UNIT 1. Cell - Structure and Function PDF

UNIT 1 THE CELL – STRUCTURE AND FUNCTION Introduction All the living things are composed of cells. Cells are the body’s smallest functional units. Cell is defined as the structural and functional unit of the living body. They are grouped together to form tissues, each o...

UNIT 1 THE CELL – STRUCTURE AND FUNCTION Introduction All the living things are composed of cells. Cells are the body’s smallest functional units. Cell is defined as the structural and functional unit of the living body. They are grouped together to form tissues, each of which has a specialised function, e.g. blood, muscle, bone. Different tissues are grouped together to form organs, e.g. the heart, stomach and brain. Organs are grouped together to form systems, each of which performs a particular function that maintains homeostasis and contributes to the health of the individual. For example, the digestive system is responsible for taking in, digesting and absorbing food, which involves a number of organs, including the stomach and intestines. General Characteristics of Cell Each cell in the body: 1. Needs nutrition and oxygen. 2. Produces its own energy necessary for its growth, repair and other activities 3. Eliminates carbon dioxide and other metabolic wastes 4. Maintains the medium, i.e. the environment for its survival 5. Shows immediate response to the entry of invaders like bacteria/toxic substances into the body. 6. Reproduces by division. There are some exceptions like neuron, which do not reproduce. STRUCTURE OF THE CELL Each cell is formed by a cell body and a membrane covering the cell body called the cell membrane. Cell body has two parts, namely nucleus and cytoplasm surrounding the nucleus. Thus, the structure of the cell is studied under three headings: 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. This membrane separates the fluid outside the cell called extracellular fluid (ECF) and the fluid inside the cell called intracellular fluid (ICF). The cell membrane is a semipermeable membrane. Composition Of Cell Membrane Cell membrane is composed of three types of substances: 1. Proteins (55%) 2. Lipids (40%) 3. Carbohydrates (5%) Structure Of Cell Membrane On the basis of structure, cell membrane is called a unit membrane or a three-layered membrane. The two electron-dense layers are placed one on either side of the central layer. The central layer is a lipid layer formed by lipid substances. The other two layers are protein layers formed by proteins. Cell membrane contains some carbohydrate molecules also. I. Lipid Layers of the Cell Membrane The central lipid layer is a bilayered structure. This is formed by a thin film of lipids. It is fluid in nature and not a solid structure. Major lipids are: 1. Phospholipids 2. Cholesterol. 1. Phospholipid: Phospholipid molecules are arranged in two layers. Each phospholipid molecule resembles the headed pin in shape. The outer part of the phospholipid molecule is called the head portion - polar end and it is soluble in water and has strong affinity for water (hydrophilic) facing outer layer ECF. The inner portion is called the tail portion - non-polar end. It is insoluble in water and repelled by water (hydrophobic) - meet in the center of the membrane (ICF - cytoplasm). 2. Cholesterol: Cholesterol molecules are arranged in between the phospholipid molecules. Functions of Lipid Layer in Cell Membrane Lipid layer of the cell membrane is a semipermeable membrane and thus, allows only the fat- soluble substances like oxygen, carbon dioxide and alcohol to pass through this lipid layer. The water-soluble substances such as glucose, urea and electrolytes cannot pass through this layer II. Functions of Proteins in Cell Membrane 1. Integral proteins provide the structural integrity of the cell membrane 2. Channel proteins help in the diffusion of water-soluble substances like glucose and electrolytes 3. Carrier or transport proteins help in the transport of substances across the cell membrane by means of active or passive transport 4. Pump: Some carrier proteins act as pumps, by which ions are transported actively across the cell membrane 5. Receptor proteins serve as the receptor sites for hormones and neurotransmitters 6. Enzymes: Some of the protein molecules form the enzymes and control chemical (metabolic) reactions within the cell membrane  7. Antigens: Some proteins act as antigens and induce the process of antibody formation 8. Cell adhesion molecules or the integral proteins are responsible for attachment of cells to their neighbors or to basal lamina. III. Carbohydrates of the Cell Membrane Some of the carbohydrate molecules present in cell membrane are attached to proteins and form glycoproteins (proteoglycans). Some carbohydrate molecules are attached to lipids and form glycolipids. Carbohydrate molecules form a thin and loose covering over the entire surface of the cell membrane called glycocalyx. Functions of Carbohydrates in Cell Membrane 1. Carbohydrate molecules are negatively charged and do not permit the negatively charged substances to move in and out of the cell 2. Glycocalyx from the neighboring cells helps in the tight fixation of cells with one another Functions Of Cell Membrane 1. Protective function: Cell membrane protects the cytoplasm and the organelles present in the cytoplasm 2. Selective permeability: Cell membrane acts as a semipermeable membrane, which allows only some substances to pass through it and acts as a barrier for other substances 3. Absorptive function: Nutrients are absorbed into the cell through the cell membrane 4. Excretory function: Metabolites and other waste products from the cell are excreted out through the cell membrane 5. Exchange of gases: Oxygen enters the cell from the blood and carbon dioxide leaves the cell and enters the blood through the cell membrane 6. Maintenance of shape and size of the cell: Cell membrane is responsible for the maintenance of shape and size of the cell Cytoplasm Cytoplasm of the cell is the jellylike material formed by 80% of water. It contains a clear liquid portion called cytosol and various particles of different shape and size. These particles are proteins, carbohydrates, lipids or electrolytes in nature. Cytoplasm also contains many organelles with distinct structure and function. Endoplasmic Reticulum Endoplasmic reticulum is a network of tubular and microsomal vesicular structures which are interconnected with one another. The lumen of endoplasmic reticulum contains a fluid medium called endoplasmic matrix. The endoplasmic reticulum forms the link between nucleus and cell membrane by connecting the cell membrane with the nuclear membrane. Types of Endoplasmic Reticulum Rough endoplasmic reticulum and smooth endoplasmic reticulum. Both the types are interconnected and continuous with one another. Depending upon the activities of the cells, the rough endoplasmic reticulum changes to smooth endoplasmic reticulum and vice versa.  a. Rough Endoplasmic Reticulum: It is the endoplasmic reticulum with rough, bumpy or bead-like appearance. Rough appearance is due to the attachment of granular ribosomes to its outer surface. Hence, it is also called the granular endoplasmic reticulum. Functions of Rough Endoplasmic Reticulum (RER) 1. Synthesis of proteins: RER is concerned with the synthesis of proteins in the cell. 2. Degradation of worn-out organelles: RER also plays an important role in the degradation of worn-out cytoplasmic organelles like mitochondria. b. Smooth Endoplasmic Reticulum: It is the endoplasmic reticulum with smooth appearance and are formed by many interconnected tubules. Thus, it is also called agranular reticulum or tubular endoplasmic reticulum. Functions of Smooth Endoplasmic Reticulum 1. Synthesis of non-protein substance: SER is responsible for synthesis of non-protein substances such as cholesterol and steroid. 2. Role in cellular metabolism: Outer surface of SER contains many enzymes which are involved in various metabolic processes of the cell. 3. Storage and metabolism of calcium: SER is the major site of storage and metabolism of calcium. In skeletal muscle fibers, it releases calcium which is necessary to trigger the muscle contraction. 4. Catabolism and detoxification: SER is also concerned with catabolism and detoxification of toxic substances like some drugs and carcinogens (cancer-producing substances) in the liver. 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. 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. The processed substances make their exit from Golgi apparatus through trans face. Functions of Golgi Apparatus: Major functions of Golgi apparatus are processing, packing, labeling and delivery of proteins and other molecules like lipids to different parts of the cell. Lysosomes Lysosomes are the membrane-bound vesicular organelles found throughout the cytoplasm. The lysosomes are formed by Golgi apparatus. The enzymes synthesized in rough endoplasmic reticulum are processed and packed in the form of small vesicles in the Golgi apparatus. Then, these vesicles are pinched off from Golgi apparatus and become the lysosomes. Among the organelles of the cytoplasm, the lysosomes have the thickest covering membrane. It has many small granules which contain hydrolytic enzymes. 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. Functions of Lysosomes: Lysosomes are often called ‘garbage system’ of the cell because of their degradation activity. About 50 different hydrolytic enzymes, known as acid hydroxylases are present in the lysosomes, through which lysosomes execute their functions. Important lysosomal enzymes 1. Proteases, which hydrolyze the proteins into amino acids 2. Lipases, which hydrolyze the lipids into fatty acids and glycerides 3. Amylases, which hydrolyze the polysaccharides into glucose 4. Nucleases, which hydrolyze the nucleic acids into mononucleotides. Specific functions of lysosomes 1. Degradation of macromolecules 2. Degradation of worn-out organelles 3. Removal of excess secretory products in the cell 4. Secretory function – secretory lysosomes Mitochondrion Mitochondrion (plural = mitochondria) is a membrane bound cytoplasmic organelle concerned with production of energy. It is a rod-shaped or oval- shaped structure. The outer membrane is smooth and encloses the contents of mitochondrion. The inner membrane is folded in the form of shelf-like inward projections called cristae and it covers the inner matrix space. Functions of Mitochondrion 1. Production of energy: Mitochondrion is called the ‘power house’ or ‘power plant’ of the cell because it produces the energy required for cellular functions. 2. Synthesis of ATP: The components of respiratory chain in mitochondrion are responsible for the synthesis of ATP. Whenever energy is needed for cellular activity, the ATP molecules are broken down 3. Other functions: Other functions of mitochondria include storage of calcium and detoxification of ammonia in liver. Nucleus Nucleus is the most prominent and the largest cellular organelle and it occupies about 10% of total volume of the cell. Nucleus is present in all the cells in the body except the red blood cells. The cells with nucleus are called eukaryotes and those without nucleus are known as prokaryotes. Presence of nucleus is necessary for cell division. Generally, the nucleus is located in the center of the cell. It is mostly spherical in shape. However, the shape and situation of nucleus vary in some cells. Structure Of Nucleus Nucleus is covered by a membrane called nuclear mem - brane and contains many components. Major components of nucleus are nucleoplasm, chromatin and nucleolus. Nuclear Membrane: Nuclear membrane is double layered and porous in nature. This allows the nucleoplasm to communicate with the cytoplasm. 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. It contains dense fibrillar network of proteins called the nuclear matrix and many substances such as nucleotides and enzymes. 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. All the dividing cells of the body except reproductive cells contain 23 pairs of chromosomes. 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 (similar to those found in ribosomes). The RNA is synthesized by 5 different pairs of chromosomes and stored in the nucleolus. Later, it is condensed to form the subunits of ribosomes. All the subunits formed in the nucleolus are transported to cytoplasm through the pores of nuclear membrane. In the cytoplasm, these subunits fuse to form ribosomes, which play an essential role in the formation of proteins. Functions Of Nucleus 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. Centrosome And Centrioles: Centrosome is the membrane-bound cellular organelle situated almost in the center of cell, close to nucleus. Centrioles are responsible for the movement of chromosomes during cell division. Secretory Vesicles: Secretory vesicles are the organelles with limiting membrane and contain the secretory substances. These vesicles are formed in the endoplasmic reticulum and are processed and packed in Golgi apparatus. Secretory vesicles are present throughout the cytoplasm. When necessary, these vesicles are ruptured and secretory substances are released into the cytoplasm.