AP Biology. The Cell Membrane

Transcription

1 The Cell Membrane

2 Updated model of an animal cell s plasma membrane Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE

3 Phospholipids: Phosphate head hydrophilic Fatty acid tails hydrophobic Arranged as a bilayer Phosphate attracted to water Fatty acid repelled by water Aaaah, one of those structure function examples

4 The Fluidity of Membranes: Phospholipids can move within the bilayer Lipids, and some proteins, drift laterally Rarely, a lipid may flip-flop transversely across the membrane Membranes are like a jelly sandwich What makes-up the jelly and the bread?

5 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

6 Figure 7.5 (a) Unsaturated versus saturated hydrocarbon tails Fluid Viscous Unsaturated tails prevent packing. Saturated tails pack together. (b) Cholesterol within the animal cell membrane Cholesterol reduces membrane fluidity at moderate temperatures, but at low temperatures hinders solidification. Cholesterol

7 Cryopreservation Of gametes??

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9 Effects of Freezing Rates on Cells:

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11 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.

12 Arranged as a Phospholipid bilayer: Serves as a cellular barrier / border polar hydrophilic heads sugar H 2 O salt nonpolar hydrophobic tails impermeable to polar molecules polar hydrophilic heads waste lipids

13 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)

14 Permeability to polar molecules? Membrane becomes semi-permeable via protein channels specific channels allow specific material across cell membrane inside cell H 2 O aa sugar NH 3 salt outside cell

15 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

16 The membrane looks like chicken skin!

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

18 Classes of amino acids: What do these amino acids have in common? nonpolar & hydrophobic

19 Classes of amino acids: What do these amino acids have in common? I like the polar ones the best! polar & hydrophilic

20 Protein domains anchor molecule: Within membrane nonpolar amino acids hydrophobic anchors protein into membrane On outer surfaces of Hey,these are Amphiphilic Molecules! membrane in fluid polar amino acids hydrophilic extend into extracellular fluid & into cytosol Polar areas of protein Nonpolar areas of protein

21 Examples: aquaporin = water channel in bacteria Retinal chromophore H + H + NH 2 Porin monomer H 2 O β-pleated sheets Bacterial outer membrane Nonpolar (hydrophobic) COOH α-helices in the cell membrane H + Cytoplasm proton pump channel protons H 2 O function through conformational change = protein changes shape

22 Many Functions of Membrane Proteins: Outside Plasma membrane Channel Inside Transporter Enzyme activity Cell surface receptor Antigen Cell surface identity marker Cell adhesion Attachment to the cytoskeleton

23 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)

24 Cell membrane must be 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!

25 Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Glycoprotein Extracellular fluid Glycolipid Phospholipids Peripheral protein Cholesterol Cytoplasm Transmembrane proteins Filaments of cytoskeleton 1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model

26 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

27 Cell to Cell Recognition (cellular communication): Outside the cell Inside the cell Carbohydrates Integral protein Cholesterol Molecule Peripheral proteins Phospholipid Bilayer

28 How HIV Invades Cells And How To Stop It 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

29 The genetic basis for HIV resistance: HIV Receptor (CD4) Co-receptor (CCR5) Receptor (CD4) but no CCR5 Plasma membrane (a) (b)

30 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.

31 Synthesis and Sidedness of Membranes: Membranes have distinct polarity inside and outside faces: The asymmetrical distribution of proteins, lipids, and carbs in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus.

32 Any Questions??