1 Cell Membranes Mark Mayo Cypress College Last update 9/10/13

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Cell MembranesCell Membranes

Mark MayoCypress College

Last update 9/10/13

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Fluid Mosaic ModelFluid Mosaic Model

The cell membrane is composed of two layers of The cell membrane is composed of two layers of simple lipid moleculessimple lipid molecules

They are commonly depicted as a ball with two They are commonly depicted as a ball with two dangling stringsdangling strings

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The heads face outward – away from The heads face outward – away from the middle of the the middle of the lipid bilayerlipid bilayer

The heads are The heads are hydrophilichydrophilic while the while the tails are tails are hydrophobichydrophobic

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The evidence for The evidence for our model of cell our model of cell membrane membrane structure has been structure has been verified by verified by numerous electron numerous electron micrographs that micrographs that show a dual layershow a dual layer

Cell #1

Cell #2

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Fluid Mosaic Model Fluid Mosaic Model ** Membrane components includeMembrane components include::

• Lipids (gray)Lipids (gray)• Proteins (purple)Proteins (purple)• Cholesterol (yellow)Cholesterol (yellow)

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Membranes Membranes regulate the regulate the materials that materials that can enter and can enter and leave the cellleave the cell

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Fluid Mosaic ModelFluid Mosaic Model Carbohydrates are frequently attached to Carbohydrates are frequently attached to

the proteins on the external portionthe proteins on the external portion

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Membrane ProteinsMembrane Proteins There are several types of proteins There are several types of proteins

associated with cell membranes:associated with cell membranes:

• IntegralIntegral – embedded in the membrane and are – embedded in the membrane and are difficult to removedifficult to remove

• Integral TransmembraneIntegral Transmembrane – spans the entire – spans the entire membrane from inside to the outside of the membrane from inside to the outside of the cell membrane cell membrane **

• PeripheralPeripheral – proteins loosely attached to the – proteins loosely attached to the cell membrane on the inside or outside of the cell membrane on the inside or outside of the cellcell

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Membrane ProteinsMembrane Proteins Important **(spans membrane)(spans membrane)

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Membrane ProteinsMembrane Proteins

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Membrane ProteinsMembrane Proteins

Membrane proteins have a variety of Membrane proteins have a variety of functions they performfunctions they perform• Transport proteinsTransport proteins allow the movement of allow the movement of

materials into and out of the cellmaterials into and out of the cell

• Receptor proteinsReceptor proteins receive a signal from outside receive a signal from outside the cell and pass it insidethe cell and pass it inside

• Recognition proteinsRecognition proteins display “self” signals to display “self” signals to identify foreign cellsidentify foreign cells

• Adhesion proteinsAdhesion proteins permit cell-cell junctions permit cell-cell junctions

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Membrane TransportMembrane Transport

The cell membrane is both passively The cell membrane is both passively and actively involved in the and actively involved in the movement of materials into and out movement of materials into and out of the cellof the cell

Transport of water, ions and other Transport of water, ions and other substances will be treated separatelysubstances will be treated separately

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Phagocytosis **

A form of cell eating where large particles or cells are engulfed by a white blood cell **

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EndocytosisEndocytosisthe process the process where materials where materials are brought into are brought into the cellthe cell

vs. vs.

ExocytosisExocytosisthe process the process where materials where materials are removed are removed from the cellfrom the cell

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Membrane TransportMembrane Transport EndocytosisEndocytosis is a is a

general term for the general term for the movement of movement of substances into the cellsubstances into the cell

Outside Inside

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ExocytosisExocytosis is the is the removal of removal of substances from substances from within the cell to within the cell to the outsidethe outside

This may be in the This may be in the form of waste form of waste removal or the removal or the secretion of secretion of substances needed substances needed outside the celloutside the cell

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Contractile vacuolesContractile vacuoles are used by protists are used by protists to remove internal waterto remove internal water

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Contractile vacuole in actionContractile vacuole in action

Full

Empty

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Membrane TransportMembrane Transport Passive transportPassive transport

• Simple diffusionSimple diffusion• Facilitated diffusion using a carrier moleculeFacilitated diffusion using a carrier molecule

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Membrane TransportMembrane Transport Passive transport – Passive transport – SimpleSimple DiffusionDiffusion

• DiffusionDiffusion is the passive movement of materials from an is the passive movement of materials from an area of greater concentration to an area of lesser area of greater concentration to an area of lesser concentration concentration **

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Another view of facilitated diffusionAnother view of facilitated diffusion

MembraneProtein

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Membrane TransportMembrane Transport Active transportActive transport**

requires energy, requires energy, usually in the usually in the form of ATP form of ATP **,,to move materials to move materials across the cell across the cell membranemembrane

Note that the Note that the pump works in the pump works in the reverse of diffusion reverse of diffusion due to the added due to the added energy (from lower energy (from lower to higher to higher concentration) concentration) **

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The sodium (Na /K ) pumpThe sodium (Na /K ) pump+ +

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Isotonic, Hypotonic & Hypertonic Isotonic, Hypotonic & Hypertonic SolutionsSolutions

SoluteSolute – the dissolved solids in a – the dissolved solids in a solution solution (the salt in a salt/water solution)(the salt in a salt/water solution)

SolventSolvent - the liquid that dissolves the - the liquid that dissolves the solids in a solutionsolids in a solution(the water in a salt/water solution)(the water in a salt/water solution)

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DiffusionDiffusion is the passive movement of is the passive movement of materials from an area of greater materials from an area of greater concentration to an area of lesser concentration to an area of lesser concentration concentration **

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Diffusion Diffusion **

This This animation animation shows the shows the diffusion of diffusion of small small molecules molecules from a from a concentrated concentrated source source **

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A cell placed in an A cell placed in an isotonic solutionisotonic solutionIsotonic means EQUAL SALTIsotonic means EQUAL SALT

• In the SolutionIn the Solution:: Salt concentration that is equal to that of the

cell (ie. 1.0% salt). Water concentration that is equal to that of

the cell (ie. 99.0%)

• In the Cell: Salt concentration is usually 1.0% Water concentration is usually 99.0%

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A cell placed in an A cell placed in an isotonic solutionisotonic solution

solutioncell

What happens and why...Since the water concentration outside of the cell is exactlyequal to the water concentration inside the cell, water will move in and out WITHOUT ANY NET CHANGE in the cell by simple diffusion. The cell will not change in size and is said to be in equilibrium. Salt = 0.9 %Salt = 0.9 %

Water = 99.1 %Water = 99.1 %Salt = 0.9 %Salt = 0.9 %

Water = 99.1 %Water = 99.1 %

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A cell placed in a A cell placed in a hypotonic solutionhypotonic solution hypotonic means LESS SALThypotonic means LESS SALT

• Solution:Solution: Salt concentration that is less than that of

the cell (ie. 0.1% or 0.0% salt) Water concentration that is greater than

that of the cell (ie. 99.9% or 100%)

• Cell: Salt concentration is usually 1.0% Water concentration is usually 99.0%

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solutioncell


A cell placed in a A cell placed in a hypotonic solutionhypotonic solution

What happens and why... Since the water concentration outside of the cell is greaterthan the water concentration inside the cell, water will enterthe cell by simple diffusion. The cell will expand and possibly burst generally called cytolysis or hemolysis for red blood cells. **

Cells get LARGER & HEAVIER Salt = 0 %Salt = 0 %

Water = 100 %Water = 100 %Salt = 0.9 %Salt = 0.9 %

Water = 99.1 %Water = 99.1 %

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A cell placed in a A cell placed in a hypertonic solutionhypertonic solution hypertonic means MORE SALThypertonic means MORE SALT

• Solution:Solution: Salt concentration that is greater than that

of the cell (ie. 2.0% or 10.0% salt) Water concentration that is less than that of

the cell (ie. 98.0% or 90%)• Cell:

Salt concentration is usually 1.0% Water concentration is usually 99.0%

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A cell placed in a A cell placed in a hypertonic solutionhypertonic solution**

solutioncell

What happens and why... Since the water concentration outside of the cell is less than the water concentration inside the cell, water will leave the cell** by simple diffusion. The cell will shrink and distort generally called plasmolysis or for red blood cells - crenation.

Cells get SMALLER & LIGHTER Salt = 10 %Salt = 10 %

Water = 90 %Water = 90 %Salt = 0.9 %Salt = 0.9 %

Water = 99.1 %Water = 99.1 %

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1 Cell Membranes Mark Mayo Cypress College Last update 9/10/13