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Law_Sherrice_Block4_MM3 (Cellular membranes are fluid mosaics of lipids…
Law_Sherrice_Block4_MM3
Cellular membranes are fluid mosaics of lipids and proteins
Phospholipid is amphipathic molecule
hydrophilic region
hydrophobic
Fluid mosaic model
membrane is fluid structure attached to bilayer of phospholipids
Sandwich model
phospholipids between heads (protein)
Membrane proteins are not very soluble bc amphipathic
Proteins are individually inserted into bilayer
Fluidity
Rarely flip-flop, but they move laterally fast
Membrane remains fluid as temp decreases until closely packed and solidifies
unsaturated=more fluid; saturated=viscous
Steroid cholesterol between phospholipids in animal cells
Warm: makes membrane less fluid
Lowers temp required for membrane to solidify
Temp buffer
Membrane Proteins
Proteins determine membrane’s specific functions
Integral proteins penetrate hydrophobic core of bilayer
Peripheral proteins are not in bilayer, loosely bound to surface of membrane
Functions
Transport
through hydrophilic channels or change shape (use ATP) to pump
Enzymatic activity: may be enzyme with active site
Signal transduction
binding site with shape; chemical messenger changes shape and relays message
Cell-cell recognition
glycoproteins serve as ID tags
Intercellular joining
adjacent membrane proteins hook together in junctions, like gap to tight
Attachment to cytoskeleton and ECM
microfilaments or cytoskeleton may be bonded to proteins that helps maintain cell shape and stabilize location
The Role of Membrane Carbs in Cell-Cell Recognition
Basis of rejecting foreign cells in immune system
Membrane carbs: short, branched chains, fewer than 15 sugar units
Covalently bond with lipids: glycolipid
Glycoproteins
Synthesis and Sidedness of Membranes
Synthesis of membrane proteins/lipids in ER: glycoproteins
Golgi: glycolipids
Transmembrane proteins, membrane glycolipids, secretory proteins are transported in vesicles to plasma
Vesicles fuse with membrane
Membrane structure results in selective permeability
Hydrophobic nonpolar molecules like hydrocarbons, CO2, O2 can dissolve in lipid bilayer
Transport Proteins
Hydrophilic pass through transport proteins
Channel proteins (hydrophilic channel)
aquaporins to help water transport
Carrier proteins: change shape
Passive transport is diffusion of a substance across a membrane with no energy investment
Thermal motion(heat): diffusion: spread out evenly
More concentrated to less concentrated
Diffuse down to its concentration gradient
Osmosis
Water diffuses from lower to higher concentration
Water Balance of Cells Without Walls
Tonicity (ability of a solution to cause a cell to gain or lose water)
Depends on non-penetrating solutes
Isotonic: stable, no net movement of water
Hypertonic(more non-penetrating solutes): lose water and shrivel
Hypotonic: water enter cell faster than it leaves and will burst
Osmoregulation: control of water balance
Water Balance of Cells with Walls
In hypotonic, plant cells will maintain water balance
Cell is turgid (very firm)=healthy state of plant cells
If plant’s cells and their surroundings are isotonic, no net tendency for water to enter, and cell becomes flaccid(limp)
In hypertonic, plasmolysis causes plant to wilt and can be lethal
Facilitated Diffusion: Passive Transport Aided by Proteins
Channel proteins allow water/small ion molecules to flow fast
Ion channels function as gated channels (stimulus of electric or chemical causes them to open or close)
Carrier proteins change shape from binding/releasing molecules
Active transport uses energy to move solutes against their gradients
Cell must use energy; uses carrier proteins
Allows maintaining internal concentrations of small molecules that differ from environment
Maintenance of Membrane Potential by Ion Pumps
All cells have voltages(electric potential energy) across plasma membrane
Cytoplasm is negative
Voltage across a membrane is membrane potential (-50 to -200 mV)
Acts like a battery
Electrochemical gradient (ion’s concentration gradient and effect of membrane potential on the ion’s movement)
Sodium-potassium pump: 3 sodium ions out and 2 potassium in
Transport protein generates voltage across a membrane is called electrogenic pump
Proton pump: transport protons out of the cell (plants, fungi, bacteria)
Cotransport: Coupled Transport by a Membrane Protein
Single ATP powered pump that transports a specific solute can indirectly drive the active transport of several other solutes
Proteins can move sucrose into cell against concentration gradient WITH a hydrogen ion
Plants use this process to load sucrose by photosynthesis in specialized cells
Found treatments for dehydration from diarrhea
High concentrations of glucose and salt
Solutes taken up by transport proteins on intestinal cell surface and pass through cells into blood
Osmotic pressure causes a flow of water from intestine through cells into blood
Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
Exocytosis
Cell secretes macromolecules by fusion of vesicles with plasma
Endocytosis
Cell takes in macromolecules and matter by forming new vesicles from plasma membrane
Phagocytosis (cellular eating)
Cell engulfs particle by wrapping pseudopodia around it and packaging it within a membrane closed sac large enough to be a vacuole; particle is digested after vacuole fuses with lysosome containing hydrolytic enzymes
Pinocytosis (cellular drinking)
Cell “gulps” droplets of extracellular fluid into tiny vesicles
All solutes are taken in so nonspecific
Receptor-mediated endocytosis
Enables cell to store bulks even though not very concentrated in extracellular fluid
Receptor proteins are clustered called coated pits
Extracellular substances (ligands) bind to receptors and pit forms vesicle
Receptors are recycled back by same vesicle
Human cells use receptor-mediated endocytosis to take in cholesterol to synthesize membranes and as a precursor for synthesis of other steroids
Cholesterol travels in particles called low-density lipoproteins (LDL) that act as ligands by binding to receptor and enter cell by endocytosis