Medical brain notes 25

  LIVING CELL Introduction The cell is the basic structural and functional unit in all living organisms. Living forms vary in size but they are all made up of cells. In unicellular organisms, the cell is the organ. As an organism grow in complexity, the cells increase in number and similar cells aggregate into tissues and organs with specialized functions. The shape of the cell is often determined by its function and the size of the cell is determined by the optimum dimensions that will enable it to perform its duties more effectively . The human body is composed of a multiple of specialized tissues which inturn consist of vast clusters of cells differentiated into specialised chemical factories which perform various biochemical reactions. Types of cells Two general types are recognised in nature. They are prokaryotic and eukaryotic cells. Prokaryotic cells Prokaryotes were the first cells to arise in biological evolution. They are very small and simple having only a single membrane. T

  Types of cells Two general types are recognised in nature. They are prokaryotic and eukaryotic cells. Prokaryotic cells Prokaryotes were the first cells to arise in biological evolution. They are very small and simple having only a single membrane. The cell membrane, is usually surrounded by a rigid cell wall. They are devoid of nucleus and membranous organelles such as mitochondria and endoplasmic reticulum etc. (Fig. 1.1). Eukaryotic cells Eukaryotic cells are presumably derived from prokaryotes. They are much larger and much more complex than prokaryotic cells (Fig. 1.2). They have nucleus and membrane bound subcellular organelles. Many of their metabolic reactions are segregated within structural compartments. The significant differences between prokaryotic and enkaryotic cells are: The eukaryotic cell structure is composed of (i) cell membrane, (ii) nucleus (iii) mitochondria (iv) endoplasmic reticulum (v) golgi apparatus (vi) ribosomes (vii) lysosomes and others. These speciali

  Sub cellular Organelles   Cell Membrane 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 in

  Cell Membrane 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

  Cell Wall Plant and bacterial cell membranes are surrounded by a thick cell wall. Bacterial 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. Plant cell wall 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

  Nucleus Nucleus is the heaviest particulate component of the cell. Except matured mammalian erythrocytes, nucleus is found in almost all cells. The nucleus about 4-6 µ m 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 eac

  Mitochondria 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 enzyme

  Endoplasmic reticulum 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). Functions 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 apparatus Golgi complex is a smooth membrane system consists of flattened, single membrane vesicles which are often stacked (Fig. 1.7). Functions 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.

  Ribosomes 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. Functions ·          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 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). Functions 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 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. Functions Microbodies participate in the oxidation of certain nutrients. Hydrogen peroxide, the toxic reduction product of oxygen is decomposed to form water in these organelles.

  Cytoplasm The intracellular cell content that posses both soluble and insoluble constituents is called cytoplasm. Cytosol 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). Functions Some important metabolic processes occur in the cytosol are glycolysis, gluconeogenesis, activation of amino acids and biosynthesis of fatty acids.