Post on 25-Jan-2016
description
1
CHAPTER 5
MEMBRANE
STRUCTURE
AND TRANSPORT
Prepared by
Brenda Leady, University of Toledo
2
Biological Membranes
Basic framework of the membrane is the phospholipid bilayer
Phospholipids are amphipathic moleculesHydrophobic (water-fearing) region faces inHydrophilic (water-loving) region faces out
Membranes also contain proteins and carbohydratesRelative amount of each vary
3
Fluid-mosaic model
Membrane is considered a mosaic of lipid, protein, and carbohydrate molecules
Membrane exhibits properties that resemble a fluid because lipids and proteins can move relative to each other within the membrane
4
5
Proteins bound to membranes Integral membrane proteins
Transmembrane proteins One or more regions that are physically embedded in the
hydrophobic region of the phospholipid bilayerLipid anchors
Covalent attachment of a lipid to an amino acid side chain within a protein
Peripheral membrane proteinsNoncovalently bound to regions of integral membrane
proteins that project out from the membrane, or they are bound to the polar head groups of phospholipids
6
Approximately 25% of All Genes Encode Membrane Proteins
Membranes are important biologically and medically Computer programs can be used to predict the number
of membrane proteins Estimated percentage of membrane proteins is
substantial: 20–30% of all genes may encode membrane proteins
This trend is found throughout all domains of life including archaea, bacteria, and eukaryotes
Function of many genes unknown- study may provide better understanding and better treatments
9
Membranes are semifluid
Fluidity- individual molecules remain in close association yet have the ability to readily move within the membrane
Semifluid- most lipids can rotate freely around their long axes and move laterally within the membrane leaflet
“Flipflop” of lipids from one leaflet to the opposite leaflet does not occur spontaneouslyFlippase requires ATP to transport lipids from one
leaflet to another
10
11
Factors affecting fluidity
Length of fatty acyl tailsShorter acyl tails are less likely to interact, which
makes the membrane more fluid Presence of double bonds in the acyl tails
Double bond creates a kink in the fatty acyl tail, making it more difficult for neighboring tails to interact and making the bilayer more fluid
Presence of cholesterolCholesterol tends to stabilize membranesEffects depend on temperature
12
Experiments on lateral transport
Larry Frye and Michael Edidin conducted an experiment that verified the lateral movement of membrane proteins
Mouse and human cells were fused Temperature treatment- 0°C or 37°C Mouse membrane protein H-2 fluorescently
labeled 0°C cells- label stays on mouse side 37°C cells- label moves over entire cell
13
14
FRAP
Watt Webb and colleagues used fluorescence recovery after photobleaching (FRAP)
Proteins on the surface of a cell were covalently labeled with a fluorescent chemical
Small area of cell photobleached leaving white spot Over time, bleached molecules within the white spot
spread outward, and the white region filled in with red fluorescent molecules
Indicates that proteins can laterally move in the membrane
15
16
Not all integral membrane proteins can move Depending on the cell type, 10–70% of
membrane proteins may be restricted in their movement
Integral membrane proteins may be bound to components of the cytoskeleton, which restricts the proteins from moving laterally
Also, membrane proteins may be attached to molecules that are outside the cell, such as the interconnected network of proteins that forms the extracellular matrix
17
18
Glycosylation
Process of covalently attaching a carbohydrate to a protein or lipidGlycolipid – carbohydrate to lipidGlycoprotein – carbohydrate to protein
Can serve as recognition signals for other cellular proteins
Often play a role in cell surface recognition Protective effects
Cell coat or glycocalyx - carbohydrate-rich zone on the cell surface shielding cell
19
20
Electron microscopy
Transmission electron microscopy (TEM), uses a biological sample that is thin sectioned and stained with heavy-metal dyes
Dye binds tightly to the polar head groups of phospholipids, but it does not bind well to the fatty acyl chains
21
FFEM
Freeze fracture electron microscopy, specialized form of TEM, can be used to analyze the interiors of phospholipid bilayers Sample is frozen in liquid nitrogen and fractured with
a knife Due to the weakness of the central membrane region,
the leaflets separate into a P face (the protoplasmic face that was next to the cytosol) and the E face (the extracellular face)
Can provide significant three-dimensional detail about membrane protein form and shape
22
23
Selectively permeable
Structure ensures …Essential molecules enterMetabolic intermediates remainWaste products exit
24
Phospholipid bilayer is a barrier
Hydrophobic interior makes formidable barrier
DiffusionMovement of solute from an area of higher
concentration to an area of lower concentrationPassive diffusion- without transport protein
Solutes vary in their rates of penetration
25
26
Cells maintain gradients
Transmembrane gradientConcentration of a solute is higher on one
side of a membrane than the other Ion electrochemical gradient
Both an electrical gradient and chemical gradient
27
Passive transport
Passive transport does not require an input of energy
2 typesPassive diffusion
Diffusion of a solute through a membrane without transport protein
Facilitated diffusion Diffusion of a solute through a membrane with the aid of
a transport protein
28
29
Tonicity Isotonic
Equal water and solute concentrations on either side of the membrane
HypertonicSolute concentration is higher (and water
concentration lower) on one side of the membrane Hypotonic
Solute concentration is lower (and water concentration higher) on one side of the membrane
30
Isotonic
Hypertonic
Hypotonic
Outside the cell Inside the cell
The solution and cell are isotonic
The solution is hypertonic to the cell
The solution is hypotonic to the cell
31
Osmosis
Water diffuses through a membrane from an area with more water to an area with less water
If the solutes cannot move, water movement can make the cell shrink or swell as water leaves or enters the cell
Osmotic pressure- the tendency for water to move into any cell
32
Animal cells must maintain a balance between extracellular and intracellular solute concentrations to maintain their size and shape
Crenation- shrinking in a hypertonic solution
33
A cell wall prevents major changes in cell size
Turgor pressure- pushes plasma membrane against cell wallMaintains shape and
size Plasmolysis- plants wilt
because water leaves plant cells
34
Agre Discovered That Osmosis Occurs More Quickly in Cells with Transport Proteins That Allow the Facilitated Diffusion of Water Water passively diffuses across plasma membranes Certain cell types allow water to move across the plasma membrane
at a much faster rate than would be predicted by passive diffusion Peter Agre and his colleagues first identified a protein that was
abundant in red blood cells and kidney cells, but not found in many other cell types
CHIP28 Striking difference was observed between frog oocytes that
expressed CHIP28 versus the control Aquaporins Agre was awarded the Nobel Prize in 2003 for this work
37
Transport proteins
Transport proteins enable biological membranes to be selectively permeable
2 classesChannelsTransporters
38
Channels
Form an open passageway for the direct diffusion of ions or molecules across the membrane
Aquaporins
39
Most are gated- open or closeLigand-gated Intracellular
regulatory proteinsPhosphorylationVoltage-gatedMechanosensitive
channels
40
Transporters
Also known as carriers Conformational change
transports solute Principal pathway for the
uptake of organic molecules, such as sugars, amino acids, and nucleotides
Key role in export
41
Transporter types Uniporter
single molecule or ion
Symporter/ cotransporter
2 or more ions or molecules transported in same direction
Antiporter 2 or more ions or
molecules transported in opposite directions
42
PumpCouples
conformational changes to an energy source, such as
ATP-driven pumps ATP hydrolysis can be uniporters,
symporters, or antiporters
Active transport
43
Active transport
Movement of a solute across a membrane against its gradient from a region of low concentration to higher concentration
Energetically unfavorable and requires the input of energy
Primary active transport Directly use energy to transport solute
Secondary active transport Use pre-existing gradient to drive transport of solute
44
45
ATP-Driven Ion Pumps Generate Ion Electrochemical Gradients Na+/K+-ATPase
Actively transport Na+ and K+ against their gradients by using the energy from ATP hydrolysis
3 Na+ exported for 2 K+ imported into cell Antiporter Electrogenic pump- export 1 net positive charge
46
47
Exocytosis/ Endocytosis
Transport larger molecules such as proteins and polysaccharides, and even very large particles
Exocytosis Material inside the cell, which is packaged into
vesicles, is excreted into the extracellular medium Endocytosis
Plasma membrane invaginates, or folds inward, to form a vesicle that brings substances into the cell
Receptor-mediated endocytosis Pinocytosis Phagocytosis
48
49