Cell MembranesChapt 5

www.cellsalive.com/

The Cell Membrane

Cell Membrane:

At Very High Magnification & in color

Membrane Structure

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellMembranes.html

Cell Membrane

Every cell is encircled by a membrane and most cells contain an extensive intracellular membrane system. Membranes fence off the cell's interior from its surroundings. Membranes let in water, certain ions and substrates and they excrete waste substances. They act to protect the cell. Without a membrane the cell contents would diffuse into the surroundings, information containing molecules would be lost and many metabolic pathways would cease to work:

The cell would die!

www.biologie.uni-hamburg.de/b-online/e22/22.htm

Cell Membranes: • Surround all cells

• Fluid-like composition…like soap bubbles• Composed of:

– Lipids in a bilayer– Proteins embedded in lipid layer (called

transmembrane proteins)– And, Proteins floating within the lipid sea

(called integral proteins)– And Proteins associated outside the lipid

bilayer (peripheral).

Membrane Lipids• Composed largely of phospholipids

• Phospholipids composed of….glycerol and two fatty acids + PO4 group

• P-Lipids are polar molecules…

P-Lipids are represented like this

Text pg. 81

Membrane Lipidsform a Bilayer

Outside layer

Inside Layer

Quiz

• If Phospholipids are polar, which end seeks out water and which avoids water?

Phospholipid Molecule Model

glycerol

fatty acids (hydrophobic)

phosphate (hydrophilic)

Membrane Proteins

• Integral: embedded within

bilayer

• Peripheral: reside outside

hydrophobic region of lipids

Text pg. 80

Membrane Proteins

Text pg 80

Integral membrane proteins

Peripheral membrane proteins

Integral

Membrane Models Fluid Mosaic Model - lipids arranged in bilayer with proteins embedded or associated with the lipids.

Fluid Mosaic Membrane

Text pg 80

Evidence for the Fluid Mosaic Model (Cell Fusion)

More Evidence for the Fluid Mosaic Model

Membrane Functions

• allows for different conditions between

inside and outside of cell

• subdivides cell into compartments with

different internal conditions

• allows release of substances from cell via

vesicle fusion with outer membrane: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/exocyt.gif

Membrane Permeability

• Biological membranes are physical barriers..but which allow small uncharged molecules to pass…

• And, lipid soluble molecules pass through

• Big molecules and charged ones do NOT pass through

How to get other molecules across membranes??

There are two ways that the molecules typically move through the membrane:

passive transport and active transport

•Active transport requires that the cell use energy that it has obtained from food to move the molecules (or larger particles) through the cell membrane. •Passive transport does not require such an energy expenditure, and occurs spontaneously.

Membrane Transport MechanismsI. Passive Transport

• Diffusion- simple movement from regions of high concentration to low concentration

• Osmosis- diffusion of water across a semi-permeable membrane

• Facilitated diffusion- protein transporters which assist in diffusion

Text pg 89

Membrane Transport MechanismsII. Active Transport

• Active transport- proteins which transport against concentration gradient.

• Requires energy input

Text pg 89

Diffusion

Movement generated by random motion of particles. Caused by internal thermal energy.

Movement always from region of high free energy(high concentration) to regions of low free energy (low conc.)

Text pg 86

Osmosis

Movement of water across a semi-permeable barrier.

Example: Salt in water, cell membrane is barrier. Salt will NOT move across membrane, water will.

Text pg 87

Osmosis in Hypertonic medium

cell

Hypertonic solutions- shrink cells

Osmosis in Hypotonic medium

Hypotonic solutions- swell cells

Endocytosis

• Transports macromolecules and large particles into the cell.

• Part of the membrane engulfs the particle and folds inward to “bud off.”

– Fig. 5.16

Endocytosis

Putting Out the Garbage

• Vesicles (lysosomes, other secretory vesicles) can fuse with the membrane and open up the the outside…

Exocytosis (Cellular Secretion)

Movies!

Membrane Permeability

1) lipid soluble solutes go through faster

1) smaller molecules go faster

1) uncharged & weakly charged go faster

2) Channels or pores may also exist in

membrane to allow transport1

2

Cellular Membranes

REVIEW• Importance of Membranes•Membrane Structure•Proteins•Fluid Mosaic model•Permeability•Types of Transport

– Passive and Active

Types of Protein Transporters: Ion Channels

• work by facilitated diffusion No E!

• deal with small molecules... ions

• open pores are “gated”- Can change shape.– How?– How much gets in?

• important in cell communication

Ion Channels

• Work fast: No conform. changes needed

• Not simple pores in membrane:– specific to different ions (Na, K, Ca...)– gates control opening– Toxins, drugs may affect channels

• saxitoxin, tetrodotoxin

• cystic fibrosis

Toxins…how they work

Cystic Fibrosis• Fatal genetic disorder

• Mucus build-up results in lung and liver failure

• Patients die between 4 and 30 yrs.

• Single gene defect

• 1 in 25 Caucasians carry 1 bad gene copy

• 1 in 2500 kids has it in Canada

• Testing

CF Cont…

• ~Proteins for diffusion of salt into the airways don't work.     

• ~Less salt in the airways means less water in the airways. 

• ~ Less water in the airways means mucus layer is very             sticky (viscous).

• ~Sticky mucus cannot be easily moved to clear particles from the lungs.    

• ~Sticky mucus traps bacteria and causes more lung infections.

http://www.the-aps.org/education/lot/cell/HotT.htm

Transport ProteinsFacilitated Diffusion & Active Transport

• move solutes faster

across membrane

• highly specific to

specific solutes

• can be inhibited by

drugs

Types of Protein Transporters

A. Facilitated Diffusion

Assist in diffusion process.

Solutes go from High conc to Low conc.

Examples: Glucose transporters

Text pg 88http://bio.winona.msus.edu/berg/ANIMTNS/FacDiff.htm

Facilitated DiffusionThe Glucose Transporters

• Transport of glucose into cells mediated by proteins in the GLUT (GLUcose Transport) family of transporters. There are 7 different, but related, proteins. But, only four (GLUT1-4) are known to be involved in glucose transport.

• All GLUT proteins share a set of similar structural features and are all about 500 amino acids in length (giving them a predicted molecular weight of about 55,000 Daltons)

• Glucose uptake shows saturation and glucose uptake can be inhibited by drugs

A classic Membrane Transport protein

Glucose Transporter Characteristics:

• integral protein: spans the membrane

• 12 alpha helices woven into membrane

• 55,000 mol. wt.

• Text pg. 88

Glucose Transporter:How it works..

• glucose binds to outside of transporter (exterior side with higher glucose conc.)

• glucose binding causes a conform. change in protein

• glucose drops off inside cell

• protein reassumes 1st configuration

Types of Protein Transporters:

Active Transport• carrier proteins

• go against the concentration gradients Low to High

• require Energy to function (ATP, PEP, light energy, electron transport)

Membrane Transport:Active transport

• Movement from region of low free energy(low concentration) to regions of high free energy (high conc.)

• Requires energy input

Active Transport:Sodium-Potassium

Pump

Na+high

K+low

Na+low

K+high

Balance of the two ions goes hand-in-hand

ATP required for maintenance of the pump

The sodium/potassium pump• All nerve and muscle cells have a high internal potassium ion

concentration and a low internal sodium ion concentration. [Ki=166 mM; Ko=5 mM; Nai=18 mM; Nao=135 mM].

• Early on, it was thought that the nerve and muscle membranes were relatively impermeable to these ions and that the difference in ionic concentration was set up in early development of the cells. The membrane then became impermeable.

• The later availability and use of radioactive Na and K ions showed that this was not true and that there was a metabolic pump that pumped Na out of the cell and K in; the ratio being 3 Na pumped out of the cell for every 2 K pumped into the cell.

Is a Protein Involved ?

• Experiments showed a dependency of both Na and K ions for pump to work

• Pump was inhibited by ouabain (a drug)

• 1957: an ATPase enzyme was found to be associated with Na/K pumping

• Studies showed this ATPase capable of pumping Na/K ions

• Text pg 90

Sodium/PotassiumATPase Protein

• Made of 2 large and 2 small subunits

• 2 large units span membrane – inside region: contains ATP binding site– inside: binding sites for Na– outside: binding site for K

• How does it work??

Sodium-Potassium Pump

• Text pg. 90

http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/sppump.html

Na-K Pump Model: Part I

• 3 Na+ bind to inner region of protein

• Na+ binding triggers phosphorylation of protein. ATP ADP + Pi

• Phosphorylation causes conformation change and Na+ binding site faces outside

• 3 Na+ released to outside

Na-K Active Pump: Part II

• 2 K+ ions on outside are able to bind

• K+ binding causes dephosphorylation and new conformation change

• 2K+ ions exposed to inside and released

Cyclic process uses ATP energy to drive Na & K ion transport against conc. Gradient

Cell Junctions

• Allow specific types of cells to stay together to perform special jobs

• Layers of these types of cells…– Line body cavities– Cover body surfaces

3 Types of Cell Junctions

1. Tight Junctions

2. Desmosomes

3. Gap Junctions

Tight Junctions

• Seal tissues and prevent leaks

• Link epithelial cells together

• Prevent things from moving through the intercellular space

• Restrict migration of proteins and phospholipids

Desmosomes• Like spot welds!

• Dense plaques with fibers attached- Anchor cells together from one side to the other.

• These cells withstand lots of abuse!

Gap Junctions

• Cell to cell communication.

• Protein channels- (what type?) “connexons”

• Plasmodesmata

Put Them All Together…

Membrane Structure The cell is highly organized with many functional units or organelles inside.

Most of these units are limited by one or more membranes. To perform the functions of an organelle, the membrane is specialized in that it contains specific proteins and lipid components that enable it to perform its unique roles. 

In essence membranes are essential for the integrity and function of the cell. Membrane functions:• be protective • regulate transport in and out of cell or organelle• allow selective receptivity and signal transduction by providing

transmembrane receptors that bind signaling molecules• allow cell recognition• provide anchoring sites for cytoskeletal components. This allows the cell to

maintain its shape and perhaps move to distant sites.• provide a stable site for the binding and catalysis of enzymes.• regulate the fusion of the membrane with other membranes in the cell via

specialized junctions • provide a passageway across the membrane for certain molecules• allow directed cell or organelle motility