Membranes consists of a mosaic of proteins/lipids in a fluidic state
Why is membrane fluidity important
Vesicle formation, fusion, secretion
Cell division
Muscle contraction
Cell migration
Signalling mechanisms
What is membrane fluidity
Ability for the bilayer components to rotate about their axis and for lateral movement of particles through the bilayer
What happens when the membrane is too rigid
Membrane components can’t organize properly, as proteins can’t move and interact with binding partners.
What happens when the membrane is too viscous
Lose mechanical support (the ability to orientate properly), with excessive fluidity resulting in an increase in membrane permeability
transition temperature
Temperature at which the lipid bilayer undergoes a phase transition from solid to liquid
Factors that lead to high fluidity of the membrane
Unsaturated acyl tails,shorter tails,
Cholesterol function in the lipid bilayer
1)Sterols decrease the permeability of membranes to ions and small polar molecules
2)Cholesterol acts as a fluidity buffer (broadens the temperature range of transition)
Cholesterol effect on unsaturated fatty acids
cholesterol decrease fluidity by wedging itself in between the packing of the lipids
Cholesterol affect on saturated fatty acids
Increases fluidity by disrupting the tightly packed acyl tails
Homeoviscous Adaptation
The ability of a cell to regulate membrane fluidity in response to temperature changes by altering lipid composition and maintain membrane fluidity
Lipid rafts
Localized regions of membrane lipids in association with specific proteins These structures are believed to serves as floating platforms that concentrate proteins into “compartments” on the membrane
Flip-Flop
Movement of a lipid through the hydrophobic sheet reorienting itself 180 degrees, typically very slow but will be aided by flippase
FRAP
Flouresent recovery after bleaching, membranes that have been dyed with flouresecnt materiel once bleached of colour after a period of time will patch the bleach hole.
Different levels of membrane mobility
A.Random Movement
B.Immobile (intracellular tethering)
C.Directed Movement
D.Reduced Mobility (crowding)
E.Fenced or Corralled
F.Extracellular Entanglement
Passive transport
Movement of molecules down the concentration gradient without costing energy or specifically requiring a protien
Active transport
Goes against a concentration gradient and requires ATP and a protein pump to move the solutes
Three factors that affect permeability
1) Molecule size, smaller= faster
2)Partition coefficient, the greater lipid solubility= faster
3)Charge,Charged molecules will not pass the membrane
Two components of electrochemical gradient
Concentration gradient: Non charged Molecules move down a concentration gradient
Electric Potential gradient:Charged molecules want to move to their opposite charge
Simple diffusion
Unassisted Movement of gases, non polar molecules and small polar molecules down a concentration gradient at a speed proportional to concentration gradient and permeability
Example of passive diffusion
Movement of O2 diffusing into RBC in the lungs and diffusing out once at the tissues
Osmosis
Movement of water to areas of high solute to low solute, Hypertonic:Movement into cell, Hypo tonic: water moves out the cell, Isotonic, NO movement
Facilitated transport
Requires a protein to help get the molecule across the membrane using integral proteins, Made of two types Carriers and Channels
Carriers
Transporters that move molecules via an alternation of two conformations
Channels
Water filled pores which ions or small molecules can diffuse through
Three types of channels
1)Ion Channels: Highly specific conducting ions
2) Porins: Allows movement of hydrophillic solutes based on size
3)Aquaporins: Allows water to flow through but blocks any ions of solutes
Ion Channels
Channels which allow for bidirectional movement of ions based on chemical gradients, they are highly specific due to their cores acting as a size filter
Porins
Low specificity water filled beta barrels that have a nonpolar side embedded in the membrane and a polar side facing the water
Three types of Ion Channels
1)Voltage gated: Open and close in response to changes in charge
2)Ligand gated:Open and close via the binding of a specific ligand
3)Mechanosensitive gated: Respond to mechanical changes in the membrane
Carrier Proteins
Allow movement of solute though an alternation of conformation
Steps in solute transport of a carrier protein
1)Solute binding side is open at one end
2)Solute binding cause a conformational change
3) The protein changes conformation so that the solute binding side now opens to the opposite side of the membrane
4)Solute is released
Similarity of carrier activity to enzyme activity
1)High specificity
2)Regulated activity
3)Exhibit saturation kinetics
Types of carrier proteins
1)Uniporters:Transport a single solute
2)Symporters: Transports two solutes in teh same direction
3)Antiporters: Transport solutes to opposite sides across the membrane
GLUT1
Uniporter carrier protein that can carry glucose in and out of the cell depending on the concentration gradient of Na+ with an exchange rate of 2 Na+ per glucose
Chlorine Bicarbonate exchanger
Antiporter that allows movement of chlorine and HCO3- in and out of the RBC, Chlorine will move out in the lungs and in at the tissues
Active transport
Movement of solute against its concentration gradient