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8/14/2019 CMB Lect 3 2011 colour 2 slides per page.pdf
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LECTURE 3
Cell membranes, cell
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unc ons an ce u ar
interactions
Cell Membranes
•
- encloses the cell, defines its boundaries andmaintains differences between the cytosol andthe extracellular environment.
• Other membranes in the cell
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- surround organelles (eg mitochondria,chloroplasts, nuclei), form ER and Golgi
- control passage of materials among intracellularcompartments
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Cell membranes (contd)
• Chemical composition of membranes
a li ids
- most abundant lipids in membranes arephospholipids
- amphipathic ie hydrophilic heads andhydrophobic tails
- lipid bilayer is the basic structure of cellmembranes, 7-8 nm thick
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(b) proteins
- may be detected chemically and seen inelectron micrographs using freeze-fracturetechniques
The phospholipid bilayer
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Freeze fracturing membranes
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Cell membranes (contd)
•
- 1930’s (Davson and Danielli) - bilayer withlayer of protein on each surface.
- 1972 (Singer and Nicholson) - Fluid Mosaicmodel. The membrane is described as a mosaic
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o pro e n mo ecu es oa ng n a p osp o pbilayer
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The fluid mosaic model formembrane structure
Phospholipid
bilayer
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Hydrophobic regionsof protein Hydrophilicregions of protein
Cell membranes (contd)
Important features of the fluid mosaic model:
- lipid bilayer is fluid; molecules are able todiffuse rapidly within their own monolayer butvery rarely transfer spontaneously from onemonolayer to the other
- in eukaryotes, cholesterol molecules moderatethe fluidit of the bila er
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- the two halves of the bilayer are asymmetricalie may contain different species of phospholipids.Glycolipids are found on the outer half of thebilayer
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Movement of phospholipids inmembranes
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107 times per second.
-
is rare ( once per month).
Fluidity of membranes
Fluid Viscous
Unsaturated hydrocarbon
tailsSaturated hydrocarbon tails
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(a) Unsaturated versus saturated hydrocarbon tails
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Cholesterol in the membrane
(b) Cholesterol w ithin the animal
cell membrane
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Cell membranes (contd)
-hydrophobic core of the bilayer, peripheralmembrane proteins are loosely bound to thesurface
- proteins float in the lipid bilayer and some mayextend right through it
-
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as receptors
- proteins can diffuse laterally in the bilayer (cellfusion expts)
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Evidence for movement ofmembrane proteins
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The plasma membrane
Fibers of extra-
cellular matrix (ECM)
Glyco-
proteinCarbohydrate
GlycolipidEXTRACELLULAR
SIDE OF
MEMBRANE
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Microfilaments
of cytoskeleton
CYTOPLASMIC SIDE
OF MEMBRANE
Integral
protein
Peripheral
proteins
Cholesterol
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Cell membranes (contd)•
- intercellular junctions (gap junctions)
- transport (Na/K Atp'ase)
- cellular recognition (histocompatibility antigens)
- enzymes (mitochondria)
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- receptors (hormones)
- anchors (cytoskeleton)
Functions of membrane proteins (1)
Signaling molecule
Enzymes
ATP
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(a) Transport (b) Enzymatic activity (c) Signal transduction
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Functions of membrane proteins (2)
Glyco-
protein
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(d) Cell-cell recogniti on (e) Intercellular joining (f) Attachment to
the cytoskeleton
and extracellular matrix (ECM)
Functions of membrane proteins
• a) Intercellular Junctions
i) Desmosomes - provide structural strength(spot welds)
ii) Tight junctions - water-tight seal
iii) Gap junctions - communication channels, 6subunits around a 2nm diameter hole.
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iv)Plasmodesmata - channels which traverseplant cell walls thereby directly connecting thecytoplasm of adjacent cells
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Tight junctions prevent
fluid from moving
across a layer of cellsTight junction
TEM0.5 m
Intercellular junctions inanimal cells
Tight junction
TEM1 m
Desmosome
Intermediate
filaments
Gap
jun cti on
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T E M
0.1 m
Extracellular
matrixPlasma membranes
of adjacent cells
Space
between cells
Ions or small
molecules
Gap junction
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Plasmodesmata form channels through
the cell walls of plants
Cell walls
Interior
of cell
Interior
of cell
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0.5 m Plasmodesmata Plasma membranes
Functions of membrane proteins (contd)
• b) Membrane Transport
- membranes are selectively permeable., 2
and small, uncharged polar molecules (CO2)can pass rapidly through the cell membranebut it is relatively impermeable to large,uncharged polar molecules (glucose) and ions
(H+, Na+, K+, Cl-)
- water moves to equalise concentrations of
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solutes on each side of the membrane(osmosis)
- compounds can cross the membrane bymoving through the bilayer, through a proteinor by endo/exocytosis.
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Passive and active transport
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Functions of membrane proteins (contd)
• b) Membrane Transport (contd)
- represents a movement of molecules fromhigh concentration to low concentration(requires a favourable gradient to bemaintained)
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Functions of membrane proteins (contd)
• b) Membrane Transport (contd)
transport proteins
Transport proteins:
- provide great selectivity
- remain unchanged
- are recycled after each assisted entry and/or
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exit
Functions of membrane proteins (contd)
• in facilitated diffusion no energy is expended egentry of glucose into an animal cell. Glucose israpidly broken down when it enters the cell so asteep concentration gradient is maintained but rateof entry is limited by the number of specificlucose-trans ortin molecules in the membrane
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• water is also now known to diffuse through achannel protein (aquaporin) rather than the lipidbilayer itself
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Two models for facilitated diffusion
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Functions of membrane proteins (contd)
• (ii) facilitated diffusion and active transport (contd)
- for active transport, energy is required in theform usually of ATP eg the sodium /potassium pump. This allows cells to maintaina low internal concentration of Na and high K.
- three Na pumped out for every two K in andthe protein responsible is known as the Na /K stimulated ATPase (consumes 30% of the
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ATP supply of a resting cell)
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Active transport by the Na/K pump
EXTRACELLULAR
FLUID [Na ] high
[K ] low
Na
Na
Na
Na
Na
Na
Na
[Na ] low
[K ] high
CYTOPLASM Na
1 2 3
Na
K
PP
ATP
ADP
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456
K
K
K
K K
PP i
Functions of membrane proteins (contd)
• b) Membrane Transport (contd)
(iii) Exo- and endocytosis
- exocytosis is associated with secretion
- involves fusion of vesicles with the plasmamembrane eg vesicles from Golgi
- endocytosis is associated with entry ofmacromolecules and particulate matter
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- achieved by phagocytosis, pinocytosis orreceptor-mediated endocytosis
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Pseudopodium
Solutes
EXTRACELLULAR
FLUID
Pseudopodiumof amoeba
Phagocytosis
Phagocytosis“Food”
or other
particle
Bacterium
Food vacuole
An amoeba engul fin g a bacter ium
via phagocytosi s (TEM).
1 m
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Foodvacuole
CYTOPLASM
Plasma 5 m
Pinocytosis
PinocytosisPinocytosis vesicles forming
in a cell lining a small blood
vessel (TEM).
0 .
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Vesicle
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Receptor-mediated endocytosis
Receptor-Mediated Endocytosis
Receptor
m
Ligand
Coat proteins
Coated
pit
Coat
proteins
Plasma
membrane
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Top: A coated p it. Bottom: Acoated vesicle forming duringreceptor-mediated endocytosi s(TEMs).
0 . 2
Coated
vesicle
Past question
a n s o mo ecu es pass roug a cemembrane most easily?
1. ionic2. small and hydrophobic3. large and hydrophobic
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.5. large and polar