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Membranes Chapter 5

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Page 1: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

Membranes

Chapter 5

Page 2: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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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

Page 3: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Page 4: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Page 5: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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• Cellular membranes have 4 components1. 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

Page 6: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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• Both transmission electron microscope (TEM) and scanning (SEM) used to study membranes

• One method to embed specimen in resin– 1µm shavings– TEM shows layers

Page 7: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

• Freeze-fracture visualizes inside of membrane

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Page 8: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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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

Page 9: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Page 10: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

• Bilayers are fluid• Hydrogen bonding

of water holds the 2 layers together

• Individual phospholipids and unanchored proteins can move through the membrane

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Page 11: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

• 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

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Page 12: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Membrane Proteins

• Various functions:1. Transporters

2. Enzymes

3. Cell-surface receptors

4. Cell-surface identity markers

5. Cell-to-cell adhesion proteins

6. Attachments to the cytoskeleton

Page 13: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Page 14: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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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

Page 15: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

• Anchoring molecules are modified lipids with1. Nonpolar regions that insert into the internal portion

of the lipid bilayer

2. Chemical bonding domains that link directly to proteins

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Page 16: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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• 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

Page 17: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

• Proteins need only a single transmembrane domain to be anchored in the membrane, but they often have more than one such domain

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Page 18: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

• Bacteriorhodopsin has 7 transmembrane domains forming a structure within the membrane through which protons pass during the light-driven pumping of protons

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Page 19: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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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

Page 20: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Page 21: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Passive Transport

• Passive transport is movement of molecules through the membrane in which– No energy is required– Molecules move in response to a

concentration gradient

• Diffusion is movement of molecules from high concentration to low concentration– Will continue until the concentration is the

same in all regions

Page 22: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Page 23: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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• Major barrier to crossing a biological membrane is the hydrophobic interior that repels polar molecules but not nonpolar molecules– Nonpolar molecules will move until the

concentration is equal on both sides– Limited permeability to small polar molecules– Very limited permeability to larger polar

molecules and ions

Page 24: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

• Facilitated diffusion– Molecules that cannot cross membrane easily

may move through proteins– Move from higher to lower concentration– Channel proteins

• Hydrophilic channel when open

– Carrier proteins• Bind specifically to molecules they assist

• Membrane is selectively permeable24

Page 25: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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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

Page 26: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Page 27: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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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

Page 28: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Page 29: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Osmosis

• Cytoplasm of the cell is an aqueous solution– Water is solvent– Dissolved substances are solutes

• Osmosis – net diffusion of water across a membrane toward a higher solute concentration

Page 30: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Page 31: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Page 32: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Osmotic concentration

• When 2 solutions have different osmotic concentrations– Hypertonic solution has a higher solute concentration– Hypotonic solution has a lower solute concentration

• When two solutions have the same osmotic concentration, the solutions are isotonic

• Aquaporins facilitate osmosis

Page 33: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

Osmotic pressure

• Force needed to stop osmotic flow• Cell in a hypotonic solution gains water causing

cell to swell – creates pressure• If membrane strong enough, cell reaches

counterbalance of osmotic pressure driving water in with hydrostatic pressure driving water out– Cell wall of prokaryotes, fungi, plants, protists

• If membrane is not strong, may burst– Animal cells must be in isotonic environments

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Page 34: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Page 35: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Maintaining osmotic balance

• Some cells use extrusion in which water is ejected through contractile vacuoles

• Isosmotic regulation involves keeping cells isotonic with their environment– Marine organisms adjust internal

concentration to match sea water– Terrestrial animals circulate isotonic fluid

• Plant cells use turgor pressure to push the cell membrane against the cell wall and keep the cell rigid

Page 36: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Active Transport

• Requires energy – ATP is used directly or indirectly to fuel active transport

• Moves substances from low to high concentration

• Requires the use of highly selective carrier proteins

Page 37: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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• Carrier proteins used in active transport include– Uniporters – move one molecule at a time– Symporters – move two molecules in the

same direction– Antiporters – move two molecules in opposite

directions– Terms can also be used to describe facilitated

diffusion carriers

Page 38: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Sodium–potassium (Na+–K+) pump

• Direct use of ATP for active transport• Uses an antiporter to move 3 Na+ out of

the cell and 2 K+ into the cell– Against their concentration gradient

• ATP energy is used to change the conformation of the carrier protein

• Affinity of the carrier protein for either Na+ or K+ changes so the ions can be carried across the membrane

Page 39: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Page 40: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Coupled transport

• Uses ATP indirectly• Uses the energy released when a molecule

moves by diffusion to supply energy to active transport of a different molecule

• Symporter is used• Glucose–Na+ symporter captures the

energy from Na+ diffusion to move glucose against a concentration gradient

Page 41: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Page 42: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

Bulk Transport

• Endocytosis– Movement of substances into the cell– Phagosytosis – cell takes in particulate matter– Pinocytosis – cell takes in only fluid– Receptor-mediated endocytosis – specific

molecules are taken in after they bind to a receptor

• Exocytosis– Movement of substances out of cell

• Requires energy

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Page 43: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

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Page 44: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

• In the human genetic disease familial hypercholesterolemia, the LDL receptors lack tails, so they are never fastened in the clathrin-coated pits and as a result, do not trigger vesicle formation. The cholesterol stays in the bloodstream of affected individuals, accumulating as plaques inside arteries and leading to heart attacks.

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Page 45: Membranes Chapter 5. 2 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model – mosaic

• Exocytosis – Movement of materials out of the cell– Used in plants to export cell wall material– Used in animals to secrete hormones,

neurotransmitters, digestive enzymes

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