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AS Biology & Human BiologyAS Biology & Human Biology

CELL MEMBRANESCELL MEMBRANESandand

TRANSPORTTRANSPORT

Colston’s School

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The CellThe Cell

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Learning ObjectivesLearning Objectives1.1. Describe the fluid mosaic model of membrane structure and explain the underlying Describe the fluid mosaic model of membrane structure and explain the underlying

reasons for this structure.reasons for this structure.

2.2. Outline the roles of phospholipids, cholesterol, glycolipids, proteins and Outline the roles of phospholipids, cholesterol, glycolipids, proteins and

glycoproteins in membranes.glycoproteins in membranes.

3.3. Outline the roles of the plasma membrane, and the roles of membranes within Outline the roles of the plasma membrane, and the roles of membranes within

cells.cells.

4.4. Describe and explain how molecules can get in and out of cells (cross cell Describe and explain how molecules can get in and out of cells (cross cell

membranes) by the processes of diffusion, facilitated diffusion, osmosis, active membranes) by the processes of diffusion, facilitated diffusion, osmosis, active

transport, endocytosis and exocytosis.transport, endocytosis and exocytosis.

5.5. Describe the effects on animal and plant cells of immersion in solutions of different Describe the effects on animal and plant cells of immersion in solutions of different

water potential.water potential.

6.6. Describe the features of the gaseous exchange surface of mammalian lung.Describe the features of the gaseous exchange surface of mammalian lung.

7.7. Describe the features of root hairs that enable the uptake of ions by active Describe the features of root hairs that enable the uptake of ions by active

transport.transport.

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Key words you should knowKey words you should know

Phospholipids Phospholipids SolutionSolution PinocytosisPinocytosis PolarPolar SoluteSolute MicropinocytosisMicropinocytosis HydrophilicHydrophilic SolventSolvent ExocytosisExocytosis HydrophobicHydrophobic Partially permeablePartially permeable Gaseous exchangeGaseous exchange MicellesMicelles Water potentialWater potential AlveoliAlveoli Phospholipid bilayerPhospholipid bilayerSolute PotentialSolute Potential Root hairRoot hair Fluid mosaic modelFluid mosaic model Pressure PotentialPressure Potential Surface areaSurface area GlycoproteinsGlycoproteins TurgidTurgid EpidermisEpidermis GlycolipidsGlycolipids PlasmolysisPlasmolysis Passive transportPassive transport CholesterolCholesterol PlasmolysedPlasmolysed ProteinsProteins Incipient plasmolysisIncipient plasmolysis Transport proteinsTransport proteins Active transportActive transport EnzymesEnzymes Carrier proteinCarrier protein Receptor moleculesReceptor molecules Bulk transportBulk transport DiffusionDiffusion EndocytosisEndocytosis Concentration gradientConcentration gradient PhagocytosisPhagocytosis Facilitated diffusionFacilitated diffusion PhagocytesPhagocytes OsmosisOsmosis Phagocytic vacuolesPhagocytic vacuoles

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Cell membraneCell membrane All living things are surrounded by a membrane.All living things are surrounded by a membrane.A cell membrane is also known as plasma A cell membrane is also known as plasma membrane.membrane.Controls exchange of materials such as nutrients Controls exchange of materials such as nutrients and waste between cells and their environment.and waste between cells and their environment.Has other important functions for example to Has other important functions for example to enable cells to receive hormones.enable cells to receive hormones.

To understand the function of anything in biology, To understand the function of anything in biology, you must study the structure first!you must study the structure first!

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Cell Membranes from Opposing Cell Membranes from Opposing Neurons (TEM x436,740). Neurons (TEM x436,740).

}} cell cell membranemembrane

7nm wide7nm wide

Cell membrane Cell membrane {{

Nerve cellNerve cell

Nerve cellNerve cell

Gap between Gap between cellscells

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HYDROPHILIC heads HYDROPHILIC heads (water liking)(water liking)-Attracted to the water -Attracted to the water

called POLARcalled POLAR

HYDROPHOBIC tails HYDROPHOBIC tails (water fearing)(water fearing)-Not attracted to the -Not attracted to the waterwater

called NON-POLARcalled NON-POLAR

Cell membranes are made Cell membranes are made of PHOSPHOLIPIDsof PHOSPHOLIPIDs

A PhospholipidA Phospholipid

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Phospholipids are important structural components Phospholipids are important structural components of cell membranes. Phospholipids are modified so of cell membranes. Phospholipids are modified so that a that a phosphate groupphosphate group (PO4-) replaces one of the (PO4-) replaces one of the

three fatty acids normally found on a lipid. The three fatty acids normally found on a lipid. The addition of this group makes a polar "head" and addition of this group makes a polar "head" and

two nonpolar "tails".two nonpolar "tails".

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A phospholipidA phospholipidHYDROPHILIC HEADAt the other end of the phospholipid

is a phosphate group and several double bonded oxygens. The atoms at this end of the molecule are not shared equally. This end of the molecule has a charge and is attracted to water. It is POLAR

HYDROPHOBIC TAILSHYDROPHOBIC TAILSThe two long chains coming off of the The two long chains coming off of the

bottom of this molecule are made bottom of this molecule are made up of carbon and hydrogen. up of carbon and hydrogen. Because both of these elements Because both of these elements share their electrons evenly these share their electrons evenly these chains have no charge. They are chains have no charge. They are NON POLAR. Molecules with no NON POLAR. Molecules with no charge are not attracted to water; charge are not attracted to water; as a result water molecules tend as a result water molecules tend to push them out of the way as to push them out of the way as they are attracted to each other. they are attracted to each other. This causes molecules with no This causes molecules with no charge not to dissolve in water.charge not to dissolve in water.

3D model of 3D model of a a PhospholipidPhospholipid

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A Phospholipid BilayerA Phospholipid BilayerPhospholipids can form: Phospholipids can form:

BILAYERSBILAYERS

-2 layers of phospholipids -2 layers of phospholipids withwith

hydrophobic tails hydrophobic tails protected inside by the protected inside by the hydrophilic heads.hydrophilic heads.

The PHOSPHOLIPIDThe PHOSPHOLIPIDBILAYER is the basicBILAYER is the basicstructure of membranes.structure of membranes.

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Structure of the cell membraneStructure of the cell membranePhospholipidsPhospholipids

Cell membranes are made mainly of Cell membranes are made mainly of phospholipids. They have:phospholipids. They have:

HYDROPHILIC heads (water liking)HYDROPHILIC heads (water liking)-Attracted to the water POLAR-Attracted to the water POLARHYDROPHOBIC tails (water fearing)HYDROPHOBIC tails (water fearing)-Not attracted to the water NON--Not attracted to the water NON-

POLARPOLAR

Phospholipids can form BILAYERSPhospholipids can form BILAYERS-2 layers of phospholipids with-2 layers of phospholipids with hydrophobic tails protectedhydrophobic tails protected inside by the hydrophilicinside by the hydrophilic heads.heads.

The PHOSPHOLIPID BILAYER isThe PHOSPHOLIPID BILAYER is the basic structure of membranes.the basic structure of membranes.

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Diagram representing the cell membraneDiagram representing the cell membraneRemember the membrane is 7nm wideRemember the membrane is 7nm wide

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Fluid mosaic modelFluid mosaic model

Cell membranes also contain proteins within the phospholipid bilayer.Cell membranes also contain proteins within the phospholipid bilayer.This ‘model’ for the structure of the membrane is called the:This ‘model’ for the structure of the membrane is called the:

FLUID MOSAIC MODELFLUID MOSAIC MODELFLUID- because individual phospholipids and proteins can move FLUID- because individual phospholipids and proteins can move

around freely within the layer, like it’s a liquid.around freely within the layer, like it’s a liquid.MOSAIC- because of the pattern produced by the scattered protein MOSAIC- because of the pattern produced by the scattered protein

molecules when the membrane is viewed from above.molecules when the membrane is viewed from above.

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Diagram of a cell membrane Diagram of a cell membrane

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TEM of freeze-fractured cell membraneTEM of freeze-fractured cell membrane. .

The fracture occurs The fracture occurs between the two between the two phospholipid phospholipid layers.layers.

You can clearly see You can clearly see the exposed the exposed proteins sticking proteins sticking out of the two out of the two layers.layers.

Individual Individual phospholipids are phospholipids are too small to see.too small to see.

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Cell Membranes from Opposing Cell Membranes from Opposing Neurons (TEM x436,740). Neurons (TEM x436,740).

} } Phospholipid Phospholipid BilayerBilayer

7nm wide7nm wide

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Features of the fluid mosaic modelFeatures of the fluid mosaic model

Double layer – BILAYER of phospholipids which can move about by ………………………… in their own ……………………….

Phospholipid tails point inwards forming a ……. ………. ………………………… interior. The phospholipid heads point outwards facing the aqueous (water containing) medium surrounding the membrane.

Some phospholipids fatty acid tails are ……………………….. – straight so fit together tightly. Some are ………………………… – bent so fit together ……………………. The more unsaturated tails there are the more ……………… the membrane becomes. The lower the temp, the ……………….fluid.

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Features of the fluid mosaic modelFeatures of the fluid mosaic model Most protein molecules …………………. like icebergs in the

layers, some are fixed to ………………………. inside the cell and don’t float.

Some proteins are embedded in the outer layer, some in the inner layer and some ………………… the two layers. Hydrophobic and Hyrdophilic parts of the protein molecules sit next to the …………………………… and ………………………….. portions of the ……………………………… of the membrane. This ensures the proteins stay in the membrane.

The membrane is ……nm thick on average. Some phospholipids have carbohydrates attached to them

called- ……………………….. Some of the proteins have carbohydrates attached to them

called– …………………… The membrane also contains molecules of

………………………………..

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Features of the fluid mosaic modelFeatures of the fluid mosaic model

Double layer – BILAYER of phospholipids which can move about by DIFFUSION in their own MONOLAYER

Phospholipid tails point inwards forming a NON-POLAR HYDROPHOBIC interior. The phospholipid heads point outwards facing the aqueous (water containing) medium surrounding the membrane.

Some phospholipids fatty acid tails are SATURATED – straight so fit together tightly. Some are UNSATURATED – bent so fit together loosely. The more unsaturated tails there are the more ‘fluid’ the membrane becomes. The lower the temp, the less fluid.

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Features of the fluid mosaic modelFeatures of the fluid mosaic model Most protein molecules float like icebergs in the layers,

some are fixed to structures inside the cell and don’t float. Some proteins are embedded in the outer layer, some in

the inner layer and some span the two layers. Hydrophobic and Hyrdophilic parts of the protein molecules sit next to the Hydrophobic and Hydrophilic portions of the phospholids of the membrane. This ensures the proteins stay in the membrane.

The membrane is 7nm thick on average. Some phospholipids have carbohydrates attached to them

– GLYCOLIPIDS Some of the proteins have carbohydrates attached to them

– GLYCOPROTEINS The membrane also contains molecules of

CHOLESTEROL

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Roles of components of cell membranesRoles of components of cell membranes

Using the following headings produce a table on A4 to summarise roles of t heUsing the following headings produce a table on A4 to summarise roles of t he different types of molecules found in the cell membrane. Try not to copy, pickdifferent types of molecules found in the cell membrane. Try not to copy, pickout the relevant information and write it in note form You may want to use out the relevant information and write it in note form You may want to use bullet points, different coloured pens etc basically whatever helps you to bullet points, different coloured pens etc basically whatever helps you to

remember them.remember them.There will be a short test on this next lesson! Use pages 53-54.There will be a short test on this next lesson! Use pages 53-54.

ComponentComponent Function Function

1.1. PhospholipidsPhospholipids

2.2. CholesterolCholesterol

3.3. ProteinsProteins

4.4. Glycolipids and GlycoproteinsGlycolipids and Glycoproteins

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SummarySummary Cell membranes have a basic structure composed of a PHOSPHOLIPID BILAYER.

Phospholipds have HYDROPHOBIC (non-polar) tails and HYDROPHILIC (polar) heads.

The best model of the cell membrane is called the FLUID MOSAIC MODEL

The average thickness of the membrane is 7nm.

The fatty acid tails of phospholipids can be SATURATED (straight) or UNSATURATED (bent)

Proteins can float or be fixed and also have hydrophobic and hydrophilic portions.

Some proteins and phospholipids have carbohydrates attached to them to form

GLYCOPROTEINS AND GLYCOLIPIDS.

Phospholipids form the bilayer, act as barrier to most water soluble substances

Cholesterol regulates the fluidity of the membrane, gives mechanical stability and help to

prevent ions from passing through the membrane.

Proteins act as transport proteins to act as channels for substances to move into or out of the

cell. Some act as membrane enzymes and some have important roles in membranes of

organelles.

Glycolipids and Glycoproteins help to stabilise membrane structure, some act as receptor

molecules eg for hormones and neurotransmitters or as antigens for other cells to recognise

them.

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Movement of selected molecules Movement of selected molecules across the cell membrane across the cell membrane

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A moving AmoebaA moving Amoeba

What does its contractile What does its contractile

vacuole do?vacuole do?