In this article we will discuss about Structure of Membrane:- 1. Structure of Membrane in Prokaryotes Cell 2. Structure of Membrane in Eukaryotic Cell .
Structure of Membrane in Prokaryotes Cell:
Most prokaryotic cells have only the plasma membrane. There are no organelles in the prokaryotic cells, i.e., they have no internal membrane systems. In some bacteria, there Eire some infoldings of the plasma membrane, called Mesosomes.
Some extensive infoldings of the plasma membrane forming thylakoid vesicle have been noted in the electron micrographs of a thin section through cells of blue-green algae (Nostoc sp.).
In majority of the bacteria, the cell layers can be distinguished into an:
a. Outermost gelatinous layer (capsule),
b. A cell wall, and
c. A periplasm layer.
a. Capsule Layer:
This layer is found in some strains of bacteria. Sometimes this layer is tightly associated with the cell wall and, in some cases, it is loosely attached, then it is called Slime layer. Bacteria having a capsule show glistening appearance in a culture plate. This layer is made of polysaccharides containing sugars, glucose, galactose, mannose, fucose etc.
This layer also contains polypeptides containing amino acids of D-configuration rather than usual L-configuration. The function of the capsule layer is found to be related with its ability to cause disease. As for example, Pneumococcus can cause disease, Pneumonia, only when the capsule is present.
b. Bacterial Cell Wall:
On the basis of the reaction to stain by Gram staining technique, bacterial cell wall and even the bacteria Eire divided into two classes— Gram-negative and Gram-positive bacteria. In this technique, developed by Hans Christian Gram, bacteria are stained with Crystal Violet followed by treatment with alcohol.
Cells which retain the stain after alcohol-treatment are known as Gram-positive, while cells which cannot retain the dye after alcoholic treatment are known as Gram-negative bacteria.
In Gram-positive bacteria, the cell wall (Fig. 2.11) is (30-100 nm) thick outside the plasma membrane. The cell wall consists of peptidoglycan which is a polysaccharide- peptide complex. The polysaccharide complex of the adjacent chains are joined together by peptide bridges containing different kinds of amino acids like D and L-alanine, D-glutamic acid, L-lysine, L-serine, glycine etc.
The cross-linking of polypeptide chains by peptide bridges produces an intertwined peptidoglycan network that gives great strength to the cell wall. This network is less extensive in gram-negative bacteria.
In case of gram-negative bacteria the cell wall (Peptidoglycan layer) is thinner than gram- positive ones (3.8 nm). This difference in wall structure is responsible for the difference in staining behaviour of bacteria in the Gram technique.
The thin wall allows the removal of the dye molecules after alcoholic extraction. In this type of bacteria there is another layer composed of lipid bilayer associated with protein (Fig. 2.12). This layer is known as outer membrane.
The composition of proteins and lipid in this outer membrane is different from the plasma membrane and contains lipopolysaccharides which are also an amphipathic molecules. Actually, these lipopolysaccharides give some inflammatory responses to the host, inducing some response to trigger some toxic substances, called Endotoxins.
The outer membrane is permeable to many chemicals of molecular weight of 1,000 or more. It is composed of certain proteins called Porins or Matrix porins which are present along the pores for helping in the movement of hydrophilic solutes across the outer membrane.
There is another class of protein, called Omp A protein, which helps in maintaining the structure of the membrane. The cell wall in both the types of bacteria gives rigidity to the cell to give a definite shape. It also helps to withstand the unfavourable conditions. But the cell walls can be removed by treating bacteria with a special type of enzyme called Lysozyme.
The bacterial cell without any cell wall is called protoplasts or sphaeroplasts, these can be used for several biotechnological applications.
In nature, wall-less bacteria is also found which is known as Mycoplasmas, where membranes are strong enough to remain in the condition in which the concentrations of solutes in the external environment is low compared to that inside the cell. Cells of Mycoplasmas can swell to a large size without bursting.
c. Periplasm:
The periplasmic space is present both in the Gram-positive and Gram-negative bacteria, but the space is broad in gram-negative organisms. After disruption of the bacterial cell wall a specific group of protein is found to be released from this space.
The presence of broad periplasmic space in gram-negative organisms is to serve as a permeability barrier to prevent the loss of solute from the cell as the cell wall is thin here. The enzymes located in the periplasmic space are alkaline phosphatase 5′-nucleotides, phosphodiesterase,
ribonuclease and deooxyribonuclease. In gram-positive bacteria, some of these enzymes are present on the wall surface. All these enzymes present in the periplasmic space or on the wall surface help in degrading large macromolecules to smaller ones so that they can pass through the membrane. This function is similar to that of enzymes present in Lysosome in eukaryotic cells.
Structure of Membrane in Eukaryotic Cell :
In case of eukaryotic cells, rigid cell wall is present only in plant cells. But this type of rigid wall is absent in eukaryotic animals. Again, the composition of cell wall in eukaryotic plant cells is different from bacteria.
i. Plant Cell Wall:
The rigid wall of the plant cell is developed by the deposition of the secreted substances from the protoplast external to its plasma membrane (Esau, 1977). The cell wall gives the mechanical properties of mature tissues and organs.
In vascular plant tissues, the protoplast dies out after a certain stage of development and the cell walls that are left remain functional giving mechanical strength, and the xylem tube for the transport of water and nutrients.
In living tissues, cell walls remain thin in some cells (parenchymatous cells), or differential thickening is found as in collenchyma. In living and growing cells, cell wall is extensible and fluid like but it is rigid in non-growing cells.
The most important chemical composition of the cell wall is cellulose which is responsible for giving the fibrous infrastructure of cell walls. The mechanical behaviour of cell is due to the presence of polymer, a cellulose which has higher tensile moduli and greater tensile strength.
Cell walls are formed through apposition, i.e., the wall material is deposited by the protoplast on the plasma membrane. The first cell wall (Primary wall) is formed during the cell growth phase. When the cell elongation is stopped the secondary cell wall, formation starts (Fig. 2.13).
The first formed layer, i.e., Primary cell wall remains farthest from the protoplast and is in contact with the pectinaceous layer, the middle lamella, which adjoins the neighbouring cell. Along with the secondary wall formation, the middle lamella, primary and secondary walls are impregnated with new compounds like Lignin.
The physical properties of cell walls depend mainly on the chemistry of cellulose. Again, the stability of cellulose is due to the β 1-4 linkage between its monomeric units, glucose. Cellulose is a long-chained molecule and molecules of glucose are rotated alternately at 180°.
In natural condition, cellulose is present in the cell wall as extended chain configuration while it takes folded chain configuration when cellulose is extracted from ceil walls. In addition to cellulose there is another important constituent of cell wall which is known as hemicelluloses.
Hemicelluloses are a group of polysaccharides containing some acidic groups such as D-glucuronic, D-galactouronic residues or the methyl esters of the glucouronic acid. The amount of hemicellulose varies from plant to plant and even among cells of various organs. This long chain cellulose molecule constitutes the fibrous infrastructure of the cell wall.
The cell wall matrix may play an important role in controlling the mechanical behaviour of plant cell walls. This microscopic and macroscopic infrastructure of the cell wall is a direct physical manifestation of the developmental changes during cell morphogenesis.
Another compound that is present in the cell wall is pectin which is a polymer of the carbohydrate galactouronic acid and its derivatives. Lignins are polymerised aromatic alcohols which occur in the secondary wall of woody tissues. Besides this, cell wall also contains some proteins, lipids and minerals.
In higher plants there is a special type of protein called extension which is almost similar to collagen of animal tissue. This protein is covalently attached to cellulose micro fibrils and form a protein-polysaccharide complex.
In the external surface of the wall, waxes and other complex polymers are present—cuticle. Certain minerals such as calcium and magnesium in the form of carbonate or silicate are present in the cell wall. Generally, the cellulose and other polysaccharides of cell wall materials are synthesised in the Golgi complex.
This is then transported to the cell surface through several vesicles. After deposition of these compounds on the cell surface, the organisation of these polysaccharides into chains or micro fibrils takes place.
Fibrillar Structure of Cell Walls:
Cellulose molecules cluster together to form a rod-like structure termed micelles which are stabilized by Hydrogen bonds between the cellulose molecules. Micelles are aggregated together to form micro fibrils of about 10 nm diameter and several micrometres in length.
About 10 micro fibrils are packed together to form each macro fibril of about 50 nm diameter. This type of network, i.e., orientation of cellulose micro fibrils, is generally found in the secondary wall in densely packed manner whereas, in the primary wall, cellulose fibrils are loosely arranged.
Macro fibrils are visible in light microscope which can measure in the order of 100 to 250 nm in width. Micro fibrils are visible in the transmission electron microscope and show measurements of 3.5-8.5 nm in width. The micro fibrils are found within a matrix of glycoproteins, hemicelluloses and acidic pectin’s. Each micro fibril consists of a number of cellulose molecules.
ii. Animal Cell:
Animal cells, generally, do not contain any rigid wall but sometimes there is a coating of polysaccharide which is analogous to a cell wall. This polysaccharide layer, over the plasma membrane is called Glycocalyx. Depending on the attachment of Glycocalyx on cell surface, this layer is divided into two kinds—Attached Glycocalyx and Unattached Glycocalyx.
The former is attached with the plasma membrane in such a way that this layer cannot be separated from the plasma membrane. In electron micrographs this layer looks like a layer of filamentous material consisting of glycoproteins and glycolipids.
The second type, i.e., unattached Glycocalyx layer is located external to the membrane which can be easily removed from the cell surface. This layer is present in the most animal eggs, over the cell of Amoeba, and the Sarcolemma of muscle fibres. The function of these layers is to give protection to the cell surface, to help in recognising the cell, in cell adhesion and, lastly, to form permeability barriers.
Cell surface of certain animal cells secretes some substances which accumulate in the spaces between cells. These substances are called Extracellular matrix. This matrix is found in connective tissues like bone, tendon, cartilage and some cells of the skin.
The main components of the matrix are collages, proteoglycans and non-collagenous glycoproteins. Depending on these contents, the matrix may be gelatinous, watery, elastic, or rigid.
a. Collagens:
Collagens are a type of protein present in vertebrates. The most common amino acids present in collagens are glycine, hydroxyproline and hydroxylysine. Structurally, it consists of three intertwined polypeptide chains, termed a- chains, to form a triple-helical structure called Pro-collagen. The pro-collagen is formed first in the lumen of the endoplasmic reticulum.
Pro-collagen is then transported out of the cell into the extracellular space where it is converted into collagen by a specific enzyme. The synthesised collagen is then aggregated to form collagen fibrils and, finally, collagen fibres (Fig. 2.14) which have a specific property of having cress-striations.
b. Proteoglycans:
These are the special type of glycoproteins with a high content of carbohydrate having repeated disaccharides termed as Glycosaminoglycan’s or Mucopolysaccharides. Each carbohydrate unit contains one amino sugar and at least one acid group such as hyaluronic acid, chondroitin sulphate, keratin sulphate and heparin.
The properties of the extracellular matrix depends on the proportions of collagen and proteoglycans. The proteoglycans form a gelatinous matrix by absorbing water and solutes. Collagen fibres are embedded within the gelatinous matrix. Cartilage contains high content of proteoglycans and so it is comparatively soft. Tendon is hard due to the presence of collagen fibres in large number.
Experiments to Support Fluid-Mosaic Model of Membrane:
In this model, the concept is that proteins float freely in the lipid layer of the membrane structure. Two different cells, a mouse fibroblast and a human fibroblast, were fused. The specific antibody was prepared with a fluorescent marker for the mouse H-2 antigens.
Just after fusion the mouse antigens are present in one area of the cell. After a few hours most of the H-2 proteins or antigens were diffused freely throughout the cell showing that H-2 proteins of the membrane were not rigidly held in one place. This experiment confirmed the view that many membrane proteins are free to move in the lipid layer of the membrane and so the fluid- mosaic model holds good.
In another experiment using the vesicle of the mitochondrial inner membrane, distribution of proteins were observed on the membrane through Freeze fracture technique and electron microscopy.
When the mitochondria were subjected to a strong electric field, all the protein particles were grouped at one end showing the movement of proteins under the influence of electric field. Again, this movement is similar to the diffusion of many other proteins in a fluid- mosaic membrane.