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