2. Cell structure and function.ppt
Transcript
2.Cell as unit of life Jasmin 1 Introduction to the cell The Cell Theory is one of the basic principles of biology. Credit for the formulation of this theory is given to German scientists Theodor Schwann, Matthias Schleiden, and Rudolph Virchow. 1838- German Botanist, Matthias Sch...
2.Cell as unit of life Jasmin 1 Introduction to the cell The Cell Theory is one of the basic principles of biology. Credit for the formulation of this theory is given to German scientists Theodor Schwann, Matthias Schleiden, and Rudolph Virchow. 1838- German Botanist, Matthias Schleiden, concluded that all plant parts are made of cells. 1839- German physiologist, Theodor Schwann, who was a close friend of Schleiden, stated that all animal tissues are composed of cells. 1858- Rudolf Virchow, German physician, after extensive study of cellular pathology, concluded that cells must arise from preexisting cells. Jasmin 2 3 ideas of cell theory: All living organisms are made up of one or more cells. (Schleiden & Schwann)(1838-39) The cell is the basic unit of life in all living things. (Schleiden & Schwann)(1838-39) New cells are formed by the division of pre-existing cells. (Virchow) (1858) Jasmin 3 Modern Cell Theory contains 4 statements, in addition to the original Cell Theory: 1.The cell contains hereditary information (DNA) which is passed on from cell to cell during cell division. 2.All cells are basically the same in chemical composition and metabolic activities. 3.All basic chemical & physiological functions are carried out inside the cells. (movement, digestion, etc) 4.Cell activity depends on the activities of sub-cellular structures within the cell (organelles, nucleus, plasma membrane) Jasmin 4 Eukaryotic cells Prokaryotic cells 2 types of cells: 1) Prokaryotic cells 2) Eukaryotic cells Prokaryotic cells Bacteria and cynobacteria Prokaryote is derived from a greek word pro : before karyon: nucleus Signifying the absence of true nucleus in the cell meaning (DNA) in the cytoplasm, is not contained in a nuclear membrane or naked circular DNA, which is characteristic of prokaryotic cells. Jasmin 5 Jasmin 6 Characteristics of prokaryotic cells On the outside of the prokaryotic cell, flagella and pili project from the cell's surface. These structures are made of proteins that facilitate movement and communication between cells; Flagella are made up of protein flagellin. Pili are fine threads of the protein. Some of pili may help the bacterium to stick to a solid surface. Some bacteria have an outermost covering layer which is jellylike outer coating- called a capsules. The capsule is composed of rather amorphous polysaccharide material similar to mucous. This material is hygroscopic, meaning that it has a great capacity to retain water. Enclosing the cell is the cell envelope – generally consisting of a cell wall covering a plasma membrane. The cell wall gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. Jasmin 7 Jasmin 8 The cell wall consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. Peptidoglycan (murein) layers contain the molecules of polysaccharide cross-linked by- chains of amino acids. The wall gives strength and perform the same function as cellulose. It also prevents the cell from expanding and finally bursting (cytolysis) from osmotic pressure against a hypotonic environment. The cells are extremely small(1-10 um) in diameter. Jasmin 9 The cell membrane of bacteria is a selectively permeable barrier between the inside and outside of the cell. The cell membrane is invaginated to form mesosomes- site of respiration. Mesosomes are rosette-like clusters of folds in the plasma membrane, protruding towards the interior of the cell. These structures are important in the aerobic cellular respiration. A key part of this process requires a lot of membrane surface, and mesosomes greatly increase the membrane surface of the cell. Jasmin 10 Tubular or flattened thylakoids containing chlorophyll and other pigment are present in the cytoplasm of photosynthetic prokaryotic cells, such as the cells of blue-green algae and the purple photosynthetic bacteria. Photosynthesis is a vital function which requires a lot of membrane surface, and the long, thin thylakoid surfaces provide that area for photosynthesis to take place. Jasmin 11 Inside the cell is the cytoplasmic region that contains the cell genome (DNA) that is single circular chromosomes or a ring of deoxyribonucleic acid (DNA) in the cytoplasm, not contained in a nuclear membrane. The DNA is condensed in a nucleoid. Nucleoid- region where the cell’s DNA is located. Prokaryotes can carry extra chromosomal DNA elements called plasmids, which are usually circular. Plasmids enable additional functions, such as antibiotic resistance. Cells also contain ribosomes (site of protein synthesis). The prokaryotic cell do not contain/ lack of organelles bounded by membrane such as mitochondria, chloroplast and complex structured flagella. Jasmin 12 Jasmin 13 Eukaryotic cell 2 types of eukaryotic cells: 1) plant cell 2) animal cell Plant, animal, fungi i.e. the majority of living things The name derives from: eu =true/good karyon=nucleus Jasmin 14 eukaryotic animal cell Jasmin 15 Eukaryotic animal cell Animal cell contains protoplasm(nucleus & cytoplasm) surrounded by a thin plasma membrane. Cytoplasm contains variety of organelles such as mitochondria suspended in the cytosol (semi fluid medium) The most prominent organelle is nucleus- containing chromosomes. Jasmin 16 Jasmin 17 eukaryotic plant cell The plant cell is surrounded by a fully permeable external cell wall- made of cellulose, other polysaccharide & protein. Its also surrounded by a plasma membrane (on the inside of the cell wall) & contains a nucleus, ribosomes, ER, Golgi apparatus & microtubules. The plant cells also contain chloroplast which carry out photosynthesis. Cytoplasm, organelles and nucleus are usually pushed to the periphery due to the presence of large central vacuole. Jasmin 18 eukaryotic plant cell Jasmin 19 The differences between Prokaryotic Cell Eukaryotic Cell (Bacteria, cynobacteria) (green plants, fungi) 1) Extremely small 1) Larger cell -usually between 0.5-10 um in -usually between 10-100 um diameter. in diameter. 2) – No true nucleus 2) - Nucleus with distinct - Circular DNA helix in bounding cytoplasm membrane perforated by - DNA not supported with basic nuclear pore protein histones -DNA in linear chomosomes and combined with protein histones 3) Cell wall present containing 3) Cell wall contain cellulose peptidoglycan 4) -Few organelles. 4) - Many organelles. - No membrane-bounded - presence of single ( RER , organelle SER and such as double membrane-bounded mitochondria & chloroplast. organelles (nucleus, Jasmin mitochondria & chloroplast 20 Jasmin 21 Some eukaryotic cells have complex cillia and flagella with 9+2 arrangements of microtubules. Flagella of prokaryote Jasmin 22 Ribosomes DNA combined with protein histones in chromosome Jasmin 23 Difference between Animal cell Plant cell 1) No cellulose cell wall, only plasma 1) Have rigid cellulose cell wall & membrane and irregular in shape. plasma membrane and regular in shape. 2) No plasmodesmata & pit. 2) Plasmodesmata and pit present in cell wall. 3) No chloroplast. 3) Presence of chloroplast in photosynthetic cells for photosynthesis. 4) Small temporary vacuole. 4) Large permanent central vacuole filled with cell sap. 5) Centriole and lysosome present. 5) Centriole and lysosome not present. 6) Carbohydrate storage in form of 6) Carbohydrate storage in form of starch granules. glycogen granules. 7) Some have cilia and 7) No cilia and flagella. flagella.flagella in 8) some specialised cells/ lower plant male gamete/ antherozoid 8) Smaller in size ( 20um diameter). 8) Larger in size ( 50 um in 9) Presence of cholesterol/ absence of diameter). phytosterol Jasmin 9) Absence of cholesterol/ Presence 24 2.2 Cell structure and function Jasmin 25 At the end of this lesson, students should be able to: 1. Describe the structure & function of the following membrane system & organelles: plasma membrane, nucleus and nucleolus, nuclear envelope, mitochondria, rough & smooth endoplasmic reticulum, Golgy body, ribosomes, lysosomes, chloroplast, centerioles and vesicles. Jasmin 26 CELL MEMBRANE Objectives : 1) draw, label and describe membrane structure based on the Fluid Mosaic Model of Singer and Nicholson 2) Describe the function of the cell membrane Jasmin 27 cell membrane All cells are covered by a thin cell surface membrane. The cell membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix to help group cells together in the formation of tissues. The cell membrane is selectively permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can be either passive, occurring without the input of cellular energy, or active, requiring the cell to expend energy in moving it. Jasmin 28 Fluid mosaic model membrane Jasmin 29 Jasmin 30 The cell membrane is described as fluid mosaic model. This model is conceived by S.J. Singer and Garth Nicolson in 1972 to describe the structural features of cell membranes. The membrane is said to be dynamic. Jasmin 31 That means the membrane is not solid, but is like a fluid'. The plasma membrane is described to be fluid because of its component such as phospholipids and membrane proteins which move laterally or sideways throughout the membrane. Jasmin 32 The membrane is also described as mosaic because of the scattered arrangement of protein molecules in the phospholipid bilayer that gives the appearance of a mosaic pattern. Membrane is 7 nm thick. Jasmin 33 membrane: 1) phospholipid bilayer 2) protein 3) cholesterol The cell membrane consists primarily of a thin layer of amphipathic phospholipids which spontaneously arrange itself so that their polar, hydrophilic heads are facing towards the outer surface and attracted to water surrounding and the non polar, hydrophobic tails towards the inner surface. amphipathic molecules. – Amphipathic molecules have This forms a continuous, both hydrophobic regions and hydrophilic regions. spherical lipid bilayer. Jasmin 34 Jasmin 35 The arrangement of hydrophilic heads and hydrophobic tails of the phospholipid bilayer prevent polar solutes (e.g. amino acids, nucleic acids, carbohydrates, proteins, and ions) from diffusing across the membrane, but generally allows for the passive diffusion of hydrophobic molecules. This affords the cell the ability to control the movement of these substances via transmembrane / integral protein complexes such as pores and gates. Phospholipid can move laterally/ flip flop Jasmin 36 Protein found in the membrane falls under 2 categories: a) integral intrinsic protein b) peripheral/ extrinsic protein Some of the protein molecules are found partially or totally embedded in the phospholipid matrix. Those are the intrinsic or integral proteins, which represent nearly 70% of the membrane These proteins cannot be extracted. 1) some integral protein are embedded partially in the membrane. 2) some transmembrane intrinsic/ integral protein penetrate through the membrane Intrinsic protein can move laterally in the matrix. Jasmin 37 Some of the integral proteins have very large molecules that extend throughout the phospholipid matrix, projecting out on both surfaces. These proteins are called tunnel proteins , pore or transmembrane proteins. They are believed to have channels for the passage of water soluble substances. Jasmin 38 Peripheral proteins are not embedded in the lipid bilayer at all; they are loosely bound to the surface of the membrane, often to exposed parts of integral proteins. Some of the proteins are found at the periphery of the lipid layer. Jasmin 39 Function of protein in membrane The proteins present in the membrane may function as carriers or receptors or enzymes. Jasmin 40 Transport A protein that span a membrane may provide a hydrophylic channel across the membrane that is selective for a particular solute. Enzymatic activity A protein built into the membrane may be an enzyme with active site exposed to substances in adjacent solution. In some cases several enzymes acts as a team to carry out sequential steps of metabolic pathway. Signal transduction A membrane may have protein (receptor) that have a binding site that fits the shape of a chemical messenger such as hormone. Jasmin 41 Cell- cell recognition. Some glycoprotein serve as identification tags/ antennae that are specifically recognized by membrane protein of other cells. Intercellular joining Membrane protein of adjacent cells may hook together in various kinds of junctions such as tight junction. Attachment of cytoskeleton and extracellular matrix. Microfilament of cytoskeleton may be bound to protein membrane that helps to maintain the cell shape. Jasmin 42 There are three types of transport proteins in the cell membrane 1) channel proteins which form an aqueous channel, 2) passive transport carrier proteins which bind to specific solute molecules. 3)active transport carrier proteins which bind to specific molecules but move with the expenditure of energy. Jasmin 43 Jasmin 44 cholesterol Cholesterol molecules are found between phospholipid molecules/ tails Cholesterol increase flexibility / stability of membrane / regulate the fluidity of membrane and makes the membrane stronger and more flexible. Increase in temperature cause the cholesterol molecules to increase fluidity of membrane and promote movement of substance across the membrane. At moderate warm temperature, the cholesterol molecules reduce free movement of phospholipid molecules and make the membrane less fluid. At low temperature cholesterol molecules prevent the close packing of phospholipid molecules and slow solidification of the membrane. Finally when it is cool the phospholipid settle into a closely packed arrangement and the membrane solidifies. Thus cholesterol can be thought of as a “fluidity buffer” for the membrane, resisting changes in membrane fluidity that can be caused by changes in temperature. Jasmin 45 Jasmin 46 Membranes must be fluid to work properly; the fluidity of a membrane affects both its permeability and the ability of membrane proteins to move to where their function is needed. When a membrane solidifies, its permeability changes, and enzymatic proteins in the membrane may become inactive if their activity requires movement within the membrane. However, membranes that are too fluid cannot support protein function either. Therefore, extreme environments pose a challenge for life, resulting in evolutionary adaptations that include differences in membrane lipid composition. Jasmin 47 Carbohydrate chain attach to the protein is called glycoprotein. Carbohydrate chain attach to the phospholipid is called glycolipid. This carbohydrate layer is referred to as the glycocalyx of the cell. Jasmin 48 These short carbohydrates chain serve several important functions" Many have a negative electrical charge which repels other negatively charged ions. They serve to attach cells to one another, and to the basement membrane Many act as receptor sites for hormones such as Insulin They enter into immune reactions Jasmin 49 Jasmin 50 Function of membrane a) The plasma membrane forms a protective covering around the cell to maintain its size and shape. b) In addition, it is also responsible for the interactions that occur between the cell and its environment. c) Plasma membrane is described as a semipermeable or selectively permeable membrane since it allows only particles of a particular size, shape, texture and chemical composition to pass through. : 1) small molecules such as oxygen , carbon dioxide and hydrophobic molecules diffuse through phospholipid bilayer down concentration gradient. 2) protein channels and carrier move polar ions , molecules eg.Glucose and amino acid across membrane by facilitated diffusion. 3)protein pump actively transport substances against a concentration gradient across the membrane. Jasmin 51 Jasmin 52 Jasmin 53 Jasmin 54 Jasmin 55 Jasmin 56 Jasmin 57 Jasmin 58 Jasmin 59 Jasmin 60 Jasmin 61 Jasmin 62 Jasmin 63 cytoplasm It consists of : 1) Cytosol contains various components: a)90% of water and solutes such as sugar, amino acid , enzymes, fatty acid ,nucleotides ,ATP, and dissolved gases. b)large molecules eg. protein which form colloid. 2)Cytoskeleton made up of microfilament and microtubules which form the providing support to the cell and are involved in cell motility. Organelles are structures in cell which are suspended in the cytosol and that carry out specialised function e.g. nucleus, mitochondria and chloroplast. Jasmin 64 Cytoskeleton Jasmin 65 Jasmin 66 Jasmin 67 Jasmin 68 Organelles Jasmin 69 Nucleus Nucleus is the most conspicuous and the largest organelle in the cell. The nucleus is a highly specialized organelle that serves as the information and administrative center of the cell. This organelle stores the cell's hereditary material, or DNA, and it coordinates the cell's activities, which include intermediary metabolism, growth, protein synthesis, and reproduction (cell division). Most cells have a single nucleus and hence they are described as uninucleate. Some cells have two nuclei (binucleate) while a few may be even multinucleate. A nucleus generally occurs in all eukaryotic cells. However, a nucleus may be absent in some mature cells. Mammalian RBC and sieve tube cells in plants are enucleate (without a nucleus at maturity). Jasmin 70 Jasmin 71 Jasmin 72 Nucleus is bounded by a double-layered membrane, the nuclear envelope which , separates contents of the nucleus from the cellular cytoplasm. The envelope is perforated with holes called nuclear pores that allow specific types and sizes of molecules to pass back and forth between the nucleus and the cytoplasm. The pores are fully permeable to small molecules up to the size of the smallest proteins, but form a barrier keeping most large molecules out of the nucleus. The space between the outer and inner membranes is termed the perinuclear space and is connected with the lumen of the rough ER. Jasmin 73 The nuclear membrane also encloses a clear fluid matrix called nucleoplasm or karyolymph. The nucleus also contain a round body which is a membrane-less organelle called nucleolus in the nucleoplasm. Nucleolus manufactures ribosomal RNA. The ribosomal RNA will then be assembled with protein to form a ribosome. Besides that the nuclear envelope also Nucleus encloses a fine network of thread-like structures, called chromatin network that contain DNA and histones arranged in a specific manner. Chromatin is roughly half DNA, the genetic material of the cell, and half histone proteins. Nucleosomes are the fundamental unit of chromatin, consisting of clusters of eight histone proteins connected by a DNA molecule. Jasmin 74 The chromatin condenses to form distinct structures called chromosomes at the time of cell division. Substances from cytoplasm enter the nucleus freely and may influence the DNA and at the same time the DNA of the nucleus has its own influence on the activities inside the cytoplasm. Jasmin 75 Jasmin 76 Function of nucleus The nucleus of a cell generally serves the following functions : Nucleus is considered as the controlling centre of the cell. Nucleus contains the genetic information necessary for reproduction, development and metabolism of the organism as a whole. Nucleus maintains the cell by directing the synthesis of structural proteins. Nucleus regulates the metabolic processes in the cell by directing the synthesis of functional proteins such as enzymes. Jasmin 77 Jasmin 78 Nucleus distributes the genetic material equally through the process of replication. Nucleus is involved in the formation of ribosomes. Jasmin 79 The Endomembrane System in Eukaryotic Cells All eukaryotic cells have within them a functionally interrelated membrane system, the endomembrane system consisting of the nuclear envelope, ER and Golgi apparatus, vesicles and other organelles derived from them, and the plasma membrane. Many materials are moved around the cell by the endomembrane system, including some proteins. Jasmin 80 The endomembrane system functions together to produce, store, and export biological molecules, as well as degrade potentially harmful substances. Jasmin 81 Endoplasmic reticulum Jasmin 82 Endoplasmic Reticulum It is a system of highly branching membranes, which almost fills the space inside the cell membrane. Some of the branches of endoplasmic reticulum are connected to the plasma membrane while some of them are connected to the nuclear membrane. Two types of endoplasmic reticulum : A) Smooth endoplasmic reticulum (SER) in which the surface of the tubules lacks granules. B)Rough endoplasmic reticulum has ribosomes on outer surface. Jasmin 83 Smooth endoplasmic reticulum The smooth endoplasmic reticulum consists of tubules and vesicles that branch forming a network. The smooth endoplasmic reticulum allows for an increased surface area for the action or the storage of enzymes. Jasmin 84 Function of smooth endoplasmic reticulum Synthesis of lipids and steroids, Metabolism of carbohydrates, Store and regulate calcium concentration where calcium is released in the muscle contraction process. Detoxify drug and making them more soluble and easier to flush from the body. Jasmin 85 Rough endoplasmic reticulum RER consists of an interconnected system of membrane bound flattened sac called cisternae The surface of the rough endoplasmic reticulum (RER) is studded with ribosomes giving it a "rough" appearance. The membrane of the RER is continuous with the outer layer of the nuclear envelope. Although there is no continuous membrane between the RER and the Golgi apparatus, transport vesicles shuttle proteins between these two compartments. The RER works together with the Golgi complex to send proteins to their proper destinations. Jasmin 86 Function of rough Endoplasmic reticulum 1. It provides intracellular transportation of materials inside the cell. eg. polypeptide chain synthesized on the surface of ribosomes will then enter the cisternae of RER. Jasmin 87 The protein will then be changed to glycoprotein by addition of short polysaccharide chain before the glycoprotein is packed in transport vesicle. Transport vesicle containing protein that has been labelled are budded off/ pinched off from RER and are transported to Golgi apparatus. It forms an internal supporting framework for the cell, called cytoskeleton. Jasmin 88 Jasmin 89 Differences between SER RER Tubular in shape and has A system of flattened smooth appearance membrane bounded sac and has granular or rough appearance due to presence of ribosome found on the outer surface of RER. Site of phospholipid and Site of protein synthesis. fatty acid metabolism. Detoxify harmful Provides intracellular chemicals transportation. Involved in transport of protein to golgi Jasmin 90 Golgi Apparatus Golgi complex appears is a single layer membrane organelle and it consists of a stack of flattened sacs called cisternae. Flattened sac has an internal space or lumen. The sacs are not physically connected. The cisternae are continuously being formed at cis- face and bud off as vesicles at the trans face of the stack. Cis face is located near the endoplasmic recticulum. The trans face is closest to the plasma membrane. Cis face received materials from transport vesicles. Trans face packages molecules in vesicles and transport them out of the Golgi body. Golgi apparatus can produce vesicles full of secretion for external uses and also for internal uses ( formation of lysosomes). In animals Golgi complex is abundant in secretory cells and cells that are rapidly dividing eg. pancreatic cells, goblet cells , cell in the testis and ovaries. Jasmin 91 Jasmin 92 Function of Golgi apparatus. The cis face of Golgi apparatus receives protein/ carbohydrate/ lipid from endoplasmic reticulum through transport vesicle Golgi apparatus sort/modifies / processes/ packs the substances/ macromolecules. Golgi apparatus modifies the protein through glycosylation by adding sugar molecules or short carbohydrate chain to form glycoprotein and stores them. The trans face packages molecules in vesicles and transports substances via secretory vesicle out of the Golgi apparatus. The main function of Golgi complex is to take part in secretion. Secretory vesicles produced by Golgi apparatus contain zymogen(eg. Pepsinogen, trypsinogen) hormones and neurotransmitter. They release their content to the cell’s exterior by exocytosis process. Fusion of Golgi vesicle with the cell surface membrane maintains the membrane which is used to form phagocytic vacuoles and pinocytic vesicles. Golgi apparatus produces polysaccharides for the formation of cell wall. It is also known to be involved in the formation of lysosomes. Jasmin 93 Jasmin 94 Jasmin 95 Jasmin 96 Jasmin 97 Jasmin 98 Proteins that need to to be transported out of the cell are synthesized by ribosome on the surface of the endoplasmic reticulum (ER). At the ER membrane, proteins are inserted through (or into) the membrane via a pore called the translocon. Short carbohydrate chain are added to them to produce glycoproteins, and they are then moved to cis face of the Golgi apparatus in transport vesicles that bud off from the ER membrane. Jasmin 99 Within the Golgi, the protein may be modified further. Sugars are covalently attached in a process called glycosylation, and many different modification steps take place there. Most of these "translocated" proteins are dispatched from the trans face of the Golgi apparatus in transport vesicles that move through the cytoplasm and then fuse with the plasma membrane releasing the protein to the outside of the cell. Examples of secretory proteins are collagen, insulin, and digestive enzymes of the stomach and intestine. then be dispatched from the trans face in a new transport vesicle. Like secretory proteins and some other proteins, proteins destined for lysosomes are made on ribosomes bound to the RER and move through the endomembrane system. In this case the lysosomal protein-containing vesicle that buds from the trans face of the Golgi apparatus is the lysosome itself. Jasmin 100 Jasmin 101 lysosome Lysosomes are tiny, spherical sac-like structures evenly distributed in the cytoplasm. The membrane of lysosome, encloses a variety of hydrolytic enzymes. This membrane prevents the enzymes from digesting the cytoplasm. The substance that is to be hydrolyzed has to enter a lysosome. Lysosome are seen practically in all animal cells with the exceptions like mature mammalian RBC. Largely, they are absent in plant cells. Jasmin 102 Jasmin 103 Function of lysosome 1)Lysosomes carries out intracellular digestion /heterophagy /phagocytosis where it can digest useful organic substances taken in by the cell through endocytosis process. Jasmin 104 When the membrane has encircled the particle, projection then fused together trapping the solid particles within the vacuoles.this vacuole is called phagosome. The phagosome then fuses with lysosomes forming a secondary lysosome.= phagolysosome The ingested material is digested by hydrolytic enzymes of lysosome. Useful digested product are absorbed by the surrounding cytoplasm. Undigested material are expelled through exocytosis process. This process requires energy in the form of ATP. Jasmin 105 2) Autophagy The old and worn out organelles of cells are digested/destroyed in a process called autophagy. Double membrane structure, which grows and enclose mitochondria, ribosomes, and proteins that is non functional. The structure is called an autophagosome or autophagic vacuole. The lysosome then travels toward the autophagosome The outer membrane of the lysosome is then fused with the autophagosome membrane, leading to the release of its content which is wrapped by the inner membrane into the lysosome. The content are then degraded / digested by the various enzymes in the lysosome. The useful digested component are absorbed into the surrounding cytoplasm and useful molecules are used for the formation of new cells. Undigested product are released through exocytosis from the cell. Jasmin 106 Jasmin 107 Jasmin 108 3) Autolysis At times the lysosomal enzymes are capable of destroying the entire cell which is non functional by rupturing the membrane of the lysosome and releasing the digestive enzymes that digest the whole cell. The lysosome acts as a suicide bag that destroy the whole cell. E.g in the metamorphosis of a frog. The tail gradually disappears as it's absorbed into the body through the autolysis process that takes place in each individual cells of the tail. When the froglet is ready to live on land, it usual has a little bit of tail left, but that gradually disappears. Jasmin 109 Jasmin 110 Ribosomes Ribosomes are granule-like structures without membrane found in both prokaryotic and eukaryotic cells. They are found distributed all over the cytoplasm. In a eukaryotic cell ribosomes occur a) freely in the cytoplasm or b) attached to the surface of branches of endoplasmic reticulum (RER), or c) in the matrix of organelles like mitochondria and chloroplasts. It is also found attached to the nuclear membrane and inside the nucleolus. Ribosomes are of two types 70s type and 80s type. 70s ribosomes are found in prokaryotic cells while 80s ribosomes are found in eukaryotic cells. Jasmin 111 S stands for swedberg unit, which is a measure of particle size related to the speed at which the particles settle when subjected to centrifugation (S=1x10-13second). Each ribosome consists of two sub units that is 1)small subunit 2)large subunit. The sub units occur separately in the cytoplasm and join to form a ribosome particle only at the time of protein synthesis. The 70s ribosome has two sub units of 30s and 50s The 80s ribosome has two subunits of 60s and 40s. Jasmin 112 Composition of ribosome Ribosome consists of 50% ribosomal RNA and 50% protein Jasmin 113 Function Site of protein synthesis of ribosome Ribosome contain binding site for mRNA and tRNA molecules. The ribosome moves along the mRNA strand to translate the genetic code on mRNA and thus synthesised polypeptide chain. The protein produced by ribosomes on RER are transported to Golgi apparatus to be processed into digestive enzymes or hormone to be exported out by exocytosis process. Free lying ribosome in cytoplasm synthesis protein and enzyme for intracellular use. Jasmin 114 Jasmin 115 Each mitochondrion is bounded by a double membrane. The outer membrane is smooth while the inner membrane is thrown into numerous folds called cristae. Thus, the surface area of inner membrane is much higher than that of outer membrane. The cristae bear numerous, minute, regularly spaced particles called stalked particles or oxysomes. There may be between 100,000 to 1,000,000 stalk particles in a single mitochondrion! Oxysomes enclose The stalk particle enclose enzymes that are involved in the release of ATP molecules from the oxidation of glucose Jasmin 116 Jasmin 117 The rest of the inner membrane contains some coenzymes that are involved in this process. The inner membrane encloses a matrix, which is rich in proteins, lipids, ribosomes, RNA and one or two circular DNA molecules. The number of mitochondria in a cell varies considerably and depends on the nature of the cell. Cells with very high-energy requirements possess very large number of mitochondria. A human liver cell for example may have more than 1000 mitochondria. Other example are the flight muscle cells in insects. Mitochondria are absent in the mature human RBC. Jasmin 118 Jasmin 119 Function of mitochondria Mitochondria represent the sites of aerobic respiration in a cell. They use molecular oxygen to oxidize glucose into carbon dioxide and water. Oxidation releases energy in the form of molecules of adenosine triphosphate (ATP), which are stored in mitochondria and released in the cell. Hence, mitochondria are described as minute powerhouses of the cell. Jasmin 120 Jasmin 121 Plastid The different kinds of plastids are: The colorless leucoplasts,, are involved in the synthesis of starch, oils, and proteins. Yellow-to-red colored chromoplasts manufacture carotenoids, Chloroplasts are one of several different types of plastids, which is plant cell organelles that are involved in energy storage and the synthesis of metabolic materials. The green colored chloroplasts contain the pigments chlorophyll a and chlorophyll b, which are able to absorb the light energy needed for photosynthesis to occur. On the average, the chloroplast density on the surface of a leaf is about one-half million per square millimeter. Jasmin 122 Jasmin 123 Jasmin 124 Chloroplast One of the most widely recognized and important characteristics of plants is their ability to carry out photosynthesis, i.e. to make their own food by converting light energy into chemical energy. This process occurs in almost all plant species and is carried out in specialized organelles known as chloroplasts. All of the green structures in plants, including stems and unripened fruit, contain chloroplasts, but the majority photosynthesis activity in most plants occurs in the leaves. Jasmin 125 Chloroplast is enclosed in a double membrane ( inner and outer membrane) The area between the two layers that make up the membrane is called the intermembrane space. The outer layer of the double membrane is much more permeable than the inner layer, which features a number of embedded membrane transport proteins. Jasmin 126 Jasmin 127 Stroma is enclosed by the chloroplast membrane. Stroma is a semi-fluid matrix that contains dissolved enzymes and chloroplast DNA and special ribosomes and RNAs. Stroma comprises most of the chloroplast's volume. In higher plants the internal membranes (lamella) forms stacks of closed hollow disks called thylakoids, and inside the thylakoids is the thylakoid space or lumen. A stack of thylakoids Jasmin is called a granum (plural: grana). 128 There are intergranal membrane connecting one thylakoid of a granum to that of another granum. These intergranal membrane form a network between the grana. The membrane of the thylakoids are made up of lipoprotein. The arrangement of thylakoid increase the surface area for locating the chlorophyll. The photosynthetics pigments – the chlorophylls and carotenoids are embedded in the membrane of the thylakoids. The photosynthetic reaction takes place on the thylakoid membrane. Chloroplasts also possess their own genomes that is the chloroplast DNA and special ribosomes Jasminand RNAs. 129 Function of chloroplast Site of photosynthesis. The grana and thylakoid membrane provide large surface area for chlorophyll and accessory pigment to be located to trap light energy and carry out production of ATP and NADPH. The stroma contains enzymes to carry out light independant reaction ( calvin cycle) and production of high energy organic molecules eg. sugar molecules. Jasmin 130 Centrioles Under the electron microscope the centrioles appear as two short, hollow, cylinders usually lying at right angles to each other above the nucleus in animal cells.. Each centriole is made up of nine microtubule triplets, which lie evenly spaced in a ring. There are no microtubules in the center (9+0 arrangement). They are generally absent in plant cells, except in motile plant cells. The centrioles lie in a small mass of specialised cytoplasm called centrosphere. The centrioles and the centrosphere are together described as centrosome. Jasmin 131 Jasmin 132 Centrioles are involved in organizing the spindle fibres and astral rays during cell division in animal cells. They provide basal bodies from which cilia and flagella arise. Jasmin 133 Jasmin 134
