Download Biological Membranes

Biological Membranes
Basic framework of the membrane is the
phospholipid bilayer
Phospholipids are amphipathic molecules
Hydrophobic
(water-fearing) region faces in
Hydrophilic (water-loving) region faces out
Membranes also contain proteins and
carbohydrates
Relative
amount of each vary
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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
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Proteins bound to membranes
Integral membrane proteins
Transmembrane
One or more regions that are physically embedded
in the hydrophobic region of the phospholipid bilayer
Lipid
proteins
anchors
Covalent attachment of a lipid to an amino acid side
chain within a protein
Peripheral membrane proteins
Noncovalently
bound to regions of integral
membrane proteins that project out from the
membrane, or they are bound to the polar head
groups of phospholipids
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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
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
spontaneously
Flippase
requires ATP to transport lipids from
one leaflet to another
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Factors affecting fluidity
Length of fatty acyl tails
Shorter
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 cholesterol
Cholesterol
tends to stabilize membranes
Effects depend on temperature
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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
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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
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Glycosylation
Process of covalently attaching a
carbohydrate to a protein or lipid
Glycolipid
– carbohydrate to lipid
Glycoprotein – 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
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Phospholipid bilayer is a barrier
Hydrophobic interior makes formidable
barrier
Diffusion
Movement
of solute from an area of higher
concentration to an area of lower concentration
Passive diffusion- without transport protein
Solutes vary in their rates of penetration
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Selectively permeable
Structure ensures …
Essential
molecules enter
Metabolic intermediates remain
Waste products exit
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Cells maintain gradients
Transmembrane gradient
Concentration
of a solute is higher on one
side of a membrane than the other
Ion electrochemical gradient
Both
an electrical gradient and chemical
gradient
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Passive transport
Passive transport does not require an
input of energy
2 types
Passive
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
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Tonicity
Isotonic
Equal
water and solute concentrations on either
side of the membrane
Hypertonic
Solute
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
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Outside the cell
Inside the cell
Isotonic
The solution and
cell are isotonic
Hypertonic
The solution is
hypertonic to the cell
Hypotonic
The solution is
hypotonic to the cell
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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
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Animal cells must
maintain a balance
between
extracellular and
intracellular solute
concentrations to
maintain their size
and shape
Crenation- shrinking
in a hypertonic
solution
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A cell wall prevents
major changes in cell
size
Turgor pressurepushes plasma
membrane against cell
wall
Maintains shape
and size
Plasmolysis- plants wilt
because water leaves
plant cells
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Aquaporins
Peter Agre and his colleagues first identified,
accidentally, 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
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Aquaporin
Discovered accidently in 1992 by Dr. Peter
Agre at Johns Hopkins University while
looking for Rh factors on RBC
Dr. Agre won the Nobel Prize in 2003
…and we met him in 2009!
Left to right: Wei Dong, Robin Bagley, Nobel Laureate Dr.
Peter Agre, Samantha Mahmoud and Joanna John
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Transport proteins
Transport proteins enable biological
membranes to be selectively permeable
2 classes
Channels
Transporters
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Channels
Form an open
passageway for
the direct diffusion
of ions or
molecules across
the membrane
Aquaporins
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Most are gatedopen or close
Ligand-gated
Intracellular
regulatory proteins
Phosphorylation
Voltage-gated
Mechanosensitive
channels
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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
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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
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Pump
Couples
conformational
changes to an energy
source
ATP-driven pumps
ATP hydrolysis
can be uniporters,
symporters, or
antiporters
Active transport
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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
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ATP-Driven Ion Pumps Generate
Ion Electrochemical Gradients
Na+/K+-ATPase
transport Na+ and K+ against their
gradients by using the energy from ATP
hydrolysis
3 Na+ exported for 2 K+ imported into cell
Actively
Antiporter
Electrogenic pump- export 1 net positive charge
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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
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KU Game Day!! (brought to you by Lhasa)
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