A Closer Look at A Closer Look at MembranesMembranes

Chapter 5Chapter 5

Cystic FibrosisCystic Fibrosis

• Caused by defective protein channel Caused by defective protein channel (CFTR) in epithelial cells(CFTR) in epithelial cells

• Not enough ClNot enough Cl-- and water cross and water cross membranesmembranes

• Thick mucus clogs airways and ducts Thick mucus clogs airways and ducts throughout bodythroughout body

• Usually fatal by age 30Usually fatal by age 30

• Main component Main component of cell membranesof cell membranes

• Gives membrane Gives membrane its fluid propertiesits fluid properties

• Fatty acid tails Fatty acid tails sandwiched sandwiched between between hydrophilic headshydrophilic heads

Lipid BilayerLipid Bilayer

Figure 5.2cPage 82

lipid bilayer

fluid fluid

Fluid Mosaic ModelFluid Mosaic Model

• Membrane is a mosaic ofMembrane is a mosaic of– PhospholipidsPhospholipids– GlycolipidsGlycolipids– SterolsSterols– ProteinsProteins

• Most phospholipids and some Most phospholipids and some

proteins can drift through membraneproteins can drift through membrane

Membrane ProteinsMembrane Proteins

active transporters

receptor protein

adhesionprotein

communicationprotein

passivetransporter

recognition protein

Figure 5.5 Page 85

Membrane ExperimentsMembrane Experiments

• Split membranes reveal embedded Split membranes reveal embedded proteinsproteins

In-text figurePage 86

• Hybrid human-mouse cell shows Hybrid human-mouse cell shows some proteins drift within membranesome proteins drift within membrane

Membrane ExperimentsMembrane Experiments

human cell mouse cell hybrid cell

Figure 5.6Page 86

Selective Permeability Selective Permeability

O2, CO2, and other small, nonpolar

molecules; some water molecules

glucose and other large, polar, water-soluable molecules; ions,

water molecules

Membrane Crossing Membrane Crossing MechanismsMechanisms

Diffusion across lipid bilayerDiffusion across lipid bilayer

Passive transportPassive transport

Active transportActive transport

EndocytosisEndocytosis

ExocytosisExocytosis

Concentration Gradient Concentration Gradient

• Means the number of molecules or Means the number of molecules or ions in one region is different than ions in one region is different than the number in another regionthe number in another region

• In the absence of other forces, a In the absence of other forces, a substance moves from a region substance moves from a region where it is more concentrated to one where it is more concentrated to one where it’s less concentrated - “down” where it’s less concentrated - “down” gradientgradient

DiffusionDiffusion

• The net movement of like molecules The net movement of like molecules or ions down a concentration or ions down a concentration gradientgradient

• Although molecules collide randomly, Although molecules collide randomly, the net movement is away from the the net movement is away from the place with the most collisions (down place with the most collisions (down gradient)gradient)

Factors Affecting Factors Affecting Diffusion RateDiffusion Rate

• Steepness of concentration gradientSteepness of concentration gradient– Steeper gradient, faster diffusionSteeper gradient, faster diffusion

• Molecular sizeMolecular size– Smaller molecules, faster diffusionSmaller molecules, faster diffusion

• TemperatureTemperature– Higher temperature, faster diffusionHigher temperature, faster diffusion

• Electrical or pressure gradientsElectrical or pressure gradients

Example of DiffusionExample of Diffusion

• Span the lipid bilayerSpan the lipid bilayer

• Interior is able to open to both Interior is able to open to both sidessides

• Change shape when they interact Change shape when they interact with solute with solute

• Move water-soluble substances Move water-soluble substances across a membraneacross a membrane

Transport ProteinsTransport Proteins

Passive and Active Transport Passive and Active Transport

• Doesn’t require Doesn’t require energy inputs energy inputs

• Solutes diffuse Solutes diffuse through a channel through a channel inside the protein’s inside the protein’s interiorinterior

• Net movement is Net movement is down down concentration concentration gradientgradient

Passive Transport Active Transport

• Requires ATPRequires ATP

• Protein is an Protein is an ATPase pumpATPase pump

• Pumps solute Pumps solute against its against its concentration concentration gradientgradient

glucose transporter

solute (glucose)

high

low

Figure Figure 5.105.10Page 88Page 88

Stepped Art

Passive Transport

ATP

ADP

Pi

higher calcium concentration

lower calcium concentration

Figure 5.11Figure 5.11Page 89Page 89

Stepped Art

Active Transport

OsmosisOsmosis

• Water molecules tend to diffuse down Water molecules tend to diffuse down

water concentration gradientwater concentration gradient

• Total number of molecules or ions Total number of molecules or ions

dictates concentration of waterdictates concentration of water

• Tonicity - relative solute Tonicity - relative solute

concentrationsconcentrations

TonicityTonicity2% sucrose

solution

distilled water

10%sucrose solution

2%sucrose solution

Hypotonic Conditions

Hypertonic Conditions

Isotonic Conditions

Figure 5.13Page 90

Fluid PressureFluid Pressure

• Hydrostatic pressure Hydrostatic pressure

• Turgor pressure Turgor pressure

• Osmotic pressureOsmotic pressure

normal plant cells after plasmolysis

Endocytosis andEndocytosis and Exocytosis Exocytosis

plasma membrane

cytoplasm

cytoplasm

Exocytosis

Endocytosis

Endocytosis PathwaysEndocytosis Pathways

• Bulk phaseBulk phase

• Receptor-mediatedReceptor-mediated

• PhagocytosisPhagocytosis

clathrin Figures 5.17, 5.18Pages 92, 93

Membrane Membrane CyclingCycling

Exocytosis and Exocytosis and endocytosis endocytosis continually continually replace and replace and withdraw withdraw patches of patches of plasma plasma membranemembrane

lysosome

Golgibody

clathrin

Figure 5.19Page 93