membrane structure and function(1)

8/3/2019 Membrane Structure and Function(1)

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Membrane Structure and

FunctionChapter 8


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Plasma membrane of cell selectively permeable(allows some substances to cross more easily thanothers)

Made mostly of proteins and lipids(phospholipids).

Phospholipids and proteins create unique physicalenvironment (fluid mosaic model)


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Phospholipid


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Membrane - bilayer - hydrophilic(water loving) heads pointing

outwards, hydrophobic (waterfearing) tails pointing inwards.

Proteins help membrane to stick to

water.


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Fluid because lipids and proteinscan move laterally.

As temperatures drop, liquidmembrane can solidify.

Saturated fatty acid tails - more

solid than unsaturated fatty acidtails.


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Cholesterol found in membranehelps with fluidity of membrane.

Membranes need to be fluid towork properly - systems in place tohelp keep it fluid.


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Two different types of proteins arefound in membrane.

1

Peripheral proteins not inmembrane, bound to surface ofprotein.

2Integral proteins in membraneoften spanning entire membrane.


11/52http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/MembraneProteins.gif


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Membrane helps keep cells shape.

Also aids in cell-to-cell recognition

(ability of a cell to distinguish onetype of neighboring cell fromanother)


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Some substances move steadily acrossmembrane (sugars, ions, and wastes likeCO2)

Hydrophobic molecules (i.e. hydrocarbons,CO2, and O2) can dissolve in lipid bilayer and

cross easily.


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Charged particles and polar moleculeshave more difficulty passing.

Specific ions and polar molecules cancross lipid bilayer by passing throughtransport proteins that span

membrane.


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Diffusion - tendency for substance tospread out in open area.

Permeable membrane separating asolution with dye molecules from purewater, dye molecules will cross barrierrandomly.


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No force acting upon it - substancewill tend to move down its

concentration gradient from whereit is more concentrated to lessconcentrated (passive transport).


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Diffusion of molecules with limitedpermeability through lipid bilayer

may be assisted by transportproteins.

http://library thinkquest org/27819/media/protein channel gif


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http://library.thinkquest.org/27819/media/protein_channel.gif


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Difference in concentration - ions move from onearea to other.

Solution with higher [ ] solutes - hypertonic.

Solution with lower [ ] solutes -hypotonic. [ ] equal - isotonic.


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http://www.biologycorner.com/resources/hypertonic.gif


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http://www.biologycorner.com/resources/hypotonic.gif


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Solution hypertonic - higher solute[ ] but lower H

2O [ ].

H2O moves into solution and solutemoves out.


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Movement of H2O across selectively

permeable membrane - osmosis.

2 solutions isotonic, H2O moleculesmove at equal rates from one to theother, (no net osmosis)


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Cell placed in hypertonic solution H

20 rushes out of cell (cell shrinks).

Cell placed in hypotonic solution H

2O rushes into cell (cell swells).


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Organism does not have rigid walls musthave ability to osmoregulate and maintaininternal environment.

Plant cells expand when watered causingpressure to be exerted against cell wall.

Allows plant to stand up against gravity

(turgid cell); not watered, plant will beginto wilt (flaccid cell).


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Plant loses enough water, plasmamembrane will pull away from cell

(plasmolysis).


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http://faculty.southwest.tn.edu/jiwilliams/plasmolysis.gif


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Charged particles that cannot pass throughmembrane - proteins to pass through(facilitated diffusion - diffusion ofsubstance down its [ ] gradient with help

of transport protein) Some channel proteins (gated channels)

open/close depending on presence/absence

of physical or chemical stimulus.


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In this case, the protein actually rotatesto dump the materials to the inside

of the cell.


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Sometimes materials need to be movedagainst [ ] gradient (active transport)

Active transport requires energy of

cell to move substances from an areaof low [ ] to an area of high [ ] (i.e.sodium-potassium pump in animal cells)

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


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p p y g p g


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Sodium-potassium pump activelymaintains gradient of sodium (Na+) andpotassium ions (K+) across membrane.

Sodium-potassium pump uses energy of 1ATP to pump 3 Na+ ions out and 2 K+ ionsin.


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Cells maintain voltage across plasmamembranes.

Cytoplasm negative compared toopposite side of membrane (membranepotential - ranges from -50 to -200millivolts)


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http://bioweb.wku.edu/courses/Biol131/images/neuronions.GIF


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Membrane potential favors passivetransport of cations (positive ions)

into cell and anions (negative ions)out of cell.

Creates an electrochemical

gradient across membrane.


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Some organisms have proton pumpsthat actively pump H+ out of cell

(i.e. plants, bacteria, and fungi)


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Materials leave membrane through lipidbilayer or through transport proteins.

Exocytosis - transport vesicle buds from

Golgi apparatus - moved by cytoskeleton toplasma membrane.

When membranes meet - fuse - material islet out to outside of cell.


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Endocytosis - cell brings inmacromolecules and matter by formingnew vesicles from plasma membrane.

Membrane is inwardly pinched off andvesicle carries material to inside ofcell.


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https://reader009.{domain}/reader009/html5/0508/5af0e192d80ad/5af0e


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1Phagocytosis (cell eating) - cellengulfs particle by extending

pseudopodia around it, packaging itin a large vacuole.

Contents of vacuole are digested

when vacuole fuses with lysosome.


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2Pinocytosis (cell drinking) - cellcreates vesicle around droplet of

extracellular fluid.


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3Receptor-mediated endocytosis -specific in transported substances.

Extracellular materials bind ligands(receptors) - causes vesicle toform.

Allows materials to be engulfed inbulk (i.e. cholesterol in humans)


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membrane structure and function(1)

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