cells and their housekeeping functions cell membrane...

Shu-Ping Lin, Ph.D.

Institute of Biomedical Engineering E-mail: [email protected]

Website: http://web.nchu.edu.tw/pweb/users/splin/

Cells and Their Housekeeping Functions –

Cell Membrane & Membrane Potential

mailto:[email protected]

http://web.nchu.edu.tw/pweb/users/splin/

* Cell membrane, also called plasma membrane, separates cell interior from surroundings

Thin barrier = 7~10 nm thick

Controls traffic in & out of

the cell

Selectively permeable

* Made of phospholipids, proteins, carbohydrates & other macromolecules

*Phospholipids arrange as a bilayer:

Hydrophobic fatty acid tails

Hydrophilic phosphate group head

http://en.wikipedia.org/wiki/Cell_membrane#Lipid_bilayer

Phospholipid Bilayer

polar hydrophilic heads

nonpolar hydrophobic tails

polar hydrophilic heads

http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQ-GCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafY-X0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRp-LymP07E_VLiGzjkC-wqjHTg

Several important functions:

*A. Allow nutrients to enter cell, *B. Keep out unwanted molecules and particles, *C. Transport waste out into extracellular fluid, *D. Prevent needed metabolites and ions from leaving cell

Inherently amphipathic nature: possess both hydrophilic & hydrophobic structures

More Than Lipids…

In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer

It’s like a fluid… It’s like a mosaic… It’s the Fluid Mosaic Model!


Cell membrane is composed of a double layer of phospholipid molecules (called phospholipid or lipid bilayer), protein molecules associated with lipid bilayer, and carbohydrate-containing cell coat called glycocalyx.

Extracellular fluid

Cholesterol

Cytoplasm

Glycolipid

Transmembrane proteins

Filaments of cytoskeleton

Peripheral protein

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

Transmembrane protein

Transport proteins

channels, pumps


Why are proteins the perfect molecule to build structures in the cell membrane?


Classes of Amino Acids

What do these amino acids have in common?

nonpolar & hydrophobic http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQ-GCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafY-X0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRp-LymP07E_VLiGzjkC-wqjHTg

Classes of Amino Acids What do these amino acids have in common?

polar & hydrophilic I like the polar ones the best!


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 areas of protein

Nonpolar areas of protein http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQ-GCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafY-X0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRp-LymP07E_VLiGzjkC-wqjHTg

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


Membrane Carbohydrates

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

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


Movement Across the Cell Membrane


What molecules can get through directly?

Small nonpolar molecules such as carbon dioxide, nitrogen, and oxygen diffuse freely across the bilayer.

Lipid bilayer is also permeable to small and uncharged polar molecules such as urea and ethanol.

Other small hydrophobic molecules: fats and other lipids

NOT get through directly? Ions: salts, ammonia (NH3)

Large uncharged or charged polar molecules: starches, proteins

H2O molecule has 2 pathways: Lipid pathway

Water channel-protein pathway (polar molecules)

Selectively Permeable

Diffusion Across Cell Membrane

Cell membrane is the boundary between inside & outside…

Separates cell from its environment

IN food carbohydrates sugars, proteins amino acids lipids salts, O2, H2O

OUT waste ammonia salts CO2

H2O products

cell needs materials in & products or waste out

IN

OUT

Can it be an impenetrable boundary? NO!


Diffusion

2nd Law of Thermodynamics governs biological systems universe tends towards disorder (entropy)

Diffusion

movement from high low concentration


Diffusion Move from HIGH to LOW concentration

“passive transport”

no energy needed

diffusion

osmosis

movement of water

Osmotic pressure: the pressure is required to stop the net flow of water across a membrane separating solutions of different particulate concentration

Zero flux http://www.ocvts.org/instructors/htm/asprague/Biotech/Ch06CellMembraneDiffusion.ppt+cell+membrane+ppt&hl=zh-TW&gl=tw&pid=bl&srcid=ADGEEShywZ9NflQ-GCRtlnAD5GIlGBWSBEKRBkJwIU3s-r9wakHHx6SCbBg9T-uKL7xUvpYTtYHO3iB1SQtH0eo8bD2ZhKGcUlBOZBeeFafY-X0MbAXuEr5KIF3tINko9uVQlhxFRcsI&sig=AHIEtbTYRp-LymP07E_VLiGzjkC-wqjHTg

2007-2008

The Special Case of Water: Movement of water across the cell membrane


Osmosis

Osmosis is diffusion of water

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

water

net movement of water


Effect of Osmotic Pressure Suppose an animal or a plant cell is placed in a solution of sugar or salt in

water.

If the medium is hypotonic — a dilute solution, with a higher water concentration than the cell The cell will gain water and wilt through

osmosis.

If the medium is isotonic — a solution with exactly the same water concentration as the cell There will be no net movement of water

across the cell membrane.

If the medium is hypertonic — a concentrated solution, with a lower water concentration than the cell The cell will lose water and shrink by osmosis.

http://en.wikipedia.org/wiki/Osmosis

freshwater balanced saltwater

Managing Water Balance Cell survival depends on balancing water uptake & loss

Animal cells culture: must maintain in isotonic cell culture medium (concentration of solutes is close to cell cytoplasm)


Aquaporins Cores of these channel proteins are hydrophilic

Water moves rapidly into & out of cells

Multiple water molecules pass through membrane at a rate of 108 molecules/sec

Evidence that there were water channels

1991 | 2003

Peter Agre John Hopkins

Roderick MacKinnon Rockefeller


Channels Through Cell Membrane

Membrane becomes semi-permeable with protein channels

specific channels allow specific material across cell membrane

inside cell

outside cell

sugar aa H2O

salt NH3


Facilitated Diffusion Diffusion through protein channels

Channels move specific molecules across cell membrane

No energy needed

“The Bouncer”

open channel = fast transport

facilitated = with help

high

low


Active Transport

“The Doorman”

conformational change

Cells may need to move molecules against concentration gradient

Shape change transports solute from one side of membrane to other

Protein “pump”

“Costs” energy = ATP

ATP

low

high


Many Models & Mechanisms of Active Transport

Ions and polar molecules across cell membranes include:

ATP-powered pumps

Transporter proteins

Ion channels

The arrows indicate the direction from high to low concentration of the ion or polar molecule across membrane

high

low

low low

high high

Getting through cell membrane

Passive Transport

Simple diffusion diffusion of nonpolar, hydrophobic molecules

lipids

high low concentration gradient

Facilitated transport diffusion of polar, hydrophilic molecules

through a protein channel high low concentration gradient

Active transport

diffusion against concentration gradient low high

uses a protein pump

requires ATP

ATP

Transport Summary

simple diffusion

facilitated diffusion

active transport

ATP


Active Transport- Na+-K+ Pump Sodium ion binds to transport protein in configuration 1 ATP

molecule associates with transport protein After ATP

hydrolysis, phosphate group is transferred to transport protein Causing to switch of configuration 2 Sodium ion is released to the outside of cell Potassium ion is bound to attachment site of transport protein Binding of potassium ion Result in the release of phosphate group Protein goes back to configuration 1 Potassium ion is released into cell

Cycle is completed with the attachment of sodium ion into cavity of transport protein

Proton pumps, in a lysosomal membrane, are used by plants, bacteria, and fungi to create

electrochemical gradients (sodium-potassium pumps are employed by animals for the same purpose)

www.mansfield.ohio-state.edu/~sabedon/lectures/.../campbl08.ppt

Active Transport- Proton Pump

NH2

H+

COOH

Cytoplasm

Retinal chromophore

Nonpolar (hydrophobic) a-helices in the cell membrane H+

Porin monomer

b-pleated sheets

Bacterial outer membrane

proton pump channel in photosynthetic bacteria

Water channel in bacteria

function through conformational change = shape change

Examples

Membrane Electrical Potential Membrane potential: The electrical charge across a cell membrane; the

difference in electrical potential inside and outside the cell.

Axons have two basic electrical potentials:

1. Resting membrane potential: The membrane potential of a neuron when it is not being altered by excitatory or inhibitory postsynaptic potentials.

2. Action potential: The brief electrical impulse that provides the basis for conduction of information along an axon.

www.psych.yorku.ca/desouza/PSYC3250/M/class2/Lecture2.../Lecture2.ppt

Electrochemical Gradient • An Electrochemical Gradient is a Concentration Gradient with Ions:

- These ions want to move down their concentration gradient

- These ions (particularly) also want to move towards the opposite charge found on the other side of the membrane

- This attraction for the other side of membranes (membrane potential) can be harnessed to do work

- Electrochemical gradients essentially are batteries, i.e., means of physically storing electrical energy

www.mansfield.ohio-state.edu/~sabedon/lectures/.../campbl08.ppt

http://soma.npa.uiuc.edu/courses/physl341/Lec2.html

http://bio.winona.msus.edu/berg/308s01/Lec-note/9-new.htm

cells and their housekeeping functions cell membrane...

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