cell biology resume.pdf
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Cell Biology Resume
THE FLUID
THE ST
S.J. Singe
R
PE
PASCA SARJAN
MOSAIC MODEL O
UCTURE OF CELLEMBRANE
r and Garth L. Nicolson
C
O
M
P
I
L
E
D
By :
AJA NOVI ARISKA
8136173013
DIDIKAN BIOLOGI
UNIVERSITAS NEGERI MEDAN
2013/2014
F
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I. Title : THE FLUID MOSAIC MODEL OF THE STRUCTURE
OF CELL MEMBRANE
II. Objectives :
1. Review some thermodynamics of macromolecular, particularly membrane
system, in aqueous environment.
2. Discuss some properties of protein and lipid on functional membrane.
3. Describe the fluid mosaic model in detail.
4. Analyze some recent and more direct experimental evidence for proposed
model.
5. Show that the fluid mosaic model suggest new ways of thinking about
membranes function and membrane phenomena.
III. Introduction
The article of Singer and Nicolson is talked about the fluid mosaic model of
cell membrane structure. Since the cell membranes play an important role in cell,
the molecular of its structure need to be understand.
Singer examined in considerable detail several models of the gross
structural organization of membranes, in terms of the thermodynamics of
macromolecular systems and in the light of the then available experimentalevidence. From his analysis, it was concluded that a mosaic structure of
alternating globular proteins and phospholipid-bilayer was the only membrane
model among those analyzed that was simultaneously consistent with
thermodynamic restrictions and with all the experimental data available.
This article present and discuss a fluid mosaic model of membrane
structure, its components properties, and propose that it is applicable to most
biological membranes, such as plasmalemmal and intracellular membranes,
including the membranes of different cell organelles, such as mitochondria and
chloroplasts. These membranes are henceforth referred to as functional
membranes.
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IV. Discussion
a. Thermodynamics a
The fluid mosaic mo
version. It indicates that the
covalent interactions are mo
amphipathic.
Hydrophobic as tail
thermodynamic factors that a
non-polar groups away fr
hydrocarbons and water. T
kilocalories of free energy to
to water at 25C.
Hydrophilic as head
thermodynamic factors that
groups for an aqueous rather
required to transfer a mole o
6.0 kcal at 25C, showing t
non-polar medium.
The hydrophilic ar
wateryenvironment. The hy
When phospholipids are pla
because of the chemical prop
Figure 1. pho
d Membrane Structure
el has evolved by a series of stages from ea
membrane systems composed of two kinds of
t important, hydrophobic and hydrophilic, know
(water-fearing) interactions means a set
re responsible for the sequestering of hydrophob
m water, as, for example, the immiscibilit
o be specific, it requires the expenditure of
transfer a mole of methane from a non-polar me
(water-loving) interactions is meant a se
re responsible for the preference of ionic and
than a non-polar environment. Thus, the free en
f zwitterionic glycine from water to acetone is a
at ion pairs strongly prefer to be in water than
face outward and likely to encounte
rophobic are face inward, where there is no w
ed in water, they naturally form a spherical bil
erties of the heads and the tails (Mader.2004).
pholipid molecule (source: Freeman, 2003)
rlier
non-
n as
of
ic or
of
2.6
ium
of
olar
ergy
bout
in a
r a
ater.
ayer
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There are other non-
electrostatic interactions, w
structure. However, with r
concerned, these are very lik
and hydrophilic interactions.
The non-polar fatty
together away from conta
interactions. Furthermore, th
with the aqueous phase at th
hydrophilic interactions. In
phosphatidylcholine, dipole-
the bilayer may also contribuThe tail in cell mem
double bound in one tail. It c
affect the packing and fluidit
Figure 2. Mo
The double cis-bond
making lipid bilayer hard to
of unsaturated lipid is more s
lipid .
ovalent interactions, such as hydrogen bonding
hich also contribute to determine macromole
spect to gross structure, with which we are
ly of secondary magnitude compared to hydroph
acid chains of the phospholipids are sequest
ct with water, thereby maximizing hydroph
ionic and zwitter-ionic groups are in direct co
exterior surfaces of the bilayer, thereby maximi
the case of zwitterionic phospholipids suc
ipole interactions between ion pairs at the surfa
te to the stabilization of the, bilayer structure.brane have variable in length. It caused by the
ause small kink in the structure of cell membrane
of cell membrane. (Alberts, 2008)
lecular structure of amphipathic structure
akes more difficult to pack the chains together,
reeze. In addition, because of the hydrocarbon ch
pread apart, it has thinner structure than the satur
and
ular
now
obic
ered
obic
tact
zing
as
e of
cis-
and
thus
ains
ated
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b. Some Properties
Peripheral and integr
Peripheral protein has
(i) They require onl
strength of the
dissociate them m
(ii) They dissociate fr
(iii) In the dissociate
buffers is held to
(perhaps mainly el
with membrane lip
F
On the other hand, t
namely:
(i) They require m
detergents, bile
dissociate them f
(ii) In many instance
(iii) If completely fr
aggregated in neu
f Membrane Components
l proteins.
several characteristics:
mild treatments, such as an increase in the i
edium or the addition of a chelating agen
lecularly intact from the membrane;
ee of lipids; and
state they are relatively soluble in neutral aqu
the membrane only by rather weak non-cov
ctrostatic) interactions and is not strongly associ
id.
igure 3. Protein of lipid bilayer
e integral proteins also have several characteri
ch more drastic treatments, with reagents suc
acids, protein denaturants, or organic solvent
om membranes;
, they remain associated with lipids when isolate
eed of lipids, they are usually highly insolubl
tral aqueous buffers.
onic
, to
ous
lent
ated
tics,
h as
, to
d;
e or
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Figure
Integral proteins gros
also suggested that the integspread in a monolayer.
Lipid-anchored me
lipid molecules. The hydrop
in one leaflet of the membr
polypeptide chain itself does
Figure 3. pr
. Type of protein based on its location
sly heterogeneous and vary in molecular weig
ral proteins are largely globular in shape rather
brane proteins are bound covalently to one or
obic carbon chain of the attached lipid is embe
ane and anchors the protein to the membrane.
not enter the phospholipid bilayer.
perties of protein in cell membrane
t. It
than
ore
ded
The
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c. Fluid Mosaic Model
The characteristic of globular protein is same as the lipid bilayer, namely
amphipathic. They are structurally asymmetry, with one high polar end and one
non polar end. The highly polar region is one in which the ionic amino acid
residues and any covalently bound saccharide residues are clustered, and which is
in contact with the aqueous phase in the intact membrane; the nonpolar region is
devoid of ionic and saccharide residues, contains many of the nonpolar residues,
and is embedded in the hydrophobic interior of the membrane.
If the protein have the appropriate amino sequences and contain
amphiphatic structure, it can be included as integral protein. An integral protein
molecule with the appropriate size and structure, or a suitable aggregate of
integral proteins (below) may transverse the entire membrane that is, they haveregions in contact with the aqueous solvent on both sides of the membrane.
The reason why some proteins are membrane bound and others are freely
soluble in the cytoplasm because the amino acid sequence of the particular protein
allows it to adopt an amphipathic structure or, on the contrary, to adopt a structure
in which the distribution of ionic groups is nearly spherically symmetrical, in the
lowest free energy state of the system.
If the ionic distribution on the protein surface were symmetrical, the protein
would be capable of interacting strongly with water all over its exterior surface,that is, it would be a monodisperse soluble protein.
At body temperature, the phospholipid bilayer is a liquid; it has the
consistency of olive oil, and the proteins are able to change their positions by
moving laterally. The fluid-mosaic model, a working description of membrane
structure, suggests that the protein molecules have a changing pattern (form a
mosaic) within the fluid phospholipid bilayer. Our plasma membranes also
contain a substantial number of cholesterol molecules. These molecules lend
stability to the phospholipid bilayer and prevent a drastic decrease in fluidity atlow temperatures.
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Figu
Short chains of sugarand lipid molecules (called
carbohydrate chains, specifi
particular individual and acc
patients system sometimes r
Some glycoproteins h
a receptor for a chemical me
proteins form channels thro
others are carriers involved(Fremann.2003)
e 5. phospholipid bilayer structure
s are attached to the outer surfaces of some pr glycoproteins and glycolipids, respectively). T
c to each cell, mark the cell as belonging
ount for such characteristics as blood type or w
jects an organ transplant.
ave a special configuration that allows them to a
ssenger such as a hormone. Some plasma memb
gh which certain substances can enter cells,
in the passage of molecules through the memb
teinhese
o a
hy a
t as
rane
hile
rane
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Figure 6. Dynamic of Phospholipid
Matrix of the mosaic, lipid or protein?
It is not well determine which components are arrange the matrix of the
mosaic. This question must be answered when the third dimension of the mosaicstructure is specified.
If the membrane proteins formed the matrix of the mosaic, defined by
specific contacts between the molecules of different integral proteins, protein A
might be distributed in a highly ordered, two dimensional array on the surface. On
the other hand, if lipid formed the matrix of the mosaic, there would be no long-
range interactions intrinsic to the membrane influencing the distribution of A
molecules, and they should there-fore be distributed in an aperiodic random
arrangement on the membrane surface.If a membrane consisted of integral proteins dispersed in a fluid lipid
matrix, the membrane would in effect be a two-dimensional liquid-like solution of
mon-omeric or aggregated integral proteins (or lipoproteins) dissolved in the lipid
bilayer. The mosaic structure would be a dynamic rather than a static one. The
integral proteins would be expected to undergo translational diffusion within the
membrane, at rates determined in part by the effective viscosity of the lipid, unless
they were tied down by some specific interactions intrinsic or extrinsic to the
membrane. However, because of their amphipathic structures, the integral proteinswould maintain their molecular orientation and their degree of intercalation in the
membrane while undergoing translational diffusion in the plane of the membrane.
In contrast, if the matrix of the mosaic were constituted of integral
proteins, the long-range structure of the membrane would be essentially static.
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Large energies of activation would be required for a protein component to diffuse
in the plane of the membrane from one region to a distant one because of the
many non-covalent bonds between the proteins that would have to be
simultaneously broken for exchange to take place. Therefore, a mosaic membrane
with a protein matrix should make for a relatively rigid structure with essentially
no translational diffusion of its protein components within the membrane.
d. Some evidence related to mosaic model
1. Evidence for proteins embedded in membranes
The results of recent freeze-etching experiments with membranes strongly
suggest that a substantial amount of protein is deeply embedded in many
functional membranes.Methods : a frozen specimen is fractured with a microtome knife; some of the
frozen water is sub-limed (etched) from the fractured surface if desired; the
surface is then shadow cast with metal, and the surface replica is examined in the
electron microscope. A characteristic feature of the exposed surface of most
functional membranes examined by this technique, including plasmalemmal,
vacuolar, nu-clear, chloroplast, mitochondrial, and bacterial membranes is a
mosaic-like structure consisting of a smooth matrix interrupted by a large number
of particles. These particles have a fairly characteristic uniform size for aparticular membrane, for example, about 85 diameter for erythrocyte
membranes. Such surfaces result from the cleavage of a membrane along its
interior hydrophobic face.
2. Distribution of component in the plain of membrane.
The distribution of component in plain of membrane can be visualized by
using electron microscopy. It is shown the distribution of membrane antigen over
large area surface membrane.
Thus, analyzing the distribution of Rho(D) antigen of human erythrocytemembrane cell. The O Rh positive react with saturating amount 125I labeled
purified human antibody against Rho(D)/anti Rho(D). The cell is lysed at air-
water interface causing the cell membrane flattened. The cell then picked up on
electron microscope grid. Staining method is then occupied using indirect stain
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(ferritin conjugated goat spe
whenever anti Rho(D)/ boun
goat attached. ( globulin
This indicates that there rel
radius 300 . The cluster/un
human and an Rho(D) mole
ferritin bound to anti Rho
molecules bound to a single
conclude that Rho(D)/ anti
distributed randomly in two
3. Protein is consist
There present tightl
rhodopsin). Earlier studies
particles distribution. But remembrane showed that (1) o
the spacing of Rhodopsin
arrangement of particles is
were consistent with idea t
cific for human globulins). The results show
d to Rho antigen on surface membrane, the fer
uman antibody became antigen for goat antib
ation between clusters of 2-8 ferritin molecule
it area is in balance between the total of125I la
ules bound/unit area. Or in other word every cl
D) and the cluster is the total of goat anti
human Y globulin molecules (one cluster),a we
gen which possess integral protein properties
imensional array.
luid state in intact internal membrane
packages ordered array particles (the particl
howed that there was a long range order of t
ent X-ray diffraction data of wet pellet receptornly a few order of reflection was observed relat
in membranes. It means, no crystalline aperi
resent in membranes. (2) X-ray diffraction ma
at particles are in planar liquid-like state in i
that
ritin
dy).
s on
eled
ster
ody
can
are
s in
hese
diskd to
odic
ima
tact
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membranes. (3) Foreign protein absorption (Bovine serum albumin) to the
membrane, alter x-ray spacing of rhodopsin because albumin weakly bind to
membrane to rhodopsin non-rigid structures causing aggregates in membrane,
indicating the liquid-like state.
4. The asymmetry of membrane
Two surfaces of membranes are not identical in structure composition or
function. This asymmetry is come from the distribution of oligosaccharide on
the two surface of membrane. Technique : There exist plant proteins, called
lectins or plant agglutinins, which bind to specific sugar residues, and, as a
result, can cause the agglutination of cells bearing the sugar residues on their
surfaces. The authors have been able to visualize the distribution of
oligosaccharides on membranes in the electron microscope.
e. The Fluid Mosaic Model and membrane functions
The structure of cell membrane which viscous with amphipathic solution
and lipid instantaneous thermodynamic equilibrium is lead to several functions
related to transportation within and to the cell. Physical and chemical perturbation
can affect and change the components of cell membrane, thus provide new
thermodynamic interaction of altered components.
1. Malignant transformation of cells and the "exposure of cryptic sites.
Malignant trans-formation is closely correlated with a greatly increased
capacity for the trans-formed cells to be agglutinated by several saccharides
binding plant agglutinins. Mild treatment of normal cells with proteolytic enzymes
can render malignant also more readily agglutinable by these protein agglutinins.
Burger has suggested, therefore, that the agglutinin-binding sites are pres-ent on
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the membrane surfaces of nor-mal cells but are "cryptic" and that proteolytic
digestion of normal cells or the processes of malignant transformation "exposes"
these cryptic sites on the membrane surface. In some cases, quantitative binding
studies have indeed indicated that no significant change in the numbers of
agglutinin-binding sites on the membrane accompanies either mild proteolysis of
normal cells or malignant transformation.
2. Cooperative phenomena in membrane
Trans effects refer to cooperative (allosteric) changes that have been
postulated to operate at some localized region on the membrane surface, to
transmit an effect from one side of the membrane to the other. For example, fan
integral protein may exist in the membrane as an aggregate of two (or more)
subunits, one of which is exposed to the aqueous solution at the outer surface ofthe membrane, and the other is exposed to the cytoplasm at the inner surface. The
specific binding of a drug or hormone molecule to the active site of the outward-
oriented subunit may induce a conformational rearrangement within the
aggregate, and thereby change some functional property of the aggregate or of its
inward-oriented subunit. By cis effects, on the other hand, we refer to cooperative
changes that may be produced over the entire membrane, or at least large areas of
it, as a consequence of some event or events occurring at only one or a few
localized points on the membrane surface. For example, the killing effects ofcertain bacteriocins on bacteria, the lysis of the cortical granules of egg cells upon
fertilization of eggs by sperm, and the interaction of growth hormone with
erythrocyte membranes are cases which may involve transmission and
amplification of local-ized events over the entire surface of a membrane.
V. Conclusion
1. The structure of phospholipid bilayer in aqueous environment is in
amphipathic structure, which means, the lipid bilayer consist ofhydrophilic head and hydrophobic tails. The hydrophilic is consist of
covalent bond (high polar) of phosphate and glycerol, while in hydropobic
(non-polar) the tails is composed of fatty acid. Both of this structure is
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works cooperatively makes the structure for its proper function and its
composition affect the fluidity of lipid bilayer.
2. In lipid bilayer, there are two major proteins arrange its structure,
peripheral and integral. The peripheral is located on the side of the bilayer
and easier to breaks by mild treatments, while the integral is embedded in
the bilayer. The integral is assumed as the main protein in the formation of
lipid bilayer. The lipid itself has a major role in the case of the fluidity of
the cell membrane. Since the tail is consist of the fatty acid, the structure
of tail effect the fluidity of bilayer, especially the unsaturated, kinky
structure.
3. The structure offluid-mosaic at body temperature is a liquid; a working
description of membrane structure, suggests that the protein moleculeshave a changing pattern (form a mosaic) within the fluid phospholipid
bilayer. Our plasma membranes also contain a substantial number of
cholesterol molecules. These molecules lend stability to the phospholipid
bilayer and prevent a drastic decrease in fluidity at low temperatures.
4. There are four some recent evidence of cell membrane structures.
a. protein are embedded in membrane
b. the distribution of component in membrane
c. Protein is consist fluid state in intact internal membraned. The asymmetry of membrane
5. The structure of cell membrane which viscous with amphipathic solution
and lipid instantaneous thermodynamic equilibrium is lead to several
function, related to transportation within and to the cell. Physical and
chemical perturbation can affect and change the components of cell
membrane, thus provide new thermodynamic interaction of altered
components.
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REFERENCES
Alberts, Bruce et al. 2008. Molecular Biology of the Cell, Fifth Edition. GarlandScience, Taylor and Francis Group. USA
Freeman.2003.Molecular Biology. (downloading from 4shared.com on January,2013)
Mader, Sylvia S. 2004. Understanding Human Anatomy and Physiology, FifthEdition. Mc. Graw-Hill. USA
Singer, S.J and Garth L. Nicholson. 1972.The Fluid Mosaic Model of the
Structure of Cell Membranes.Published by: American Association for the
Advancement of Science.Vol. 175pp. 720-731.