lect 3: membranes I Flashcards
what are the general function of biological membranes?
all these functions are influenced by membrane structure/chemistry
1. compartmentalization
2. scaffold for biochemical activities
3. selectively permeable barrier
4. transport solutes
5. response to external stimuli
6. cell-cell communication (junctions)
7. energy transduction (mitochondria membrane)
8. cell growth/mobility
what are the learning objectives of this lecture?
-outline the general structure and functions of the plasma membrane
-identify the basic structures and functions of the carbohydrates and lipids within cellular membranes
-describe the three classes of membrane proteins and their function within the lipid bilayer
-explain the importance of fluidity to plasma membrane properties
what is the graph of a cell?
what is the introduction of the plasma membrane?
-outer boundary of cell, separates cell from enviro
-thin, fragile, 5-10 nm thick
-electron microscope to visualize
-container of cells; a protein studded fatty film
-without protection of outside world=no cells and thus no life
-does much more than protect; nutrients must enter and wastes must leave
same membrane ultrastructure (conserved structures across species)
-plants
-animals
-microorganisms
what is the structure of the plasma membrane?
made up of phospholipids
-integral membrane proteins which can be transmembrane or monolayer associated
-sugar moidies
what is the structures of membrane lipids?
amphipathic, so form bilayers in water
types of membrane lipids
1. phosphoglycerides
2. sphingolipids
3. cholesterol
what are the most abundant lipids in cell membranes?
phospholipids (ex: phosphatidylcholine which has phosphate, which always carries a negative charge)
-hydrophilic=water soluble, so charged groups or uncharged polar groups
-hydrophobic=insoluble so uncharged and nonpolar (fats molecules, triglycerides, are entirely hydrophobic)
what does the fluidity of a lipid bilayer depend on?
its composition: 2 properties of tail affect how tightly they pack together
-their length (shorter=more fluid because less van der waals forces)
-number of double bonds (creates kink so more double bonds=more fluid)
-unsaturated (in respect to H) when they contain double bonds
what are phosphoglycerides?
phospholipid (general)
-fatty acid + phosphate group joined by alcohol reside
phosphoglyceride (specific)
-type of phospholipid
-alcohol residue=glycerol
fatty acids chains can be variable, may be
-fully saturated (no C=C) (most hydrogens it can have)
-monounsaturated (one C=C)
-polyunsaturated (multiple C=C)
what are sphingolipids?
build with sphingosine (instead of glycerol)
-amino alcohol with a long hydrocarbon chain
what are the general structures of sphingolipids?
what is cholesterol?
-a sterol
-carbon rings are flat and rigid
-compared to other membrane lipids: smaller, and less amphipathic
-comprises up to 50% of lipids of animal membranes
-cholesterol can fill spaces left by kinks and can stiffen the bilayer (much less flexible and less permeable)
how does cholesterol stiffen regions?
-cholesterol is smaller and less amphipathic (most of it is nonpolar and a small amount is polar)
-carbon rings are flat and rigid allows it to sandwich itself in between phospholipid tails
what are the functions of cholesterol?
-insertion into plasma membrane
-functions: structural integrity and membrane fluidity
what is the asymmetric distribution of phospholipids?
-SM= sphingomyelin
-PC=phosphatidylcholine
-PS= phosphatidylserine
-PE= phosphatidylethanolamine
-PI= phosphatidylinositol
-CI= cholesterol (fairly evenly distributed)
what is the graph of the asymmetric distribution?
phospholipids and glycolipids are distributed asymmetrically in the lipid bilayer of animal cell plasma membrane
membrane assembly begins in ____
ENDOPLASMIC RETICULUM
-in euks, new phospholipids are manufactured by enzymes bound to the cytosolic side of the ER (exclusively the cytosolic half of the bilayer)
how do new phospholipids make it to the opposite monolayer?
-flip flops (not spontaneous)
-phospholipids are transferred by a protein called scramblase, a type of transporter protein that removes randomly selected phospholipids from one half and inserts it into the other
what is scramblase?
transfers random phospholipids between monolayers
–>symmetric bilayer growth
what is the golgi apparatus a source of?
membrane leaflet asymmetry
-flippases: transfers specific phospholipids between monolayers
-distinct inside vs outside faces
but if membranes emerge from the ER with an evenly assorted set of phospholipids, where does asymmetry arise?
it begins in the Golgi apparatus (most cell membranes are asymmetric)
-golgi has another family of phospholipid handling transporters, called flippases, which use ATP to transfer specific phospholipids from one layer to another
what is the nature and importance of plasma membrane?
-lipid bilayer can self-assemble
-can also form into micelles and liposomes
-liposomes have proven invaluable in membrane research
-membrane proteins can be inserted into liposomes
-liposomes are vehicles to deliver drugs, DNA, RNA (covid vaccine)
what are membrane carbohydrates?
-short (<15 sugars per chain)
-CHO content of eukaryotic cell membrane 2-10% by weight (species, cell type)
-located extracellularly bc form glycocalyx (sugary layer outside of cell)
-important role in interactions of cell with its environment
-interact with either lipid or protein components of the membrane
-glycocalyx: plays a role as protective barrier, provides a way to interact with ECM and also provides a lubrication which can affect cell mobility
how are membrane carbohydrates linked?
->90% membrane CHO covalently linked to protein= glycoprotein (called glycosylation)
-rest linked to lipid=glycolipid
variability (e.g. same protein)
-different chains of sugars in different cells/tissues
what are glycolipids? in regard to blood groups?
-CHO attached to lipid by glycosidic (covalent) bond (R-O-R, with R being an alkyl group)
-located in outer layer of plasma membrane
glycolipid CHOs of red blood cell plasma membrane=blood type
-A, B, AB or O
-A: enzyme adds N-acetylgalactosamine to chain terminus
-B: enzymes adds galactose to the chain terminus
-AB: both enzymes present
-O: lack enzymes to attach either terminal sugar
what are glycoproteins?
-CHO attached to amino acid
2 types of linkages
1. N-linkages (nitrogen of amino group): asparagine, arginine
2. O-linkages (O of -OH): serine, threonine
what percentage of proteins are membrane proteins in animal’s genome?
what are integral proteins?
-function: receptors, channels, transporters
-amphipathic
-not necessarily fixed in location
-60% of all current drug targets
transmembrane domain
-van der waal interactions with fatty acyl chain
-preserves the permeability barrier of membrane
what are peripheral proteins?
-associate with membrane via weak noncovalent bonds
-one-sided localization
-dynamic relationship with membrane
-can be extra or intracellular
cytosolic peripheral proteins
-enzymes
-factors that transmit transmembrane signals
what are lipid-anchored proteins?
-covalently bonded (unlike integral and peripheral proteins) to lipid group located within membrane
-can be either intracellular or extracellular
what is the plasma membrane structure?
cell cortex or membrane skeleton
-reinforces plasma membrane
-in animal cells
-spectrin is a major component of the internal membrane skeleton
what is the example of plasma membrane structure (human red blood cell)?
-plasma membrane of human RBCs extensively studied/best understood
-membrane proteins can be purified
spectrin: major component internal membrane skeleton
-attached to membrane surface via ankyrin
-also linked to other cytoplasmic proteins
-detect proteins with western blot.
what is the fluid mosaic model?
mosaic (bunch of different components)
-phospholipids, cholesterol, proteins, carbohydrates
fluid (dynamic and fluidity of plasma membrane)
-components can freely move laterally
-crossing between leaflets is energetically unfavorable
what are the factors influencing membrane fluidity?
- temperature
- cholesterol (when less cholesterol=closer together so more rigid, less fluidity)
- saturated and unsaturated fatty acids
what are membrane lipids and membrane fluidity?
-internal temperatures of most organisms fluctuate with environmental temperature
cells respond by altering phospholipid composition
-desaturation catalyzed by specific enzymes, desaturases
-change to types of phospholipids being synthesized (make more unsaturated fatty acids)
what is the dynamic nature of the plasma membrane?
phospholipids can move laterally
-diffusion from one end of a bacterium to the other in 1-2 sec
possible for phospholipid to move from one leaflet to another
-flippases (requires energy, must less energetically favourable)
what is cell fusion?
reveals mobility of membrane proteins
-cell fusion: technique whereby two different types of cells, or cells from two different species, can be fused to produce one cell
-labeled proteins show that membrane proteins can move throughout fused cell
what is the dynamic nature of plasma membrane (membrane lipid mobility)?
phospholipid movement also restricted
-no Brownian movement
-confined for brief periods to certain areas
integral protein “fences” attached to membrane skeleton
what is the dynamic nature of plasma membrane (control of membrane protein mobility)?
protein movement slower than predicted based on
-protein size (smaller=move more)
-membrane viscosity (how fluid the membrane is)
protein movements limited by various interactions
-membrane skeleton
-neighboring proteins
-extracellular interactions
what is the restriction of membrane protein fluidity?
-proteins do not just freely flow around in lipid “sea”
-membrane domains: functionally specialized regions
what is the dynamic nature of plasma membrane (membrane domain and polarity, e.g. epithelial cells)?
membranes vary in protein
-composition
-mobility
intestinal/kidney epithelial cells
-highly polarized (diff parts have diff composition and functions)
-apical: absorption
-basal: basement membrane contact (support)
-lateral: cell contact/communication
what is the dynamic nature of plasma membrane (membrane domain and polarity, e.g. mammalian sperm cell)?
-distinct parts (head, midpiece, tail)
-continuous plasma membrane
-localized domains: specific protein distribution
-determined via fluorescent antibodies
