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Structure & function of cell
1.
©Goutam Mallik, Assistant
Professor, Pharmacology Page 1 CHAPTER – 2 CELLULAR LEVEL OF ORGANISATION Structure and Functions of cell The smallest unit of protoplasm, capable of carrying out independent existence, The word cell (L. cella-a storeroom, a chamber) was first Introduced in the biology by Robert Hooke. CELL The cell is the structural and functional unit of the living matter and is capable of carrying on the processes of life independently. The cells are not of same size and shape only due to the presence of differentiation of functions. But there are certain structural characteristic features which are common to them all. The structure of the cell is consisting of following three important parts.: 1. Cell membrane 2. Cytoplasm 3. Nucleus CELL MEMBRANE Cell membrane is a protective sheath, enveloping the cell body. It is also known as plasma membrane or plasmalemma. Cell membrane is composed of three types of substances: 1. Proteins (55%) 2. Lipids (40%) 3. Carbohydrates (5%) STRUCTURE OF CELL MEMBRANE All membranes are composed of lipids (mainly phospholipids, cholesterol and glycolipids) and proteins, in approximately equal ratios. Plasma membrane lipids form a lipid bilayer, a layer two molecules thick. The hydrophobic ends of each lipid molecule face the interior of the membrane and the hydrophilic ends face outwards. Most proteins are embedded within, or float in, the lipid bilayer as a fluid mosaic. Some proteins, because of extensive hydrophobic regions of their polypeptide chains, span the entire width of the membrane (transmembrane proteins), whereas others are only superficially attached to the bilayer by lipid groups.
2.
©Goutam Mallik, Assistant
Professor, Pharmacology Page 2 Both are integral (intrinsic) membrane proteins, as distinct from peripheral (extrinsic) membrane proteins, which are membrane-bound only through their association with other proteins. Carbohydrates in the form of oligosaccharides and polysaccharides are bound either to proteins (glycoproteins) or to lipids (glycolipids), and project mainly into the extracellular domain. Fig: Structure of cell membrane as seen under electron microscope (diagrammatic representation) Fig: The molecular organization of the plasma membrane According to the fluid mosaic model of membrane structure. Intrinsic or integral membrane proteins include diffusion or transport channel complexes, receptor proteins and adhesion molecules. These may span the thickness of the membrane (transmembrane proteins) and can have both extracellular and cytoplasmic domains. Transmembrane proteins have hydrophobic zones, which cross the phospholipid bilayer and allow the protein to ‘float’ in the plane of the membrane.
3.
©Goutam Mallik, Assistant
Professor, Pharmacology Page 3 Some proteins are restricted in their freedom of movement where their cytoplasmic domains are tied to the cytoskeleton. Properties of Membrane Carbohydrates—The Cell “Glycocalyx.” The “glyco” portions of these molecules almost invariably protrude to the outside of the cell, hanging outward from the cell surface. Many other carbohydrate compounds, called proteoglycans—which are mainly carbohydrate substances bound to small protein cores—are loosely attached to the outer surface of the cell as well. Thus, the entire outside surface of the cell often has a loose carbohydrate coat called the glycocalyx. The carbohydrate segments attached to the outer surface of the cell have several important functions: 1. Many of them have a negative electrical charge, which gives most cells an overall negative surface charge that repels other negatively charged objects. 2. The glycocalyx of some cells attaches to the glycocalyx of other cells, thus attaching cells to one another. 3. Many of the carbohydrates act as receptor substances for binding hormones, such as insulin; when bound, this combination activates attached internal proteins that, in turn, activate a cascade of intracellular enzymes. 4. Some carbohydrate fragments enter into immune reactions. Properties of Lipid Layers of the Cell Membrane The characteristic feature of lipid layer is that, it is fluid in nature and not a solid structure. So, the portions of the membrane move from one point to another point along the surface of the cell. Major lipids are: 1. Phospholipids 2. Cholesterol. Phospholipids are mainly Amino-phospholipids, sphingomyelins, phosphatidylcholine, phosphatidyl-ethylamine, phosphatidylglycerol, phosphatidylserine and phosphatidyl- inositol. Cholesterol molecules are arranged in between the phospholipid molecules. Phospholipids are soft and oily structures and cholesterol helps to ‘pack’ the phospholipids in the membrane. So, cholesterol is responsible for the structural integrity of lipid layer of the cell membrane. Properties Protein Layers of the Cell Membrane Protein layers give protection to the central lipid layer. The protein substances present in these layers are mostly glycoproteins. Protein molecules are classified into two categories: 1. Integral proteins or transmembrane proteins (intrinsic proteins).
4.
©Goutam Mallik, Assistant
Professor, Pharmacology Page 4 2. Peripheral proteins or peripheral membrane proteins(extrinsic proteins). Integral proteins provide the structural integrity of the cell membrane. Channel proteins help in the diffusion of water-soluble substances like glucose and electrolytes. Carrier or transport proteins help in the transport of substances across the cell membrane by means of active or passive transport. Pump: Some carrier proteins act as pumps, by which ions are transported actively across the cell membrane. Receptor proteins serve as the receptor sites for hormones and neurotransmitters. Enzymes: Some of the protein molecules form the enzymes and control chemical (metabolic) reactions within the cell membrane. Antigens: Some proteins act as antigens and induce the process of antibody formation. Cell adhesion molecules or the integral proteins are responsible for attachment of cells to their neighbours or to basal lamina. Function of Plasma Membrane Transport: It facilitates the transport of materials across it. It is selectively permeable to certain substance and helps transports of substances needed for survival. The various transport mechanisms are: A. Diffusion ( oxygen, carbon dioxide, small molecules, etc.) and passive osmosis (water). B. Transmembrane protein channels and transporters Aquaporins for water transport, ion channels for sodium and potassium transport, etc. C. Endocytosis: It is a process by which cell absorbs molecules by engulfing them: Example: Pinocytosis: Small vesicles, by a process of pinocytosis (drinking by cells), encircle and carry fluid within it across the membrane. By the pinocytic process fluid of smaller molecules (0.01-2.0 µm) can be engulfed in. Phagocytosis: The phagocytosis (eating by cells) is similar to engulfing of solid materials by the amoeba. D. Exocytosis in which cells removed undigested products brought in by endocytosis, or to secrete enzymes and hormones or to excrete substances outside the cell. Helps in the protection of cell. It surrounds cytoplasm of cell and forms a physical barrier between intracellular component and extracellular compartment. It anchors to the cytoskeleton to the extracellular matrix and thereby provide shape to the cell and maintains its structural integrity. Receives stimuli from the outside. The protein component of cell membrane acts as ligand receptors. The cell membrane contains receptor site for some hormones, immune proteins and neurotransmitters thus the cell recognizes and process these signals. Takes in food and excretes waste products.
5.
©Goutam Mallik, Assistant
Professor, Pharmacology Page 5 They aid in cell recognition (identifiers). Example: Glycoproteins (e.g., major histocompatibility complex, ABO blood group antigens). The surface protein markers which are embedded in the cell identify the cells, thus helps neighbouring cell to communicate with each other. The proteins in cell membrane act as enzymes and catalyse reactions and thus involved in metabolic process. Fig: Different phases of pinocytosis; the particle in solution approaching the cell surface (a and b) and gradually being drunk (c to f) Cytoplasm The cytoplasm is the protoplasm, which surrounds the nucleus and is bounded peripherally by the cell membrane. Cytoplasm may be homogeneous, vacuolated, granular, reticular or fibrillar. The cytoplasm can be classified into two groups. 1. Cytoplasmic organelles 2. Cytoplasmic inclusions
6.
©Goutam Mallik, Assistant
Professor, Pharmacology Page 6 Fig: Structure of Human cell under Light Microscope Fig: 3D structure of Human Cell
7.
©Goutam Mallik, Assistant
Professor, Pharmacology Page 7 Endoplasmic Reticulum These consist of network of canals (tubules) and vesicles (cisternae). These are three-dimensional and bounded by membrane of about 80 A in thickness. The elements of the endoplasmic reticulum may connect intermittently with the plasma membrane at one hand and on the other hand with the outer nuclear membrane. Two types of endoplasmic reticulum have been recognized: 1. Rough-surfaced endoplasmic reticulum: This reticulum is studded with osmiophilic granules-the ribosomes lying in rows in contact with the membranes of the endoplasmic reticulum . The roughness of the membrane is due to the presence of these granules- Palade granules. 2. Smooth-surfaced endoplasmic reticulum: This type of endoplasmic reticulum does not possess osmiophilic granules-the ribosomes at the outer border of the membrane. This is why it is smooth. (A) (B) Fig: (A) Endoplasmic reticulum (B) Structure of the endoplasmic reticulum. (Modified from DeRobertis EDP, Saez FA, DeRobertis EMF: Cell Biology, 6th ed. Philadelphia: WB Saunders, 1975.)
8.
©Goutam Mallik, Assistant
Professor, Pharmacology Page 8 Fig: A typical Golgi apparatus and its relationship to the endoplasmic reticulum (ER) and the nucleus. Function of Endoplasmic Reticulum 1. As the smooth-surfaced endoplasmic reticulum is very abundant in the interstitial (Leydig) cells of the testis and in cells of the corpus luteum, this reticulum is concerned with the synthesis of steroid hormones. 2. In the parietal cells of the gastric mucosa, it is concerned with secretion of hydrochloric acid. 3. In the skeletal muscle, it (sarcoplasmic reticulum) is concerned in some way with binding of the Ca++ ions and also plays role in conducting impulses in the substances of muscle cells. 4. In the liver cells both types of reticula are concerned with the synthesis of protein and carbohydrate. 5. Smooth endoplasmic reticulum is also concerned with catabolism and detoxification of toxic substances like some drugs and carcinogens (cancer-producing substances) in the liver. 6. Outer surface of smooth endoplasmic reticulum contains many enzymes which are involved in various metabolic processes of the cell. Golgi Apparatus Golgi apparatus or Golgi body or Golgi complex is a membrane-bound organelle, involved in the processing of proteins. It is present in all the cells except red blood cells. It is named after the discoverer Camillo Golgi. Usually, each cell has one Golgi apparatus. Some of the cells may have more than one Golgi apparatus. Each Golgi apparatus consists of 5 to 8 flattened membranous sacs called the cisternae. Golgi apparatus is situated near the nucleus. It has two ends or faces, namely cis face and trans face. The cis face is positioned near the endoplasmic reticulum. Reticular vesicles from endoplasmic reticulum enter the Golgi apparatus through cis face. The trans face is situated near the cell membrane.
9.
©Goutam Mallik, Assistant
Professor, Pharmacology Page 9 The processed substances make their exit from Golgi apparatus through trans face. Fig: Golgi Apparatus Function of Golgi Apparatus 1. Processing of materials: Vesicles containing glycoproteins and lipids are transported into Golgi apparatus. Here, the glycoproteins and lipids are modified and processed. 2. Packaging of materials: All the processed materials are packed in the form of secretory granules, secretory vesicles and lysosomes, which are transported either out of the cell or to another part of the cell. Because of this, Golgi apparatus is called the ‘post office of the cell’. 3. Labelling and deliveryof materials: Finally, the Golgi apparatus sorts out the processed and packed materials and labels them (such as phosphate group), depending upon the chemical content for delivery (distribution) to their proper destinations. Hence, the Golgi apparatus is called ‘shipping department of the cell’. Mitochondria These are relatively solid bodies, granular, rod-shaped or filamentous in form and remain scattered throughout the cytoplasm (dimensions varying from 0.5 to 5.0 microns). In humans, mitochondria mDNA is inherited solely from the mother. They are surrounded by a trilaminar double membrane, the inner one of which remains folded and forms a number of partitions, the cristae mitochondriales. These cristae may be complete, septate or incomplete. Numerous projecting particles known as elementary particles are present on the inner mitochondrial membrane and cristae. The fluid of the intramitochondrial space is called matrix. ( A ) ( B )
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©Goutam Mallik, Assistant
Professor, Pharmacology Page 10 Fig: (A) Mitochondria (B) Electron micrographic representation of the basal portion of the renal tubular epithelium (rat) embedded on basement lamina showing part of the, Golgi apparatus, numerous nucleus (C) Mitochondria showing internal structures. (C) Function of Mitochondria Production of energy Mitochondrion is called the ‘power house’ or ‘power plant’ of the cell because it produces the energy required for cellular functions. The energy is produced during the oxidation of digested food particles like proteins, carbohydrates and lipids by the oxidative enzymes in cristae. During the oxidative process, water and carbon dioxide are produced with release of energy. The released energy is stored in mitochondria and used later for synthesis of ATP. Synthesis of ATP The components of respiratory chain in mitochondrion are responsible for the synthesis of ATP by utilizing the energy by oxidative phosphorylation. ATP molecules diffuse throughout the cell from mitochondrion. Whenever energy is needed for cellular activity, the ATP molecules are broken down. Cytochrome C and second mitochondria-derived activator of caspases (SMAC)/diablo secreted in mitochondria are involved in apoptosis. Other functions Other functions of mitochondria include storage of calcium and detoxification of ammonia in liver. Lysosome Lysosomes are vesicular organelles that form by breaking off from the Golgi apparatus and then dispersing throughout the cytoplasm. The lysosome is quite different in various cell types, but it is usually 250 to 750 nanometers in diameter. Among the organelles of the cytoplasm, the lysosomes have the thickest covering membrane. The membrane is formed by a bilayered lipid material.
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Professor, Pharmacology Page 11 Fig: Digestive function of lysosomes Types of Lysosomes Lysosomes are of two types: 1. Primary lysosome, which is pinched off from Golgi apparatus. It is inactive in spite of having hydrolytic enzymes. 2. Secondary lysosome, which is the active lysosome. It is formed by the fusion of a primary lysosome with phagosome or endosome. Lysosomes are often called ‘garbage system’ of the cell because of their degradation activity.it is also called suicidal sac of cell. About 50 different hydrolytic enzymes, known as acid hydroxylases are present in the lysosomes, through which lysosomes execute their functions. Lysosomal functions involve two mechanisms: 1. Heterophagy: Digestion of extracellular materials engulfed by the cell via endocytosis. 2. Autophagy: Digestion of intracellular materials such as worn-out cytoplasmic organelles. Function of Lysosome 1. Degradation of macromolecules like protein, lipid and carbohydrate. 2. The rough endoplasmic reticulum wraps itself around the worn-out organelles like mitochondria and form the vacuoles called autophagosomes.The enzymes in the secondary lysosome are activated. Now, these enzymes digest the contents of autophagosome. 3. Removal of excess secretory products in the cells Lysosomes in the cells of the secretory glands remove the excess secretory products by degrading the secretory granules. 4. Secretory function – secretory lysosomes Recently, lysosomes having secretory function called secretory lysosomes are found in some of the cells, particularly in the cells of immune system.
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Professor, Pharmacology Page 12 Centrosomes &Centrioles It consists of another specialised part of clear cytoplasm, the centrosphere, containing in its interior two or more deeply staining particles-the centriole (generally arranged in pairs, i.e. diplosome) lying close to the nucleus in the resting cell. The centriole is an empty cylinder which is 3 to 5 micron long and the compact walls of centrioles are made of thin parallel nine tubular structures longitudinally arranged. Each tubule consists, in turn, of three subunits or triplets. The pericentriolar bodies or satellites which are round in shape are attached to centriolar tubules by a filament of chromatid. Centriole is closely related to spindle formation during mitosis (normal cell division) and also in sustaining other fibrillar structure like cilia and flagella. At the beginning of cell division, the centriole and centrosome divide. A system of radiating lines, made up of microtubules, then grows out from each of the two newly formed centrioles and the whole structure, due to its star-like shape, is called aster. The two asters grow in size and repel each other till they occupy the opposite poles of the elongated cell. Centrioles control polarisation of spindle fibres and play some part in their formation. Fig: Arrangement of centriolar tubule, nine in number, each tubule consisting of three centriolar subunits in right sided inset. Nucleus Nucleus is covered by a membrane called nuclear membrane and contains many components. Major components of nucleus are nucleoplasm, chromatin and nucleolus.
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Professor, Pharmacology Page 13 The nucleus is generally a round body occupying the centre of the cell. Its shape, size, position and number vary. The nucleus may contain many lobes. Nucleoplasm Nucleoplasm is a highly viscous fluid that forms the ground substance of the nucleus. It is similar to cytoplasm present outside the nucleus . Nucleoplasm surrounds chromatin and nucleolus. Chromatin Chromatin is a thread-like material made up of large molecules of DNA. Chromosomes Chromosome is the rod-shaped nuclear structure that carries a complete blueprint of all the hereditary characteristics of that species. A chromosome is formed from a single DNA molecule coiled around histone molecules. Each DNA contains many genes. Nucleolus Nucleolus is a small, round granular structure of the nucleus. Each nucleus contains one or more nucleoli. The nucleolus contains RNA and some proteins, which are similar to those found in ribosomes. (A) (B) Fig: (A) Diagram shows chromosomes extending through amorphous matrix and innermost limit of cytoplasm and porous nuclear membrane (B)Structure of nucleus. Functions of Nucleus Major functions of nucleus are the control of cellular activities and storage of hereditary material. Several processes are involved in the nuclear functions.
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Professor, Pharmacology Page 14 1. Control of all the cell activities that include metabolism, protein synthesis, growth and reproduction (cell division). 2. Synthesis of RNA. 3. Formation of subunits of ribosomes. 4. Sending genetic instruction to the cytoplasm for protein synthesis through messenger RNA (mRNA). 5. Control of the cell division through genes. 6. Storage of hereditary information (in genes) and transformation of this information from one generation of the species to the next. Ribosome Ribosomes are the organelles without limiting membrane. These organelles are granular and small dot-like structures with a diameter of 15 nm. Ribosomes are made up of 35% of proteins and 65% of ribonucleic acid (RNA). RNA present in ribosomes is called ribosomal RNA (rRNA). Ribosomes are concerned with protein synthesis in the cell. Ribosomes are called ‘protein factories’ because of their role in the synthesis of proteins. Messenger RNA (mRNA) carries the genetic code for protein synthesis from nucleus to the ribosomes. The ribosomes, in turn arrange the amino acids into small units of proteins. ***************************** References 1. CC Chaterjee’s Human Physiology 12th Ed Vol.1 2. Essentials of Human Physiology for Pharmacy by Laurie Kelly 3. Essentials of Human Anatomy & Physiology 5th Ed by Valerie C. Scanlon & Tina Sanders 4. Gray’s Anatomy, The Anatomical Basis of Clinical Practice 41st Ed by Elsevier 5. Guyton and Hall Textbook of Medical Physiology 13th Ed by John E. Hall 6. Essentials of Medical Physiology 6th Ed by K Sembulingam and Prema Sembulingam
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