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