AP Biology

The Cell Membrane

AP Biology

Updated model of an animal cell’s plasma membrane:

Glyco- protein Carbohydrate Glycolipid

EXTRACELLULAR SIDE OF

MEMBRANE

Microfilaments of cytoskeleton

Fibers of extra- cellular matrix (ECM)

Cholesterol

Peripheral proteins Integral

protein CYTOPLASMIC SIDE OF MEMBRANE

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Phospholipids:

Fatty acid

Phosphate  Phosphate head

 hydrophilic   Fatty acid tails

 hydrophobic  Arranged as a bilayer

Aaaah, one of those

structure–function examples

“repelled by water”

“attracted to water”

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Phospholipid bilayer

Hydrophlic region of protein

Hydrophobic region of protein

It’s like a fluid…

It’s like a mosaic…

It’s the

Fluid Mosaic Model!

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The Fluidity of Membranes:   Phospholipids can move within the bilayer   Lipids, and some proteins, drift laterally/bend/flex   Rarely, a lipid may flip-flop transversely across the

membrane Membranes are like a jelly sandwich… What makes-up the jelly and the

bread?

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  As temperatures cool, membranes:  switch from a fluid state to a solid state  depends on the types of lipids  unsaturated fatty acids are more fluid  saturated fatty acids are less fluid (cold bacon grease)

  Membranes must be fluid to work properly; they are usually about as fluid as salad oil

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Figure 7.5

Fluid Viscous

Unsaturated tails prevent packing.

Saturated tails pack together.

Cholesterol

Cholesterol reduces membrane fluidity at

moderate temperatures, but at low temperatures hinders solidification.

(a) Unsaturated versus saturated hydrocarbon tails

(b) Cholesterol within the animal cell membrane

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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

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Evolution of Differences in Membrane Lipid Composition:   Variations in lipid composition of cell membranes of many

species appear to be adaptations to specific environmental conditions

  Ability to change the lipid compositions, and membrane protein position in response to temperature changes, has evolved in organisms that live where temperatures vary

Hibernating Squirrels Membrane proteins

Mixed proteins after 1 hour

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Cryopreservation Of gametes??

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Effects of Freezing Rates on Cells:

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Flash Freezing Food in Industry:

Clarence Birdseye found a way to flash-freeze foods and deliver them to the public---one of the most important

steps forward ever taken in the food industry. If the fish were frozen too quickly for ice crystals to form this would

ruin their cellular structure. Birdseye the businessman, saw that the public back home would gladly pay for such frozen foods, if he could deliver them. He returned to New

York, and in 1924 founded Birdseye Seafoods, Inc.

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Arranged as a Phospholipid bilayer:

polar hydrophilic

heads

nonpolar hydrophobic

tails

polar hydrophilic

heads

 Serves as a cellular barrier / border

H2O sugar

lipids

salt

waste

impermeable to polar molecules

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Cell membrane defines cell:  Cell membrane separates living cell from

aqueous environment   thin barrier = 8nm thick

 Controls traffic in & out of the cell  allows some substances to cross more

easily than others   hydrophobic (nonpolar) vs. hydrophilic (polar)

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Permeability to polar molecules?  Membrane becomes semi-permeable via

protein channels  specific channels allow specific material

across cell membrane inside cell

outside cell

sugar aa H2O

salt NH3

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Cell membrane is more than lipids…   Transmembrane (integral) proteins

embedded in phospholipid bilayer  create semi-permeabe channels

lipid bilayer membrane

protein channels in lipid bilyer membrane

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The membrane

looks like

chicken skin!

AP Biology 2007-2008

Why are proteins the perfect

molecule to build structures in the cell membrane?

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Classes of amino acids: What do these amino acids have in common?

nonpolar & hydrophobic

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Classes of amino acids: What do these amino acids have in common?

polar & hydrophilic

I like the polar ones the best!

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Protein domains anchor molecule:   Within membrane

  nonpolar amino acids  hydrophobic  anchors protein into membrane

  On outer surfaces of membrane in fluid   polar amino acids

 hydrophilic  extend into extracellular fluid & into cytosol

Hey,these are

“Amphiphilic

Molecules!

Polar areas of protein

Nonpolar areas of protein

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NH2

H+

COOH

Cytoplasm

Retinal chromophore

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

Porin monomer β-pleated sheets

Bacterial outer membrane

proton pump channel protons

function through conformational change = protein changes shape

Examples: aquaporin = water channel in bacteria

H2O

H2O

H+

H+

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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

“Antigen”

“Channel”

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Membrane Proteins:   Proteins determine membrane’s specific functions

  cell membrane & organelle membranes each have unique collections of proteins

  Classes of membrane proteins:   peripheral proteins

  loosely bound to surface of membrane   ex: cell surface identity marker (antigens)

  integral proteins   penetrate lipid bilayer, usually across whole membrane   transmembrane protein   ex: transport proteins

  channels, permeases (pumps)

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Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer

Extracellular fluid

Phospholipids

Cholesterol

Cytoplasm

Glycolipid

Transmembrane proteins

Filaments of cytoskeleton

Peripheral protein

Glycoprotein

1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model

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Membrane carbohydrates:  Play a key role in cell-cell recognition

 ability of a cell to distinguish one cell from another   Antigens – human A, B, O blood grps.

  important in organ & tissue development

 basis for rejection of foreign cells by immune system

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Cell to Cell Recognition (cellular communication):

Inside the cell

Outside the cell

Carbohydrates

Cholesterol Molecule

Integral protein

Peripheral proteins

Phospholipid Bilayer

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  HIV must bind to the immune cell surface protein CD4 and a “co-receptor” CCR5 in order to infect a cell

  HIV cannot enter the cells of resistant individuals that lack CCR5

How HIV Invades Cells—And How To Stop It

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The genetic basis for HIV resistance:

HIV

Receptor (CD4)

Co-receptor (CCR5)

Receptor (CD4) but no CCR5 Plasma

membrane

(a) (b)

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How HIV Invades Cells—And How To Stop It Immune cells that

carry a mutant form of CCR5 don’t allow HIV

to bind. Several research groups are trying to mimic this

natural resistance by introducing mutant

versions of CCR5 into HIV-infected people.

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Synthesis and Sidedness of Membranes:   Membranes have

distinct “polarity” inside and outside faces:

  The asymmetrical distribution of proteins, lipids, and carbos in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus.

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Any Questions??