Cell membranes Flashcards
Functions of cell membranes
Barriers Transport Receptors Adhesion: cells joined into tissues, cells interacting with different types of cells Movement Localization of functions of organelles
Fluid mosaic model
Eukaryotic cells: 40% lipid, 50% protein, 10% carbohydrates
Bacterial cells: more proteins-greater ability for transportation
Amphipathic molecule
Having both polar and non-polar portions
Bilayer formation
Hydrophilic heads want to be near water and each other
Hydrophobic tails want to be away from water
Liposome
Sealed compartment formed by phospholipid bilayer
Movement of phospholipids in membrane
Lateral diffusion (movement right and left)
Flexion of tails
Rotation of phospholipid
Flip-flop (rarely occurs)
Phospholipids
Phosphoglycerides: glycol base
Phosphatidyl choline: most common phospholipid
Phosphatidylserine
Glycolipids
Lipids with sugar attached
No phosphate: sugar instead
Sugar is polar
Sterol
Ring structure: hydrophilic and hydrophobic sections
Fills in gaps in membrane: makes less fluid
-OH groups
Cholesterol: animals
Phytosterol: plants
Hopanoid: prokaryotes
Fatty acid chains
Length of chain is variable (typical: 18-20 hydrocarbons)
Number of saturations is variable (usually 1 or 2 double bonds)
Increase in double bonds: increase in fluidity
Decrease in chain length: decrease in melting point
Asymmetry of plasma membrane
Cell positions different molecules on different sides of membrane
Flipase
Enzyme that transfers phospholipid from cytosolic face of membrane to the other side
Lipids made on ER
Homeoviscous adaptation
Alteration of lipid composition to alter membrane fluidity
Some organisms can shorten chain length or add double bonds in response to cooling
Organisms who do so: bacteria, poikilotherms (organisms whose internal temps vary greatly)
Lipid rafts
Membrane regions with more sterols and longer chain length (more saturation)
Less fluidity
Involved in:
Detection of signals and response (ex- T-cell signals in immunity)
Entry sites of pathogens
Types of proteins in plasma membrane
Transporters (in: sugars, ions, water, peptides; out: waste, manufactured products)
Anchors
Receptors
Enzymes
Protein connection to plasma membrane
Transmembrane: all the way through membrane, alpha helices, 23-30 amino acids to span membrane
Membrane-associated: top part non-polar, bottom part polar
Lipid-linked
Protein-attached: quaternary structure (2 proteins attached), bonds (H-bonds, disulfide, non-polar, etc.)
Transmembrane proteins
Alpha helices and beta barrels: H-bonding covers up polar parts, exposed R-groups are non-polar
Beta barrel: found in outer membranes of mitochondria and bacteria
Glycosylation
Covalent addition of sugars to proteins outside of plasma membrane
Typically mannose, sialic acid, or galactose
Varies in terms of which sugars, number of sugars, and linkage of sugars
Glycoprotein
Protein with a few sugars covalently bonded to it
Proteoglycan
Protein with long sugar chains covalently bonded to it
N-linked glycosylation
Carbohydrate group linked to amino group of asparagine
O-linked glycosylation
Carbohydrate group linked to hydroxyl group of serine or threonine
Roles of glycoproteins
Recognition by other cells (bind to protein that recognizes)
Hydration: sugars are polar- attract water
Tissue formation: getting like cells to associate
Recognition by pathogens
Glycocalyx
Glycoproteins on outside of epithelial cells
Purpose: water absorption, protection, identification
Determine fluidity of proteins in membrane
Label membrane proteins (fluorescence, etc.)
Fuse cell with labeled proteins with cell with differently labeled proteins
Track movement of proteins using microscopy
Cortex
Protein structure
Location of anchorage of other proteins
Spectrin: linking protein
Detergents
Remove lipids from proteins: non-polar parts compete with lipids
Sodium dodecyl sulfate (SDS): give proteins negative charge
Triton X-100
Detergent mechanism
Membrane protein in lipid bilayer + detergent micelles -> water-soluble complexes of transmembrane proteins and detergent molecules + water-soluble mixed lipid-detergent micelles
