The Cell Membrane

Cell Membranes:

• 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 amphipathic molecules, with polar and nonpolar regions…

phosphate (hydro philic) polarglycerol

fatty acids (hydro phobic) nonpolar

Membrane Proteins

Text pg 80

• Integral: embedded within bilayer• Peripheral: reside outside hydrophobic

region of lipids

Fluid Mosaic Membrane

Fluid Mosaic MembraneFluid Mosaic Model - lipids arranged in

bilayer with proteins embedded or associated with the lipids.

Evidence for the Fluid Mosaic Model

Frey and Edidin

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 • Semi-permeable / selectively permeable• 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 Mechanisms

I. Passive Transport• Diffusion- simple movement from regions of high concentration to low concentration by random motion of particles caused by internal thermal energy.

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

• Facilitated diffusion- protein transporters which assist in diffusion

Diffusion Rates• Factors affecting diffusion rate through a

membrane

– temperature - temp., motion of particles

– molecular weight - larger molecules move slower

– steepness of concentrated gradient - difference, rate

– membrane surface area - area, rate

– membrane permeability - permeability, rate

Concentration Gradient of Ions

across a Membrane and the Resulting

Electrochemical Potential

Tonicity• Tonicity - ability of a solution to affect fluid

volume and pressure within a cell– depends on solute concentration and

permeability• Hypotonic solution

– low concentration of nonpermeating solutes (high water concentration)

– cells absorb water, swell and may burst (lyse)• Hypertonic solution

– has high concentration of nonpermeating solutes (low water concentration)

– cells lose water + shrivel (crenate)• Isotonic solution = normal saline

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.

How Osmosis Works

Osmosis in Hypertonic medium

cell

• Hypertonic solutions- shrink cells

Osmosis in Hypotonic medium

• Hypotonic solutions- swell cells

“Hypos make hippos”

For more animations view: http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm

For Osmosis in ActionView frozen frogs at:

http://www.pbs.org/wgbh/nova/sciencenow/3209/05.html

How did the frog use the principles of osmosis and diffusion to survive the

winter? Make sure you use the following terms appropriately in your description: hypertonic, hypotonic, solute, solvent, diffusion, osmosis,

cytolysis, crenation, isotonic and semi-permeable membrane.

Osmosis & Food Preservation• Food can be preserved by causing

any microorganism that comes in contact with it to become plasmolysed and, therefore, shrivel and die. To do this food is placed in a high salt or sugar medium. The salt or sugar concentration is higher than the cytoplasm of bacteria or fungi. Bacteria or fungi, that contaminate the food, will lose water by osmosis and their metabolism will decline. Many will die but some bacteria may survive by forming dormant resistant endospores. Meat and fish are often preserved in salt. Fruit is commonly preserved in sugar as in jam or syrup.

Membrane Transport: Active Transport (Direct &

Indirect)

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

Requires energy input

•Cotransport •Sodium-Potassium Exchange Pump •Endocytosis and Exocytosis•Proton Pump

EndocytosisPart of the membrane engulfs the particle and folds inward to “bud off.”•Phagocytosis•Pinocytosis •Receptor Mediated Endocytosis

Exocytosis: Cellular Secretion• Vesicles (lysosomes,

other secretory vesicles) can fuse with the membrane and open up the the outside…

Vesicular Transport(Active Transport)

EndocytosisPhagocytosis, 2PinocytosisReceptor Mediated Endocytosis

Exocytosis

Membrane Permeability

1) lipid soluble solutes go

through faster

2) smaller molecules go faster

3) uncharged & weakly charged

go faster

4) Channels or pores may also

exist in membrane to allow

transport

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Functional Roles of

Membrane Protein

Molecules

Types of Protein Transporters: Ion Channels

• Work fast: No conformational 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

• work by facilitated diffusion No E! • deal with small molecules... ions• open pores are “gated”- Can change shape.• important in cell communication

Receptors Linked to a Channel Protein

Ion Channels• Channel proteins or carrier

proteins allow the facilitated diffusion of solutes down their concentration gradients or electrochemical gradients

• Carrier proteins allow the active transport of solutes up their concentration gradients or electrochemical gradients.

Cystic Fibrosis• 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.

Transport ProteinsFacilitated Diffusion & Active Transport

• move solutes

faster across

membrane

• highly specific to

specific solutes

• can be inhibited

by drugs

Facilitated Diffusion: Glucose Transporters• Transport of glucose into cells mediated by proteins in

the GLUT (GLUcose Transport) family of transporters. • 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

• These are uniporters, different from the Sodium-dependent glucose cotransporters (SGLT)

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

Active Transport:Sodium-Potassium

Pump

Na+high

K+low

Na+low

K+high

Balance of the two ions goes hand-in-handATP required for maintenance of the pump

How it Works

Sodium-Potassium Pump• 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 • 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

1.Tight Junctions2. Desmosomes3. Gap Junctions4. Plasmodesmata

Intercellular Junctions

Tight Junctions• intimate physical

connections linking cells that line the inner or outer surface of organs or body cavities

• Leak proof barriers that prevent the movement of molecules through the spaces located b/t cells, must diffuse to get by and are therefore subject to the precise control mechanisms inherent to transport through cell mem

• e.g. bladder

Desmosomes• junctions exhibiting

mechanical strength• found in organs/tissues

exposed to mechanical forces that subject cells to much stretching and distortion

• maintains integrity of cell

Pemphigus is an autoimmune disease in which the patient has developed antibodies against proteins (cadherins) in desmosomes. The loosening of the adhesion between adjacent epithelial cells causes blistering.

Carcinomas are cancers of epithelia. However, the cells of carcinomas no longer have desmosomes. This may partially account for their ability to metastasize.

Gap Junctions• permit small

molecules to move b/t cells w/o passing thru mem

• six dumbbell shaped protein units in mem, adjacent in the cells

The action potential in cardiac muscle provides the rhythmic contraction of the heartbeat.

At some electrical synapses in the brain, gap junctions permit the arrival of an action potential at the synaptic terminals to be transmitted across to the postsynaptic cell without the delay needed for release of a neurotransmitter.

As the time of birth approaches, gap junctions between the smooth muscle cells of the uterus enable coordinated, powerful contractions to begin.

Plasmodesmata• similar to gap

junctions, but in plant cells

• allows continuous flow of cytoplasm through cells

Resources• Directory of Animations: • Anatomy & Physiology Chapter 3 Animations• Cell Membrane: Just Passing Through• Absorption in the Small Intestine• GLUT4 & Diabetes (Monogenetic disorder)