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Sub cellular Organelles
The cell is enveloped and thus separated from its surroundings by a thin wall contains a rigid framework of polysaccharide chains crosslinked with short peptide chains. Its outer surface is coated with lipopolysaccharide. Cell membrane is also called as plasma membrane (or) plasma lemma. The pili, not found in all bacteria have extensions of the cell wall. The cell membrane contains about 45% lipid and 55% protein. The cell membrane or plasma membrane have an average thickness of 75A°. The principal lipids are phospholipids, sphingolipids and cholesterol. An important feature of these lipids is they are composed of hydrophobic (water - insoluble) hydrocarbon sections and hydrophilic (water soluble) units. The latter include charged units (eg. phosphate or amino groups) and uncharged units (eg. hexose). In water, such compounds orient themselves in such a way that only the hydrophilic section is exposed to water. The hydrophobic components of individual molecules tend to contact with other; this is accomplished either by arrangement into micelles or by the formation of bilayers.
Two types of poteins are found in cell membrane; viz. intrinsic or integral and extrinsic or peripheral (Fig. 1.3).Integral proteins are either partially or totally immersed in the lipid bilayer and difficult to remove by any means other than the distruption of the membrane with a detergent. Peripheral proteins are bound only to the surface of membrane and interact only with the hydrophilic groups and therfore are readily removed by extraction with an aqueous medium. The model of arrangement of lipids and proteins in the memberane is known as fluid mosaic model.
· The cell membrane performs a number of important functions It holds the cell together
· The membrane is a selectively permeable boundary which allows water, certain required nutrients and metal ions to pass freely It secrete waste products
· It keeps out toxic materials
· It contain receptors to bind certain regulatory substances such as hormones which regulate the various metabolic activities.
Plant and bacterial cell membranes are surrounded by a thick cell wall.
The bacterial cell is enclosed within a wall that differs chemically from the cell wall of plants. The cell wall contains a rigid framework of polysaccharide chain cross linked with short peptide chains and its outer surface is coated with lipopolysaccharide. The pili, found in some bacteria are extensions of the cell wall. In some bacteria the cell wall is surrouned by an additional structure called a capsule.
The cell wall and capsule confer shape and form of the bacteriam and also act as a physical barrier to the cell membrane. In the absence of cell wall and capsule is mechanically fragile and the bacteria would rupture.
The cell wall is a thick polysaccharide containing structure immediately surrounding the plasma membrane. In multicelllar plants, the plasma membrane of neighboring cells are separated by these walls, and adjacent plant cell have their walls fused together by a layer called the middle lamella. The cell wall serves both as a protective and a supportive unit for the plant. The degree to which the cell wall may be involved in the regulation of the exchange of materials between the plant cell and its surroundings is difficult to assess but is most likely restricted to macromolecules of considerable size. As in animal cells,most of the regulation of exchanges between the cytoplasm and the extracellular surrounding of plant cells is a function of the plasma membrane
The cell wall protects bateria against swelling in hypotonic media. It is porous and allows most small molecules to pass. Some of the pili are hollow and serve to transfer DNA from sexual conjugation.
Nucleus is the heaviest particulate component of the cell. Except matured mammalian erythrocytes, nucleus is found in almost all cells. The nucleus about 4-6µm in diameter is surrounded by a perinuclear envelope. At various position the outer membrane of the envelope fuses with the inner membrane to form pores (Fig. 1.4). Nuclear pores provide continuity between the cytosol and the contents of the nucleus (nucleoplasm). The electron microscope reveals that the nuclear content consist of granular or fibrillar structures. The nucleolus, a discrete body within the nucleus, contains ribonucleic acids (RNA). The most important component of the nucleus is an organised clumps of threadwork known as chromatin which is distributed throughout the nucleus and contains most of the cellular deoxy ribonucleic acids (DNA). Immediately before the cell division the chromatin organises into simple thread like structures known as chromosomes which will eventually be distributed equally to each daughter cell.
Take part in cell division Contain DNA molecules which are heriditary carriers.
These are the largest particulate components of the cytoplasm and represent upto 15% -20% of the dry weight of the cell. They vary in shape (spherical, filamentous, sausage shaped) and size (0.5 to 3μ long 0.1 to 0.6μ wide).The number varies with the size and energy requirements of the cell. For eg. flight muscles in birds contain rich amount of mitochondria when compared to any other parts of the body
Electron microscopic studies show that a mitochondrion has two membranes inner and outer which are separated from each other by 50 to 100oA. The outer and inner membranes differ in lipid composition and in enzyme content.
The inner membrane is very much folded to form shelf - like structures of varying width. These shelf - like structures, known as internal ridgs or cristae, extent into matrix of the mitochondrion structure. Thus two structurally different space can be distinguished, the intracristae space and the matrix space (Fig. 1.5). The matrix space is rich in enzymes. The inner membrane shows the existance of knob like structures, which are proteins involved in biological oxidations.
Functions : The mitochondria are the ‘power houses’ of the cell, where carbohydrates, lipids and amino acids are oxidised to CO2 and H2O by molecular oxygen, and the energy set free is stored in the form of adenosine triphosphate (ATP). The enzymes involved in this energy conversion are located in the inner membrane.
The endoplasmic reticulum consist of flattened single membrane vesicles. These have the same lipid bilayer structure but thinner than the cell membrane (about 7mm). The endoplasmic reticulum is of two types; rough (RER) and smooth (SER). Only the rough type has small granules known as ribosomes (Fig. 1.6).
RER is concerned with protein synthesis while SER is concerned with lipids and glycoprotein synthesis. The cisternae (enclosed spaces) of the endoplasmic reticulum play a role in the exchange of material between the cell and the extra cellular fluid. The exchange of material takes place by the process of pinocytosis.
Golgi complex is a smooth membrane system consists of flattened, single membrane vesicles which are often stacked (Fig. 1.7).
These are organelles to which the newly synthesized proteins are transferred and temporarily stored. Small vesicles arise peripherally by a pinching - off process. Some become vacuoles in which secretory products are concentrated.
The outer membrane of the endoplasmic reticulum contain small granules commonly known as ribosomes, which are the smallest particulate components of the cytoplasm. They are rich in ribonucleic acids. Each ribosome has a large and a small subunit with a sedimentation constant of 50s and 30s respectively (Fig. 1.8). Each subunit contains about 65% RNA and 35% protein.
· Ribosomes are the sites of protein synthesis. Messenger RNA binds in the groove between the subunits and specifies the sequence of amino acids in the growing polypeptide chains. The proteins synthesized on membrane bound ribosomes must pass successively through each of cytomembrane system.
· Secretion may involve the fusion of the vacuoles with the plasma membrane followed by a discharge of the contents into the extra cellular space. This process is called exocytosis.
Lysosomes are single membrane vesicles, having intermediate size between microsomes and mitochondria. These are surrounded by a lipoprotein membrane (Fig.1.9). Lysosomes are rich in many hydrolytic enzymes such as phosphatase and ribonuclease and because of this, they are named as lysosomes (Lyso means lytic action).
The hydrolytic enzymes of lysosome completely destroy the foreign materials like pathogenic microorganism. They also serve to digest cell components after cell death. Inside the macrophages these lysosomes combine with vecuole which has engulfed the foreign particles and form phagolysosomes. Inside these phagolysosomes foreign particles are degraded or killed. The pathogen engulfed lysosomes are destroyed by the reticulo endothelial system. Due to this action lysosomes are called as ‘Suicidal Bags’.
Peroxisomes are otherwise known as microbodies. They are single - membrane vesicles of about 0.5 mm in diameter. They contain catalase, D-amino acid oxidase, urate oxidase and other oxidative enzymes.
Microbodies participate in the oxidation of certain nutrients. Hydrogen peroxide, the toxic reduction product of oxygen is decomposed to form water in these organelles.
The intracellular cell content that posses both soluble and insoluble constituents is called cytoplasm.
The soluble liquid portion of the cytoplasm is known as cytosol in which the organelles are bathed. Cytosol is also known as cell sap. Cell sap contains water, proteins, lipids and numerous other solutes and is highly viscous (Fig.1.10).
Some important metabolic processes occur in the cytosol are glycolysis, gluconeogenesis, activation of amino acids and biosynthesis of fatty acids.
Plant cells have cell wall made up of cellulose and the cytoplasm consists of big vacuoles and chloroplasts (Fig.1.11).
The ability to use light as a source of energy for sugar synthesis from water and carbon dioxide is a special feature of certain plant cells. This process, termed photosynthesis is carried out in organelles called chloroplasts. These organelles are commonly ellipsoidal structures bounded by an outer membrane but also containing a number of internal membranes. Internally, the chloroplast consists of a series of membranes arranged in parallel sheets called lamellae and supported in a homogeneous matrix called the stroma. The membranes are arranged as thin sacs called thylakoids that contain chlorophyll and may be stacked on top of one another, forming structures called grana. Lamellar membranes connecting the grana are called stroma lamellae (Fig.1.12).
Although vacuoles are present in both animal and plant cells, they are particularly large and abundant in plant cells, often occupying a major portion of the cell volume and forcing the remaining intracellular structures into a thin peripheral layer. These vacuoles are bound by a single membrane and are formed by the coalescence of smaller vacuoles during the plant’s growth and development. Vacuoles serve to expand the plant cell without diluting its cytoplasm and also function as sites for the storage of water and cell products or metabolic intermediates.
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