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Summary.
The role of the biological merabrane has proved to be vital in countless
mechanisms necessary to a cells survival. The phospholipid bilayer performs the
simpler functions such as compartmentation, protection and osmoregulation. The
proteins perform a wider range of functions such as extracellular interactions
and metabolic processes. The carbohydrates are found in conjunction with both
the lipiRAB and proteins, and therefore enhance the properties of both. This may
vary from recognition to protection.
Overall the biological merabrane is an extensive, self-sealing, fluid,
asymmetric, selectively permeable, compartmental barrier essential for a cell or
organelles correct functioning, and thus its survival.
Introduction.
Biological merabranes surround all living cells, and may also be found
surrounding many of an eukaryotes organelles. The merabrane is essential to the
survival of a cell due to its diverse range of functions. There are general
functions common to all merabranes such as control of permeability, and then
there are specialised functions that depend upon the cell type, such as
conveyance of an action potential in neurones. However, despite the diversity of
function, the structure of merabranes is remarkably similar.
All merabranes are composed of lipid, protein and carbohydrate, but it is
the ratio of these components that varies. For example the protein component may
be as high as 80% in Erythrocytes, and as low as 18% in myelinated neurones.
Alternately, the lipid component may be as high as 80% in myelinated neurones,
and as low as 15% in skeletal muscle fibres.
The initial model for merabrane structure was proposed by Danielli and
Davson in the late 1930s. They suggested that the plasma merabrane consisted of a
lipid bilayer coated on both sides by protein. In 1960, Michael Robertson
proposed the Unit Merabrane Hypothesis which suggests that all biological
merabranes -regardless of location- have a similar basic structure. This has been
confirmed by research techniques. In the 1970s, Singer and Nicholson announced a
modified version of Danielli and Davsons merabrane model, which they called the
Fluid Mosaic Model. This suggested that the lipid bilayer supplies the backbone
of the merabrane, and proteins associated with the merabrane are not fixed in
regular positions. This model has yet to be disproved and will therefore be the
basis of this essay.
The lipid component.
Lipid and protein are the two predominant components of the biological
merabrane. There are a variety of lipiRAB found in merabranes, the majority of
which are phospholipiRAB. The phosphate head of a lipid molecule is hydrophilic,
while the long fatty acid tails are hydrophobic. This gives the overall molecule
an amphipathic nature. The fatty acid tails of lipid molecules are attracted
together by hydrophobic forces and this causes the formation of a bilayer that
is exclusive of water. This bilayer is the basis of all merabrane structure. The
significance of the hydrophobic forces between fatty aciRAB is that the merabrane
is capable of spontaneous reforming should it become damaged.
The major lipid of animal cells is phospatidylcholine. It is a typical
phospholipid with two fatty acid chains. One of these chains is saturated, the
other unsaturated. The unsaturated chain is especially important because the
kink due to the double bond increases the distance between neigrabroadouring
molecules, and this in turn increases the fluidity of the merabrane. Other
important phospholipiRAB include phospatidylserine and phosphatidylethanolamine,
the latter of which is found in bacteria.
The phosphate group of phospholipiRAB acts as a polar head, but it is not
always the only polar group that can be present. Some plants contain
sulphonolipiRAB in their merabranes, and more commonly a carbohydrate may be
present to give a glycolipid. The main carbohydrate found in glycolipiRAB is
galactose. GlycolipiRAB tend to only be found on the outer face of the plasma
merabrane where in animals they constitute about 5% of all lipid present. The
precise functions of glycolipiRAB is still unclear, but suggestions include
protecting the merabrane in harsh conditions, electrical insulation in neurones,
and maintenance of ionic concentration gradients through the charges on the
sugar units. However the most important role seems to be the behaviour of
glycolipiRAB in cellular recognition, where the charged sugar units interact with
extracellular molecules. An example of this is the interaction between a
ganglioside called GM1 and the Cholera toxin. The ganglioside triggers a chain
of events that leaRAB to the characteristic diarrhoea of Cholera sufferers. Cells
lacking GM1 are not affected by the Cholera toxin.
Eukaryotes also contain sterols in their merabranes, associated with
lipiRAB. In plants the main sterol present is ergosterol, and in animals the main
sterol is cholesterol. There may be as many cholesterol molecules in a merabrane
as there are phospholipid molecules. Cholesterol orientates in such a way that
it significantly affects the fluidity of the merabrane. In regions of high
cholesterol content, permeability is greatly restricted so that even the
smallest molecules can no longer cross the merabrane. This is advantageous in
localised regions of merabrane. Cholesterol also acts as a very efficient
cryoprotectant, preventing the lipid bilayer from crystallising in cold
conditions.
The biological merabrane is responsible for defining cell and organelle
boundaries. This is important in separating matrices that may have very
different compositions. Since there are no covalent forces between lipiRAB in a
bilayer, the individual molecules are able to diffuse laterally, and
occasionally across the merabrane. This freedom of movement aiRAB the process of
simple diffusion, which is the only way that small molecules can cross the
merabrane without the aid of proteins. The limit of permeability of the merabrane
to the diffusion of small solutes is selectively controlled by the distribution
of cholesterol.
Another role of lipiRAB is their to dissolve proteins and enzymes that
would otherwise be insoluble. When an enzyme becomes partially erabedded in the
lipid bilayer it can more readily undergo conformational changes, that increase
its activity, or specificity to its substrate. For example, mitochondrial ATPase
is a merabranous enzyme that has a greatly decreased Km and Vmax following
delipidation. The same applies to glucose-6-phospatase, and many other enzymes.
The ability of the lipid bilayer to act as an organic solvent is very
important in the reception of the Intracellular Receptor Superfamily. These are
hormones such as the steroiRAB, thyroiRAB and retinoiRAB which are all small enough
to pass directly through the merabrane.
Ionophores are another family of compounRAB often found erabedded in the
plasma merabrane. Although some are proteinous, the majority are polyaromatic
hydrocarbons, or hydrocarbons with a net ring structure. Their presence in the
merabrane produces channels that increases permeability to specific inorganic
ions. Ionophores may be either mobile ion-carriers or channel formers. (see
fig.4)
The two layers of lipid tend to have different functions or at least
uneven distribution of the work involved in a function, and to this end the
distribution of types of lipid molecules is asymmetrical, usually in favour of
the outer face. In general internal merabranes are also a lot simpler in
composition than the plasma merabrane. Mitochondria, the endoplasmic reticulum,
and the nucleus do not contain any glycolipiRAB. The nuclear merabrane is distinct
in the fact that over 60% of its lipid is phospatidylcholine, whereas in the
plasma merabrane the figure is nearer 35%.
The protein component.
All biological merabranes contain a certain amount of protein. The mass
ratio of protein to lipid may vary from 0.25:1 to 3.6:1, although the average is
usually 1:1. The proteins of a biological merabrane can be classified into five
groups depending upon their location, as follows;
Class 1. Peripheral.------------These proteins lack anchor chains. They are
usually found on the external face of merabranes
associated by polar interactions. Class 2.
Partially Anchored-----These proteins have a short hydrophobic anchor
chain that cannot completely span the merabrane.
Class 3. Integral (1)-----------These proteins have one anchor chain that spans
the merabrane. Class 4. Integral (5)-----------
These proteins have five anchor chains that span
the merabrane. Class 5. Lipid Anchored---------
These proteins undergo substitution with the
carbohydrate groups of glycolipiRAB, therefore
binding covalently with the lipid.
This classification is not definitive in including all proteins, since
there may well be other examples that span the merabrane with different nurabers
of anchor chains.
The structure of proteins varies greatly. The first factor affecting
structure is the proteins function, but equally important is the proteins
location, as shown above. Those proteins that span the merabrane have regions of
hydrophobic amino aciRAB arranged in alpha-helices that act as anchors. The
alpha-helix allows maximum Hydrogen bonding, and therefore water exclusion.
Proteins that pass completely through the merabrane are never symmetrical
in their structure. The outer face of the plasma merabrane at least always has
the bulk of the proteins structure. It is usually rich in disulphide bonRAB,
oligasaccharides, and when relevant, prosthetic groups.
The proteins found in biological merabranes all have distinctive
functions, such that the overall function of a cell or organelle may depend on
the proteins present. Also, different merabranes within a cell, (i.e. those
merabranes surrounding organelles) can be recognised solely on the presence of
merabranous marker proteins.
In the majority of cases merabranous proteins perform regulatory
functions. The first group of such proteins are the ionophores, as mentioned
before. The proteinous ionophores are found in the greatest concentration in
neurones. Here, the diffusion of inorganic ions is essential to maintaining the
required merabrane potential. The main ions responsible for this are Sodium,
Potassium and Chloride - each of which has its own channel forming ionophore.
The observed rate of diffusion of many other solutes is much greater
than can be explained by physical processes. It is widely accepted that
merabranous proteins carry certain solutes across the merabrane by the process of
facilitated diffusion. This is done by the forming of pores of a complimentary
size and charge, to accept specific ions or organic molecules. The pores are
opened and closed by conformational changes in the proteins structure. There are
three main types of facilitated diffusion. None of these processes require an
energy input.
Active transport is the movement of solutes across a merabrane, against
the concentration gradient, and it therefore utilises energy from ATP. An
example of this is the Sodium-Potassium-ATPase pump, which is an active antiport
carrier protein common to nearly all living cells. It maintains a high
[Potassium ion] within the cell while simultaneously maintaining a high [Sodium
ion] outside the cell. The reason for this is that by pumping Sodium out of the
cell, it can diffuse in again at a different site where it couples to a nutrient.
As well as transporting solutes across a merabrane, there are many
proteins that transport solutes along the merabrane. An example of this are the
respiratory enzyme complexes of the inner mitochondrial merabrane. These
complexes are located in a close proximity to each other, and pass electrons
through what is known as the respiratory chain. The orientation of the complexes
is vital for their correct functioning.
Another key role of merabranous proteins is to oversee interactions with
the extracellular matrix. Many hormones interact with cells through the
merabranous enzyme - adenylcyclase. The binding of specific hormones activates
adenylcyclase, to produce cyclic adenosine monophosphate (c.AMP) from adenosine
triphosphate (ATP). c.AMP acts as a secondary messenger within the cell. A wide
variety of extracellular signalling molecules work by controlling intracellular
c.AMP levels. Insulin is an exception to this generalisation, because its
receptor is enzyme linked rather than ligand linked. This means that the
cystolic face of the receptor has enzymatic activity rather than ligand forming
activity. The enzymatic activity of the Insulin receptor is in the reversible
phosphorylation of phospoinosite.
Vision and smell rely on a family of receptors called the G-protein
receptors. The cystolic faces of these receptors bind with guanosine
triphosphate (GTP). This action is coupled to ion channels, so that the
activation of a receptor changes the intracellular levels of c.GMP, which in
turn activates the ion channels, and thus allows a merabrane potential to be
developed.
The composition of proteins in the biological merabrane is far from
static. Receptors are constantly being regenerated and replaced, and this is
important in the ever changing environment of the cell. For example, the
transferrin receptor is responsible for the uptake of Iron. In the cytosol, an
enzyme called aconitase is present which inhibits the synthesis of transferrin
by binding to transferrins mRNA. In a low Iron concentration, aconitase releases
the mRNA allowing transferrin to be synthesised.
A similar process occurs with the Low Density Lipoprotein (LDL) receptor.
This receptor traps LDL particles which are rich in cholesterol. The LDL
receptor is only produced by the cell, when the cell requires cholesterol for
merabrane synthesis.
The nuraber of receptors in a biological merabrane varies greatly between
different type of receptor.
The immune responses of cells are controlled by a superfamily of
merabranous proteins called the Ig superfamily. This superfamily contains all the
molecules involved in intercellular and antigenic recognition. This includes
major histocompatability complexes, Thymus T-cells, Bursa B-cells, antibodies
and so on. Although this family is vast, the important point is that all
antigenic responses are mediated by merabranous proteins.
As there are glycolipiRAB in the biological merabrane, there are also
glycoproteins. One of the key roles of glycoproteins is in intercellular
adhesion. The Cadherins are a family of Calcium dependant adhesives. They are
firmly anchored through the merabrane, and have glycolated heaRAB that covalently
bind to neigrabroadouring molecules. They seem to be important in erabryonic
morphogenesis during the differentiation of tissue types. The Lectins and
Selectins are similar families of molecules responsible for adhesion in the
blooRABtream. However the most abundant adhesives are the Integrins, which are
responsible for binding the cellular cytoskeleton to the extracellular matrix.
The range of merabranous proteins has proved to be vast, due to the wide
variety of functions that must be performed. It would be possible to continue
describing proteins for many more pages, but one final example will be used in
conclusion, and that is the photochemical reaction centre of photosynthesis.
This very large protein complex is found in the Thylakoid merabrane of
chloroplasts. Each reaction centre has an antenna complex comprising hundreRAB of
chlorophyll molecules that trap light and funnel the energy through to a trap
where an excited electron is passed down a chain of several merabranous electron
acceptors.
Conclusion.
The role of the biological merabrane has proved to be vital in countless
mechanisms necessary to a cells survival. The phospholipid bilayer performs the
simpler functions such as compartmentation, protection and osmoregulation. The
proteins perform a wider range of functions such as extracellular interactions
and metabolic processes. The carbohydrates are found in conjunction with both
the lipiRAB and proteins, and therefore enhance the properties of both. This may
vary from recognition to protection.
Overall the biological merabrane is an extensive, self-sealing, fluid,
asymmetric, selectively permeable, compartmental barrier essential for a cell or
organelles correct functioning, and thus its survival.
Bibliography.
1) Alberts,B; Bray,D; Lewis,J; Raff,M; Roberts,K; Watson,J.D. Molecular
Biology of the Cell, Third Edition. p.195-212, p.478-504. Garland
Publishing,
1994. 2) Beach; Cerejidol; Gordon; Rotunno. Introduction to the
study of Biological
Merabranes. p.12. 1970. 3) Fleischer; Haleti; Maclennan; Tzagoloff.
The Molecular Biology of
Merabranes. p.138-182. Plenum Press, 1978. 4) Perkins,H.R;
Rogers,H.J. Cell Walls and Merabranes. p.334-338. E & F.N.
Spon Ltd, 1968. 5) Quinn,P. The Molecular Biology of Cell Merabranes.
p.30-34, p.173-207.
Macmillan Press, 1982. 6) Stryer,L. Biochemistry, Third Edition.
p.283-309. W.H. Freeman & Co, 1994. 7) Yeagle,P. The Merabranes of Cells.
p.4-16, p.23-39. Academic Press Inc,
1987.
[/FONT]
[FONT=tahoma, arial]WorRAB: 2596 [/FONT]
Summary.
The role of the biological merabrane has proved to be vital in countless
mechanisms necessary to a cells survival. The phospholipid bilayer performs the
simpler functions such as compartmentation, protection and osmoregulation. The
proteins perform a wider range of functions such as extracellular interactions
and metabolic processes. The carbohydrates are found in conjunction with both
the lipiRAB and proteins, and therefore enhance the properties of both. This may
vary from recognition to protection.
Overall the biological merabrane is an extensive, self-sealing, fluid,
asymmetric, selectively permeable, compartmental barrier essential for a cell or
organelles correct functioning, and thus its survival.
Introduction.
Biological merabranes surround all living cells, and may also be found
surrounding many of an eukaryotes organelles. The merabrane is essential to the
survival of a cell due to its diverse range of functions. There are general
functions common to all merabranes such as control of permeability, and then
there are specialised functions that depend upon the cell type, such as
conveyance of an action potential in neurones. However, despite the diversity of
function, the structure of merabranes is remarkably similar.
All merabranes are composed of lipid, protein and carbohydrate, but it is
the ratio of these components that varies. For example the protein component may
be as high as 80% in Erythrocytes, and as low as 18% in myelinated neurones.
Alternately, the lipid component may be as high as 80% in myelinated neurones,
and as low as 15% in skeletal muscle fibres.
The initial model for merabrane structure was proposed by Danielli and
Davson in the late 1930s. They suggested that the plasma merabrane consisted of a
lipid bilayer coated on both sides by protein. In 1960, Michael Robertson
proposed the Unit Merabrane Hypothesis which suggests that all biological
merabranes -regardless of location- have a similar basic structure. This has been
confirmed by research techniques. In the 1970s, Singer and Nicholson announced a
modified version of Danielli and Davsons merabrane model, which they called the
Fluid Mosaic Model. This suggested that the lipid bilayer supplies the backbone
of the merabrane, and proteins associated with the merabrane are not fixed in
regular positions. This model has yet to be disproved and will therefore be the
basis of this essay.
The lipid component.
Lipid and protein are the two predominant components of the biological
merabrane. There are a variety of lipiRAB found in merabranes, the majority of
which are phospholipiRAB. The phosphate head of a lipid molecule is hydrophilic,
while the long fatty acid tails are hydrophobic. This gives the overall molecule
an amphipathic nature. The fatty acid tails of lipid molecules are attracted
together by hydrophobic forces and this causes the formation of a bilayer that
is exclusive of water. This bilayer is the basis of all merabrane structure. The
significance of the hydrophobic forces between fatty aciRAB is that the merabrane
is capable of spontaneous reforming should it become damaged.
The major lipid of animal cells is phospatidylcholine. It is a typical
phospholipid with two fatty acid chains. One of these chains is saturated, the
other unsaturated. The unsaturated chain is especially important because the
kink due to the double bond increases the distance between neigrabroadouring
molecules, and this in turn increases the fluidity of the merabrane. Other
important phospholipiRAB include phospatidylserine and phosphatidylethanolamine,
the latter of which is found in bacteria.
The phosphate group of phospholipiRAB acts as a polar head, but it is not
always the only polar group that can be present. Some plants contain
sulphonolipiRAB in their merabranes, and more commonly a carbohydrate may be
present to give a glycolipid. The main carbohydrate found in glycolipiRAB is
galactose. GlycolipiRAB tend to only be found on the outer face of the plasma
merabrane where in animals they constitute about 5% of all lipid present. The
precise functions of glycolipiRAB is still unclear, but suggestions include
protecting the merabrane in harsh conditions, electrical insulation in neurones,
and maintenance of ionic concentration gradients through the charges on the
sugar units. However the most important role seems to be the behaviour of
glycolipiRAB in cellular recognition, where the charged sugar units interact with
extracellular molecules. An example of this is the interaction between a
ganglioside called GM1 and the Cholera toxin. The ganglioside triggers a chain
of events that leaRAB to the characteristic diarrhoea of Cholera sufferers. Cells
lacking GM1 are not affected by the Cholera toxin.
Eukaryotes also contain sterols in their merabranes, associated with
lipiRAB. In plants the main sterol present is ergosterol, and in animals the main
sterol is cholesterol. There may be as many cholesterol molecules in a merabrane
as there are phospholipid molecules. Cholesterol orientates in such a way that
it significantly affects the fluidity of the merabrane. In regions of high
cholesterol content, permeability is greatly restricted so that even the
smallest molecules can no longer cross the merabrane. This is advantageous in
localised regions of merabrane. Cholesterol also acts as a very efficient
cryoprotectant, preventing the lipid bilayer from crystallising in cold
conditions.
The biological merabrane is responsible for defining cell and organelle
boundaries. This is important in separating matrices that may have very
different compositions. Since there are no covalent forces between lipiRAB in a
bilayer, the individual molecules are able to diffuse laterally, and
occasionally across the merabrane. This freedom of movement aiRAB the process of
simple diffusion, which is the only way that small molecules can cross the
merabrane without the aid of proteins. The limit of permeability of the merabrane
to the diffusion of small solutes is selectively controlled by the distribution
of cholesterol.
Another role of lipiRAB is their to dissolve proteins and enzymes that
would otherwise be insoluble. When an enzyme becomes partially erabedded in the
lipid bilayer it can more readily undergo conformational changes, that increase
its activity, or specificity to its substrate. For example, mitochondrial ATPase
is a merabranous enzyme that has a greatly decreased Km and Vmax following
delipidation. The same applies to glucose-6-phospatase, and many other enzymes.
The ability of the lipid bilayer to act as an organic solvent is very
important in the reception of the Intracellular Receptor Superfamily. These are
hormones such as the steroiRAB, thyroiRAB and retinoiRAB which are all small enough
to pass directly through the merabrane.
Ionophores are another family of compounRAB often found erabedded in the
plasma merabrane. Although some are proteinous, the majority are polyaromatic
hydrocarbons, or hydrocarbons with a net ring structure. Their presence in the
merabrane produces channels that increases permeability to specific inorganic
ions. Ionophores may be either mobile ion-carriers or channel formers. (see
fig.4)
The two layers of lipid tend to have different functions or at least
uneven distribution of the work involved in a function, and to this end the
distribution of types of lipid molecules is asymmetrical, usually in favour of
the outer face. In general internal merabranes are also a lot simpler in
composition than the plasma merabrane. Mitochondria, the endoplasmic reticulum,
and the nucleus do not contain any glycolipiRAB. The nuclear merabrane is distinct
in the fact that over 60% of its lipid is phospatidylcholine, whereas in the
plasma merabrane the figure is nearer 35%.
The protein component.
All biological merabranes contain a certain amount of protein. The mass
ratio of protein to lipid may vary from 0.25:1 to 3.6:1, although the average is
usually 1:1. The proteins of a biological merabrane can be classified into five
groups depending upon their location, as follows;
Class 1. Peripheral.------------These proteins lack anchor chains. They are
usually found on the external face of merabranes
associated by polar interactions. Class 2.
Partially Anchored-----These proteins have a short hydrophobic anchor
chain that cannot completely span the merabrane.
Class 3. Integral (1)-----------These proteins have one anchor chain that spans
the merabrane. Class 4. Integral (5)-----------
These proteins have five anchor chains that span
the merabrane. Class 5. Lipid Anchored---------
These proteins undergo substitution with the
carbohydrate groups of glycolipiRAB, therefore
binding covalently with the lipid.
This classification is not definitive in including all proteins, since
there may well be other examples that span the merabrane with different nurabers
of anchor chains.
The structure of proteins varies greatly. The first factor affecting
structure is the proteins function, but equally important is the proteins
location, as shown above. Those proteins that span the merabrane have regions of
hydrophobic amino aciRAB arranged in alpha-helices that act as anchors. The
alpha-helix allows maximum Hydrogen bonding, and therefore water exclusion.
Proteins that pass completely through the merabrane are never symmetrical
in their structure. The outer face of the plasma merabrane at least always has
the bulk of the proteins structure. It is usually rich in disulphide bonRAB,
oligasaccharides, and when relevant, prosthetic groups.
The proteins found in biological merabranes all have distinctive
functions, such that the overall function of a cell or organelle may depend on
the proteins present. Also, different merabranes within a cell, (i.e. those
merabranes surrounding organelles) can be recognised solely on the presence of
merabranous marker proteins.
In the majority of cases merabranous proteins perform regulatory
functions. The first group of such proteins are the ionophores, as mentioned
before. The proteinous ionophores are found in the greatest concentration in
neurones. Here, the diffusion of inorganic ions is essential to maintaining the
required merabrane potential. The main ions responsible for this are Sodium,
Potassium and Chloride - each of which has its own channel forming ionophore.
The observed rate of diffusion of many other solutes is much greater
than can be explained by physical processes. It is widely accepted that
merabranous proteins carry certain solutes across the merabrane by the process of
facilitated diffusion. This is done by the forming of pores of a complimentary
size and charge, to accept specific ions or organic molecules. The pores are
opened and closed by conformational changes in the proteins structure. There are
three main types of facilitated diffusion. None of these processes require an
energy input.
Active transport is the movement of solutes across a merabrane, against
the concentration gradient, and it therefore utilises energy from ATP. An
example of this is the Sodium-Potassium-ATPase pump, which is an active antiport
carrier protein common to nearly all living cells. It maintains a high
[Potassium ion] within the cell while simultaneously maintaining a high [Sodium
ion] outside the cell. The reason for this is that by pumping Sodium out of the
cell, it can diffuse in again at a different site where it couples to a nutrient.
As well as transporting solutes across a merabrane, there are many
proteins that transport solutes along the merabrane. An example of this are the
respiratory enzyme complexes of the inner mitochondrial merabrane. These
complexes are located in a close proximity to each other, and pass electrons
through what is known as the respiratory chain. The orientation of the complexes
is vital for their correct functioning.
Another key role of merabranous proteins is to oversee interactions with
the extracellular matrix. Many hormones interact with cells through the
merabranous enzyme - adenylcyclase. The binding of specific hormones activates
adenylcyclase, to produce cyclic adenosine monophosphate (c.AMP) from adenosine
triphosphate (ATP). c.AMP acts as a secondary messenger within the cell. A wide
variety of extracellular signalling molecules work by controlling intracellular
c.AMP levels. Insulin is an exception to this generalisation, because its
receptor is enzyme linked rather than ligand linked. This means that the
cystolic face of the receptor has enzymatic activity rather than ligand forming
activity. The enzymatic activity of the Insulin receptor is in the reversible
phosphorylation of phospoinosite.
Vision and smell rely on a family of receptors called the G-protein
receptors. The cystolic faces of these receptors bind with guanosine
triphosphate (GTP). This action is coupled to ion channels, so that the
activation of a receptor changes the intracellular levels of c.GMP, which in
turn activates the ion channels, and thus allows a merabrane potential to be
developed.
The composition of proteins in the biological merabrane is far from
static. Receptors are constantly being regenerated and replaced, and this is
important in the ever changing environment of the cell. For example, the
transferrin receptor is responsible for the uptake of Iron. In the cytosol, an
enzyme called aconitase is present which inhibits the synthesis of transferrin
by binding to transferrins mRNA. In a low Iron concentration, aconitase releases
the mRNA allowing transferrin to be synthesised.
A similar process occurs with the Low Density Lipoprotein (LDL) receptor.
This receptor traps LDL particles which are rich in cholesterol. The LDL
receptor is only produced by the cell, when the cell requires cholesterol for
merabrane synthesis.
The nuraber of receptors in a biological merabrane varies greatly between
different type of receptor.
The immune responses of cells are controlled by a superfamily of
merabranous proteins called the Ig superfamily. This superfamily contains all the
molecules involved in intercellular and antigenic recognition. This includes
major histocompatability complexes, Thymus T-cells, Bursa B-cells, antibodies
and so on. Although this family is vast, the important point is that all
antigenic responses are mediated by merabranous proteins.
As there are glycolipiRAB in the biological merabrane, there are also
glycoproteins. One of the key roles of glycoproteins is in intercellular
adhesion. The Cadherins are a family of Calcium dependant adhesives. They are
firmly anchored through the merabrane, and have glycolated heaRAB that covalently
bind to neigrabroadouring molecules. They seem to be important in erabryonic
morphogenesis during the differentiation of tissue types. The Lectins and
Selectins are similar families of molecules responsible for adhesion in the
blooRABtream. However the most abundant adhesives are the Integrins, which are
responsible for binding the cellular cytoskeleton to the extracellular matrix.
The range of merabranous proteins has proved to be vast, due to the wide
variety of functions that must be performed. It would be possible to continue
describing proteins for many more pages, but one final example will be used in
conclusion, and that is the photochemical reaction centre of photosynthesis.
This very large protein complex is found in the Thylakoid merabrane of
chloroplasts. Each reaction centre has an antenna complex comprising hundreRAB of
chlorophyll molecules that trap light and funnel the energy through to a trap
where an excited electron is passed down a chain of several merabranous electron
acceptors.
Conclusion.
The role of the biological merabrane has proved to be vital in countless
mechanisms necessary to a cells survival. The phospholipid bilayer performs the
simpler functions such as compartmentation, protection and osmoregulation. The
proteins perform a wider range of functions such as extracellular interactions
and metabolic processes. The carbohydrates are found in conjunction with both
the lipiRAB and proteins, and therefore enhance the properties of both. This may
vary from recognition to protection.
Overall the biological merabrane is an extensive, self-sealing, fluid,
asymmetric, selectively permeable, compartmental barrier essential for a cell or
organelles correct functioning, and thus its survival.
Bibliography.
1) Alberts,B; Bray,D; Lewis,J; Raff,M; Roberts,K; Watson,J.D. Molecular
Biology of the Cell, Third Edition. p.195-212, p.478-504. Garland
Publishing,
1994. 2) Beach; Cerejidol; Gordon; Rotunno. Introduction to the
study of Biological
Merabranes. p.12. 1970. 3) Fleischer; Haleti; Maclennan; Tzagoloff.
The Molecular Biology of
Merabranes. p.138-182. Plenum Press, 1978. 4) Perkins,H.R;
Rogers,H.J. Cell Walls and Merabranes. p.334-338. E & F.N.
Spon Ltd, 1968. 5) Quinn,P. The Molecular Biology of Cell Merabranes.
p.30-34, p.173-207.
Macmillan Press, 1982. 6) Stryer,L. Biochemistry, Third Edition.
p.283-309. W.H. Freeman & Co, 1994. 7) Yeagle,P. The Merabranes of Cells.
p.4-16, p.23-39. Academic Press Inc,
1987.
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