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Unit 2. Structure of the Cell

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Unit 2. Structure of the Cell FYJC Jacob’s Notes of Biology 1 Unit 2. Structure of the Cell. You have probably seen that bricks placed one above the other help in the construction of a building. Therefore, we can say that the basic unit of a building is a brick. Living beings are called organisms. Organisms display an immense diversity in form and size. Yet they have an underlying unity in their basic structure and functions. They are made up of one or many units called cells. The body of organisms may be made up of a single cell (unicellular) or many cells (multicellular). In unicellular organisms, the characteristic features of life are expressed by the single cell. Eg. Amoeba, Chlamydomonas, Paramaecium etc. In multicellular organisms, all the functions of life are carried out by one or other group of cells. In fact, the cells are the binding blocks of life. A cell is a structural and functional unit of life. There is division of labour in the cells constituting the body of multicellular organisms. Therefore, in their body tissues, organs, and organ systems have developed. Group of cells with similar structure and functions are known as tissues. Many types of tissues together constitute organs which perform one or many specific functions. In higher organisms several organs which are closely related with each other constitute organ system which is responsible for a typical life process. Digestive system, as important life process, is constituted of many organs as salivary glands, stomach, intestine, pancreas, liver, gall bladder etc. These organs are further made up of tissues like epithelial, connective, and muscular. CELLS TISSUES ORGANS ORGAN SYSTEMS BODY Characteristics of cell 1. Cell is the basic unit of all organisms. 2. The cell is surrounded by plasma membrane in all organisms. 3. The basic structure and functional properties cell membranes are similar in all types of cells. 4. The genetic material in all the cells consists of nucleic acids which is responsible for transmission of hereditary characters. 5. All cells arise from preexisting cells. 6. The basic biochemical reactions like nucleic acids synthesis, protein synthesis, enzyme actions, etc. are similar in all the cells. Types of Cells Cells can be grouped into two main types: a) Prokaryotic cells & b) Eukaryotic cells. a. Prokaryotic Cell & its organization The prokaryotic cells are small, simple and most primitive. The prokaryotes consist of the bacteria and blue green algae. They multiply rapidly and their cells vary in shape and size. A prokaryotic cell is a one envelope system. Every prokaryotic cell is bound by a complex cell wall. A plasma membrane is present below the cell wall. The plasma membrane invaginates to form internal membrane structures. Membrane bound organelles are absent. The genetic material is called nucleoid. It is not separated from the surrounding cytoplasm by any limiting membrane. Ribosomes are scattered in the cytoplasmic matrix. Each ribosome is composed of two subunits – 50S (larger) and 30S (smaller), that make up 70S for the complete ribosome. The ribosome is the site of protein synthesis. Many bacteria are motile and possess one or many flagella for locomotion. b. Eukaryotic Cell & its organization Eukaryotic cells have a well-defined nucleus in which nuclear materials are enclosed in a double layered membrane. In addition they possess all cell organelles such as endoplasmic reticulum, Golgi complex, Jacob Thomas / 09322380093 / FYJC / B O T A N Y / 1 FYJC Jacob’s Notes of Biology 2 mitochondria, lysosomes, centrioles etc. Cytoplasm has a high complex organization. Motile cells have additional appendages for locomotion. Plant cells have a well-defined rigid cell wall. The size of the cells varies in plants and animals. The smallest cells are of pleuropneumonia-like organisms (PPLO) as Mycolasma gallisseptecum. The Ostrich egg is the largest animal cell. In plant algae have large sized cells. Ex. Acetabularia. Units of Measurement: The cells and its organelles are measured in millimeter. A millimeter is one thousandth of a meter. One thousandth of a millimeter is known as micron or micrometer. One thousandth of micron is called millimicron or nanometer. Angstrom is one tenth of millimicron. 1. CELL WALL The outer, rigid, porous, non-living covering of plant cells is known as ‘cell wall’. Cell wall is present only in plant cells. The cell walls are composed of carbohydrate known as cellulose. Besides this, various chemical substances like hemi cellulose, pectin, lignin, cutin and chitin also occur with cellulose. The cell wall is complex in nature and is differentiated into: i. Primary cell wall – This thin outer most layer is composed of intricate network of micro fibrils of cellulose, pectic polysaccharides etc. ii. Secondary Cell wall - This layer is placed just inside the primary wall. It is composed of compactly arranged micro fibrils of the cellulose. It is thick and near the plasma membrane. iii. Tertiary Cell wall - The cell wall layer present beneath the secondary cell wall of some plants is known as tertiary cell wall. It is relatively thin and lack cellulose micro fibrils. It is made up of cellulose and a chemical substance known as xylan. The cell wall is perforated by narrow pores or pits at many places. Through these narrow pores, fine strands of cytoplasm are extending. These strands are called plasmodesamta. Plasmodesmata connect the cytoplasm of adjacent cells and facilitate the movement of substances between cells. Functions of cell wall: 1. It maintains the shape and gives mechanical support to the cells. 2. it allows the materials to pass in and out of cells. 3. It helps to maintain the balance of intracellular osmotic pressure with that of its surroundings. 4. It helps to check loss of water from the cells surface. 2. CELL MEMBRANE (PLASMA MEMBRANE) The thin, flexible, semi permeable living membrane that surrounds the protoplasm of a cell is called ‘cell membrane’ or ‘plasma membrane’ or ‘plasma lemma’. The term cell membrane was coined by Nageli and Cramer in 1885. Cell membrane is present in all plant and animal cells, it forms the boundary of the cell, while in plant cells it is located between the cell wall and protoplasm. Electron microscopic study reveals that the membrane consists of a double lipid layer sandwiched between two protein layers and hence it is termed as unit membrane. The membrane is selectively permeable and hence it controls the entry into and exit of selected molecules from the cell. 3. CYTOPLASM An amorphous, translucent, homogenous colloidal ground substance lying between the plasma membrane and the nucleus is known as cytoplasm. The cytoplasm is a complex material composed of membranes, particles and various organelles excluding nucleus. The fluid part of the cytoplasm is known as hyaloplsam or cytoplasmic matrix. This part contains various inorganic molecules like water, salts etc. and organic compounds like carbohydrates, lipids, nuclic acids etc. Jacob Thomas / 09322380093 / FYJC / B O T A N Y / 2 FYJC Jacob’s Notes of Biology 3 The cytoplasm shows two distinct regions – the outer non-granular, transparent thin layer being known as ectoplasm and the inner granular viscous portion called endoplasm, where cell organelles are located. Functions : 1. The nucleus, various cell organelles and non-living cytoplasmic inclusion remain enclosed within the cytoplasm. 2. It supplies proper nutrition to the living parts of a cell. 3. It is the seat of many vital chemical reactions which are essential for life, eg. Protein synthesis, respiration, etc. 4. It is the seat of many catabolic pathways e.g. Glycolysis. 4. CELL ORGANELLES The cell organelles are sub-cellular protoplasmic structures. They are usually covered by membrane. These are of two types: extracytoplasmic cell organelles like nucleus and cytoplasmic cell organelles like Endoplasmic reticulum, Golgi complex, Mitochondria, Plastid, Ribosomes, etc. i. Endoplasmic reticulum (ER) It is an interconnected system of membrane bound channels in the cytoplasm. It was first reported by Keith Porter in 1953. The whole structure of ER freely communicates with one another and their lumen is filled with a fluid. The membrane of the ER has a lipoprotein composition. The ER is of two types: smooth or agranular and rough or granular. Rough ER possess rough walls because the Ribosomes attached to its membranes. They are found in those cells which are active in protein synthesis. Smooth ER possess smooth walls because of the absence of the absence of Ribosomes. Functions: i. ER acts as an intracellular structural framework and gives mechanical support to the cytoplasm. ii. It acts as a transporting system carrying various substances to various organelles. iii. ER performs various syntheitic and metabolic activities. iv. They provide surface area for various enzyme activities. v. They regulate the movement and exchange of materials within the cell. vi. It involves in the formation of new nuclear membrane after each cell division. II. Golgi Apparatus Golgi complex or Golgi bodies or Golgi apparatus are a group of membrane bound bodies occur abundantly in glandular cells. The Golgi Complex was discovered by Camillo Golgi in 1898. The Golgi Complex is a closely packed disc shaped structures formed of three components namely: Cisternae – Parallel and flattened plate like components; Tubules – a network of interconnected tubules arise from periphery of cisternae and Vesicles – small droplet like sacs found attached to the tubules. Functions: i. It is involved in cellular secretion. ii. It plays an important role in the formation of cell walls in plant cells. iii. It has a role in formation of plasma membrane. During cellular secretion, the membrane of the secretary vesicles is added to the plasma membrane. iv. It helps in the formation of primary lysosomes. v. The Golgi complex gives rise to the acrosome in sperms. III. Lysosomes The lysosomes are small membrane bound vesicular bodies.
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