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The Cell Membrane
Usman Sumo Friend TambunanArli Aditya Parikesit
Bioinformatics GroupFaculty of Mathematics and Science
University of Indonesia
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Overview
• Cell membrane separates living cell from nonliving surroundings• thin barrier = 8nm thick
• Controls traffic in & out of the cell• selectively permeable• allows some substances to cross more easily
than others• hydrophobic vs hydrophilic
•Made of phospholipids, proteins & other macromolecules
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Phospholipids• Fatty acid tails• hydrophobic
• Phosphate group head • hydrophilic
• Arranged as a bilayer
Fatty acid
Phosphate
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Phospholipid bilayer
polarhydrophilicheads
nonpolarhydrophobictails
polarhydrophilicheads
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Phospholipid bilayer
polarhydrophilicheads
nonpolarhydrophobictails
polarhydrophilicheads
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More than lipids… • In 1972, S.J. Singer & G. Nicolson proposed that
membrane proteins are inserted into the phospholipid bilayer
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Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer
Extracellular fluid
Cholesterol
Cytoplasm
Glycolipid
Transmembraneproteins
Filaments ofcytoskeleton
Peripheralprotein
Glycoprotein
Phospholipids
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Membrane fat composition varies• Fat composition affects flexibility• membrane must be fluid & flexible
• about as fluid as thick salad oil
• % unsaturated fatty acids in phospholipids• keep membrane less viscous
• cold-adapted organisms, like winter wheat • increase % in autumn
• cholesterol in membrane
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Membrane Proteins• Proteins determine membrane’s specific
functions• cell membrane & organelle membranes each
have unique collections of proteins•Membrane proteins:• peripheral proteins • loosely bound to surface of membrane
• cell surface identity marker (antigens)
• integral proteins • penetrate lipid bilayer, usually across whole membrane
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Proteins domains anchor molecule• Within membrane• nonpolar amino acids
• hydrophobic • anchors protein
into membrane
• On outer surfaces of membrane• polar amino acids
• hydrophilic
• extend into extracellular fluid & into cytosol
Polar areasof protein
Nonpolar areas of protein
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NH2
H+
COOH
Cytoplasm
Retinalchromophore
Nonpolar(hydrophobic)a-helices in thecell membrane H+
Porin monomerb-pleated sheets
Bacterialoutermembrane
proton pump channel in photosynthetic bacteria
water channel in bacteria
function through conformational change = shape change
Examples
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Many Functions of Membrane ProteinsOutside
Plasmamembrane
InsideTransporter Cell surface
receptorEnzymeactivity
Cell surface identity marker
Attachment to thecytoskeleton
Cell adhesion
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Membrane carbohydrates • Play a key role in cell-cell recognition• ability of a cell to distinguish one cell from another
• antigens
• important in organ & tissue development
• basis for rejection of foreign cells by immune system
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The Cell Membrane
Active and Passive Transport
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Cell Membrane
• Phospholipid bilayer: hydrophilic heads and hydrophobic tails
• Semi-permeability
• Transport proteins (passive transport channels)
• Ion pumps (active transport pumps)
•Receptor proteins (neurons, hormones, immune system)
•
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•What is passive transport? What are the three types of passive transport?
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Passive Transport
• Diffusion - process by which molecules tend to move from an area where they are more concentrated to an area where they are less concentrated
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Passive Transport
• In Passive Transport - Molecules move down the concentration gradient (no energy required)• 1) simple diffusion - molecules are small enough and soluble
can pass directly through the lipid bilayer• 2) facilitated diffusion – need transport proteins (molecules
are either to large or can’t pass through the lipid bilayer themselves)• 3) osmosis – molecules can’t pass through lipid bilayer at all,
but water can(movement of water)• Depends on molecule size, lipid solubility, and concentration
gradient
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Passive Diffusion
• http://www.youtube.com/watch?v=JShwXBWGMyY • http://www.youtube.com/watch?v=s0p1ztrbXPY
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Solutions
• A mixture of solid particles (solute) and liquid (solvent)• Most common solvent is water• Cells are surrounded by solutions• Three types: isotonic, hypotonic, and hypertonic solutions
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Isotonic Solutions
• Particles are equal inside and outside of the cell• Water molecules are equal inside and outside of the cell
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Hypotonic Solutions
• There are more particles inside than outside• There is more water outside the cell• Water will move INTO the cell
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Hypertonic Solutions
• There are more particles outside• There is more water inside• Water will move OUT OF the cell
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Active Transport
• Molecules move against the concentration gradient• Requires energy• 1. Solute Pumps (Na+-K+-ATPase pump)• 2. Bulk Transport
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• Na+-K+-ATPase pump• https://mail.xavierhs.org/exchange/UngerJ/Inbox/No%20Subject-748.EML/07_16ActiveTransport_A.swf/C58EA28C-18C0-4a97-9AF2-036E93DDAFB3/07_16ActiveTransport_A.swf?attach=1
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Transport Across The Membrane
• Endocytosis (pinocytosis, phagocytosis): phagocytes (macrophages)- molecules are coming into cells• Exocytosis: release of proteins, hormones,
neurotransmitters- release of proteins, hormones, neurotransmitters• http://www.youtube.com/watch?v=4gLtk8Yc1Zc&NR=1
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Transport summary
simplediffusion
facilitateddiffusion
activetransport
ATP
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How about large molecules?• Moving large molecules into & out of cell• through vesicles & vacuoles
• endocytosis• phagocytosis = “cellular eating”
• pinocytosis = “cellular drinking”
• exocytosis
exocytosis
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Endocytosis
phagocytosis
pinocytosis
receptor-mediated endocytosis
fuse with lysosome for digestion
non-specificprocess
triggered bymolecular signal
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The Special Case of Water
Movement of water across the cell membrane
2007-2008
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Osmosis is diffusion of water
• Water is very important to life, so we talk about water separately• Diffusion of water from
high concentration of water to low concentration of water• across a
semi-permeable membrane
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Concentration of water• Direction of osmosis is determined by
comparing total solute concentrations• Hypertonic - more solute, less water
• Hypotonic - less solute, more water
• Isotonic - equal solute, equal water
hypotonic hypertonic
net movement of water
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freshwater balanced saltwater
Managing water balance• Cell survival depends on balancing water
uptake & loss
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Managing water balance• Isotonic
• animal cell immersed in mild salt solution• example:
blood cells in blood plasma
• problem: none
• no net movement of water• flows across membrane equally, in
both directions
• volume of cell is stablebalanced
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Managing water balance• Hypotonic
• a cell in fresh water• example: Paramecium
• problem: gains water, swells & can burst• water continually enters
Paramecium cell
• solution: contractile vacuole • pumps water out of cell
• ATP
• plant cells• turgid freshwater
ATP
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Water regulation• Contractile vacuole in Paramecium
ATP
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Managing water balance• Hypertonic• a cell in salt water• example: shellfish
• problem: lose water & die
• solution: take up water or pump out salt
• plant cells• plasmolysis = wilt
saltwater
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Aquaporins• Water moves rapidly into & out of cells• evidence that there were water channels
1991 | 2003
Peter AgreJohn Hopkins
Roderick MacKinnonRockefeller
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Cell (compared to beaker) hypertonic or hypotonicBeaker (compared to cell) hypertonic or hypotonicWhich way does the water flow? in or out of cell
.05 M .03 M
Osmosis…
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References
• http://bioap.wikispaces.com/file/view/Ch%207%20Cell%20Membrane.ppt• http://
www.xavierhs.org/s/717/images/editor_documents/faculty/tanzmanj/Cell%20Membrane%20Transport.ppt• http://
www.lf3.cuni.cz/opencms/export/sites/www.lf3.cuni.cz/en/applicant/premedical/study-materials/biology/8_Premedical_Cell_communication.ppt