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What Are The Major Components Of Biological Membranes And How Do They Contribute
To Membrane Function?. Essay, Research Paper

What Are The Major Components of Biological Membranes And How Do They Contribute

To Membrane Function?.

Summary.

The role of the biological membrane 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 lipids and proteins, and therefore enhance the properties of both. This may

vary from recognition to protection.

Overall the biological membrane 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 membranes surround all living cells, and may also be found

surrounding many of an eukaryotes organelles. The membrane is essential to the

survival of a cell due to its diverse range of functions. There are general

functions common to all membranes 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 membranes is remarkably similar.

All membranes 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 membrane structure was proposed by Danielli and

Davson in the late 1930s. They suggested that the plasma membrane consisted of a

lipid bilayer coated on both sides by protein. In 1960, Michael Robertson

proposed the Unit Membrane Hypothesis which suggests that all biological

membranes -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 membrane model, which they called the

Fluid Mosaic Model. This suggested that the lipid bilayer supplies the backbone

of the membrane, and proteins associated with the membrane 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

membrane. There are a variety of lipids found in membranes, the majority of

which are phospholipids. 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 membrane structure. The

significance of the hydrophobic forces between fatty acids is that the membrane

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 neighbouring

molecules, and this in turn increases the fluidity of the membrane. Other

important phospholipids include phospatidylserine and phosphatidylethanolamine,

the latter of which is found in bacteria.

The phosphate group of phospholipids acts as a polar head, but it is not

always the only polar group that can be present. Some plants contain

sulphonolipids in their membranes, and more commonly a carbohydrate may be

present to give a glycolipid. The main carbohydrate found in glycolipids is

galactose. Glycolipids tend to only be found on the outer face of the plasma

membrane where in animals they constitute about 5% of all lipid present. The

precise functions of glycolipids is still unclear, but suggestions include

protecting the membrane 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

glycolipids 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 leads to the characteristic diarrhoea of Cholera sufferers. Cells

lacking GM1 are not affected by the Cholera toxin.

Eukaryotes also contain sterols in their membranes, associated with

lipids. 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 membrane

as there are phospholipid molecules. Cholesterol orientates in such a way that

it significantly affects the fluidity of the membrane. In regions of high

cholesterol content, permeability is greatly restricted so that even the

smallest molecules can no longer cross the membrane. This is advantageous in

localised regions of membrane. Cholesterol also acts as a very efficient

cryoprotectant, preventing the lipid bilayer from crystallising in cold

conditions.

The biological membrane 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 lipids in a

bilayer, the individual molecules are able to diffuse laterally, and

occasionally across the membrane. This freedom of movement aids the process of

simple diffusion, which is the only way that small molecules can cross the

membrane without the aid of proteins. The limit of permeability of the membrane

to the diffusion of small solutes is selectively controlled by the distribution

of cholesterol.

Another role of lipids is their to dissolve proteins and enzymes that

would otherwise be insoluble. When an enzyme becomes partially embedded 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 membranous 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 steroids, thyroids and retinoids which are all small enough

to pass directly through the membrane.

Ionophores are another family of compounds often found embedded in the

plasma membrane. Although some are proteinous, the majority are polyaromatic

hydrocarbons, or hydrocarbons with a net ring structure. Their presence in the

membrane 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 membranes are also a lot simpler in

composition than the plasma membrane. Mitochondria, the endoplasmic reticulum,

and the nucleus do not contain any glycolipids. The nuclear membrane is distinct

in the fact that over 60% of its lipid is phospatidylcholine, whereas in the

plasma membrane the figure is nearer 35%.

The protein component.

All biological membranes 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 membrane 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 membranes

associated by polar interactions. Class 2.

Partially Anchored These proteins have a short hydrophobic anchor

chain that cannot completely span the membrane.

Class 3. Integral (1) These proteins have one anchor chain that spans

the membrane. Class 4. Integral (5)

These proteins have five anchor chains that span

the membrane. Class 5. Lipid Anchored

These proteins undergo substitution with the

carbohydrate groups of glycolipids, 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 membrane with different numbers

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 membrane have regions of

hydrophobic amino acids 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 membrane are never symmetrical

in their structure. The outer face of the plasma membrane at least always has

the bulk of the proteins structure. It is usually rich in disulphide bonds,

oligasaccharides, and when relevant, prosthetic groups.

The proteins found in biological membranes all have distinctive

functions, such that the overall function of a cell or organelle may depend on

the proteins present. Also, different membranes within a cell, (i.e. those

membranes surrounding organelles) can be recognised solely on the presence of

membranous marker proteins.

In the majority of cases membranous 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 membrane 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

membranous proteins carry certain solutes across the membrane 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 membrane, 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 membrane, there are many

proteins that transport solutes along the membrane. An example of this are the

respiratory enzyme complexes of the inner mitochondrial membrane. 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 membranous proteins is to oversee interactions with

the extracellular matrix. Many hormones interact with cells through the

membranous 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 membrane potential to be

developed.

The composition of proteins in the biological membrane 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

membrane synthesis.

The number of receptors in a biological membrane varies greatly between

different type of receptor.

The immune responses of cells are controlled by a superfamily of

membranous 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 membranous proteins.

As there are glycolipids in the biological membrane, 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 membrane, and have glycolated heads that covalently

bind to neighbouring molecules. They seem to be important in embryonic

morphogenesis during the differentiation of tissue types. The Lectins and

Selectins are similar families of molecules responsible for adhesion in the

bloodstream. However the most abundant adhesives are the Integrins, which are

responsible for binding the cellular cytoskeleton to the extracellular matrix.

The range of membranous 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 membrane of

chloroplasts. Each reaction centre has an antenna complex comprising hundreds 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 membranous electron

acceptors.

Conclusion.

The role of the biological membrane 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 lipids and proteins, and therefore enhance the properties of both. This may

vary from recognition to protection.

Overall the biological membrane 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

Membranes. p.12. 1970. 3) Fleischer; Haleti; Maclennan; Tzagoloff.

The Molecular Biology of

Membranes. p.138-182. Plenum Press, 1978. 4) Perkins,H.R;

Rogers,H.J. Cell Walls and Membranes. p.334-338. E & F.N.

Spon Ltd, 1968. 5) Quinn,P. The Molecular Biology of Cell Membranes.

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 Membranes of Cells.

p.4-16, p.23-39. Academic Press Inc,

1987.


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