Introduction to Plasma Membrane
All cells and many subcellular organelles are bounded by thin membranes of phospholipid bilayer. With the help of light microscopy, it is not possible to identify the cell membrane or plasma membrane. Scientists were able to identify the membrane with the advent of the electron microscope.
It shows that every cell is surrounded by a membrane and also the cell has a complex internal membranous structure. Again, membranes make some compartments inside the cytoplasm to perform some specific functions as in mitochondria, chloroplasts, lysosomes etc.
Membrane-bound enzymes also perform certain specific reactions which are needed for certain cellular activities. Proteins present in the membrane help in the transport of certain molecules from inside and outside of the cell. Proteins also help in anchoring some cytoskeletal fibres to give the cell its shape. So, the membrane is a highly differentiated dynamic structure that controls the behaviour of the cell. It is the most multifunctional cellular structure
Important Functions of Plasma Membrane
- Interactions of series of enzymatic processes for performing several cellular events and in the production of chemical energy (ATP) by confining macromolecules in a small space.
- It acts as a receptor site for some agents like hormones, neurotransmitters, immune proteins.
- It helps in the conversion of signal conveyed by some extracellular agents as stated above.
- It prevents the loss of different macromolecules.
- It protects the cell from the uptake, of some harmful materials.
- The cell membrane interacts with other adjacent cells in forming tissues and organs during organogenesis and embryonic development.
Composition of Plasma Membrane
Plasma membranes or biological membranes are composed of lipids, proteins and small amounts of carbohydrate. The ratio of proteins to lipid varies considerably among different membranes (Table ). Phospholipids are present in almost all the membranes such as Phosphatidylcholine, Phosphatidylserine, Phosphatidylethanol-amine, Sphingomyelin.
Cholesterol is common in the membrane of mammalian cells. Cardiolipin is found only in the inner mitochondrial membrane. The plant plasma membrane has a high sterol to phospholipid molar ratio. Cholesterol and various sterol esters are found in the plant plasma membrane.
Carbohydrates are bound to the membrane in the form of glycoproteins when attached to proteins or glycolipids when attached to lipids. Carbohydrates are found in the membrane of eukaryotic cells. They are not present in the chloroplast lamellae, mitochondrial membrane and other membranes of cell organelles.
The major component of the plant plasma membrane is carbohydrate in the form of glycolipids, glycoproteins and various cell wall polysaccharides. Although the structure and function of the plant plasma membranes is fundamentally similar, but little work has been done on plant plasma membrane as compared to the animal system.
Hence the nature of lipids and proteins is not clearly known in plant system. The plant cell membrane has to perform some other functions than in animal cell, particularly in mediating the transport of solutes into and out of the cell.
Further, it has to perform in synthesizing the cell wall micro fibrils and to transmit hormonal and environmental signals during growth and differentiation. The knowledge of the membrane is based mainly on cells of prokaryotic and animal systems.
Morphology of the Plasma Membrane
An eukaryotic cell of 20 µm diameter has a membrane of less than 10 nm thickness, i.e., 1/2000 of the cell diameter. Before the invention of electron microscopy, cytologists observe, a very thin refractile outline of the cell through a light microscope.
Actually, the existence of the membrane was taken into account on the basis of experiment on cell osmosis in plant cells as early as 1877 by Pfeffer. At that time only the semipermeable properties of plasma membrane was known.
The structure and function of the plasma membrane is known with the refinement of the techniques used in the Transmission Electron Microscope (TEM). The knowledge of many aspects of the plasma membrane in plants is scanty as compared to the plasma membrane of animal and bacteria.
However, the basic structure and function of the plasma membrane is similar to that in animals, fungi and bacteria. The plasma membrane is composed primarily of proteins and lipids in all cases. The plasma membrane is seen as a thin wavy line around the surface of the protoplast under the electron microscope.
The tripartite structure of the plasma membrane with dark-light-dark structures can be seen at higher magnification. The lighter structure of the membrane is about 35A° thick while the two dark layers show thickness of 30- 35A° in each case.
Numerous small vesicles and cell organelles are also bounded by membranes. There are other cellular bodies which are invaginations of the plasma membrane. In plant cells, the plasma membrane has a continuity throughout the tissues by plasmodesmata.
Models of Plasma Membrane
Previously, membranes were thought to be a static structure functioning only to separate the cell from the external environment. It has now been established that membranes are involved almost in most of the cellular activities.
Thus, the knowledge of how the different components of the membranes are organised in the plasma membrane of different cell and cellular organelles is essential in understanding the mechanism of cell function.
We already know that membranes are composed of lipids, proteins and small amounts of carbohydrate. The chemical composition of the membrane is not constant for all cell types. There is considerable variation in the amount of proteins and lipids present in the membrane structure of different organisms.
The ratio of protein to lipid varies from 80 : 20 in bacteria to 20 : 80 in some nerve cells. But in most of the membranes the ratio is about 50 : 50. The lipid components of the membrane consists of phospholipids, glycolipids or steroids. Due to this diversity in membrane-composition, different ideas or models have been proposed to show the structure and organisation of membrane.
Lipid Monolayer Model of Langmuir
The first scientific attempt to know the structure of membrane was made by Langmuir (1881-1957) who suggested that the membrane was composed of phospholipids one molecule thick. It was shown by an experiment in which the phospholipid was spread on water. This formed a layer one molecule thick on water surface. Phospholipids are known to be amphipathic molecule which contains both hydrophilic and hydrophobic regions.
Langmuir interpreted from his model that the hydrophilic or ‘head’ groups of the lipid molecules remain attached to the water surface and the hydrophobic ‘tails’ remain free towards the air.
Lipid Bilayer Model of Gorter and Grendel (1925):
E Gorter and F Grendel proposed a lipid bilayer model (Fig.) of membrane structure from their experiments on Red Blood Cells. When lipids extracted from Red Blood Cells were spread on the water surface, it was found that lipids were also spread as one layer on water. But it covers twice the area on the water surface than that of the surface area of the cell from which the lipid is extracted.
From these observations they came to the following conclusions
- Lipids are present in the membrane as a bilayer.
- Hydrophilic head groups are towards the aqueous environments of the two membrane surfaces.
- Hydrophobic tails are away from the water and present in the interior of the membrane. iv. These types of structure of lipid bilayer would be most stable.
The model of Gorter and Grendel gives a new impetus to membrane research as they first tried to describe the structure of membrane at the molecular level.
The Danielli-Davson Model (1934)
Harvey and Danielli’s observations on surface tension experiment led doubt on the model of Gorter and Grendel. Their results showed that the surface tension of cell membranes was higher than that of pure lipids. Hence they concluded that biological membranes could not be of lipids alone.
Later, Danielli and Davson proposed a molecular model (Fig. 2.3) of the membrane in which hydrophilic head groups of the lipid molecule is covered on both sides by protein layer. The proteins are attached to the hydrophilic head groups of lipid bilayer by ionic bonds. But, in this model, the distance between ends of the fatty acid chains (hydrophobic tails) is not specified. Later, the observations made through polarised light and Xray diffraction on myelin membrane by Schmidt and others (1936, 1941) confirmed the existence of lipids as bilayer.
With the advent of electron microscopy, first visible structure of plasma membrane was noted. But the detailed analysis of membrane structure was not possible at that time as Osmium tetroxide was used as the only fixative in electron microscopy.
As Osmium tetroxide did not preserve membrane structure, only a single line was found on the cell surface. Later, Robertson (1964, 1966) used Permanganate as a fixative instead of Osmium tetroxide and showed the trilamellar structure of the biological membrane.
With the appearance of permanganate fixed membranes in all cell systems, a general idea has propounded that there is a basic identical general membrane structure in all cell forms. Again, it has been noted through electron microscope that there are two electron-dense lines separated by a lightly stained zone.
As the three layers of the membrane were observed, membranes were said to have a trilaminar arrangement. These trilaminar appearance of the membrane are found in prokaryotic and eukaryotic plasma membrane, endoplasmic reticulum, mitochondrial, chloroplast and nuclear membrane. The presence of common structure in almost all biological membranes led Robertson to postulate Unit membrane hypothesis.
For detailed study of the membrane structure and its molecular organisation, Robertson selected myelin as his experimental sample. He selected myelin rather than a typical membrane because, in case of myelin, multiple layers of membrane are present which forms a quasi- crystalline structure.
This structure facilitates the analysis by X-ray diffraction (Fig. 2.4). Thus, X-ray diffraction analysis can be correlated with electron microscopic observations.
Myelin surrounds the axons of certain nerve cells and is composed of multiple layers of membrane coming from Schwann cells (Fig. 2.5). This myelin is formed from the plasma membrane of Schwann cells that surrounds the axon of a neuron.
In the early stages of myelin formation, axons become surrounded by Schwann cells. The plasma membranes of the Schwann cells starts wrapping around the axon in a spiral manner. At a later stage of development, the plasma membranes of the Schwann cells becomes closely stacked upon each other and the cytoplasmic contents of the cell become disorganized from the space between the membranes.
This stack of closely packed membranes around the axon is called Myelin.
Robertson carried out investigations by electron microscope using different stains for lipids and proteins. He found that both lipids and proteins are present in the membrane. Lipids are present in two layers covered by protein with lipid head groups projecting outwards toward both membrane surfaces.
Robertson’s observation corroborates the structure proposed by Danielli and Davson. The appearance of the lipid bilayer with an electron microscope looks like a railway track. The electron-opaque stain binds to the polar head groups of phospholipid.
X-ray diffraction analysis of myelin membrane also show that the membrane consists of lipid bilayers with a layer of protein between them.
Thus, the electron microscopic observations and X-ray diffraction data confirmed the Danielli-Davson model of membrane structure. With the refinement of techniques, further research was carried out on membrane structure and this led to the view against Danielli-Davson-Robertson model.
The significant points against their view were the following:
- The point was raised against the use of myelin as a model for membrane structure. The myelin has no metabolic and enzymatic activities. The protein content in myelin is also low.
- Robertson showed the thickness of the membrane as 7-8 nm in electron micro-graphs. About 4-5 nm is for the thickness of lipid bilayer. The remaining 3 nm is not sufficient for the protein layer as most of the proteins are present as globular proteins. These globular proteins are too large to accommodate in this 3 nm thickness.
- Danielli-Davson stated in their model that membranes are held together by electrostatic attraction between the lipid layers and by the ionic side-chains in protein molecules. If it is so, then the membranes will be disrupted by increasing the ionic strength of the solution. But most of the proteins cannot be removed in this way. This observation suggests that the arrangement of proteins in the lipid bilayer is different.
- Electron micrographs—through freeze- etching—showed the presence of proteins in the interior of lipid layers, thus indicating that proteins may not be present as a continuous sheet.
- Studies on thermodynamic stability showed that both lipids and proteins should be sequestered from contact with water.
But Danielli-Davson model does not fulfill these criteria.
Fluid Mosaic Model
In this model (Fig. 2.6) also, the main component is the lipid bilayer with hydrophilic groups oriented toward outside and the hydrophilic groups toward inside of the layer. The basic requirement for the molecular organisation of the membrane is the free energy.
The term ‘fluid’ is given because the lipid layer is present in the fluid state. The transition of the lipid layer from non-fluid (gel) condition to a liquid-crystalline (fluid) state depends on the temperature of the cell. In most of the membranes, lipids remain in a liquid-crystalline (fluid) state.
The temperature, again, depends on the composition of lipids. An increased proportion of lipids with straight chain saturated fatty acids increases the transition temperature which changes the state of the membrane from the gel to the liquidcrystalline state.
According to this model, proposed by S.J. Singer and G Nicholson, the principle of membrane-organisation is as follows:
- Lipids are present in two layers.
- Proteins are arranged in two ways:
- Some jure embedded in the lipid layer, called Integral proteins, and
- Some are present on surface of the lipid bilayer, called the Peripheral proteins.
iii. The lipid layer is usually in liquid-crystal line, i.e., fluid state.
With the use of different sophisticated techniques, it has been established that lipid exists in the membrane as a bilayer. This has been further confirmed by comparing the properties of natural membrane with artificial membranes.
These artificial lipid layers can be made in two ways:
- By placing a drop of lipid in a small hole separating two compartments containing water, and
- By vibrating the suspension of lipid and water ultrasonically (Fig. 2.7).
The first method produces lipid bilayer membrane at one plane and, in the second method, an enclosed vesicle with lipid bilayer is found.
This enclosed vesicle is called Liposomes. Now the artificial membranes and natural membranes were analysed in electron-spin-resonance spectra and X-ray diffraction apparatus. The data showed similarity in the two membranes and the bilayer arrangement of lipid is confirmed. In majority of the membranes, phospholipids is the most predominant but in myelin and chloroplast glycolipids are predominant.
Again, various types of phospholipids are found in different membranes. But the lipids have one common property, i.e., they are amphipathic which means lipids have hydrophilic and hydrophobic portions in a single molecule. Most of the phospholipids are again phosphoglycerides, the structure of which is based on a glycerol backbone.
Different types of phosphoglycerides are present in membranes such as Phosphatidylcholine, Phosphatidyl-ethanolamine etc. Besides glycerol derivatives, there are other derivatives of phospholipid like derivatives of Sphingosine.
When the substitute is a carbohydrate, the molecule is known as Glycolipid. Cholesterol is also present in some membranes. The steroids sitosterol and stigma sterol are present in the plasma membrane of plant cells
The fatty acid chains present in the membranes also vary. In case of myelin membrane very few fatty acids are unsaturated (10%). Again, about 50% of the fatty acids are unsaturated in mitochondrial and chloroplast membranes. Some chains are also branched.
Although there are variations in fatty acid compositions in different types of membranes, one common point is that lipids are present in bilayer in all biological membranes.
On the basis of their orientation in lipid bilayer, membrane proteins are divided into two types:
Integral or intrinsic proteins, which are embedded in the lipid layer, and the Peripheral proteins which are attached to the membrane surface by weak ionic bonds.
The integral proteins are attached to the membrane by hydrophobic interactions with the tails or fatty acid chains of the lipid layer (Fig. 2.8). As a result of their hydrophobic interactions, the integral proteins are buried inside the membrane and are, thus, difficult to extract.
It can be extracted only with the help of a detergent, i.e., Sodium dodecyl sulphate. In addition to hydrophobic associations, integral proteins also possess hydrophilic amino acid residues which are exposed at the surface of the membrane. For this reason, interactions with ions, hormones, antigens can occur on the membrane surface.
Schematic representation of transmembrane proteins:
- a single transmembrane α-helix (bitopic membrane protein)
- a polytopic transmembrane α-helical protein
- a polytopic transmembrane β-sheet protein
Schematic representation of the different types of interaction between monotopic membrane proteins and the cell membrane:.
- interaction by an amphipathic α-helix parallel to the membrane plane (in-plane membrane helix)
- nteraction by a hydrophobic loop
- interaction by a covalently bound membrane lipid (lipidation)
- electrostatic or ionic interactions with membrane lipids (e.g. through a calcium ion)
In addition to the integral proteins, there are other class of proteins called Peripheral proteins which are attached to the membrane by weak ionic bonds. These proteins are not much involved in the architecture of membrane.
Peripheral proteins are bound to the hydrophilic head groups of the lipid or to the hydrophilic portions of the integral proteins protruding from the lipid layer. In this model, they showed that, although the peripheral proteins are present on the membrane surface they do not form a continuous layer as in Davson- Danielli model.
The distribution of integral and peripheral proteins in the fluid-mosaic model satisfies many of the problems raised against the previous model. As for example, the reasons for not able to extract proteins from the membrane can be easily explained as due to the embedding of the integral proteins deep into the lipid bilayer.
Again, the existence of the globular proteins in the membrane structure (a-helical structure) can be explained by this model. Freeze-etching micrographs of the membrane structure also correlated with this model.
Lastly, the fluid-mosaic model is thermodynamically stable, as the hydrophilic areas of the lipid and membrane proteins are exposed to the aqueous environment while the hydrophobic regions are away from the water surface.