Post on 29-Jan-2016
Chapter 5: Warm-Up 11. Is the plasma membrane
symmetrical? Why or why not?2. What types of substances cross the
membrane the fastest? Why?3. Explain the concept of water
potential. (Hint: Refer to Lab 1)
Chapter 5: Warm-Up 21. What are glycoproteins and
glycolipids and what is their function?
2. How do hydrophilic substances cross the cell membrane?
3. Why does water move through the bi-layer quickly?
Chapter 5: Warm-Up 31. Explain membrane potential and how it
affects the cell.2. In a U-tube, side A has 4 M glucose and 2
M NaCl. Side B has 2M glucose and 6 M NaCl. Initially, side A is ____ to side Band side B is ____ to side A. What happens if the membrane is permeable to both solutes? Only permeable to water and NaCl?
Chapter 5: Warm-Up 41. Side A in a U tube has 5M sucrose and 3
M glucose. Side B has 2 M sucrose and 1 M glucose. The membrane is permeable to glucose and water only. What happens to each side?
Chapter 5: Warm-Up 51. Side A in a U tube has 3 M sucrose and 1
M glucose. Side B has 1 M sucrose and 3 M glucose. The membrane is permeable to glucose and water only. What happens to each side?
Membrane Structure and Function
What You Must Know: Why membranes are selectively
permeable. The role of phospholipids, proteins, and
carbohydrates in membranes. How water will move if a cell is placed in
an isotonic, hypertonic, or hypotonic solution.
How electrochemical gradients are formed.
Cell MembraneA. Plasma membrane is selectively
permeableAllows some substances to cross more
easily than othersB. Fluid Mosaic Model
Fluid: membrane held together by weak interactions
Mosaic: phospholipids, proteins, carbs
Early membrane model(1935) Davson/Danielli –
Sandwich modelphospholipid bilayer
between 2 protein layersProblems: varying
chemical composition of membrane, hydrophobic protein parts
The freeze-fracture method: revealed the structure of membrane’s interior
Fluid Mosaic Model
PhospholipidsBilayerAmphipathic =
hydrophilic head, hydrophobic tail
Hydrophobic barrier: keeps hydrophilic molecules out
Membrane fluidityLow temps: phospholipids
w/unsaturated tails (kinks prevent close packing)
Cholesterol resists changes by:limit fluidity at high
tempshinder close packing at
low tempsAdaptations: bacteria in
hot springs (unusual lipids); winter wheat ( unsaturated phospholipids)
Membrane ProteinsIntegral Proteins Peripheral ProteinsEmbedded in
membraneDetermined by freeze
fractureTransmembrane with
hydrophilic heads/tails and hydrophobic middles
Extracellular or cytoplasmic sides of membrane
NOT embeddedHeld in place by the
cytoskeleton or ECMProvides stronger
framework
Integral & Peripheral proteins
Hydrophobic interior
Hydrophilic ends
Some functions of membrane
proteins
CarbohydratesFunction: cell-cell recognition; developing
organismsGlycolipids, glycoproteinsEg. blood transfusions are type-specific
Synthesis and sidedness of membranes
Selective PermeabilitySmall molecules Small molecules (polar or nonpolar) cross
easily (hydrocarbons, hydrophobic molecules, CO2, O2)
Hydrophobic core prevents passage of ionsions, , large polar molecules large polar molecules
Passive TransportNO ENERGY needed!Diffusion down concentration gradientconcentration gradient (high
low concentration)Eg. hydrocarbons, CO2, O2, H2O
Facilitated DiffusionTransport proteinsTransport proteins (channel or carrier
proteins) help hydrophilic substance cross(1) Provide hydrophilic channel or (2) loosely bind/carry molecule across
Eg. ions, polar molecules (H2O, glucose)
Aquaporin: channel protein that allows passage of H2O
Active TransportRequires ENERGYENERGY (ATP)Proteins transport substances against concentration gradientconcentration gradient (low high conc.)
Eg. Na+/K+ pump, proton pump
Electrogenic Pumps: generate voltage across membraneNa+/K+ Pump Proton PumpPump Na+ out, K+ into
cellNerve transmission
Push protons (H+) across membrane
Eg. mitochondria (ATP production)
Cotransport: membrane protein enables “downhill” diffusion of one solute to drive “uphill” transport of other
Eg. sucrose-H+ cotransporter (sugar-loading in plants)
Passive vs. Active TransportLittle or no EnergyHigh low
concentrationsDOWN the
concentration gradient
eg. diffusion, osmosis, facilitated diffusion (w/transport protein)
Requires Energy (ATP)
Low high concentrations
AGAINST the concentration gradient
eg. pumps, exo/endocytosis
Bulk TransportTransport of proteins, polysaccharides,
large molecules
Endocytosis: take in macromolecules, form new vesicles
Exocytosis: vesicles fuse with cell membrane, expel contents
Phagocytosis:“cellular eating” - solids
Pinocytosis:“cellular drinking” - fluids
Receptor-Mediated Endocytosis:Ligands bind to specific receptors on cell surface
Membrane Transport