Membrane Structure and Membrane Structure and FunctionFunction

Chapter 7Chapter 7

Plasma MembranePlasma Membrane

The boundary that separates the living cell The boundary that separates the living cell from its nonliving surroundings. from its nonliving surroundings.

Surrounds the cell and controls chemical Surrounds the cell and controls chemical traffic into and out of the cell.traffic into and out of the cell.

Selectively permeable: Selectively permeable: allows some allows some substances to cross more easily than substances to cross more easily than others.others.

Phospholipid BilayerPhospholipid Bilayer Polar (hydrophilic) heads of phospholipids are oriented Polar (hydrophilic) heads of phospholipids are oriented

towards the outside of the membrane.towards the outside of the membrane. The nonpolar (hydrophobic) tails are oriented towards The nonpolar (hydrophobic) tails are oriented towards

the inside of the membrane, away from the cell contents the inside of the membrane, away from the cell contents and the outside environment.and the outside environment.

Proteins are imbedded in the bilayer.Proteins are imbedded in the bilayer.

Fluid Mosaic ModelFluid Mosaic Model Membranes must be fluid to work properly. Membranes must be fluid to work properly. Membranes are held together by weak hydrophobic Membranes are held together by weak hydrophobic

interactions, which are weak attractions. interactions, which are weak attractions. Lipids and proteins drift and bob within the bilayer.Lipids and proteins drift and bob within the bilayer. Membranes can solidify if the temperature drops too low. Membranes can solidify if the temperature drops too low.

Critical temperature is lower in membranes with a greater Critical temperature is lower in membranes with a greater concentration of unsaturated phospholipids.concentration of unsaturated phospholipids.

Unsaturated hydrocarbon tails enhance membrane fluidity, Unsaturated hydrocarbon tails enhance membrane fluidity, because kinks in the hydrocarbon tails hinder close packing because kinks in the hydrocarbon tails hinder close packing of phospholipids.of phospholipids.

Cholesterol, found in plasma membranes of eukaryotes, Cholesterol, found in plasma membranes of eukaryotes, makes the membrane:makes the membrane:

Less fluid at warmer temperatures by restraining Less fluid at warmer temperatures by restraining phospholipid movement.phospholipid movement.

More fluid at lower temperatures by preventing close packing More fluid at lower temperatures by preventing close packing of phospholipids.of phospholipids.

Fluid Mosaic Model Cont.Fluid Mosaic Model Cont. Cells may alter membrane lipid concentration in response Cells may alter membrane lipid concentration in response

to changes in temperature. Cold tolerant plants increase to changes in temperature. Cold tolerant plants increase the unsaturated phospholipid concentration in autumn, the unsaturated phospholipid concentration in autumn, which prevents the plasma membranes from solidifying in which prevents the plasma membranes from solidifying in winter.winter.

A membrane is a A membrane is a mosaic mosaic of different proteins embedded of different proteins embedded and dispersed in the phospholipid bilayer. and dispersed in the phospholipid bilayer.

1. 1. Integral proteins: Integral proteins: inserted into the membrane so their inserted into the membrane so their hydrophobic regions are surrounded by hydrocarbon hydrophobic regions are surrounded by hydrocarbon portions of phospholipids. They may be:portions of phospholipids. They may be: Unilateral, reaching only partway across the membrane.Unilateral, reaching only partway across the membrane. Transmembrane, with hydrophobic midsections Transmembrane, with hydrophobic midsections

between hydrophilic ends exposed on both sides of the between hydrophilic ends exposed on both sides of the membrane.membrane.

2. Peripheral proteins: 2. Peripheral proteins: not embedded but attached to the not embedded but attached to the membrane's surface. May be attached to integral proteins membrane's surface. May be attached to integral proteins or held by filaments of cytoskeleton.or held by filaments of cytoskeleton.

Fluid Mosaic Model Cont.Fluid Mosaic Model Cont. Additional cell markers (carbohydrates) are found on the Additional cell markers (carbohydrates) are found on the

external surface of the plasma membrane.external surface of the plasma membrane. Usually branched Usually branched oligosaccharides oligosaccharides (< 15 monomers).(< 15 monomers). Some covalently bonded to lipids Some covalently bonded to lipids (glycolipids).(glycolipids). Most covalently bonded to proteins Most covalently bonded to proteins (glycoproteins).(glycoproteins).

Cell-cell recognitionCell-cell recognition -- Ability of a cell to determine if -- Ability of a cell to determine if other cells it encounters are alike or different from itself.other cells it encounters are alike or different from itself.

Cell-cell recognition is crucial in the functioning of an Cell-cell recognition is crucial in the functioning of an organism. organism.

Sorting of an animal embryo's cells into tissues and Sorting of an animal embryo's cells into tissues and organs.organs.

Rejection of foreign cells by the immune system.Rejection of foreign cells by the immune system. Variation from species to species, between individuals of Variation from species to species, between individuals of

the same species, and among cells in the same the same species, and among cells in the same individual.individual.

Permeability of the MembranePermeability of the Membrane Nonpolar (Hydrophobic) MoleculesNonpolar (Hydrophobic) Molecules • • Dissolve in the bilayer and cross it with ease. (hydrocarbons, ODissolve in the bilayer and cross it with ease. (hydrocarbons, O22).). • • Smaller molecules will cross the membrane faster.Smaller molecules will cross the membrane faster. Polar (Hydrophilic) MoleculesPolar (Hydrophilic) Molecules • • Small, polar molecules (HSmall, polar molecules (H22O, COO, CO22) are small enough to pass slowly ) are small enough to pass slowly

between membrane lipids.between membrane lipids. • • Larger, polar molecules (glucose) will Larger, polar molecules (glucose) will not not easily pass through.easily pass through. • • Ions (Na+, H+) have difficulty penetrating the hydrophobic layer due to Ions (Na+, H+) have difficulty penetrating the hydrophobic layer due to

charge.charge.

Biological membranes Biological membranes are are permeable to specific ions and certain polar permeable to specific ions and certain polar molecules of moderate size. These hydrophilic substances avoid the molecules of moderate size. These hydrophilic substances avoid the hydrophobic core of the bilayer by passing through hydrophobic core of the bilayer by passing through transport proteins.transport proteins.

Transport proteins:Transport proteins: • • May provide a hydrophilic tunnel through the membrane.May provide a hydrophilic tunnel through the membrane. • • May bind to a substance and physically move it across the membrane.May bind to a substance and physically move it across the membrane. • • Are specific for the substance they transport.Are specific for the substance they transport.

Passive Movement Across the Passive Movement Across the MembraneMembrane

Concentration gradientConcentration gradient -- concentration change -- concentration change over a distance in a particular direction.over a distance in a particular direction.

Net directional movementNet directional movement -- Overall movement -- Overall movement away from the center of concentration.away from the center of concentration.

DiffusionDiffusion -- The -- The net net movement of a substance movement of a substance down a down a concentration gradient concentration gradient (from area of high (from area of high conc to area of low conc)conc to area of low conc)..

Passive transportPassive transport – Spontaneous diffusion of a – Spontaneous diffusion of a substance across a biological membrane; does substance across a biological membrane; does not require the cell to expend energy; driven by not require the cell to expend energy; driven by potential energy stored in a concentration potential energy stored in a concentration gradient.gradient.

Osmosis is the passive transport of Osmosis is the passive transport of waterwater

OsmosisOsmosis -- Diffusion of water across a selectively -- Diffusion of water across a selectively permeable membrane from an area of higher water permeable membrane from an area of higher water potential to an area of lower water potential.potential to an area of lower water potential.

Hypertonic solutionHypertonic solution -- (hyperosmotic) A solution with a -- (hyperosmotic) A solution with a greater solute concentration than that inside a cell; water greater solute concentration than that inside a cell; water will move out from cell and it may shrivel and die. will move out from cell and it may shrivel and die.

Hypotonic solutionHypotonic solution – (hypoosmotic) A solution with a – (hypoosmotic) A solution with a lower solute concentration compared to that inside a cell; lower solute concentration compared to that inside a cell; water will move into the cell and it may burst.water will move into the cell and it may burst.

Isotonic solutionIsotonic solution – (isosomotic) A solution with an equal – (isosomotic) A solution with an equal solute concentration compared to that inside a cell; equal solute concentration compared to that inside a cell; equal water flow in both directions.water flow in both directions.

Water Potential / Osmotic PressureWater Potential / Osmotic Pressure

Pure water = 0.Pure water = 0. The more solute is The more solute is

dissolved, the lower dissolved, the lower the water potential the water potential (negative #).(negative #).

Equals 0 if both sides Equals 0 if both sides of membrane have of membrane have equal solute equal solute concentration.concentration.

Greater solute conc Greater solute conc (lower water potential) (lower water potential) = greater osmotic = greater osmotic pressure.pressure.

Water Balance of Cells With WallsWater Balance of Cells With Walls

Prokaryotes, some protists, fungi and plants have cell Prokaryotes, some protists, fungi and plants have cell walls outside the plasma membrane.walls outside the plasma membrane.

PlasmolysisPlasmolysis -- a walled cell shrivels and the plasma -- a walled cell shrivels and the plasma membrane pulls away from the cell wall as the cell loses membrane pulls away from the cell wall as the cell loses water to a hypertonic environment; usually lethal.water to a hypertonic environment; usually lethal.

TurgidTurgid -- Firmness or tension found in walled cells that -- Firmness or tension found in walled cells that are in a hypoosmotic environment where water enters are in a hypoosmotic environment where water enters the cell by osmosis.the cell by osmosis.

Ideal state for most plant cells.Ideal state for most plant cells. Turgid cells provide mechanical support for plants.Turgid cells provide mechanical support for plants. Requires cells to be hyperosmotic to their environment.Requires cells to be hyperosmotic to their environment.

In an isotonic environment, there is no net movement of In an isotonic environment, there is no net movement of water into or out of the cell; loss of structural support water into or out of the cell; loss of structural support from turgor pressure causes plants to wilt.from turgor pressure causes plants to wilt.

Facilitated DiffusionFacilitated Diffusion Movement of solutes across a membrane with the help Movement of solutes across a membrane with the help

of transport proteins of transport proteins (permeases).(permeases). Is passive because solute is transported down its Is passive because solute is transported down its

concentration gradient.concentration gradient. Transport proteins are Transport proteins are specific specific for the solutes they for the solutes they

transport. Specific binding site analogous to an transport. Specific binding site analogous to an enzyme's active site.enzyme's active site.

Transport proteins can be inhibited by molecules that Transport proteins can be inhibited by molecules that resemble the solute normally carried by the protein resemble the solute normally carried by the protein (similar to competitive inhibition in enzymes).(similar to competitive inhibition in enzymes).

Uniport Transport ProteinUniport Transport Protein – carries a single molecule – carries a single molecule across.across.

SymportSymport – moves 2 different molecules in same – moves 2 different molecules in same direction.direction.

Antiport Antiport – moves 2 molecules in opposite directions.– moves 2 molecules in opposite directions.

http://www.sp.uhttp://www.sp.uconn.edu/~terryconn.edu/~terry/images/anim/s/images/anim/symport.gifymport.gif

http://http://www.sp.uconn.www.sp.uconn.edu/~terry/edu/~terry/images/anim/images/anim/antiport.gifantiport.gif

Active TransportActive Transport

Energy-requiring process during which a transport Energy-requiring process during which a transport protein pumps a molecule across a membrane, protein pumps a molecule across a membrane, against against its concentration gradient.its concentration gradient.

Transport proteins involved in active transport harness Transport proteins involved in active transport harness energy from ATP to pump molecules “uphill”.energy from ATP to pump molecules “uphill”.

An example of an active transport system that An example of an active transport system that translocates ions against steep concentration gradients translocates ions against steep concentration gradients is the is the sodium-potassium pump.sodium-potassium pump.

ATP powers the shape change in the protein from Na-ATP powers the shape change in the protein from Na-receptive to K-receptive.receptive to K-receptive.

Three Na+ ions out of the cell for every two K+ ions Three Na+ ions out of the cell for every two K+ ions pumped into the cell. pumped into the cell.

http://www.brookscole.com/chemistry_d/tehttp://www.brookscole.com/chemistry_d/templates/student_resources/shared_resourmplates/student_resources/shared_resources/animations/ion_pump/ionpump.htmlces/animations/ion_pump/ionpump.html

Vesicle-Mediated TransportVesicle-Mediated Transport Large molecules (proteins and polysaccharides) cross membranes Large molecules (proteins and polysaccharides) cross membranes

by the processes of by the processes of exocytosis exocytosis and and endocytosis.endocytosis. ExocytosisExocytosis -- Process of exporting materials from a cell; vesicle -- Process of exporting materials from a cell; vesicle

usually budded from the ER or Golgi and migrates to and fuses with usually budded from the ER or Golgi and migrates to and fuses with plasma membrane .plasma membrane .

Used by secretory cells (insulin from pancreas, or neuro-transmitter Used by secretory cells (insulin from pancreas, or neuro-transmitter from neuron).from neuron).

EndocytosisEndocytosis -- Process of importing materials into a cell; vesicle -- Process of importing materials into a cell; vesicle forms from a region of plasma membrane that sinks inward and forms from a region of plasma membrane that sinks inward and pinches off. pinches off.

PhagocytosisPhagocytosis -- (cell eating) Endocytosis of solid particles; forms a -- (cell eating) Endocytosis of solid particles; forms a food vacuole which fuses with a lysosome.food vacuole which fuses with a lysosome.

PinocytosisPinocytosis -- (cell drinking) Endocytosis of fluid droplets. -- (cell drinking) Endocytosis of fluid droplets. Receptor-mediated endocytosisReceptor-mediated endocytosis – Large molecules attach to specific – Large molecules attach to specific

receptors on the cell's surface; causes vesicle to form around the receptors on the cell's surface; causes vesicle to form around the substance.substance.

Membrane-embedded proteins cluster in regions called Membrane-embedded proteins cluster in regions called coated pits.coated pits. Molecule that binds to the receptor site is called a Molecule that binds to the receptor site is called a ligand.ligand.

http://student.ccbcmd.edu/~gkaiser/biotutohttp://student.ccbcmd.edu/~gkaiser/biotutorials/eustruct/exocyt.htmlrials/eustruct/exocyt.html

http://bio.winona.edu/berg/ANIMTNS/Secrthttp://bio.winona.edu/berg/ANIMTNS/Secrtion.htmion.htm

http://student.ccbcmd.edu/~gkaiser/biotutohttp://student.ccbcmd.edu/~gkaiser/biotutorials/eustruct/phagocyt.htmlrials/eustruct/phagocyt.html