Membranes Chapter 5 Membrane Structure Phospholipids arranged in

Membranes Chapter 5

Membrane Structure • Phospholipids arranged in a bilayer • Globular proteins inserted in the lipid bilayer • Fluid mosiac model – mosaic of proteins floats in or on the fluid lipid bilayer like boats on a pond 2

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• Cellular membranes have 4 components 1. Phospholipid bilayer • Flexible matrix, barrier to permeability 2. Transmembrane proteins • Integral membrane proteins 3. Interior protein network • Peripheral membrane proteins 4. Cell surface markers • Glycoproteins and glycolipids 5

• Both transmission electron microscope (TEM) and scanning (SEM) used to study membranes • One method to embed specimen in resin – 1µm shavings – TEM shows layers 6

• Freeze-fracture visualizes inside of membrane 7

Phospholipids • Structure consists of – Glycerol – a 3 -carbon polyalcohol – 2 fatty acids attached to the glycerol • Nonpolar and hydrophobic (“water-fearing”) – Phosphate group attached to the glycerol • Polar and hydrophilic (“water-loving”) • Spontaneously forms a bilayer – Fatty acids are on the inside – Phosphate groups are on both surfaces 8

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• Environmental influences – Saturated fatty acids make the membrane less fluid than unsaturated fatty acids • “Kinks” introduced by the double bonds keep them from packing tightly • Most membranes also contain sterols such as cholesterol, which can either increase or decrease membrane fluidity, depending on the temperature – Warm temperatures make the membrane more fluid than cold temperatures • Cold tolerance in bacteria due to fatty acid desaturases 10

Membrane Proteins • Various functions: 1. 2. 3. 4. 5. 6. Transporters Enzymes Cell-surface receptors Cell-surface identity markers Cell-to-cell adhesion proteins Attachments to the cytoskeleton 11

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Structure relates to function • Diverse functions arise from the diverse structures of membrane proteins • Have common structural features related to their role as membrane proteins • Peripheral proteins – Anchoring molecules attach membrane protein to surface 13

• Anchoring molecules are modified lipids with 1. Nonpolar regions that insert into the internal portion of the lipid bilayer 2. Chemical bonding domains that link directly to proteins 14

• Integral membrane proteins – Span the lipid bilayer (transmembrane proteins) • Nonpolar regions of the protein are embedded in the interior of the bilayer • Polar regions of the protein protrude from both sides of the bilayer – Transmembrane domain • Spans the lipid bilayer • Hydrophobic amino acids arranged in α helices 15

Membrane Proteins • Pores – Extensive nonpolar regions within a transmembrane protein can create a pore through the membrane – Cylinder of sheets in the protein secondary structure called a -barrel • Interior is polar and allows water and small polar molecules to pass through the membrane 16

Channel proteins • Ion channels – Allow the passage of ions – Gated channels – open or close in response to stimulus (chemical or electrical) – 3 conditions determine direction • Relative concentration on either side of membrane • Voltage differences across membrane • Gated channels – channel open or closed 17

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Carrier proteins • Can help transport both ions and other solutes, such as some sugars and amino acids • Requires a concentration difference across the membrane • Must bind to the molecule they transport – Saturation – rate of transport limited by number of transporters 19

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