Exam 4 - Lecture 8 Flashcards
Membranes define
the boundaries of a cell and its internal
compartments
The 5 Functions of Membranes
- Define boundaries of a cell and organelles and act as
permeability barriers - Serve as sites for biological functions, such as electron
transport - Possess transport proteins that regulate the movement of
substances into and out of cells and organelles - Contain protein molecules that act as receptors to detect
external signals - Provide mechanisms for cell-to-cell contact, adhesion,
and communication
Membranes are effective permeability barriers because
their interior is hydrophobic
what surrounds the whole cell?
the plasma membrane
intracellular membranes do what
compartmentalize functions within the cell
Membranes are associated with specific functions
because
the molecules responsible for the functions are embedded in or localized on membranes
The specific enzymes associated with particular
membranes can be used to characterize a specific
membrane
Membrane Proteins Regulate
the transport of solutes
Membrane proteins carry out and regulate the
transport of substances across the membrane
Cells and organelles take up nutrients, ions, gases,
water, and other substances, and they expel
products and wastes
what are the two ways substances can move into or out of the cell
Some substances diffuse directly across
membranes, whereas others must be moved by
specific transporters
Membrane Proteins Detect and Transmit
Electrical and Chemical Signals
A cell receives information from its environment as
electrical or chemical signals at its surface
Signal transduction describes the mechanisms by which
signals are transmitted from the outer
surface to the interior of a cell
Chemical signal molecules usually bind to
membrane proteins, known as receptors, on the
outer surface of the plasma membrane
describe signal transduction
Binding of signal molecules to their receptors
triggers chemical events on the inner membrane
surface that ultimately lead to changes in cell
function
Membrane receptors allow cells to recognize,
transmit, and respond to a variety of specific
signals in nearly all types of cells
Membrane Proteins Mediate
Cell Adhesion
and Cell-to-Cell Communication
Cell-to-cell contacts, critical in animal development,
are often mediated by
cadherins
cadherins
mediate cell to cell contact
Cadherins have extracellular sequences of amino
acids that bind calcium and promote adhesion
between similar types of cells in a tissue
4 types of junctions
adhesive junctions
tight junctions
gap junctions
plasmodesmata
what do Adhesive junctions do
hold cells together
Tight junctions form
seals that block the passage of
fluids between cells
Gap junctions allow for
communication between
adjacent animal cells
plasmodesmata are present in
plants
describe the fluid mosaic model
The model envisions a membrane as two fluid
layers of lipids with proteins within and on the
layers
Overton and Langmuir
Lipids Are Important
Components of Membranes
Overton: cell surface had some kind
of lipid “coat” on it
Langmuir: phospholipids
areamphipathic
Gorter and Grendel:
The Basis of Membrane
Structure Is a Lipid Bilayer
Structure is a lipid bilayer, with the
nonpolar regions of the lipids facing
inward
Davson and Danielli: Membranes Also Contain
Proteins
go look at the research
did not make flashcards
Electron microscopy revealed that
there was not enough space
on either side of the bilayer for an additional layer of protein ( not ssure of in new slides)
The Davson–Danielli model also did not account for
not in updated slides
Membranes are susceptible to digestion by
phospholipases, suggesting that membrane lipids
are exposed ( not sure if in new slides)
Scientists were unable to isolate “surface” proteins
from membranes unless
organic solvents or
detergents were used
The fluid mosaic bilayer model
accounts
or all the
inconsistencies with previous
models
the fluid mosaic model has two key features
A fluid lipid bilayer
A mosaic of proteins attached
to or embedded in the bilayer
Transmembrane Segments
Most Membrane
Proteins Contain Transmembrane Segments
Most integral membrane proteins
have one or more hydrophobic
segments that span the lipid
bilayer
These transmembrane segments
anchor the protein to the
membrane
the first
membrane protein shown to
possess this structural feature
was Bacteriorhodopsin
How are membranes ordered and are the homogenous or heterogenous?
Not homogenous, freely mixing structures
Ordered through dynamic microdomains called lipid rafts
Most cellular processes that involve membranes
depend on
structural complexes of specific lipids
and proteins
Membrane lipids are important components of the
“fluid” part of the fluid mosaic model
Membranes contain several types of lipids
what are the main classes of lipids
phospholipids, glycolipids, and sterols
most abundant lipids in membranes
phospholipids
what are the two different bases for phospholipids?
glycerol-based phosphoglycerides and the
sphingosine-based sphingolipids
Glycolipids are formed by the addition of
carbohydrates to lipids
what are two variations of glycerol
Some are glycerol based (the glycoglycerolipids),
and some are sphingosine based (the
glycosphingolipids)
The most common glycosphingolipids are
cerebrosides and gangliosides
Cerebrosides are
are neutral glycolipids; each molecule has an
uncharged sugar as its head group
A ganglioside has
an oligosaccharide head group with one or more
negatively charged sialic acid residues
Cerebrosides and gangliosides are especially prominent
in brain and nerve cells
The membranes of most eukaryotes contain significant
amounts of
sterols
The main sterol in animal cell membranes is
cholesterol
cholesterol function
needed to stabilize and maintain membranes
Plant cell membranes contain what type of sterol?
phytosterols
fungal cell membranes contain
ergosterol, similar to
cholesterol
Fatty acids are components of all membrane lipids
except
the sterols
the long hydrocarbon tails provide as a
barrier to diffusion of polar solutes
The sizes of membrane fatty acids range between
12 and 20 carbons long, which is optimal for bilayer
formation and dictates the usual thickness of
membranes (6–8 nm)
Fatty Acids Vary in Degree of
saturation
Palmitate has
16 carbons
stearate has how many carbons
18 carbons
palmitate and stearate are common
saturated fatty acids
Oleate has how many double bonds
one double bond
linoleate has how many double bonds
two double bonds
Oleate (one double bond) and linoleate (two
double bonds) are both
18-carbon unsaturated fatty acids
Polyunsaturated fatty acids have more than one
double bond
Omega-3 fatty acids are
polyunsaturated fatty acids
that are essential for normal human development
Omega-3 fatty acids may also reduce the risk of
heart disease
Lipids can be isolated, separated, and studied
using
nonpolar solvents such as acetone and
chloroform
Thin-layer chromatography (TLC) is used to
separate different kinds of lipids based on their
relative polarities
the bottom of the
TLC plate is called the
origin
A nonpolar organic solvent moves up the plate by
capillary action taking
different lipids with it to varying degrees
Nonpolar lipids have little affinity for
silicic acid on the plate, so they
move readily with the solvent, near the solvent front
in reference to TLC Polar lipids will interact variably (depending on how polar they are) with
the
silicic acid, and their movement will be slowed proportionately
Membrane asymmetry describes the difference in degree of what component?
is the difference between
the monolayers regarding the kind of lipids present
and the degree of saturation of fatty acids in the
phospholipids
Most of the glycolipids in the plasma membrane of
animal cells are in what layer
outer layer
Membrane asymmetry is established during
the synthesis of the membrane
does membrane asymmetry change?
Once established, membrane asymmetry does not
change much
transverse diffusion
The movement of lipids from one monolayer to
another requires their hydrophilic heads to move all
the way through the hydrophobic interior of the
bilayer
This transverse diffusion (or “flip-flop”) is
relatively rare
describe lipid mobility
Lipids Move Freely Within Their Monolayer
Lipids are mobile within their monolayer
Movements are rapid and random
types of lipids motion
Rotation - Rotation of phospholipids about their axes
can occur
lateral diffusion - Phospholipids can also move within the monolayer, via lateral
diffusion
which membranes tend to have transverse diffusion / flip-flop?
Some membranes, in particular the smooth ER
membrane
why are some membranes prone to transverse diffusion?
what is the substance called?
because they have proteins that catalyze the flip-flop of
membrane lipids
These proteins are called phospholipid
translocators, or flippases
proteins catalyze the lip flop
phospholipid
translocators, or flippases
The lipid bilayer behaves as a fluid that permits the
movement of both
lipids and Proteins
How far can lipids move
Lipids can move as much as several μm per
second within the monolayer
Lateral diffusion can be demonstrated using what method
Fluorescence recovery after photobleaching
(FRAP)
FRAP measures lipid ______
mobility
how does FRAP work
Investigators label lipid molecules in a membrane with a fluorescent dye.
A laser beam is used to bleach the dye in a small area, creating a dark spot
on the membrane.
The membrane is observed afterward to determine how long it takes for the
dark spot to disappear, a measure of how quickly new fluorescent lipids
move in
Membranes Function Properly Only in the________ state
Fluid
what affects membrane fluidity
Membrane fluidity changes with temperature,
decreasing as temperature falls and vice versa
what is the transition temperature?
a characteristic of every lipid bilayer- the transition
temperature Tm, the temperature at which it
becomes fluid
The Tm is the point of maximum heat absorption as the membrane changes
from the gel to the fluid state
change is state of the membrane is called
Phase transition ( solid to liquid)
what happens to memrane functions when the temp is below Tm ?
Below the Tm , any functions that rely on membrane
fluidity will be disrupted
The transition temperature can be measured by
differential scanning
calorimetry
- The membrane is placed inside a calorimeter, and the uptake of heat is
measured as temperature is increased
Fluidity of a membrane depends
mainly on
that fatty acids that it contains
what two characteristics about fatty acids affect membrane fluidity
The length of fatty acid chains and
the degree of saturation both affect
the fluidity of the membrane
what has a higher Tm? Long chains and saturated fats or short chains and unsaturated?
Long-chain and saturated fatty
acids have higher Tm values,
whereas short-chain and
unsaturated fatty acids have lower Tm values
how do saturated farts sit together in the membrane
they pack well together in the membrane ( linear)
how do double bonds affect fatty acid shape
Fatty acids with one or
more double bonds have
bends in the chains that
prevent them from
packing together neatly
which is more fluid saturated or unsaturated
Because saturated bonds pack together and unsaturated bonds have a bend - – unsaturated fatty
acids are more fluid than
saturated fatty acids and
have lower Tm values
Why do Most plasma membrane fatty acids vary in chain length and
degree of saturation?
to ensure that membranes are fluid at physiological
temperatures
what type of double bonds do unsaturated fatty acids typically have ? and what type of bonds do trans fats normally have?
Most unsaturated fatty acids have cis double bonds
commercially produced trans fats, which pack together like
saturated fats do
how do sterols impact membrane fluidity ?
The intercalation of rigid cholesterol
molecules into a membrane decreases its
fluidity and increases the Tm
However, cholesterol also prevents
hydrocarbon chains of phospholipids from
packing together tightly and so reduces the
tendency of membranes to gel upon cooling
Therefore, cholesterol is a fluidity buffer;
sterols in other organisms may function
similarly
Other Effects of Sterols on Membranes
Sterols decrease the permeability of membranes to
ions and small polar molecules
This is likely because they fill spaces between the
hydrocarbon chains of phospholipids
This effectively blocks the routes that ions and
small molecules would take through the membran
cholesterol’s impact on the membrane gives it the name….
fluidity buffer
how do sterols affect permeability of membranes to ions and small polar molecules??? Explain why
Sterols decrease the permeability of membranes to
ions and small polar molecules
This is likely because they fill spaces between the
hydrocarbon chains of phospholipids
This effectively blocks the routes that ions and
small molecules would take through the membrane
How do organisms regulate membrane fluidity?
Most organisms can regulate membrane fluidity by
varying the lipid composition of the membranes
poikilotherms
organisms that cannot regulate their body temperature
use homeoviscous adaptation,
compensating for changes in temperature by
altering the length and degree of saturation of fatty
acids in their membranes
desaturase enzyme
Some organisms have a desaturase enzyme, which
introduces double bonds into fatty acids as needed
In plants and yeasts, temperature-related fluidity
changes are tied to
the increased solubility of
oxygen at lower temperatures
More oxygen is available at low temperatures, and
oxygen acts as a substrate for desaturase, allowing
membrane fluidity to be maintained at lower
temperatures
Lipid Rafts Are
Localized Regions of Membrane
Lipids That Are Involved in Cell Signaling
they are associated with specific proteins
they are also called lipid microdomains
These are dynamic structures,
changing composition as lipids and
proteins move into and out of them
how do lipid rafts in the outer membrane compare to those in the rest of the membrane?.
Lipid rafts in the outer monolayer of
animal cells have elevated levels of
cholesterol and glycosphingolipids
and are less fluid than the rest of the
membrane
Lipid Raft Formation
Early models of raft formation proposed that localized regions
of tightly associated cholesterol and glycosphingolipid
molecules attracted particular proteins to them
Raft-associated proteins sometimes are lipoproteins, with fatty
acids attached to them
Some of the more than 200 known raft-associated proteins
capture and organize particular lipid rafts
Lipid rafts contain actin-binding proteins, suggesting that the
cytoskeleton may play a role in their formation and organization
Functions of Lipid Rafts
Lipid rafts are thought to have roles in detecting
and responding to exracellular signals
For example, lipid rafts have roles in
Transporting nutrients and ions across
membranes
Binding activated immune system cells to their
microbial targets
Transporting cholera toxin into intestinal cells
Receptors in Lipid Rafts
When a receptor molecule on the outer surface of
the plasma binds its ligand, it can move into lipid
rafts also located in the outer monolayer
Lipid rafts containing receptors are coupled to lipid
rafts on the inner monolayer
Some lipid rafts contain kinases, enzymes that
generate second messengers in a cell via
phosporylation of target molecules
Some lipid rafts contain kinases which are
enzymes that
generate second messengers in a cell via
phosporylation of target molecules
what is the main component of the “mosaic” part of the mebrane
The mosaic part of the fluid mosaic model includes
lipid rafts and other lipid domains
However, membrane proteins are the main
components
Support for the fluid mosaic model came from
studies involving
freeze fracturing - a bilayer or membrane is frozen and then hit
sharply with a diamond knife
The resulting fracture often follows the plane
between the two layers of membrane lipid
Freeze-Fracture Analysis of Membranes
(fix)
When a fracture plane splits a membrane into its two layers,
particles the size and shape of globular proteins can be seen
The E surface is the exoplasmic face, and the P surface is the
protoplasmic face
The protein/lipid ratio varies among cell types
Membrane proteins fall into three categories:
Integral, peripheral, and lipid anchored
Membrane proteins have different _________ and so occupy different positions in or on
membranes
hydrophobicites
^ which determines how easily such proteins
can be extracted from membranes
Integral membrane proteins
are embedded in the lipid bilayer
because of their hydrophobic regions
Lipid-anchored proteins are
hydrophilic and attached to the bilayer
by covalent attachments to lipid molecules embedded in the bilayer
Peripheral proteins are
hydrophilic and located on the surface of the
bilayer
Integral Membrane Proteins in depth (fix)
Most membrane proteins possess one or more hydrophobic regions
with an affinity for the interior of the lipid bilayer
These are integral membrane proteins, with hydrophobic regions
embedded in the interior membrane bilayer
They are difficult to remove from membranes by standard isolation
procedures
Some integral membrane proteins, called integral monotropic
proteins, are embedded in just one side of the bilayer
However, most are transmembrane proteins that span the membrane
and protrude on both sides
Transmembrane proteins cross either once (singlepass proteins) or
several times (multipass proteins)
Most transmembrane proteins are anchored to the
lipid bilayer by one or more hydrophobic
Transmembrane segments
conformation and length of transmembrane segments
the polypeptide chain appears to
span the membrane in an α-helical conformation
about 20–30 amino acids long
Some are arranged as a closed β sheet called a
β barrel
Singlepass membrane proteins have the______ terminus extending from
one surface of the membrane and the ________ from the other
Singlepass membrane proteins have the C-terminus extending from
one surface of the membrane and the N-terminus from the other
For example, glycophorin is a singlepass protein on the erythrocyte
plasma membrane that is oriented so the C-terminus is on the inner
surface and the N-terminus is on the outer
describe Multipass Membrane Proteins
Multipass membrane proteins have 2–20 (or more) transmembrane
segments
For example, bacteriorhodopsin has seven transmembrane
segments positioned to form a channel
Membrane proteins that lack discrete hydrophobic
regions do not penetrate the lipid bilayer are called
peripheral membranes
peripheral membranes lack what ?
discrete hydrophobic
regions do not penetrate the lipid bilayer
peripheral membrane proteins are bound
to membrane surfaces through
weak electrostatic forces and hydrogen bonds
Some hydrophobic residues play a role in
anchoring them to the membrane surface
The polypeptide chains of lipid-anchored
membrane proteins are located on the
surfaces of
membranes
They are covalently bound to lipid molecules
embedded in the bilayer
Proteins bound to the inner surface of the plasma
membrane are linked to fatty acids, or isoprenyl
groups
Types of Lipid-Anchored Membrane Proteins
Fatty acid-anchored membrane proteins
Isoprenylated membrane proteins
GPI-anchored membrane proteins
Fatty acid-anchored membrane proteins
are
attached to a saturated fatty acid, usually myristic
acid (14C) or palmitic acid (16C)
Isoprenylated membrane proteins are
synthesized in the
cytosol and then modified by
addition of multiple isoprenyl groups (5C) usually
farnesyl (15C) or geranylgeranyl (20C) groups
GPI-anchored membrane proteins are _________ linked to __________
are covalently
linked to glycosylphosphatidylinositol
Isolation of Membrane Proteins
Peripheral membrane proteins are usually easy to
isolate by altering pH or ionic strength
Chelating (cation-binding) agents are also used to
solubilize peripheral membrane proteins
Lipid-anchored proteins are isolated by
similar means
Isolating Integral Membrane Proteins
Integral membrane proteins are difficult to isolate
from membranes
Often detergents are used that disrupt hydrophobic
interactions and dissolve the lipid bilayer
Electrophoresis is a
group of techniques that use
an electric field to separate charged molecules
How quickly a molecule moves during
electrophoresis depends on both charge and size
Electrophoresis uses various support media, most
commonly polyacrylamide or agarose
How does Electrophoresis of Membrane Proteins work?
Membrane fragments are solubilized in sodium
dodecyl sulfate (SDS), which disrupts protein-
protein and protein-lipid associations
The proteins are thus coated with negatively
charged detergent molecules
The proteins are loaded onto a polyacrylamide gel
and an electrical potential is applied
Two-dimensional SDS-polyacrylamide gel
electrophoresis (SDS-PAGE) separates….
Following electrophoresis, polypeptides can be
identified by
Two-dimensional SDS-polyacrylamide gel
electrophoresis (SDS-PAGE) separates proteins in
two dimensions, first by charge and then by size
Following electrophoresis, polypeptides can be
identified by Western blotting
In this technique, proteins are transferred to a
membrane and bound by specific antibodies
Some integral membrane proteins, called integral monotropic
proteins, are embedded
in just one side of the bilayer
However, most are transmembrane proteins that span the membrane
and protrude on both sides
Affinity labeling
utilizes radioactive molecules that
bind to certain proteins based on function
For example, cytochalasin B is an inhibitor of
glucose transport
membrane reconstitution
proteins are extracted
from membranes and separated individually
Membranes exposed to radioactive cytochalasin B
will likely have
the radioactivity bound to proteins
involved in glucose transport
purified proteins
are mixed with phospholipids
to form vesicles call liposomes
Liposomes can be loaded with
particular molecules
and tested for their ability to carry out certain
functions
X-ray crystallography
an be used to determine the
structure of proteins that can be isolated in
crystalline form
Membrane proteins are hard to isolate and
crystallize
X-ray crystallography is widely used to determine
three-dimensional structure of proteins
An alternative approach to X-ray crystallography
hydropathy analysis
Integral membrane proteins are difficult or easy to isolate
and crystallize
difficult
hydropathy (or hydrophobicity) plot
the number and location of transmembrane
segments in a membrane protein can be inferred if
the protein sequence is known
A computer program identifies clusters of
hydrophobic residues, calculating a hydropathy
index for successive “windows” along the protein
Site-specific mutagenesis allows determination of
how certain amino acids affect protein function
hydropathy (or hydrophobicity) plot is used for
The number and location of transmembrane segments in a
membrane protein can be inferred if the protein sequence is
known
hydropathy index
calculating hydrophobic residues
protein sequence is hard to determine unlike DNA bc
different amino acids are encoded with different codes. Histodine has 6 codes so you have to know which code was responsible
Membrane Proteins Are Oriented
Asymmetrically Across the Lipid Bilayer
Site-specific mutagenesis
allows determination of
how certain amino acids affect protein function
Once in place, in or on one of the monolayers,
proteins ( can or cannot) move across the membrane from
one surface to the other
proteins cannot move across the membrane from
one surface to the other
All the molecules of a particular protein are oriented
the same way in the membrane
The enzyme lactoperoxidase (LP) can be used to
The enzyme galactose oxidase (GO) can be used to
attach (125^I) to proteins
label carbohydrate
side chains attached to membrane proteins and lipids
Radioactive labeling procedures are used to
distinguish between proteins on
inner and outer surfaces of membrane vesicles
Glycoproteins
are membrane proteins with
carbohydrate chains covalently linked to amino acid
side chains ( covalent = strong)
The PROCESS of adding a carbohydrate side chain to a
protein is called
glycosylation
glycosylation
The addition of a carbohydrate side chain to a
protein
where does Glycosylation occurs ?
ER and Golgi
compartments
Glycosylation involves linkage of the
carbohydrate to
The nitrogen atom of an amino group
or
The oxygen atom of a hydroxl group
N-linked glycosylation
Glycosylation involves linkage of the
carbohydrate to The nitrogen atom of an amino group (N-linked glycosylation)
of an asparagine residue
O-linked glycosylation
Glycosylation involves linkage of the
carbohydrate to The oxygen atom of a hydroxl group (O-linked glycosylation)
of a serine, threonine, or modified lysine or proline residue
explain the statement : Membrane Proteins Vary in Their Mobility
Membrane proteins are more variable than lipids in
their ability to move freely within the membrane
Some proteins can move freely, whereas others are
constrained because they are anchored to protein
complexes
Experimental Evidence for Protein Mobility
Evidence for mobility of some membrane proteins
comes from cell fusion experiments
These experiments were performed by David Frye
and Michael Edidin
The Frye and Edidin Experiments
Frye and Edidin fused human and mouse cells and used two fluorescent
antibodies, each with a differently colored dye linked to it
The anti-mouse antibodies were linked to fluorescein, a green dye; and the
anti-human antibodies were linked to rhodamine, a red dye
Within a few minutes of fusion, the red and green region proteins began to
intermix
Protein distribution on memnbranes are different
image on slide 25
When plasma membranes are examined in freeze-fracture
micrographs, the embedded proteins appear to be randomly
distributed
The same is true for other types of membranes
Overcoming the permeability barrier of cell
membranes is crucial to proper functioning of the cell.
What does this mean inte rms on the transports in and out?
Specific molecules and ions need to be selectively
moved into and out of the cell or organelle
Membranes are protective barriers described as
selectively permeable or
semipermeable
Homeostasis
Cells and cellular compartments are
able to accumulate a variety of substances in
concentrations that are very different from those of the
surroundings
solutes
Most of the substances that move across membranes
are dissolved gases, ions, and small organic
molecules
A central aspect of cell function is selective
transport
Three quite different mechanisms are involved in
moving solutes across membranes:
Simple
Diffusion,
Facilitated Diffusion,
Active
Transport
intrinsic directionality
opposite direction requirers active transport?
The movement of a molecule that
has no net charge is determined
by its
concentration gradient
Simple diffusion and facilitated
diffusion involves (ender or exergonic?) movement
exergonic
movement “down” the
concentration gradient
(negative ΔG)
Active transport involves ( ender or exergonic) movement
endergonic movement “up” the
concentration gradient
(positive ΔG)
The movement of an ion is
determined by its
electrochemical potential
electrochemical potential
the combined effect of its
concentration gradient and the
charge gradient across the
membrane
The active transport of ions
across a membrane creates a
charge gradient, or
membrane potential (Vm),
across the membrane
Simple Diffusion
Unassisted Movement
Down the Gradient
the unassisted net movement of a
solute from
high to lower
concentration
Simple Diffusionis only possible for…
gases, nonpolar molecules, or
small polar molecules such as
water, glycerol, or ethanol
Diffusion always moves solutes
toward
equilibrium: solutes will
move toward regions of lower
concentration until the
concentrations are equal
Osmosis Is
the Diffusion of Water Across a
Selectively Permeable Membrane
Water molecules, being
uncharged are not affected by
the membrane potential
Water concentration is not
appreciably different on opposite
sides of a membrane
Osmosis: water will move toward the region of
higher solute concentration
If the solute
concentration is higher
outside the cell, the
SOLUTION is called
hypertonic
Osmolarity
Is the total solute
concentrations inside
versus outside of the cel
hypotonic in plants is called ( see figure and know differnet names)
turgid
If the solute concentration is lower outside the cell, the solution is called
hypotonic
isotonic solution
solute concentration inside and outside the cell is the
same
Animal cells vs cells with cell walls (plants, algae, fungi, and many bacteria )
act differently
Facilitated Diffusion:
Protein-Mediated
Movement Down the Gradient
Most substances in the cell are too large or too polar to
cross membranes by simple diffusion
These can move in and out of cells only with the assistance
of
Transport proteins
movment using transport protein is ( exergonic or endetgonic)
This process is exergonic: the solute diffuses as dictated by
its concentration gradient
The role of the transport proteins is just to provide
a path
through the lipid bilayer, allowing the “downhill” movement
of a polar or charged solute
Carrier proteins aka
transporters or permeases
Carrier proteins bind
bind solute
molecules on one side of a membrane, undergo a
conformation change, and release the solute on the other side
of the membrane
Channel proteins form
form hydrophilic channels through the
membrane to provide a passage route for solutes
The alternating conformation model states that
a
carrier protein is allosteric protein and alternates
between two conformational states
Carrier Proteins Alternate Between
Two
Conformational States
what are the two conformational states
n one state, the solute-binding site of the protein
is accessible on one side of the membrane
The protein shifts to the alternate conformation,
with the solute-binding site on the other side of
the membrane, triggering solute release
Carrier Proteins Are Analogous to Enzymes in
Their
Specificity and Kinetics
Facilitated diffusion involves binding
a substrate
on a specific solute-binding site
The carrier protein and solute form an
intermediate
After conformational change, the “product” is
released (the transported solute)
Carrier proteins are regulated by
external factors
Carrier Proteins Transport how many solutes
1-2 solutes
When a carrier protein transports a
single solute across the membrane, the
process is called
uniport
uniport
When a carrier protein transports a
single solute across the membrane
A carrier protein that transports a single
solute is called a
uniporter
uniporter
A carrier protein that transports a single
solute
When two solutes are transported
simultaneously, and their transport is
coupled, the process is called
coupled transport
coupled transport
When two solutes are transported
simultaneously, and their transport is
coupled
symport (or cotransport)
If the two solutes are moved
across a membrane in the same
direction
If the two solutes are moved
across a membrane in the same
direction, the process is referred
to as
symport (or cotransport)
antiport (or
countertransport)
the solutes are moved in
opposite directions,
If the solutes are moved in
opposite directions, the process
is called
antiport (or
countertransport)
Transporters that mediate these ( symport and antiport)
processes are
symporters and antiporters
pores
large and
nonspecific channels on the outer membranes of
bacteria, mitochondria, and
chloroplasts
Pores are formed by
transmembrane proteins called
porins that allow passage of
solutes up to a certain
molecular weight to pass
Most channels are
smaller and
highly selective
Most of the smaller
channels are involved in
ion transport and are
called ion channels
ion channels are involved in
involved in ion transport
The movement of solutes
through ion channels is
much ( faster OR SLOWER) than
transport by carrier
proteins
faster
because
conformation changes
are not required
There are three types of channels: ???
ion channels, porins, and aquaporins
Channel Proteins Facilitate Diffusion by Forming
Hydrophilic
Transmembrane Channels
Ion Channels
transmembrane proteins that allow rapid passage of
specific ions
typically gated meaning they open and close in response to some stimulus
Voltage-gated channels
open and close in response to changes in
membrane potential
Ligand-gated channels
are triggered by the binding of certain substances
to the channel protein
Mechanosensitive channels
respond to mechanical forces acting on the
membrane
porins:
transmembrane proteins that allow rapid passage of
various solutes
the transmembrane segments of porins cross the membrane
as
β barrels
Polar side chains line the
inside of the pore, allowing
passage of
many hydrophilic
solutes
The outside of the barrel
contains many
nonpolar side
chains that interact with the
hydrophobic interior of the
membrane
Aquaporins (AQPs)
transmembrane channels that
allow rapid passage of water
All aquaporins are
tetrameric integral
membrane proteins
The identical monomers
associate with their 24
transmembrane segments
oriented to form four central
channels
The channels, lined with
hydrophilic side chains, are
just large enough for water
molecules to pass through
one at a time
active transport moves what direction on a gradient
protein-mediated movement up the gradient
Active transport is used to move
solutes up a
concentration gradient, away from equilibrium
Active transport couples
endergonic transport to an
exergonic process, usually ATP hydrolysis
Active transport performs three important cellular functions
- Uptake of essential nutrients
- Removal of wastes
- Maintenance of nonequilibrium concentrations of certain
ions
Active Transport Is
Unidirectional
Active transport differs from diffusion (both simple
and facilitated) in
the direction of transport
Diffusion is nondirectional with respect to the
membrane and proceeds as directed by the
concentrations of the transported substances
Active transport has an intrinsic directionality
The Coupling of Active Transport to an Energy
Source May Be
Direct or Indirect
describe direct active transport
(reference slide)
(primary active transport),
the accumulation of solute
molecules on one side of the
membrane is coupled
directly to an exergonic
chemical reaction
This is usually hydrolysis of
ATP
Transport proteins driven by
ATP hydrolysis are called
transport ATPases or
ATPase pumps
Indirect active transport
depends on the simultaneous
transport of two solutes
Favorable movement of one
solute down its gradient drives
the unfavorable movement of
the other up its gradient
This can be a symport or an
antiport, depending on
whether the two molecules
are transported in the same or
different directions
