The Cell Membrane The Cell Membrane FunctionsFunctions

FunctionFunction Regulates the movement of materials Regulates the movement of materials

from one environment to the other.from one environment to the other. Transports raw materials into the cell and Transports raw materials into the cell and

waste out of the cell.waste out of the cell. Prevents the entry of unwanted matter Prevents the entry of unwanted matter

and the escape of needed materials.and the escape of needed materials. Maintain a steady environment: Maintain a steady environment:

HomeostasisHomeostasis

Structure:Structure:

Composed of a phospholipid bilayer with Composed of a phospholipid bilayer with a collage of many different proteins, lipids a collage of many different proteins, lipids and carbohydrates.and carbohydrates.

A Phospholipid is composed of 1 glycerol A Phospholipid is composed of 1 glycerol molecule, 2 fatty acids and 1 phosphate molecule, 2 fatty acids and 1 phosphate group. This structure causes hydrophilic group. This structure causes hydrophilic and hydrophobic regions.and hydrophobic regions.

Phospholipid bilayerPhospholipid bilayer

polarhydrophilicheads

nonpolarhydrophobictails

polarhydrophilicheads

The Fluid-Mosaic The Fluid-Mosaic Membrane Model Membrane Model

Membranes are not static; they have a fluid Membranes are not static; they have a fluid consistency.consistency.

Most membrane lipids and proteins can drift Most membrane lipids and proteins can drift about laterally in the plane of the membrane.about laterally in the plane of the membrane.

Cholesterol enhances membrane fluidity, Cholesterol enhances membrane fluidity, allows animal membranes to function in a wide allows animal membranes to function in a wide range of temperatures and also makes the range of temperatures and also makes the membrane less permeable to biological membrane less permeable to biological molecules.molecules.

Membrane Proteins:Membrane Proteins:Two Types:Two Types: Integral:Integral: Proteins that insert into the Proteins that insert into the

membrane (transmembrane proteins)membrane (transmembrane proteins) Peripheral:Peripheral: Proteins attached to the surface Proteins attached to the surface

of the cell membrane.of the cell membrane.Function:Function: TransportationTransportation EnzymesEnzymes Receptor sitesReceptor sites Cell adhesionCell adhesion Attachment to the cytoskeletonAttachment to the cytoskeleton

Carbohydrates:Carbohydrates: Usually branched molecules of 15 or less Usually branched molecules of 15 or less

sugar units.sugar units. Some are bonded to lipids: Some are bonded to lipids: Glycolipids.Glycolipids. Most are bonded to proteins: Most are bonded to proteins:

Glycoproteins.Glycoproteins. Function: Function: Cell-cell recognition.Cell-cell recognition.

Membrane carbohydrates Membrane carbohydrates Play a key role in Play a key role in cell-cell recognitioncell-cell recognition

ability of a cell to distinguish one cell from ability of a cell to distinguish one cell from anotheranother antigensantigens

important in organ & important in organ & tissue developmenttissue development

basis for rejection of basis for rejection of foreign cells by foreign cells by immune systemimmune system

Membrane is a collage of proteins & other Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid molecules embedded in the fluid matrix of the lipid bilayerbilayer

Extracellular fluid

Cholesterol

Cytoplasm

Glycolipid

Transmembraneproteins

Filaments ofcytoskeleton

Peripheralprotein

Glycoprotein

Phospholipids

Through the Cell Through the Cell MembraneMembrane

1.1. DiffusionDiffusion2.2. OsmosisOsmosis3.3. Facilitated DiffusionFacilitated Diffusion4.4. Active TransportActive Transport5.5. Bulk TransportBulk Transport

1. Diffusion:1. Diffusion: PassivePassive movement of molecules from a region of movement of molecules from a region of

high concentration to a region of low high concentration to a region of low concentration.concentration.

((Concentration gradientConcentration gradient is the difference in is the difference in concentration between the two regions)concentration between the two regions)

Small, uncharged molecules like OSmall, uncharged molecules like O22, CO, CO22 and and HH22O can move easily through the membrane.O can move easily through the membrane.

Works well over short distances. Once Works well over short distances. Once molecules enter the cell the rate of diffusion molecules enter the cell the rate of diffusion slows.slows.

Limits cell size.Limits cell size.

Move from Move from HIGHHIGH to to LOWLOW concentration concentration ““passive transport”passive transport” no energy neededno energy needed

diffusion osmosis

movement of water

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AquaporinsAquaporins Water ChannelsWater Channels Protein pores used during OSMOSISProtein pores used during OSMOSIS

WATERMOLECULES

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Isotonic Solution

NO NET MOVEMENT OF H2O (equal amounts entering & leaving)

Hypotonic Solution

CYTOLYSIS

Hypertonic Solution

PLASMOLYSIS

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Cytolysis & PlasmolysisCytolysis & Plasmolysis

Cytolysis Plasmolysis

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Three Forms of Transport Across the MembraneThree Forms of Transport Across the Membrane

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2. Osmosis2. Osmosis Diffusion of the solvent across a semi-permeable Diffusion of the solvent across a semi-permeable

membrane separating two solutions. (Diffusion of water)membrane separating two solutions. (Diffusion of water) Water molecules move from a region of high Water molecules move from a region of high

concentration to a region of low concentration.concentration to a region of low concentration. Direction depends on the relative concentration of water Direction depends on the relative concentration of water

molecules on either side of the cell membrane.molecules on either side of the cell membrane. Isotonic:Isotonic: Water inside the cell equals the water outside Water inside the cell equals the water outside

the cell and equal amounts of water move in and out of the cell and equal amounts of water move in and out of the cell.the cell.

Hypotonic:Hypotonic: Water outside the cell is greater than that Water outside the cell is greater than that inside the cell, water moves into the cell, may cause cell inside the cell, water moves into the cell, may cause cell to burst (lysis)to burst (lysis)

Hypertonic:Hypertonic: Water inside the cell is greater than Water inside the cell is greater than outside. Water moves out of the cell, may cause the cell outside. Water moves out of the cell, may cause the cell to shrink (plasmolysis)to shrink (plasmolysis)

Osmosis: HypotonicOsmosis: Hypotonic

3. Facilitated Diffusion3. Facilitated Diffusion Assists with the movement of large molecules Assists with the movement of large molecules

like glucose.like glucose. PassivePassive movement of a substance into or out of movement of a substance into or out of

the cell by means of carrier proteins or channel the cell by means of carrier proteins or channel proteins.proteins.

Moves molecules from high to low regions of Moves molecules from high to low regions of concentration.concentration.

Carrier proteins:Carrier proteins: Transports non-charged Transports non-charged molecules with a specific shape.molecules with a specific shape.

Channel proteins:Channel proteins: Tunnel shape that Tunnel shape that transports small charged molecules.transports small charged molecules.

4. Active Transport4. Active Transport The process of moving substances against their The process of moving substances against their

concentration gradients. Requires energy.concentration gradients. Requires energy. Examples:Examples: Kidney cells pump glucose and amino acids out of the Kidney cells pump glucose and amino acids out of the

urine and back into the blood.urine and back into the blood. Intestinal cells pump in nutrients from the gut.Intestinal cells pump in nutrients from the gut. Root cells pump in nutrients from the soil.Root cells pump in nutrients from the soil. Gill cells in fish pump out sodium ions.Gill cells in fish pump out sodium ions. Active Transport Pump:Active Transport Pump:

Sodium-potassium pumpSodium-potassium pump 3 sodium ions inside the cell and 2 potassium ions 3 sodium ions inside the cell and 2 potassium ions

outside the cell bind to the pump.outside the cell bind to the pump. This allows the release of energy from ATP and This allows the release of energy from ATP and

causes the protein complex to change shape.causes the protein complex to change shape. The change in shape allow the Na + and K+ ions to The change in shape allow the Na + and K+ ions to

move across and be released.move across and be released.

Active Transport PumpActive Transport Pump

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Active transportActive transportExamples: Pumping Examples: Pumping

NaNa++ (sodium ions) out (sodium ions) out and Kand K++ (potassium (potassium ions) in against ions) in against strong concentration strong concentration gradientsgradients..

Called Na+-K+ PumpCalled Na+-K+ Pump

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Sodium-Potassium PumpSodium-Potassium Pump

3 Na+ pumped in for every 2 K+ pumped out; creates a membrane potential

5. Bulk Transport5. Bulk Transport1. Endocytosis:1. Endocytosis: The cell membrane folds The cell membrane folds

inward, traps and encloses a small amount of inward, traps and encloses a small amount of matter from the extracellular fluid. 3 types:matter from the extracellular fluid. 3 types:

EndocytosisEndocytosis Pinocytosis: Pinocytosis: The intake of a small droplet of The intake of a small droplet of

extracellular fluid. This occurs in nearly all cell types.extracellular fluid. This occurs in nearly all cell types. Phagocytosis:Phagocytosis: The intake of a large droplet of The intake of a large droplet of

extracellular fluid. This occurs in specialized cells.extracellular fluid. This occurs in specialized cells. Receptor-assisted endocytosis:Receptor-assisted endocytosis: The intake of The intake of

specific molecules that attach to special proteins in specific molecules that attach to special proteins in the cell membrane. These proteins are uniquely the cell membrane. These proteins are uniquely shaped to fit the shape of a specific molecule.shaped to fit the shape of a specific molecule.

Bulk TransportBulk Transport

2. Exocytosis:2. Exocytosis: The reverse of endocytosis: A The reverse of endocytosis: A vesicle from inside the cell moves to the cell vesicle from inside the cell moves to the cell membrane. The vesicle fuses to the membrane membrane. The vesicle fuses to the membrane and the contents are secreted and the contents are secreted