chapter 5 membranes. 2 membrane structure phospholipids arranged in a bilayer globular proteins...
TRANSCRIPT
Chapter 5 Membranes
2
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
4
5
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
6
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
9
Membrane Proteins
TransportersEnzymesCell-surface receptorsCell-surface identity markersCell-to-cell adhesion proteinsAttachments to the cytoskeleton
Various functions:
10
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
11
12
• 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
14
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
15
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
16
• 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
17
18
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
19
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
20
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
21
Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
22
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
23
24
25
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
26
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
27
• 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
28
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
29
30
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
31
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
32
33
• 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.
34
• 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
35