Chapter 5 Membranes

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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

One method to embed specimen in resin

1µm shavingsTEM shows

layers

• 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

• Freeze-fracture visualizes inside of membrane

A sandwich modelby Hugh Davson and James Danielli

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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

• 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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Fig. 7-5

Lateral movement(~107 times per second)

Flip-flop(~ once per month)

(a) Movement of phospholipids

(b) Membrane fluidity

Fluid Viscous

Unsaturated hydrocarbontails with kinks

Saturated hydro-carbon tails

(c) Cholesterol within the animal cell membrane

Cholesterol

The fluidity of membrane

• Environmental influences– Saturated fatty acids make the membrane less

fluid than unsaturated fatty acids• “Kinks 糾結” 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 (37 )℃ temperatures make the membrane more fluid than cold temperatures

• At cool temperatures, it maintains fluidity by preventing tight packing

• Cold tolerance in bacteria due to fatty acid desaturases

– http://en.wikipedia.org/wiki/Phytosterol8

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Membrane Proteins

TransportersEnzymesCell-surface receptorsCell-surface identity markersCell-to-cell adhesion proteinsAttachments to the cytoskeleton

Various functions:

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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

• Some membrane proteins are attached to the surface of the membrane by special molecules hat associate strongly with phospholipids.

• 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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• 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

• Bacteriorhodopsin – has 7 transmembrane domains forming a structure within the

membrane through which protons pass during the light-driven

pumping of protons

13archaea 最著名地鹽桿菌

Bacteriorhodopsin (bR)為 Halobacterium salinarium 嗜鹽菌紫色細胞膜上的獨一光驅動質子泵之光能轉換蛋白。

視黃醛發色團 (retinal chromophore) 位於蛋白質的正中間

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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

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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

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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

• 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 permeable

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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

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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

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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

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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 (Water channel)– 11 kinds– Specific for only water– Allow other small hydrophlic molecules, such as glycerol or urea – Hereditary (Nephrogenic) diabetes insipidus (NDI) caused by a

nonfunctional aquaporin protein.

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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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

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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

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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

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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

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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

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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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• 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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• 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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