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The Cell Membrane
Usman Sumo Friend TambunanArli Aditya Parikesit
Bioinformatics GroupFaculty of Mathematics and Science
University of Indonesia
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
Phospholipids• Fatty acid tails• hydrophobic
• Phosphate group head • hydrophilic
• Arranged as a bilayer
Fatty acid
Phosphate
Phospholipid bilayer
polarhydrophilicheads
nonpolarhydrophobictails
polarhydrophilicheads
Phospholipid bilayer
polarhydrophilicheads
nonpolarhydrophobictails
polarhydrophilicheads
More than lipids… • In 1972, S.J. Singer & G. Nicolson proposed that
membrane proteins are inserted into the phospholipid bilayer
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
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
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
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
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
Many Functions of Membrane ProteinsOutside
Plasmamembrane
InsideTransporter Cell surface
receptorEnzymeactivity
Cell surface identity marker
Attachment to thecytoskeleton
Cell adhesion
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
The Cell Membrane
Active and Passive Transport
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)
•
•What is passive transport? What are the three types of passive transport?
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
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
Passive Diffusion
• http://www.youtube.com/watch?v=JShwXBWGMyY • http://www.youtube.com/watch?v=s0p1ztrbXPY
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
Isotonic Solutions
• Particles are equal inside and outside of the cell• Water molecules are equal inside and outside of the cell
Hypotonic Solutions
• There are more particles inside than outside• There is more water outside the cell• Water will move INTO the cell
Hypertonic Solutions
• There are more particles outside• There is more water inside• Water will move OUT OF the cell
Active Transport
• Molecules move against the concentration gradient• Requires energy• 1. Solute Pumps (Na+-K+-ATPase pump)• 2. Bulk Transport
• 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
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
Transport summary
simplediffusion
facilitateddiffusion
activetransport
ATP
How about large molecules?• Moving large molecules into & out of cell• through vesicles & vacuoles
• endocytosis• phagocytosis = “cellular eating”
• pinocytosis = “cellular drinking”
• exocytosis
exocytosis
Endocytosis
phagocytosis
pinocytosis
receptor-mediated endocytosis
fuse with lysosome for digestion
non-specificprocess
triggered bymolecular signal
The Special Case of Water
Movement of water across the cell membrane
2007-2008
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
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
freshwater balanced saltwater
Managing water balance• Cell survival depends on balancing water
uptake & loss
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
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
Water regulation• Contractile vacuole in Paramecium
ATP
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
Aquaporins• Water moves rapidly into & out of cells• evidence that there were water channels
1991 | 2003
Peter AgreJohn Hopkins
Roderick MacKinnonRockefeller
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…
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
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