Chapter 6

Chapter 6 – Cell Membranes

Fluid Mosaic ModelFluid Mosaic Model

Phospholipids form a bilayer which is like a Phospholipids form a bilayer which is like a “lake” in which a variety of proteins “float.”“lake” in which a variety of proteins “float.”

Phospholipids BilayerPhospholipids BilayerForms Spontaneously in waterForms Spontaneously in water

Vary in fatty acid chain length, degree of saturation, and phosphate Vary in fatty acid chain length, degree of saturation, and phosphate groupsgroups

Membranes may be up to 25 percent cholesterolMembranes may be up to 25 percent cholesterol

Flexible allowing lateral movementFlexible allowing lateral movement

of moleculesof molecules

FluidityFluidity depends on temperature depends on temperature

and lipid composition.and lipid composition.

Components of the Fluid Components of the Fluid Mosaic ModelMosaic Model

• LipidsLipids– Phospholipids/ CholesterolPhospholipids/ Cholesterol

• ProteinsProteins– IntergralIntergral– PeripheralPeripheral

• CarbohydratesCarbohydrates

Membrane Bounds ProteinsMembrane Bounds ProteinsTwo types of membrane proteins:Two types of membrane proteins:

• Peripheral membrane proteinsPeripheral membrane proteins lack exposed lack exposed hydrophobic groups and do not penetrate hydrophobic groups and do not penetrate the bilayer.the bilayer.

• Integral membrane proteinsIntegral membrane proteins have have hydrophobic and hydrophilic regions or hydrophobic and hydrophilic regions or domainsdomains..

Transmembrane Proteins Transmembrane Proteins

• Type of integral Type of integral proteinprotein

• Extend all the way Extend all the way through the through the phospholipid phospholipid bilayer.bilayer.

Selective PermeabilitySelective Permeability

Some substances can Some substances can pass through, but not pass through, but not othersothers

What can pass:What can pass:- Small Small - UnchargedUncharged

Examples?Examples?

PassivePassive transport:transport:

no outside energy no outside energy required (diffusion).required (diffusion).

ActiveActive transport:transport:

energy requiredenergy required

DiffusionDiffusion

The process of The process of random movement random movement toward equilibriumtoward equilibrium

It is the net movement It is the net movement from regions of from regions of greater concentration greater concentration to regions of lesser to regions of lesser concentration.concentration.

What Controls the Speed of What Controls the Speed of Diffusion?Diffusion?

- Diameter of the molecules or ions- Diameter of the molecules or ions

- Temperature of the solution- Temperature of the solution

- Concentration Gradient- Concentration Gradient

- Electrochemical Gradient Electrochemical Gradient

- When a cell is involved you also have When a cell is involved you also have to consider movement across a to consider movement across a membranemembrane. .

Simple Diffusion Simple Diffusion

Small molecules pass directly through Small molecules pass directly through the lipid bilayer.the lipid bilayer.

Water and lipid-soluble molecules Water and lipid-soluble molecules can can diffuse across the membrane.diffuse across the membrane.

Electrically charged and polar molecules Electrically charged and polar molecules can NOT pass through easily.can NOT pass through easily.

Osmosis Osmosis

The Diffusion of WaterThe Diffusion of Water

Osmosis depends on the Osmosis depends on the number number of of solute particles present, not the type of solute particles present, not the type of particles.particles.

Hypertonic Solutions: High Hypertonic Solutions: High Solute ConcentrationsSolute Concentrations

Cell ShrinksCell Shrinks

Hypotonic Solutions: Low Hypotonic Solutions: Low Solute ConcentrationsSolute Concentrations

Cell Lyses in Animal CellsCell Lyses in Animal CellsSwells in Plant Cells (Turgor Pressure)Swells in Plant Cells (Turgor Pressure)

Isotonic Solutions: Equal Isotonic Solutions: Equal solute/water concentrationsolute/water concentration

Cell Remains the SameCell Remains the Same

Facilitated DiffusionFacilitated Diffusion

Passive Transport across a membrane using Passive Transport across a membrane using an integral membrane proteinan integral membrane protein

Channel proteinsChannel proteins: have a central pore lined : have a central pore lined with polar amino acids.with polar amino acids.

Carrier proteinsCarrier proteins: : membrane proteins that bind membrane proteins that bind some substances and speed their diffusion some substances and speed their diffusion through the bilayer.through the bilayer.

Channel ProteinsChannel ProteinsSpecificitySpecificity

Best Example are Best Example are

Ion ChannelsIon Channels

Most are “Gated”Most are “Gated”

Ligand-GatedLigand-Gated

Voltage GatedVoltage Gated

Rate and direction of ion movement through channels depends on the concentration gradient and the distribution of electrical charge.

Membrane PotentialMembrane Potential

A charge imbalance across a membrane.A charge imbalance across a membrane.

Represents lots of potential energy Represents lots of potential energy

Signals volatge-gated channels to openSignals volatge-gated channels to open

Critical to ATP production during Cellular Critical to ATP production during Cellular Respiration or PhotosynthesisRespiration or Photosynthesis

Carrier ProteinsCarrier Proteins

Passive TransportPassive Transport

Directly binds to Directly binds to molecule molecule

Then Changes shape Then Changes shape to move the to move the molecule across the molecule across the membranemembrane

Active TransportActive Transport

Require EnergyRequire Energy

Move molecules Move molecules AGAINSTAGAINST the the concentration gradientconcentration gradient

UniportersUniporters

SymportersSymporters

AntiportersAntiporters

Large Molecule TransportLarge Molecule Transport

Endocytosis: bring Endocytosis: bring material into the cell material into the cell using vesiclesusing vesicles

Exocytosis: Move Exocytosis: Move material out of the cell material out of the cell using vesiclesusing vesicles

EndocytosisEndocytosis3 TYPES3 TYPES

PhagocytosisPhagocytosis: Engulf large : Engulf large molecules or even whole molecules or even whole cells making hagosomescells making hagosomes

PinocytosisPinocytosis: Cellular : Cellular Drinking (small vesicles)Drinking (small vesicles)

Receptor Mediated Receptor Mediated EndocytosisEndocytosis: binds specific : binds specific targetstargets