Download 2-Cell and Molecular Biology (Plasma Membrane)

 Plasma Membrane:
 Models:
 In 1915, working with red blood cells by Evert Gorter and Grendel, it was
discovered that the membranes consisted of lipids and proteins
 In 1935, Davson and Danielli proposed the first membrane model widely
accepted, in which the cell membrane consisted of a lipid bilayer with
hydrophilic coating of proteins on both sides which could also form pores
• This model was called “sandwich” model of Davson and Danielli
 In 1957, Roberts by using the electron microscopy, proposed a modified
version of the “sandwich” model named “unit
• Under the electron microscope, he could identify a trilaminar structure
of the membrane, which he interpreted as being an internal lipid bilayer
with two flanking protein layers on the exterior. This model was
accepted between 1960-1970
 In 1972, Singer and Nicholson proposed proteins were embedded in the
bilayer with only their hydrophillic ends exposed to the external aqueous
solution and their hydrophobic regions embedded inside the membrane –
Fluid mosaic model
 The "freeze-fracture" method of examining cells microscopically showed that the
fluid mosaic model was accurate
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 This model retains the bilayer structure proposed by the two Dutch scientists Gorter
and Grendel
•
The proteins are, however, in the bilayer and can move due to the membrane
fluidity
•
They proposed that the lipid bilayer is organized in such a way that the
hydrophylic part of the phospholipids are on the exterior of the lipid bilayer in
contact with the water
•
While their hydrophobic tails face inward
•
The proteins float in this bilayer and
•
can form pores and channels
 The fluid mosaic model continues to be refined. For example
•
The existence of membrane compartmentalization is also important for cell
function
•
It is now well established that the plasma membranes contain:
 Protein-protein complexes
 Lipid rafts and
 pickets and fences formed by the actin-based cytoskeleton and
 can be polarized (apical and latero-basal compartmentalization in the
epithelial cells)
 So this is the Fences and pickets model of plasma membrane - a
concept of cell membrane structure suggesting that the fluid plasma
membrane is compartmentalized by actin-based membrane-skeleton
“fences” and anchored transmembrane protein “pickets”
 The fences and pickets model was proposed to explain observations of
molecular traffic made due to recent advances in single molecule
tracking techniques
http://www.google.com search for
membranes
pickets and fences of plasma
 Plasma Membrane:
 Cell membranes are crucial to the life of the cell
 Plasma membrane
•
Encloses the cell
•
Defines its boundaries and
•
Maintains the essential differences between the cytosol and the
extracellular environment
 Inside eukaryotic cells, the membranes of the endoplasmic reticulum, Golgi
apparatus, mitochondria and other membrane-enclosed organelles
•
maintain the characteristic differences between the contents of each
organelle and cytosol
•
Ion gradient across the membranes, established by activities of
specialized membrane proteins, can be used
 to synthesize ATP
 to drive the transmembrane movement of selected solutes or
 as in nerve and muscle cells
o
to produce and transmit electrical signals
 In all cells, plasma membrane also contains proteins that act as sensors of
external signals
•
allowing the cell to change its behavior in response to environmental
cues including signals from other cells
•
These protein sensors or receptors transfer information rather than
molecules across the membrane
 Despite their different functions, all biological membranes have a common
general structure:
•
Each is very thin film of lipid and protein molecules held together mainly
by non covalent interaction
Fig 10.1 Alberts 3rd Ed / 5th Ed
•
Cell membranes are dynamic, fluid structures and most of their
molecules move about in the plane of membrane
•
The lipid molecules are arranged as a continuous double layer of about 5
nm thick
•
This lipid bilayer provides the basic fluid structure of the membrane and
•
Serve as a relatively impermeable barrier to the passage of most water
soluble molecules
 Protein molecules that span the lipid bilayer mediate nearly all of other
functions of membrane. For example:
•
Transporting specific molecules across it or
•
Catalyzing membrane-associated reactions such as ATP synthesis
•
In the plasma membrane, some trans-membrane proteins serve as
structural links that connect cytoskeleton through lipid bilayers to either
 the extracellular matrix or
 An adjacent cell while
 Others serve as receptors to detect and transduce chemical signals in
the cell’s environment
•
Many different proteins to enable a cell to function and
•
Interact with its environment
•
It is estimated that about 30% of the proteins encoded in an animal cell’s
genome are membrane proteins
•
We will discuss here the structure and organization of the two main
constituents of biological membranes:
 The lipids and
 The proteins
•
Although we will focus mainly on plasma membrane but most concepts
discussed apply to the various internal membranes in cells as well
 The Lipid Bilayer;
•
As discussed earlier, the lipid bilayer provides the basic structure for all
cell membrane
•
It is easily seen by electron microscopy and
•
Its structure is attributable exclusively to the special properties of the
lipid molecules
•
Which assemble spontaneously in to bilayers even under simple artificial
conditions
 Phosphoglycerides, Spingolipids and Sterols are the Major Lipids in
Cell Membranes
•
Lipid molecules constitute about 50% of the mass of most animal cell
membranes, nearly all of the reminder being protein
•
All the lipid molecules in cell membranes are amphiphilic/ amphiphatic
i.e. They have
 a hydrophilic - water loving or polar end and
 a hydrophobic - water hating or non-polar end
•
The most abundant lipid are phospholipids. They have a polar head and
hydrophobic hydrocarbon tails
•
In animals, plants and bacterial cells, the tails are usually fatty acids and
they can differ in length - they normally contain between 14 and 24
carbon atoms
•
One tail typically has one or more cis - double bonds (unsaturated)
creates a small kink in the tail while other tail does not (saturated)
•
Differences in the length and saturation of the fatty acid tails influence
how phospholipid molecules pack against one another
•
thereby affecting the fluidity of the membrane
•
The main phospholipids in most animal cell membranes are the
phosphoglycerides which have a three-carbon glycerol backbone
Fig 10.2 Alberts 3rd / 5th Ed
•
By combining several different fatty acids and head groups, cells make
many different phosphoglycerides
Fig 10.3 Alberts 5th Ed/ Fig 10.10 Alberts 3rd
Table 10.1 Alberts 3rd / 5th Ed
•
In addition to phospholipids, the lipid bilayers in many cell membranes
contain cholesterol and glycolipids
•
Eukaryotic plasma membranes contain specially large amounts of
cholesterol
Fig 10.8 Alberts 3rd Ed / Fig 10.4 Alberts 5th Ed
Fig 10.9 Alberts 3rd Ed / Fig 10.5 Alberts 5th Ed
 Phospholipids Spontaneously form Bilayers
•
The shape and amphiphilic nature of the phospholipid molecules cause
them to form bilayers spontaneously in aqueous environment
Fig 10.6 Alberts 5th Ed
•
The hydrophobic and hydrophilic regions of lipid molecules behave in
the same way
•
Thus lipid molecules spontaneously aggregate to bury their hydrophobic
hydrocarbon tails in the interior and expose their hydrophilic heads to
water
•
Depending on their shape, they can do this in either of two ways;
 they can form spherical micelles with the tail inward or
 they can form double layered sheets or bilayers with hydrophobic
tails sandwiched between the hydrophilic head groups
Fig 10.7 Alberts 5th Ed
Fig 10.8 Alberts 5th Ed
 The Lipid Bilayer is a Two-Dimensional Fluid
•
Around 1970, researchers first recognized the individual lipid molecules
are able to diffuse freely within lipid bilayers
•
The initial demonstration came from studies of synthetic lipid bilayers
•
Two types of preparations have been very useful in such studies:
 Bilayers made in the form of spherical vesicles called liposomes
Fig 10.4 Alberts 3rd Ed / Fig 10.9 Alberts 5th Ed
 Planar bilayers called black membrane formed across a hole in a
partition between two aqueous compartments
Fig 10.5 Alberts 3rd Ed / Fig 10.10 Alberts 5th Ed
• Studies showed that phospholipid molecules in synthetic bilayers very
rarely migrate from the monolayer (leaf let) on one side to that on the
other - this process is called flip flop
• These studies have also shown that individual lipid molecules rotate very
rapidly about their long axis and have flexible hydrocarbon chains
• Computer simulation show that lipid molecules in membrane are very
disordered
Fig 10.11 Alberts 5th Ed
 The Fluidity of a Bilayer Depends on its Composition
• The fluidity of cell membranes has to be precisely regulated. For
example:
 Certain membrane transport processes and enzyme activities cease
when the bilayer viscosity is experimentally increased beyond
threshold level
• The fluidity of a lipid bilayer depends on both:
 Its composition and
 Its temperature as is readily demonstrated in studies of synthetic
bilayers
Fig 10.7 Alberts 3rd Ed / Fig 10.12 Alberts 5th Ed
•
Bacteria, yeast and other organisms whose temperature fluctuates with
that of their environment adjust the fatty acid composition of their
membrane lipids to maintain a relatively constant fluidity
•
As the temperature falls, the cells of those organisms synthesize fatty
acids with more cis-double bonds and
•
They avoid the decrease in bilayer fluidity that would otherwise result
from the temperature drop
•
Cholesterol modulates the properties of lipid bilayers
•
When mixed with phospholipids, it enhances the permeability barrier
properties of lipid bilayer
•
It inserts in to the bilayer with its hydroxyl group close to the polar head
groups of the phospholipids
•
So that its rigid, plate like steroid rings interact with and partly
immobilize those regions of the hydrocarbon chain closest to the polar
head groups
Fig 10.9 Alberts 3rd Ed / Fig 10.5 Alberts 5th Ed
•
By decreasing the mobility of the first few CH2 groups of the
hydrocarbon chains of the phospholipids molecules, cholesterol makes
the lipid bilayer less deformable in this region and
•
thereby decreases the permeability of the lipid bilayer to small watersoluble molecules
•
Though cholesterol tightens the packing of the lipids in a bilayer, it does
not make membrane any less fluid
•
At the high concentrations found in most eukaryotic plasma membranes,
cholesterol also prevents the hydrocarbon chains from coming together
and crystallizing
•
Besides major phospholipids (500-1000 different lipid species present in
a cell), membranes also contain many structurally distinct minor lipids –
some of which have important functions. For example:
 The inositol phospholipids are present in small quantities but have
crucial functions in guiding membrane traffic and in cell signaling
 Their local synthesis and degradation are regulated by a large number
of enzymes
o
which create both small intracellular signaling molecules and
lipid docking sites on membrane that recruit specific proteins
from the cytosol
 The Asymmetry of the Lipid Bilayer is Functionally Important
•
The lipid composition of the two monolayers of the lipid bilayer in many
membranes are strikingly different. For example:
 In human red blood cell membrane, almost all the phospholipid
molecules that have choline in their head group (phosphatidylcholine
and spingomyelin) are in outer monolayer
 Whereas almost all that contain terminal primary amino group
(phosphatidylethanolamine and phosphatidylserine) are in inner
monolayer
Fig 10.11 Alberts 3rd Ed / Fig 10.16 Alberts 5th Ed
•
Lipid asymmetry is functionally important specially in converting
extracellular signals in to intracellular ones
•
Many cytosolic proteins bind to specific lipid head groups found in the
cytosolic monolayer of the lipid bilayer
Fig 10.17 Alberts 5th Ed
•
Animals exploit the phospholipid asymmetry of their plasma membranes
to distinguish between live and dead cells
•
When animal cells undergo apoptosis - a form of programmed cell death,
phosphotidylserine which is normally confined to the cytosolic
monolayer of plasma membrane lipid bilayer, rapidly translocates to the
extracellular monolayer
•
The phosphotidylserine exposed on the cell surface signals neighboring
cells. Such as
How membrane-bound phospholipid
translocators generate and maintain
lipid asymmetry?
 Macrophages to phagocytose the dead cell and digest it
•
The translocation of phosphatidylserine in apoptotic cells is thought to
occur by two mechanisms:
i. The phospholipid translocator that normally transports this lipid from
the non cytosolic monolayer to the cytosolic monolayer is inactivated
ii. A scramblase that transfers phospholipids non specifically in both
directions between the two monolayer is activated
 Glycolipids are Found on the Surface of All Plasma Membranes
•
Sugar containing lipid molecules are called glycolipids, found
exclusively in the non cytosolic monolayer of the lipid bilayer
•
They have the most asymmetry in their membrane distribution and
•
Have important roles in interactions of the cell with its surrounding
Fig 10.12 Alberts 3rd Ed / Fig 10.18 Alberts 5th Ed
•
They may help to protect the membrane against the harsh conditions
frequently found there. Such as
 Low pH and high concentrations of degradative enzymes
•
Charged glycolipids like gangliosides, may be important because of their
electrical effects:
Provide entry point for some bacterial toxins – acts as cell
surface receptor for bacterial toxin that causes devastating
diarrhea of cholera
 Their presence alters the electrical field across the membrane and
the concentrations of ions especially Ca2+ at the membrane
surface
 Glycolipids are also thought to function in cell-recognition
processes in which
 Membrane bound carbohydrate binding proteins i.e. lectins bind
to the sugar groups on both glycolipids and glycoproteins in the
process of cell-cell adhesion
 Membrane Proteins;
•
As mentioned earlier lipid bilayer provides the basic structure of
biological membranes, the membrane proteins perform the membrane
specific task and
•
Therefore give each type of cell membrane its characteristic functional
properties
•
Membranes Proteins can be Associated with the Lipid Bilayer in
Various Ways
 Different membrane proteins are associated with the membranes in
different ways:
Fig 10.19 Alberts 5th Ed / Fig 10.13 Alberts 3rd Ed
Anchored via oligosaccharide linker to lipid
i.e. phosphotydylinositol
Anchored to cytosol surface
by amphiphilic α- helix
β-Sheet, a β- barrel
Single α helix
Multiple α-helices
Anchored by covalently
attached lipid chain
either fatty acid chain or a
prenyl group
Non-covalent interactions
Fig 10.15 Alberts 3rd Ed / Fig 10.21 Alberts 5th Ed
Fig 10.23 Alberts 5th Ed
Fig 10.26 Albnerts 5th Ed
Fig 10.14 Alberts 3rd Ed / Fig 10.20 Alberts 5th Ed
•
The Cell Surface is Coated with Sugar Residues
 As a rule, plasma proteins do not protrude naked from the exterior of
cell but are:
o Decorated
o Clothed or hidden by carbohydrates which are present on the
surface of all eukaryotic cells
 These carbohydrates occurs both as
o Oligosaccharide chains covalently bound to membrane proteins
called glycoproteins and
o To lipids called glycolipids and
o As polysaccharides chains of integral membrane proteoglycan
molecules
 Proteoglycans, which consist of long polysaccharide chains linked
covalently to a protein core, are found mainly outside the cell as part
of the extracellular matrix
 The term coat or glycocalyx is often used to describe the
carbohydrate rich zone on the cell surface
Fig 10.28 Alberts 5th Ed / Fig 10.40 and 10.41 Alberts 3rd Ed
 The oligosaccharide side chains of glycoproteins and glycolipids are
enormously diverse in their arrangement of sugars
 Although they usually contain fewer than15 sugar residues
 They are often branched and the sugars can be bonded together by a
variety of covalent linkages
 In principal, both
o
the diversity and
o
the exposed position of these oligosaccharides on the cell
membrane make them especially well suited to function in
specific cell recognition processes
 Earlier it was believed that the role of the cell coat might be merely
protect against
o
to mechanical and chemical damage and
o
to keep foreign objects and other cells at a distance
o
preventing undesirable protein-protein interaction
 Indeed , this probably is an important part of its function
 However, recently, plasma membrane bound lectins have been
identified
 that recognize specific oligosaccharides on cell surface glycolipids
and glycoproteins
o to mediate a variety of transient cell-cell adhesion processes
including those occurring in
 Sperm-egg interaction
 Blood clotting
 Lymphocyte recirculation and
 Inflammatory responses
Fig 10.42 Alberts 3rd Ed
 Membrane Transport:
 Because of its hydrophobic interior, the lipid bilayer of cell membrane serves
as a barrier to the passage of most polar molecules
 This barrier function is crucially important as it allows the cell to maintain
the concentrations of solutes in its cytosol
 That are different from those in the extracellular fluid and
 In each of the intracellular membrane bounded compartments
 To make use of this barrier, cells have had to evolve ways of transferring
specific water soluble molecules across their membranes in order to:






• ingest essential nutrients
• excrete metabolic waste products and
• regulate intracellular ion concentrations
Transport of inorganic ions and small water soluble organic molecules across
the lipid bilayer is achieved by specialized transmembrane proteins
Each of which is responsible for the transfer of specific molecule or a group
of closely related ions or molecules
Two main classes of membrane proteins that mediate the transfer are:
• Carrier proteins / transporters - which have moving parts to shift specific
molecules across the membrane
• Channel proteins - which form a narrow hydrophobic pore, allowing the
passive movement of small inorganic ions
Carrier proteins / transporters can be coupled to a source of energy to
catalyze active transport and
A combination of selective passive permeability and
Active transport creates large differences in composition of cytosol
compared with that of either
• the extracellular fluid or
• the fluid with in membrane enclosed organelles
Table11.1 Alberts 3rd / 5th Ed
 In particular, by generating the ionic concentration differences across the
lipid bilayer, cell membranes are able to store potential energy in the form of
electrochemical gradients:
•
Which drive various transport processes
 To convey electrical signals in electrically excitable cells and makes most of
the cell’s ATP in
•
Mitochondria
•
Chloroplasts and Bactria
 Principles of Membrane Transport;
•
Protein-free lipid bilayers are highly impermeable to ions
Fig 11.1 Alberts 3rd / 5th Ed
Fig 11.2 Alberts 3rd / 5th Ed
•
Like synthetic lipid bilayers, cell membranes allow water and non polar
molecules to permeate by simple diffusion
•
However, cell membranes also have to allow the passage of various polar
molecules such as
 Ions, sugars, amino acids, nucleotides and
 Many metabolites that cross synthetic bilayer very slowly
•
As discussed earlier, carrier proteins / transporters and channels are the
one who do the job
Fig 11.3 Alberts 5th Ed / 3rd Ed
•
Although water can diffuse across synthetic lipid bilayers, all cells
contain specific channel proteins called water channels or aquaporins
•
That greatly increase the permeability of these membrane to water
•
All channels and many transporters allow solute to cross the membrane
only passively – downhill; a process called passive transport or
facilitated diffusion
•
In case of transport of a single uncharged molecule, the difference in the
concentration on the two sides of the membrane – its concentration
gradient drives passive transport and determines its direction
•
Cells also require transport proteins that will actively pump certain
solutes across the membrane against their electrochemical gradient –
uphill; this process is called active transport mediated by transporters
called pumps
•
In active transport, the pumping activity of the transporter is directional
because it is tightly coupled to a source of metabolic energy. Such as
 ATP hydrolysis or
 An ion gradient
•
Thus transmembrane movement of small molecules mediated by
transporters can be either
 active or
 passive
•
Whereas that mediated by channels is always passive
Fig 11.4 Alberts 5th Ed / Fig 11.4 Alberts 3rd Ed
 Transporters and Active Membrane Transport;
•
The process by which a transporter transfers a solute molecule across the
lipid bilayer resembles an enzyme-substrate reaction and
•
In many ways transporters behave like enzymes
•
In contrast to ordinary enzyme-substrate reactions, the transporter does
not modify the transported solute
•
But instead delivers it unchanged to the other side of membrane
Fig 11.5 Alberts 5th Ed
Fig 11.9 Alberts 3rd Ed
•
While cells carry out active transport in three main ways:
An electrochemical gradient is a gradient of electrochemical
potential, usually for an ion that can move across a membrane.
The gradient consists of two parts, the electrical potential and a
difference in the chemical concentration across a membrane. The
difference of electrochemical potentials can be interpreted as a
type of potential energy available for work in a cell. The energy
is stored in the form of chemical potential, which accounts for an
ion's concentration gradient across a cell membrane, and
electrostatic energy, which accounts for an ion's tendency to
move under influence of the transmembrane potential.
Fig 11.7 Alberts 5th Ed
•
Active transport can be driven by ion gradients
Fig 11.8 Alberts 3rd and 5th Ed
•
Most animal cells, for example, take up glucose from the extracellular
fluid by passive transport through glucose carriers that operate as
uniporters
•
Where its concentration is high relative to that in cytosol
•
By contrast, intestinal and kidney cells take up glucose from the lumen
of the intestine and kidney tubule respectively where the concentration of
the sugar is low
•
These cells actively transport glucose across their plasma membrane by
symport with Na+
Fig 11.11 Alberts 5th Ed /11.13 Alberts 3rd Ed
•
We will discuss active transport by considering a carrier protein/
transporter that plays a crucial part in generating and maintaining the Na+
and K+ gradients across the plasma membrane of animal cells
•
The concentration of K+ is typically 10-20 times higher inside cells than
outside, whereas the reverse is true of Na+
Table11.1 Alberts 3rd / 5th Ed
(Bacteria and Archaea)
•
These concentration differences are maintained by a Na+ - K+ pump that
is found in the plasma membrane of virtually all animals
Fig 11.14 Alberts 5th Ed / Fig 11.10 Alberts 3rd Ed
Fig 11.15 Alberts 5th Ed / Fig 11.11 Alberts 3rd Ed
•
Na+ gradient produced by the Na+ - K+ pump drives the transport of most
nutrients in to animal cells and
•
Also has a crucial role in regulating cytosolic pH
•
A typical animal cell devotes almost one-third of its energy to fueling
this pump and
•
Pump consumes even more energy in electrically active nerve cells
which repeatedly gain small amounts of Na+ and lose small amount of
K+ during the propagation of nerve impulses
•
The Na+ - K+ pump does have a direct role in controlling the solute
concentration inside the cell and
•
Thereby helps regulate osmolarity of the cytosol
•
All cells contain specialized water channel proteins called aquaporins in
their plasma membrane to facilitate water flow across this membrane
•
Thus water moves into or out of cells down its concentration gradient – a
process called osmosis
•
As discussed in Table 11.1, cells contain high concentrations of solutes
including
 numerous negatively charged organic molecules that are confined
inside the cell and
 their accompanying cations that are required for charge balance
•
This tends to create a large osmotic gradient that tends to “pull” water
into the cell
•
Animal cells counteract this effect by an opposite osmotic gradient due to
a high concentration of inorganic ions chiefly Na+ and Cl- in extracellular
fluid
•
The Na+ - K+ pump helps maintain osmotic balance by pumping out the
Na+ that leaks in down its steep electrochemical gradient
•
The Cl- is kept out by the membrane potential
•
In the special case of human red blood cells, which lack a nucleus and
other organelles and
•
Have a plasma membrane that has an unusually high permeability to
water, osmotic water movements can greatly influence cell volume and
•
Na+ - K+ pump plays an important part in maintaining red cell volume
Fig 11.12 Alberts 3rd Ed / Fig 11.16 Alberts 5th Ed
•
Unlike carrier proteins/transporter, channel proteins form hydrophilic
pores across membranes
•
One class of channel proteins found virtually all in animals forms gap
junctions between two adjacent cell;
 Each plasma membrane contributes equally to the formation of the
channels
 Which connects the cytoplasm of the two cells
•
Both gap junctions and porins – the channel forming proteins of the outer
membranes of:
 Bacteria
 Mitochondria and
 Chloroplasts
have relatively large permissive pores which would be disastrous if
they directly connected the inside of a cell to an extracellular space
•
In actual many bacterial toxins do exactly that to kill other cells
•
In contrast, most channel proteins in the plasma membrane of animals
and plant cells
•
that connect the cytosol to the cell exterior necessarily have narrow,
highly selective pores that can open and close rapidly
•
Because these proteins are concerned specifically with inorganic ion
transport, they are called ion channels
•
For transport efficiency, ion channels have an advantage over carrier is
that up to 100 million ions can pass through open channel each second  a rate 105 times greater than the fastest rate of transport mediated by
any known carrier protein / transporter
•
However, channels can not be coupled to an energy source to perform
active transport
•
So the transport they mediates always passive - down hill
•
Thus function of ion channels is to allow specific inorganic ions
primarily
 Na+, K+, Ca2+ or Cl- to diffuse rapidly down their electrochemical
gradient across the lipid bilayer
•
Two important properties distinguish ion channels from simple aqueous
pores:
i.
They show ion selectivity, permitting some inorganic ions to pass
but not others
Fig 11.20 Alberts 5th Ed / Fig 11.17 Alberts 3rd Ed
ii.
Ion channels are not continuously open rather they are gated, which
allows them to open briefly and than close again
Fig 11.21 Alberts 5th Ed / Fig 11.18 3rd Ed
 Transport into the Cell from the Plasma Membrane: Endocytosis
•
Every cell must
 eat
 communicate with the world around it and
 quickly respond to changes in its environment
•
To help accomplish these tasks:
 cells continually adjust the composition of their plasma membrane in rapid
response to need
•
They use an elaborate internal membrane system to add and remove cellsurface proteins embedded in the membrane. Such as
 Receptors
 Ion channels and
 transporters
•
•
•
•
•
•
•
•
Through the process of exocytosis, the biosynthetic-secretory pathway
delivers newly synthesized proteins, carbohydrates and lipids to either:
 plasma membrane
 extracellular space
By the opposite process of endocytosis, cells remove plasma membrane
components and deliver them to internal compartment called endosomes
From where they can be recycled to the same or different regions of the
plasma membrane or
can be delivered to lysosomes for degradation
Fig 13.1 Alberts 5th Ed
Fig Downloaded
Cells also use endocytosis to capture important nutrients. Such as
 vitamins
 lipids
 cholesterol and iron
These are taken up together with the macromolecules to which they bind and
are then released in endosomes or lysosomes and
transported in to cytosol, where they are used in various biosynthetic
processes
Transport vesicle
•
The interior space or lumen of each membrane-enclosed compartment along
the biosynthetic-secretory and endocytic pathways is topologically
equivalent to the lumen of most other membrane-enclosed compartments
and to the exterior
•
Protein can travel in this space without having to cross a membrane, being
passed from one compartment to another by means of numerous membraneenclosed transport containers
•
Some of these containers are small spherical vesicles
•
While other are large irregular vesicles or tubules formed from the donor
compartment
•
All containers are generally called transport vesicles
•
Within a eukaryotic cell, transport vesicles continually bud off from one
membrane and fuse with another, carrying membrane components and
soluble molecules
•
Which are referred as cargo
Fig 13.2 Alberts 5th Ed
•
This membrane traffic is
 highly organized
 directional routes
•
Which allows the cell to:
 secrete
 eat and
 remodel its plasma membrane
•
The biosynthetic-secretory pathway leads outward from the endoplasmic
reticulum (ER) toward the Golgi apparatus and cell surface with a side route
leading to lysosomes
•
While the endocytic pathway leads inward from the plasma membrane
Fig 13.3 Alberts 5th Ed
•
To perform its function, each transport vesicle that buds from a compartment
must be selective
•
It must take up only the appropriate molecules and must fuse only with
appropriate target membrane. For example:
 Must exclude proteins that are to stay in the Golgi apparatus and
 It must fuse only with the plasma membrane and not with any other
organelle
•
The routes that lead inward from the cell surface to lysosomes start with the
process of endocytosis by which cells take up :
*
*
*
*
 macromolecules
 particulate substances and
 in specialized cases even other cells
•
In this process, the material to be ingested is progressively enclosed by a
small portion of plasma membrane
 which first invaginates and
 then pinches off to form an endocytic vesicle containing the ingested
substance or particles
•
Two main types of endocytosis are distinguished on the basis of the size of
endocytic vesicles formed;
i.
Phagocytosis (cell eating) - large particles are ingested via large
vesicles called phagosomes which are generally >250 nm in diameter
ii.
Pinocytosis (cell drinking) - fluid and solutes are ingested via small
pinocytic vesicle which are about 100 nm in diameter
•
Most eukaryotic cells are continually ingesting fluid and solutes by
pinocytosis while
•
Large particles are ingested most efficiently by specialized phagocytic cells
 Specialized Phagocytic Cells can Ingest Large Particles;
•
Phagocytosis is a special form of endocytosis in which a cell uses large
endocytic vesicles called phagosomes to ingest large particles such as:
 microorganisms and dead cells
•
In protozoa, phagocytosis is a form of feeding:
 Large particles taken up into phagosomes end up in lysosomes and
 The products of the subsequent digestive processes pass in to the
cytosol to be used as food
•
However, few cells in multicellular organisms are able to ingest such
large particles efficiently. For example:
 Extracellular processes break down food particles and
 Cells import small hydrolysis products
•
Phagocytosis is important in most animals for purposes other than
nutrition and
•
It is carried out mainly by specialized cells - professional phagocytes.
Which are:
 Macrophages and
 Neutrophils
•
These cells develop from hemopoietic stem cells and
•
They ingest invading microorganisms to defend us against infection
•
Macrophages also have am important role in scavenging senescent cells
and
•
Cells that have died by apoptosis
•
In quantitative terms, the clearance of senescent and dead cells is by far
the most important. For example:
 Our macrophages phagocytose more than 1011 senescent red blood
cells in each of us every day
•
Whereas the endocytic vesicles involved in pinocytosis are small and
uniform i.e. phagosomes have diameters that are determined by the size
of the ingested particle and
•
They can be almost as large as the phagocytic cell itself
Fig 13.26 Alberts 3rd Ed / Fig 13.46 Alberts 5th Ed
•
The phagosomes fuse with lysosomes inside the cell and
•
Ingested material is then degraded
•
Any indigestible substances will remain in lysosomes forming residual
bodies
•
Which can be excreted from cells by exocytosis
•
Some of the internalized plasma membrane components never reached to
lysosomes
•
Because they are retrieved from the phagosome in transport vesicles and
•
Returned to the plasma membrane
•
Particles must bind to the surface of the phagocyte to be phagocytosed
•
Phagocytes have a variety of specialized surface receptors that are
functionally linked to the phagocytic machinery of the cell
•
Phagocytosis is a triggered process
•
It requires the activation of receptors that transmit the signals to the cell
interior and initiate the response
•
The best characterized triggers of phagocytosis are antibodies
•
Which protects us by binding to the surface of infectious microorganisms
to form a coat that exposes the tail region on the exterior of each
antibody molecules
•
This tail region is called Fc region
•
The antibody coat is recognized by specific Fc receptors on the surface
of macrophages and neutrophils
•
Whose binding induces the phagocytic cells to extend pseudopods that
engulf the particle and fuse at their tips to form a phagosome
Fig 23.20 Alberts 3rd Ed
Fig Down Loaded
Fig 13.47 Alberts 5th Ed
•
In this way, the ordered generation and consumption of specific
phosphoinositides guide sequential steps in phagosome formation
•
Several other class of receptors that promote phagocytosis have been
characterized:
 Some recognize complement components which collaborate with
antibodies in targeting microbes for destruction
 Other directly recognize oligosaccharides on the surface of certain
microorganisms
 Yet others recognize cells that have died by apoptosis
 Apoptotic cells lose the asymmetric distribution of phospholipids in
their plasma membrane
 As a consequence, negatively charged phophotidylserine (normally
confined to the cytosolic leaflet of lipid bilayer) is now exposed on
the outer side of the cell
Polymerization, initiated
by Rho family GTPase,
shapes the pseudopods
 Where it helps to trigger the phagocytosis of the dead cell
•
Macrophages will also phagocytose a variety of inanimate (lifeless)
particles. Such as:
 glass or
 latex bead and
 asbestos fibers
•
Yet they do not phagocytose live animal cells
•
Living animals seems to display “do not eat me’ in the form of cell
surface proteins that bind to inhibiting receptors on the surface of
macrophages
•
Thus, like many other processes, phagocytes depends on a balance
between
 positive signals that activate the process and
 negative signals that inhibit it
•
Therefore, apoptotic cells gain “eat me” signals (extracellularly exposed
phosphatidylserine) and lose “do not eat me” signals causing them to be
very rapidly phagocytosed by macrophages - Slides 31 and 33
 Pinocytic Vesicles form from Coated Pits in the Plasma Membrane;
• Virtually all eukaryotic cells continually ingest bits of plasma membrane
in the form of small pinocytic (endocytic) vesicles which are later
retuned to cell surface
• The rate at which the plasma is internalized in this process of pinocytosis
varies between cell types but it is usually large. For example:
 A macrophage ingests 25% of its own volume of fluid each hour
 This means that it must ingest 3% of its plasma membrane each
minute or 100% in about half an hour
 Fibroblasts endocytose at a somewhat lower rate i.e. 1% of their
plasma membrane per minute
 Whereas some amoebae ingest their plasma membrane even more
rapidly
• Since a cell’s surface area and volume remain unchanged during this
process, it is clear that same amount of membrane being removed by
endocytosis is being added to the cell surface by the converse processes
of exocytosis
• In this sense, endocytosis and exocytosis are linked processes that can be
considered to constitute an endocytic-exocytic cycle
• The coupling between endocytosis and exocytosis is particularly strict in
specialized structures characterized by high membrane turnover. Such as
• Neuronal synapse
•
The endocytic part of the cycle often being at clatherin-coated pits
•
These specialized region typically occupy about 2% of the plasma
membrane area
•
The life time of a clatherin-coated pit is short:
 Within a minute or so of being formed, it invaginates in to the cell
and pinches off to form a clatherin-coated vesicle
13.28 Alberts 3rd Ed / Fig 13.48 Alberts 5th Ed
•
It has been estimated that about 2500 clatherin-coated vesicles leave
plasma membrane of a cultured fibroblast every minute
•
The coated vesicle are even more transient than the coated pits:
 Within seconds being formed, they shed their coat and
 are able to fuse with early endosomes
•
Since extracellular fluid is trapped in clatherin-coated pits as they
invaginate to form coated vesicles, any substance dissolved in the
extracellular fluid is internalized - a process called fluid-phase
endocytosis
 Not All Pinocytic Vesicles are Clatherin-Coated;
•
In addition to clatherin-coated pits and vesicles, there are another, less
well understood mechanisms by which cells can form pinocytic vesicles
•
One of these pathways initiates at caveolae (in Latin means little cavities)
originally recognized by their ability to transport molecules across
endothelial
 Which form the inner lining of blood vessels
•
Caveolae are present in plasma membrane of most cell types and in
some of these they are seen in the electron microscope as deeply
invaginated flasks
Fig 13.49 Alberts 5th Ed
•
The major structural proteins in caveolae are caveolins
 which are a family of unusual integral membrane proteins
•
These are thought to invaginate and collect cargo proteins
 Cells Use Receptor-Mediated Endocytosis to Import Selected
Extracellular Macromolecules;
•
In most animals cells, clathrin-coated pits and vesicles provide efficient
pathway for taking up specific macromolecules from the extracellular
fluid
•
This process is called receptor-mediated endocytosis
•
In this process:
 macromolecules bind to complementary transmembrane receptor proteins
 accumulate in coated pits and
 Then entered the cell as receptor-macromolecule complexes in clathrincoated vesicles
13.28 Alberts 3rd Ed / Fig 13.48Alberts 5th Ed
•
Because ligands are selectively captured by receptors
 receptor-mediated endocytosis provides a selective concentrating mechanism
that increase the efficiency of internalization of particular ligand more than a
hundred fold
•
In this way, even minor components of the extracellular fluid can be specifically
taken up in large amounts without taking in a large volume of extracellular fluid
•
A particularly well understood and physiologically important example is the
process that mammalian cells use to take up cholesterol
 Many animal cells take up cholesterol through receptor-mediated endocytosis
and
 In this way, acquire most of the cholesterol they require to make new
membrane
 If the take up is blocked, cholesterol accumulates in the blood and
 Can contribute to the formation of atherosclerotic plaques in the blood vessel
walls more specifically artery
•
•
•
•
•
 Atherosclerotic plaques are actually deposits of lipid and fibrous
tissue that can cause strokes and heart attacks by blocking arterial
blood flow
Most of the cholesterol is transported in the blood as cholesterol esters in
the for of lipid-protein particles known as low density proteins (LDLs)
Fig 13.29 Alberts 3rd Ed / Fig 13.50 Alberts 5th Ed
When a cell needs cholesterol from membrane synthesis, it makes
transmembrane receptor proteins for LDL and insert them in to its
plasma membrane
Once in plasma membrane, the LDL receptors diffuse until they associate
with clathrin-coated pits that are in process of forming
Fig 13.51A Alberts 5th Ed / Fig 13.30A Alberts 3rd Ed
Since coated pits constantly pinch off to form coated vesicles, any LDL
particles bound to LDL receptors in the coated pits are rapidly
internalized in coated vesicles
After shedding their clathrin coats, the vesicles deliver their content to
early endosomes
 which are located near the cell periphery
 Once the LDL and its receptor encounter the low pH in the
endosomes, LDL is released from its receptor and is delivered to
lysosomes via endosomes
•
If too much cholesterol accumulates in a cell, the cell shuts off both its
own cholesterol synthesis and
•
synthesis of LDL receptors so that it ceases either to make or to take up
cholesterol
•
This regulated pathway for cholesterol uptake is disrupted in individuals
who inherit defective genes encoding LDL receptors
•
The resulting high levels of blood cholesterol predispose these
individuals to develop atherosclerosis prematurely and
•
Many would die at an early stage of heart attacks resulting from coronary
artery disease if they were not treated with drugs that lower the level of
blood cholesterol
•
In some cases;
 the receptor is lacking altogether
 In others, the receptors are defective in either;
o
the extracellular binding site for LDL or
o
the intracellular binding site that attaches the receptor to the coat
of a clathrin coated pit
Fig 13.51B Alberts 5th Ed / Fig 13.30B Alberts 3rd Ed
 Endocytosed Materials that are not Retrieved from Endosomes end up
in Lysosomes;
•
The endosomal compartment can be made visible in the electron
microscope by adding a readily detectable tracer molecules
Fig 13.31 Alberts 3rd Ed
•
The endosomal compartment is kept acidic i.e. ~ pH 6 by ATP-driven H+
pumps in the endosomal membrane that pump H+ in to the lumen from
the cytosol
•
Late endosomes are more acidic than early endosomes
•
This acidic environment plays a crucial part in function of these
organelles
 Specific Proteins are Retrieved from Early Endosomes and Returned to
the Plasma Membrane;
•
Early endosomes form a compartment that acts as the main sorting
station in the endocytic pathway
•
Just as cis and trans Golgi networks serve this function in the
biosynthetic-secretory pathway
Fig 13.52 Alberts 5th Ed / Fig 13.32 Alberts 3rd Ed
•
LDL receptors follow the first pathway
Golgi Apparatus
Endosomes
ER
Fig 13.53 Alberts 5th Ed / Fig 13.33 Alberts 3rd Ed

Multivesicular Bodies form on the Pathway to Late Endosomes;
•
As discussed earlier, many of the endocytosed molecules move from early to late
endosomal compartment
•
In this process, early endosomes migrate slowly along microtubules toward the
cell interior
•
while shedding membrane tubules and vesicles that recycle materials to plasma
membrane
•
At the same time, the membrane enclosing the migrating endosomes forms
invaginating buds that pinch off and
•
form internal vesicles – called multivesicular bodies
Fig 13.56 Alberts 5th Ed / Fig 13.34 Alberts 3rd Ed
•
The multivesicular bodies carry those endocytosed proteins that are to be
degraded
Fig13.57 Alberts 5th Ed
x -------------------------- x ----------------------------- x ------------------------------x
(Acidic environment)