Cell MembranesChapt 4

www.cellsalive.com/

The Cell Membrane

Fluid Mosaic Membrane

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Cell Membrane: Fluid Mosaic Model

At Very High Magnification & in color

Membrane StructureFlash movie too..

CLICK ON THE PICTURE TO SEE AN ANIMATION OF THE CELL MEMBRANE

Cell Membrane1. Every cell is encircled by a membrane

2. 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.

3. Without a membrane the cell contents would pass into the surroundings

4. Allows the cell to maintain HOMEOSTASIS = a constant internal environment

kind of like the cell’s happy place

Cell MembranesFluid-like structure…like soap bubbles

Structure1. Lipids in a bilayer

2. Protein-coated pores extend through membrane (MOSAIC)– Pores are very small and give the membrane polar

properties– H2O penetrates membrane easily– Large polar molecules do not

3. Proteins embedded in lipid layer

4. Some proteins floating within the lipid sea (called integral proteins)

5. Some 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

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)

Fluid Mosaic Membrane

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Functional Parts of the Cell Membrane

• Cell Recognition Proteins unique markers that identify cells

• Channel Proteins allows molecules and ions to move across

• Carrier Proteins combine with substrate to move across

membrane

• Receptor Proteins bind to specific molecules (ex. hormone

receptors)

• Enzymatic Proteins proteins that run specific metabolic

reactions

• Cell membrane contains carbohydrate (CHO’s) attached to

proteins or lipids ( makes Glycoproteins and Glycolipids)

• Found on outside of cell functioning as markers to identify

cells to the immune system

Auto-Immune Disease

• Inflamed body cells in the joints or colon respectively being recognized by white blood cells as “non-self”.

• Result is an immune response that causes injury to the body’s cells – in this case, the cartilege at the joints

Membrane Proteins

• Integral: embedded within

bilayer

• Peripheral: reside outside

hydrophobic region of lipids

Membrane Proteins

Integral membrane proteins

Peripheral membrane proteins

Integral

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

Evidence for the Fluid Mosaic Model (Cell Fusion)

More Evidence for the Fluid Mosaic Model

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 three ways that the molecules typically move through the membrane:

1. Facilitated transport 2. Passive transport3. 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.

• Facilitated and Passive transport does not require such an energy expenditure, and occur 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

Membrane Transport MechanismsII. Active Transport

• Active transport- proteins which transport against concentration gradient.

• Requires energy input

Diffusion• Movement generated by random motion of particles.

• Applies to any molecule/requires NO ENERGY!! • Movement always from region of high concentration to low concentration

Diffusion continued

• Lipid soluble molecules (alcohols) and gases (O2 and CO2) pass easily thru the membrane

• H2O passes through protein channels

• Large molecules and charged ions have difficulty passing thru (charge on membrane = polarity)

Osmosis

• Movement of waterwater across across a a semi-permeable barriersemi-permeable barrier.

Example: • Salt in water, cell

membrane is barrier

• Salt will NOT move across membrane, water will.

Click Picture Above for Click Picture Above for an Osmosis Demoan Osmosis Demo

Tonicity of SolutionsTonicity = the strength of a solution that a cell is placed in

3 Tonicity scenarios you must know!!

1. Isotonic • equal conc. of particles inside / out of the cell• Cells placed in isotonic solutions do not gain or lose H2O

2. Hypertonic • outside of cell has greater particle conc.• Cells placed in hypertonic solutions lose H2O and SHRINK

3. Hypotonic • inside of cell has greater conc.• Cells placed in hypotonic solutioins gain H2O and SWELL

Osmosis in Hypertonic Medium

cell

10% NaCl = hypertonic to blood cells (accustomed to 0.9%)

CELL SHRINKS OR “CRENATES”

Solute outside cell is greater water is

drawn from inside the cell to outside

solute

Hypertonic solutions- shrink cells

Osmosis in Hypotonic medium

> 0.9 % NaCl = Hypotonic to blood cells

CELL SWELLS

Solute inside cell is less and

attracts water inside, causing

swelling

Hypotonic solutions- swell cells

Pressure on cell due to the flow of water is called OSMOTIC PRESSURE

Check out the animation ---> click me

Importance of Osmosis

• Allows for absorption of H2O by the large intestine

• Retention or shedding of H2O by kidneys

• Uptake of H2O by the blood affects our blood pressure

• Increased blood pressure creates a greater risk of heart attack and stroke

• Watch these Flash Movies --->

Endocytosis

• Transports macromolecules and large particles into the cell.

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

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!A wee flash tour of endocytosis and exocytosis – just click

the image below

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

12

3 Types of Endocytosis

• Click on active transport, then next a few times until the menu shows “endocytosis”

1. Phagocytosis = cell engulfs large amounts

2. Pinocytosis = the cell takes in (drinks) a small amount

3. Receptor-guided = when receptors must first be filled before endocytosis is allowed. This often happens with hormones.

Three Types of Transport

1. Active Transport – requires energy ATP

– Uses a transport protein in the membrane

2. Facilitated Transport = Passive– NO ATP necessary

– Like active transport, uses a transport protein in the membrane

3. Diffusion and Osmosis = Passive– no ATP needed

Transport ProteinsFacilitated Diffusion & Active Transport

• move solutes faster across membrane

• highly specific to specific solutes

• can be inhibited by drugs

ACTIVE

FACILITATED

Active Transport

• Movement of particles low concentration into high concentration (against a concentration gradient)

• Requires energy input from ATP

• Cells use active transport to build up their stores of important particles, such as vitamins, minerals, salts, etc.

Active TransportSodium-Potassium Pump

Na+high

K+low

Na+low

K+high

•Balance of the two ions is done at same time

•Helps to create a dipole inside and out of the cell

•This is necessary for nerve cells to pass an electric impulse

ATP required for maintenance of the pump

Click here to see a cool Flash Animation

Sodium-Potassium PumpClick image for an animation

Types of Protein Transporters:

Active Transport• carrier proteins

• go against the concentration gradients Low to High

• require Energy to function (ATP)

Facilitated Diffusion

• Proteins in the membrane assists in diffusion process

• Solutes go from High conc to Low conc.

Example: Glucose transporters

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

Flash Animation

Glucose Transporter:How it works..

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

• glucose binding causes a shape change in protein

• glucose drops off inside cell

• protein reverts to original shape

Facilitated Diffusion without a Shape Change

Ion Channels

• Works fast: No protein shape changes needed

• Not simple pores in membrane:

– proteins are specific to different ions (Na, K, Ca...)– gates control opening– Toxins, drugs may affect channels

Toxins…how they workThey Plug the Ion Channels