8. Cell unit of life.pdf

- A cell is the fundamental, structural and functional unit of (i) All living organisms are composed of cells and all living organisms. products of cells. - Robert Hooke: Discovered cell. (ii) All...

- A cell is the fundamental, structural and functional unit of (i) All living organisms are composed of cells and all living organisms. products of cells. - Robert Hooke: Discovered cell. (ii) All cells arise from pre-existing cells. - Anton Von Leeuwenhoek: First observed and described a AN OVERVIEW OF CELL live cell. - The invention of the compound & electron microscopes - All cells contain revealed all the structural details of the cell. o Cytoplasm: A semi-fluid matrix where cellular activities and chemical reactions occur. This keeps the CELL THEORY cell in ‘living state’. - Matthias Schleiden (1838) observed that all plants are o Ribosomes: Non-membrane bound organelles seen in composed of different kinds of cells. cytoplasm, chloroplasts, mitochondria & on rough ER. - Theodore Schwann (1839) found that cells have a thin - Cells differ in size, shape and activities. outer layer (plasma membrane). He also found that plant o Smallest cells: Mycoplasmas (0.3 µm in length). cells have cell wall. He proposed a hypothesis that animals o Largest isolated single cell: Egg of ostrich. and plants are composed of cells and products of cells. o Longest cells: E.g. Nerve cell. - Schleiden & Schwann formulated the cell theory. o Size of bacteria: 3 to 5 µm (Typical: 1 to 2 µm). - Rudolf Virchow (1855) first explained that cells divide o Human RBCs are about 7.0 µm in diameter. and new cells are formed from pre-existing cells (Omnis - Based on the functions, shape of cells may be disc-like, cellula-e cellula). He modified the cell theory. polygonal, columnar, cuboid, thread like, or irregular. - Cell theory states that: - Cells are 2 types: Prokaryotic cells & Eukaryotic cells. PROKARYOTIC CELLS - They have no membrane bound nucleus and organelles. o In distribution of chromosomes to daughter cells. - They include bacteria, blue-green algae, mycoplasma & o In respiration and secretion processes. PPLO (Pleuro Pneumonia Like Organisms). o To increase the surface area of the plasma membrane - They are generally smaller and multiply more rapidly than and enzymatic content. the eukaryotic cells. - Chromatophores are pigment-containing membranous - They vary in shape & size. E.g. Bacteria have 4 basic infoldings in some prokaryotes (e.g. cyanobacteria). shapes: Bacillus, Coccus, Vibrio and Spirillum. 1. Nucleoid Cell organelles in prokaryotic cells - It is formed of non-membranous (naked) circular genomic 1. Cell Envelope DNA (single chromosome/ Genetic material) & protein. - It is a chemically complex protective covering. - Many bacteria have small circular DNA (plasmid) outside - It is made of 3 tightly bound layers. the genomic DNA. It gives some unique phenotypic o Glycocalyx: Outer layer. Its composition and thickness characters (e.g. resistance to antibiotics) to bacteria. vary in different bacteria. It may be a slime layer (loose 1. Flagella sheath) or capsule (thick & tough). - These are thin filamentous extensions from the cell wall of o Cell wall: Middle layer. Seen in all prokaryotes except motile bacteria. Their number and arrangement are varied mycoplasma. It gives shape to the cell and provides a in different bacteria. structural support to prevent the bacterium from bursting - Bacterial flagellum has 3 parts – filament, hook and basal or collapsing. body. The filament is the longest portion and extends from o Plasma membrane: Inner layer. It is semi-permeable in the cell surface to the outside. nature and interacts with the outside. This is structurally 1. Pili and Fimbriae similar to that of the eukaryotes. - These are surface structures that have no role in motility. - Based on the types of the cell envelopes and response to - Pili (sing. Pilus) are elongated tubular structures made of a Gram staining (developed by Gram), bacteria are 2 types: special protein (pilin). o Gram positive: They take up and retain the gram stain. - Fimbriae are small bristle like fibres sprouting out of the o Gram negative: They do not retain the gram stain. cell. In some bacteria, they help to attach the bacteria to 1. Mesosomes & Chromatophores rocks in streams and to the host tissues. (Membranous structures) - Mesosome is formed by the infoldings of plasma 1. Ribosomes membrane. It includes vesicles, tubules & lamellae. - They are associated with plasma membrane of prokaryotes. - Functions: Mesosomes help - They are about 15 nm by 20 nm in size. o In cell wall formation. - They are made of 2 subunits - 50S & 30S (Svedberg’s o In DNA (chromosome) replication. unit). They together form 70S prokaryotic ribosomes. 1  (S= sedimentation coefficient; a measure of density & size). - These are non-membranous, stored reserve material seen - Function: Ribosomes are the site of translation (protein freely in the cytoplasm of prokaryotic cells. synthesis). Several ribosomes may attach to a single - E.g. phosphate granules, cyanophycean granules and mRNA to form a chain called polyribosomes (polysome). glycogen granules, gas vacuoles etc. Ribosomes translate the mRNA into proteins. - Gas vacuoles are found in blue green and purple and green 1. Inclusion Bodies photosynthetic bacteria. EUKARYOTIC CELLS - They have well organized membrane bound nucleus and o Peripheral proteins: Lie on the surface of membrane. organelles. - Fluid mosaic model of cell membrane: Proposed by - Presence of membranes gives clear compartmentalization Singer & Nicolson (1972). According to this, the quasi- of cytoplasm. fluid nature of lipid enables lateral movement of proteins - Their genetic material is organized into chromosomes. within the overall bilayer. This ability to move within the - They have complex locomotory & cytoskeletal structures. membrane is measured as its fluidity. Plant cell and Animal cell Functions: o Transport of the molecules. The membrane is selectively permeable to some molecules present on either side of it. o Due to the fluid nature, the plasma membrane can help in cell growth, formation of intercellular junctions, secretion, endocytosis, cell division etc. Types of Transport 1. Passive transport: It is the movement of molecules across the membrane along the concentration gradient (i.e., from higher concentration to the lower) without the expenditure of energy. It is 2 types: a. Simple diffusion: It is the movement of neutral solutes across the membrane. b. Osmosis: It is the movement of water by diffusion across the membrane. Polar molecules cannot pass through the non-polar lipid bilayer. So they require membrane carrier protein for Cell organelles in eukaryotic cells transport. 2. Active transport: It is the movement of molecules across 1. Cell Membrane the membrane against the concentration gradient (i.e. from - Chemical studies on human RBCs show that cell membrane lower to the higher concentration) with the expenditure of is composed of a lipid bilayer, protein & carbohydrate. energy (ATP is utilized). E.g. Na+/K+ pump. - Lipids (mainly phosphoglycerides) have outer polar head and the inner hydrophobic tails. So the non-polar tail of 2. Cell Wall saturated hydrocarbons is protected from the aqueous - It is a non-living rigid structure found outer to the plasma environment. membrane of fungi and plants. - Ratio of protein and lipid varies in different cells. E.g. In - Cell wall of Algae is made of cellulose, galactans, mannans human RBC, membrane has 52% protein and 40% lipids. and minerals like CaCO3. In other plants, it consists of - Based on the ease of extraction, membrane proteins are 2 cellulose, hemicellulose, pectins and proteins. types: - Cell wall of a young plant cell (primary wall) is capable o Integral proteins: Partially or totally buried in membrane. of growth. It gradually diminishes as the cell matures and 2  the secondary wall is formed on the inner side (towards Materials to be packaged as vesicles from the ER fuse with membrane). the cis face and move towards the trans face. This is why - The middle lamella is a layer containing calcium pectate Golgi apparatus remains in close association with the which glues the neighbouring cells together. Cell wall and endoplasmic reticulum. middle lamellae may be traversed by plasmodesmata. It o Proteins synthesized by ribosomes on the ER are modified connects the cytoplasm of neighbouring cells. in the cisternae of Golgi apparatus before they are released Functions: from its trans face. a. It gives shape to the cell. o Formation of glycoproteins and glycolipids. b. It protects the cell from mechanical damage & infection. Lysosomes c. It helps in cell-to-cell interaction. - These are membrane bound vesicular structures formed by d. It acts as barrier to undesirable macromolecules. the process of packaging in the Golgi apparatus. 3. Endomembrane System - Lysosomal vesicles contain almost all types of hydrolytic - It is a group of membranous organelles having coordinated enzymes (hydrolases– lipases, proteases, carbohydrases). functions. They are active at acidic pH. They digest carbohydrates, - They include endoplasmic reticulum (ER), Golgi complex, proteins, lipids and nucleic acids. lysosomes and vacuoles. Vacuoles Endoplasmic Reticulum (ER) - These are the membrane-bound space found in the - These are a network of cytoplasm. It contains water, sap, excretory product and tiny tubular structures other materials not useful for the cell. scattered in the - Vacuole is bound by a single membrane called tonoplast. cytoplasm. - In plant cells, the vacuoles can occupy up to 90% of the - ER divides the volume of the cell. intracellular space into 2 - In plants, the tonoplast facilitates the transport of ions and compartments: luminal other materials against concentration gradients into the (inside ER) & extra vacuole. Hence their concentration is higher in the vacuole luminal (cytoplasm). than in the cytoplasm. - Endoplasmic reticulum - In Amoeba, the contractile vacuole helps for excretion. is 2 types: - In many cells (e.g. protists), food vacuoles are formed by a. Rough endoplasmic engulfing the food particles. reticulum (RER): 4. Mitochondria Bear ribosomes on - Mitochondria are clearly visible only when stained. their surface. RER is - Number, shape and size of mitochondria per cell are frequently observed variable depending on the physiological activity. in the cells actively - It is sausage-shaped or cylindrical having a diameter of involved in protein synthesis and secretion. They 0.2-1.0 µm (average 0.5 µm) and length 1.0-4.1 µm. extend to the outer membrane of the nucleus. b. Smooth endoplasmic reticulum (SER): Ribosomes are absent. SER is the major site for synthesis of lipid. In animal cells lipid-like steroidal hormones are synthesized in SER. Golgi apparatus - Densely stained reticular structures near the nucleus. - First observed by Camillo - A mitochondrion is a double membrane-bound structure Golgi (1898). with the outer membrane and the inner membrane. It - They consist of flat, disc- divides lumen into 2 aqueous compartments, i.e., the outer shaped sacs (cisternae) of compartment and the inner compartment (matrix). 0.5- 1.0 µm diameter. These - Inner membrane forms a number of infoldings (cristae) are stacked parallelly. towards the matrix. They increase the surface area. - Cisternae are concentrically - The two membranes have their own specific enzymes arranged with convex cis associated with the mitochondrial function. (forming) face and concave trans (maturing) face. Cis & - Matrix possesses a circular DNA, a few RNA molecules, trans faces are totally different, but interconnected. ribosomes (70S) and components for protein synthesis. Function of Golgi apparatus: - The mitochondria divide by fission. o Secretes materials to intra-cellular targets or outside the cell. 3 - Function: Mitochondria are the sites of aerobic present in the cytoplasm. respiration. They produce energy in the form of ATP. So - It provides mechanical support, motility, maintenance of they are called ‘power houses’ of the cell. the shape of the cell etc. 5. Plastids 8. Cilia and Flagella - Plastids are found in all plant cells and in euglenoides. - They are hair-like outgrowths of the cell membrane. - Large sized. Easily observable under the microscope. - Cilia: Small structures which work like oars. Causes the - They contain some pigments. movement of the cell or surrounding fluid. - Based on the type of pigments, plastids are 3 types: - Flagella: Longer. Responsible for cell movement. Flagella a. Chloroplasts: Contain chlorophyll and carotenoid of prokaryotes and eukaryotes are structurally different. pigments. They trap light energy for photosynthesis. b. Chromoplasts: Contain fat soluble carotenoid pigments like carotene, xanthophylls etc. This gives a yellow, orange or red colour. c. Leucoplasts: These are colourless plastids of varied shapes and sizes with stored nutrients. They include: § Amyloplasts: Store starch. E.g. potato. § Elaioplasts: Store oils and fats. § Aleuroplasts: Store proteins. Chloroplasts: - Cilium and flagellum are covered with plasma membrane. - These are double membrane bound organelles mainly Their core (axoneme) has many microtubules running found in the mesophyll cells of the leaves. parallel to the long axis. - These are lens- - The axoneme has 9 pairs of doublets of radially arranged shaped, oval, peripheral microtubules and a pair of central microtubules. spherical, This is called 9+2 array. discoid or - The central tubules are connected by bridges and are ribbon-like enclosed by a central sheath. It is connected to one of the organelles. tubules of each peripheral doublet by a radial spoke. Thus, - Length: 5-10 µm. Width: 2-4 µm. there are 9 radial spokes. The peripheral doublets are also - Their number varies from 1 (e.g. Chlamydomonas) to 20- interconnected by linkers. 40 per cell in the mesophyll. - Cilium and flagellum emerge from centriole-like structure - Inner membrane of chloroplast is less permeable. called the basal bodies. - The space limited by the inner membrane of the chloroplast 9. Centrosome and Centrioles is called stroma. It contains many organized flattened - Centrosome is an organelle usually containing two non- membranous sacs called thylakoids. membrane bound cylindrical structures called centrioles. - Membrane of thylakoids encloses a space called lumen. - They are surrounded by pericentriolar materials. - Chlorophyll pigments are present in the thylakoids. - The centrioles lie perpendicular to each other. They are - Thylakoids are arranged in stacks called grana or the made up of 9 evenly spaced peripheral fibrils of tubulin. intergranal thylakoids. Each of the peripheral fibril is a triplet. The adjacent - There are flat membranous tubules called the stroma triplets are also linked. lamellae connecting the thylakoids of the different grana. - The central part of the centriole is also proteinaceous and - The stroma contains small, double-stranded circular DNA called the hub, which is connected with tubules of the molecules, ribosomes and enzymes for the synthesis of peripheral triplets by radial spokes made of protein. carbohydrates and proteins. - The centrioles form the basal body of cilia or flagella, and - The ribosomes of the chloroplasts are smaller (70S) than spindle fibres that give rise to spindle apparatus during cell the cytoplasmic ribosomes (80S). division in animal cells. 6. Ribosomes 10. Nucleus - They are non-membranous granular - Nucleus was first structures composed of ribonucleic described by acid (RNA) & proteins. Robert Brown - It is first observed by George (1831). Palade (1953). - The material of - Eukaryotic ribosome has 2 subunits- 60S (large subunit) the nucleus and 40S (small subunit). They together form 80S. stained by the 7. Cytoskeleton basic dyes was - It is a network of filamentous proteinaceous structures given the name chromatin by Flemming. 4 - Normally, a cell has only one nucleus. Some cells have - Based on position of centromere, chromosomes are 4 types: more than one. Some mature cells lack nucleus. E.g. o Metacentric chromosome: Middle centromere forming mammalian RBC and sieve tube cells of vascular plants. two equal arms of the chromosome. - The interphase nucleus contains o Sub-metacentric chromosome: Centromere is nearer to o Nuclear envelope: Double layered membrane with a one end forming one shorter arm and one longer arm. space between (10 - 50 nm) called perinuclear space. It o Acrocentric chromosome: Centromere is close to its is a barrier between the materials present in nucleus & end forming one very short and one very long arm. cytoplasm. Outer membrane usually remains continuous o Telocentric chromosome: Terminal centromere. with ER and also bears ribosomes on it. Nuclear envelope has minute pores formed by the fusion of its two membranes. These are the passages for the movement of RNA and protein between nucleus and cytoplasm. o Nuclear matrix (nucleoplasm) o Chromatin: A network of nucleoprotein fibres. It contains DNA and basic proteins (histones), non-histone proteins and RNA. During cell division, chromatins condense to form chromosomes. - Some chromosomes have non-staining secondary o Nucleolus: One or more non-membranous spherical constrictions at a constant location. It is called satellite. bodies. It is continuous with the nucleoplasm. It is a site 11. Microbodies for ribosomal RNA synthesis. - These are membrane bound minute vesicles that contain Chromosomes: various enzymes. - A human cell has 2 m long thread of DNA - Present in both plant and animal cells. distributed among its 46 (23 pairs) Differences between Plant and animal cells chromosomes. Plant cell Animal cell - Every chromosome has a primary 1. Cell wall present Absent constriction (centromere). On the sides 2. Plastids are present Absent of centromere, disc shaped structures 3. A large central vacuole Many small vacuoles called kinetochores are present. 4. Centrioles are absent Present COMPARISON BETWEEN PROKARYOTIC AND EUKARYOTIC CELLS Prokaryotic cells Eukaryotic cells 1. Generally smaller Larger 2. Genetic material is in the form of nucleoid Genetic material is in the form of nucleus 3. Nuclear membrane absent Present 4. Membrane bound organelles absent Present 5. Circular DNA Linear DNA 6. Ribosomes 70 S type 80 S type (70 S in plastids and mitochondria) 5  MODEL QUESTIONS 1. Match the columns A,B, & C A B C Mitochondria Sedimentation coefficient Spindle fibers Golgi bodies Hydrolytic enzyme Power house Lysosomes Axoneme Cisternae Ribosomes Centrioles Acidic PH Cilia Glycoproteins George Palade Centrosome Cristae 9+ 2 2. Bacterial cell envelope is having a complex structure. Name the layers of the envelope. 3. Types of Chromosomes based on the position of centromere are given. Name the Chromosomes. 4. In cells glycoprotein & glycolipids are secreted by a cell organelle. a. Name the cell organelle b. Neatly draw its diagram 5. Plastids are found in all plant cells a. List the three plastids found in plants. b. Name the colorless plastids and specify its role. 6. Identify the characters of prokaryotic cells from the following statements a. Endoplasmic reticulum present b. Ribosome present c. Golgi bodies absent d. Incipient nucleus e. Yeast is an example f. Mostly anaerobes 7. Golgi apparatus remains in close association with the endoplasmic reticulum. Give the reason. 8. Copy the following diagram and label the parts. 6