2.Cell Membrane

THE CELL MEMBRANE

Usman Sumo Friend Tambunan

Arli Aditya Parikesit

Bioinformatics Group

Faculty of Mathematics and Science

University of Indonesia

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OVERVIEW
Cell membrane separates living cell from nonliving surroundingsthin barrier = 8nm thickControls traffic in & out of the cellselectively permeableallows some substances to cross more easily than othershydrophobic vs hydrophilic Made of phospholipids, proteins & other macromolecules
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PHOSPHOLIPIDS
Fatty acid tailshydrophobicPhosphate group head hydrophilicArranged as a bilayer
Fatty acid

Phosphate

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PHOSPHOLIPID BILAYER

polar

hydrophilic

heads

nonpolar

hydrophobic

tails

polar

hydrophilic

heads

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

Phospholipids

Glycolipid

Transmembrane

proteins

Filaments of
cytoskeleton

Peripheral

protein

Glycoprotein

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The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins.

MEMBRANE FAT COMPOSITION VARIES
Fat composition affects flexibilitymembrane must be fluid & flexibleabout as fluid as thick salad oil% unsaturated fatty acids in phospholipidskeep membrane less viscouscold-adapted organisms, like winter wheat increase % in autumncholesterol in membrane
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MEMBRANE PROTEINS
Proteins determine membranes specific functionscell membrane & organelle membranes each have unique collections of proteinsMembrane proteins:peripheral proteins loosely bound to surface of membranecell surface identity marker (antigens)integral proteins penetrate lipid bilayer, usually across whole membrane transmembrane proteintransport proteinschannels, permeases (pumps)
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PROTEINS DOMAINS ANCHOR MOLECULE
Within membranenonpolar amino acids hydrophobic anchors protein
into membraneOn outer surfaces of membranepolar amino acids hydrophilicextend into extracellular fluid & into cytosol
Polar areas

of protein

Nonpolar areas of protein

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proton pump channel
in photosynthetic bacteria

water channel
in bacteria

function through conformational change =
shape change

EXAMPLES

NH2

H+

COOH

Cytoplasm

Retinal

chromophore

Nonpolar

(hydrophobic)

a-helices in the

cell membrane

H+

Porin monomer

b-pleated sheets

Bacterial

outer

membrane

MANY FUNCTIONS OF MEMBRANE PROTEINS

Outside

Plasma

membrane

Inside

Transporter

Cell surface
receptor

Enzyme
activity

Cell surface
identity marker

Attachment to the
cytoskeleton

Cell adhesion

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Signal transduction - transmitting a signal from outside the cell to the cell nucleus, like receiving a hormone which triggers a receptor on the inside of the cell that then signals to the nucleus that a protein must be made.

MEMBRANE CARBOHYDRATES
Play a key role in cell-cell recognitionability of a cell to distinguish one cell from anotherantigensimportant in organ &
tissue developmentbasis for rejection of
foreign cells by
immune system
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The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.

THE CELL MEMBRANE
ACTIVE AND PASSIVE TRANSPORT

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CELL MEMBRANE
Phospholipid bilayer: hydrophilic heads and hydrophobic tailsSemi-permeabilityTransport proteins (passive transport channels)Ion pumps (active transport pumps)Receptor proteins (neurons, hormones, immune system)Carbohydrate chains ( identification cards)
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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
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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 bilayer2) facilitated diffusion need transport proteins (molecules are either to large or cant pass through the lipid bilayer themselves)3) osmosis molecules cant 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 waterCells are surrounded by solutionsThree types: isotonic, hypotonic, and hypertonic solutions
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ISOTONIC SOLUTIONS
Particles are equal inside and outside of the cellWater molecules are equal inside and outside of the cell
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HYPOTONIC SOLUTIONS
There are more particles inside than outsideThere is more water outside the cellWater will move INTO the cell
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HYPERTONIC SOLUTIONS
There are more particles outsideThere is more water insideWater will move OUT OF the cell
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ACTIVE TRANSPORT
Molecules move against the concentration gradientRequires energy1. Solute Pumps (Na+-K+-ATPase pump)2. Bulk Transport
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Na+-K+-ATPase pumphttps://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 cellsExocytosis: release of proteins, hormones, neurotransmitters- release of proteins, hormones, neurotransmittershttp://www.youtube.com/watch?v=4gLtk8Yc1Zc&NR=1
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TRANSPORT SUMMARY

simple
diffusion

facilitated
diffusion

active
transport

ATP

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HOW ABOUT LARGE MOLECULES?
Moving large molecules into & out of cellthrough vesicles & vacuolesendocytosisphagocytosis = cellular eatingpinocytosis = cellular drinkingexocytosis
exocytosis

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ENDOCYTOSIS

phagocytosis

pinocytosis

receptor-mediated
endocytosis

fuse with lysosome for digestion

non-specific
process

triggered by
molecular 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 separatelyDiffusion of water from
high concentration of water to
low concentration of wateracross a
semi-permeable
membrane
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CONCENTRATION OF WATER
Direction of osmosis is determined by comparing total solute concentrationsHypertonic - more solute, less waterHypotonic - less solute, more waterIsotonic - equal solute, equal water
water

net movement of water

hypotonic

hypertonic

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freshwater

balanced

saltwater

MANAGING WATER BALANCE
Cell survival depends on balancing water uptake & loss
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Isotonicanimal cell immersed in
mild salt solutionexample:
blood cells in blood plasmaproblem: noneno net movement of waterflows across membrane equally, in both directionsvolume of cell is stable
balanced

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Hypotonica cell in fresh waterexample: Paramecium problem: gains water,
swells & can burstwater continually enters
Paramecium cellsolution: contractile vacuole pumps water out of cellATPplant cellsturgid
freshwater

ATP

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WATER REGULATION
Contractile vacuole in Paramecium
ATP

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Hypertonica cell in salt waterexample: shellfishproblem: lose water & diesolution: take up water or pump out saltplant cellsplasmolysis = wilt
saltwater

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AQUAPORINS
Water moves rapidly into & out of cellsevidence that there were water channels
1991 | 2003

Peter Agre

John Hopkins

Roderick MacKinnon

Rockefeller

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Cell (compared to beaker) hypertonic or hypotonic

Beaker (compared to cell) hypertonic or hypotonic

Which 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.ppthttp://www.xavierhs.org/s/717/images/editor_documents/faculty/tanzmanj/Cell%20Membrane%20Transport.ppthttp://www.lf3.cuni.cz/opencms/export/sites/www.lf3.cuni.cz/en/applicant/premedical/study-materials/biology/8_Premedical_Cell_communication.ppt

2.Cell Membrane

Documents

membrane carbohydrates

cellcell recognitionability

phospholipidskeep membrane

membrane proteinsproteins

cell nucleus

cell membraneactive

embedded proteins

peripheral proteins