GCE Advanced Level Biology Resource Book PDF (Grade 12)

G.C.E. (Advanced Level) Biology Grade 12 Resource Book Department of Science Faculty of Science and Technology National Institute of Education www.nie. lk Resource Book - Gr.12 G.C.E. (Advanced Level) Biology G.C.E. (Advanced Level) Biology Grade 12 Resource Book Department of Science Faculty of Science and Technology National Institute of Education www.nie. lk G.C.E. (Advanced Level) Biology Resource Book -Gr.12 Biology Resource Book Resource Book - Gr.12 G.C.E. (Advanced Level) Biology Message from the Director General Dr. (Mrs.) T. A. R. J. Gunasekara G.C.E. (Advanced Level) Biology Resource Book -Gr.12 Message from the Director From 2017, a rationalized curriculum is in effect for the G.C.E (A/L) in the education system in Sri Lanka. It means updating the curriculum that was in implementation. In this effort, revisions were made in the content, form and curricular materials of the G.C.E (A/L) subjects Physics, Chemistry and Biology and in concurrence to that, certain changes in the learning teaching methodology, evaluation and assessment were expected. The volume of the subject matter in the curriculum was largely reduced and several alterations in the learning teaching sequence were also made. A new teachers’ hand book was introduced in place of the old curricular material, the teachers’ instruction manual. The teachers’ instruction manual contained a line-up of subject matter expected to be learnt but the newly introduced teacher’s hand book doesn’t accommodate any subject matter. Yet, it provides a guideline for teachers to mould the learning events and for evaluation. Though the teacher’s hand book implicitly enunciates the limits of the subject by way of the learning outcomes, the teachers’ hand book only is not sufﬁcient to identify holistically the conﬁnes of the facts. Thus emerged the need of a resource book which simply describes the subject content. This book comes to you as a result of an attempt to fulﬁll that requirement. When implementing the previous curricula, the use of internationally recognized standard textbooks published in English has been an imperative for Advanced Level Science subjects. But, the contradictions of facts related to subject matter and inclusion of material beyond the limits of the local curriculum in them, usage of those books was not an easy task for teachers and students. This book, offers students an opportunity to study the relevant subject content in the mother tongue within the limits of the local curriculum. It also provides both students and teachers a source of reliable information expected by the curriculum dispensing with seedling information from various print media and extra classes. This book authored by specialist subject teachers and university lectures is presented to you after the approval of the Academic Affairs Board and the Council of the NIE. Thus it can be recommended as a material of high standard. Mr. R. S. J. P. Uduporuwa Director Department of Science Resource Book - Gr.12 G.C.E. (Advanced Level) Biology Guidance Dr.(Mrs.)T.A.R.J. Gunasekara Director General National Institute of Education Supervision Mr. R.S.J.P. Uduporuwa Director, Department of Science National Institute of Education Subject Leadership Miss. P.T.M.K.C. Tennakoon Assistant Lecturer, Dept. of Science National Institute of Education Internal Resource Contribution Mrs. H.M. Mapagunarathna Senior Lecturer National Institute of Education Mrs. D.A.H.U. Sumanasekara Assistant Lecturer National Institute of Education P. Atchuthan Assistant Lecturer National Institute of Education Editorial Panel Prof. B.G.D.N.K.de Silva - Senior Professor of Molecular Biology, University of Sri Jayawardanepura - (Unit 02,03) Prof. S. Abeysinghe - Head of the Department, Department of Botnay University of Ruhuna - (Unit 02,03,04) Prof. S. Hettiarachchi - Senior Professor , Department of Biological Sciences, University of Rajarata - (Unit 02,03,04) Prof. R.A.S.P. Senanayake - Head of the Department, Department of Botany, University of Kelaniya - (Unit 03,04) Prof. D.D. Wickramasinghe - Head of the Department, Department of Zoology, University of Colombo- (Unit 02) Prof. A. Pathirathna - Senior Professor, Department of Zoology and Environmental Managment University of Kelaniya (Unit 05) Dr.P.L. Hettiarachchi - Senior Lecturer,Department of Biological Sciences, University of Rajarata (Unit 04) Dr. W.A.M. Daundesekara - Senior Lecturer,Department of Botany, University of Peradeniya-(Unit 04) Dr. S. Kumburegama - Senior Lecturer, Department of Zoology, University of Peradeniya -(Unit 05) G.C.E. (Advanced Level) Biology Resource Book -Gr.12 External resource contribution Mrs.P.H.N. Kulathilake - SLTS-I, Devi Balika Vidyalaya, Colombo 08. Mrs. B. Ganeshadas - SLTS-I, D.S.Senanayake College, Colombo 08 Mrs. P.A.K.Perera - SLTS-I (Rtd), Mrs. H.L. Hemanatha - SLTS-I ,Royal College, Colombo 07 Mrs. M.S.J. Jayasooriya - SLTS- Ladies College, Colombo 07 Mrs. M.R.P.R Basnayake - SLTS-I (Rtd) , St.Anne’s College, Kurunegala Mr. A. Illaperuma - SLTS-I(Rtd) Mrs. H.A.S.G. Perera - SLTS-I , Sirimawo Bandaranayake College, Colombo -07 Mrs. A.M.S.D. N. Abeyakoon - SLTS-I (Rtd ), St. Anthony’s Girls’ College , Kandy Mrs. S.D.P.Bandara - SLTS-I Rtd), Dharamaraga College, Kandy Mrs. J.A. J.Hanee - SLTS-I Zahira College, Gampola Mr. W.G. Pathirana - SLTS-I Rahula College, Matara Mrs. C.V.S. Devotta - SLTS-I ,Dammissara College, Natthandiya Mr. R. Rupasinghum - SLTS-I, Language Editing Dr. Chandra Amarasekara Diagrams Mrs. N.R.D. Dahanayake- Teacher, Lyceum International school, Nugegoda Cover & Type setting Mrs. R. R. K. Pathirana Technical Assitant- NIE Supporting Staff Mrs. Resource Book - Gr.12 G.C.E. (Advanced Level) Biology Content Message of the Director General, NIE iii Message of the Director Department of Science , NIE iv Team of Resource Persons v-vi Unit 01-Introduction to Biology Nature scope and importance of biology with reference to 01 challenges faced by the mankind The nature and the organizational patterns of the living world 04 Unit02- Chemical and cellular basis of life Elemental composition of organisms 07 Physical and chemical properties of water important for life 07 Chemical nature and functions of main organic compounds of organisms 09 Contribution of microscope to the expansion of knowledge on cells and cellular organization 22 Historical background of the cell and analyses the structure and functions of the sub cellular units 24 The cell cycle and the process of cell division 39 The energy relationships in metabolic processes 49 The role of Enzymes in regulating metabolic reactions 51 Photosynthesis as an energy ﬁxing mechanism 58 Cellular respiration as a process of obtaining energy 67 Unit 03-Evolution and diversity of organisms The theories of origin of life and natural selection to analyze the process of evolution of life 72 Hierarchy of taxa on scientiﬁc basis 77 The diversity of organisms within the Domain Bacteria 82 The diversity of organisms within the kingdom Protista 84 The diversity of organisms within the kingdom Plantae 86 The diversity of organisms within the kingdom Fungi 94 The diversity of organisms within the kingdom Animalia 96 The characteristic features to study organisms belonging to phylum Chordata 103 Unit 04- Plant form and function Structure, growth and development of plants 106 plant tissues systems 109 Growth and development process of a plant 115 The shoot architecture and light capture 120 The process of gaseous exchange in plants 121 G.C.E. (Advanced Level) Biology Resource Book -Gr.12 Acquisition of water and minerals 126 Process involved in transport of materials in phloem 132 Processes of water loss in plants 134 The modes of nutrition of plants 137 Nutritional requirements for the optimal growth of plants 138 Trends in life cycles, to relate the adaptations of plants for a terrestrial life 141 Structures and functions associated with sexual reproduction in ﬂowering plants 146 Responses of plants to different stimuli 150 The role of plant growth substances/regulators/hormones in response to different 152 Response of plants to some biotic and abiotic stresses 154 Unit 05- Animal form and function Relate the structure of animal tissues to their functions 156 Nutrition in animals 163 Human digestive system to its functions 165 Circulatory systems in animals 181 Basic plan of blood circulatory system and lymphatic 185 Gas exchange in animals 200 Immunity 212 Osmoregulation and excretion 222 Resource Book G.C.E. (Advanced Level) Biology Introduction to Biology Nature scope and importance of biology with reference to challenges faced by the mankind B iology is a Science which is focused on studying of living organisms. (Bios- Life, logos ”study of”) The conceptof “LIFE” is not easy to deﬁne. Still scientists are unable to provide an acceptable deﬁnition for life. “Life” is something special and unique which cannot be explained using laws of chemistry and physics. Biology is a subject which is very complex and vast. Hence for the convenience of studying, it has been divided into three primary braches: Zoology (the study of animals), Botany (study of plants) and Microbiology (study of microorganisms). Some areas of study in these branches: Cell Biology (studying cells) Histology (studying tissues) Anatomy (studying about gross structure of the body) Physiology (studying function) Biochemistry (studying biological molecules) Genetics (studying inheritance) Ecology (studying environment) G.C.E. (Advanced Level) Biology Resource Book Issues pertaining to Biology Understanding biological Diversity At present our planet is rich in diversity. Life on earth formed around 3.8 billion years ago. The ﬁrst formed organisms are believed to be heterotrophic, anaerobic prokaryotes. Since then the evolutionary process resulted in the extensive biodiversity which exists in the biosphere now. Scientists assume, based on their studies that there are about 10 to over 100 million species in the world. There is a dynamic relationship between living world and the inanimate world and each and every organism has a speciﬁc role in the environment for existence of the Biosphere. The variety of life on earth, the number of species of plants, animals and microorganisms, the diversity of genes in these species, the different ecosystems on the earth such as deserts, rainforests and coral reefs are all part of a biologically diverse earth. Understanding the human Body and its functions When studying biology,especially by studying histology, anatomy of the human body,one can gain the knowledge about the structure of the organs. This results in understanding and appreciation of the organization of the human body and understanding the functions of different organ systems and the relationship between structure and functions. Sustainable use and Management of natural resources and Environment Natural resources are sources of materials and energy found naturally which are used in everyday life and for economic development. These natural resources are limited on earth. Due to the increase of growth rate of human population, overuse of natural resources is taking place. It causes threat of depletion of natural resources. Due to over exploitation of natural resources, various environmental problems arise such as; Environmental pollution Loss of Biodiversity Desertiﬁcation Hence to overcome the above problems management of natural resources and Environment should be practiced. Knowledge of Biology is useful to bring about remedies for the above problems. Sustainable Food production Sustainable food production is the production of sufﬁcient amounts of food for the Resource Book G.C.E. (Advanced Level) Biology human population using environmentally safe methods. The current human population is about 7 billion and expected to be double in less than 40 years. Therefore, for the survival of human beings sustainable food production is necessary. To maintain sustainable food production following methods can be applied, which are based on knowledge in biology. Production of high yielding varieties of plants and animals. Production of disease resistant plants and animal varieties. Improve the post harvest technological methods. Understanding plant life Plants are the primary producers in the world. All the animals depend directly or indirectly on plants. Therefore understanding plant life is important. As the time human population is increasing we need to increase the productivity. Therefore understanding plant function and biology is important to produce high yielding plants, disease resistant plants, etc. Understanding diseases and causes To maintain healthy human body one should have the knowledge of causes of the diseases and their effects. Some dangerous diseases which exist in current world are non communicable diseases such as cancers, heart diseases, diabetes, chronic renal diseases and communicable diseases such as dengue, AIDS, etc. Cancers- causes for this is not fully understood yet. Cancers are one of the leading causes of death. AIDS- is a viral disease which is a serious and growing health problem worldwide. Heart diseases- This is also a serious and growing health problem worldwide. Causes are not fully understood yet. Chronic renal diseases- In Sri Lanka, recentlyCKDu has become a serious health problem. Currently scientists are working on prevention, remedial measures and cures for such diseases. G.C.E. (Advanced Level) Biology Resource Book Solving some legal and ethical issues Knowledge and application of biological concepts is important in solving some legal issues, such as parentage testing, in criminal investigations and to solve immigration disputes. DNA ﬁngerprinting is used in above circumstances. The nature and the organizational patterns of the living world In accordance with different criteria we can see a diversity among living organisms. Organisms are diverse based on size, shape, form and habitats. Living organisms show a wide range of variation in size, shape, form and habitat. Size – Bacteria – 0.25 µm – 2 µm to Giant Sequoia (Giant Red Wood)– 100m Shape – Organisms are diverse in shape, Ex: Cylindrical (earth worm), streamline shape(birds, ﬁsh) Form –Unicellular (Amoeba), multicellular (any plant or animal) Habitat – Terrestrial (Rat), aquatic (Fish), arboreal (Loris), aerial(Birds) Characteristics of organisms In order to survive, each organism whether simple or complex must be able to perform certain functions. Following features are the characteristics of organisms. (i) Order and organization From molecular level to biosphere there is an order and organization in organisms to perform their biological activities efﬁciently. Lower level components are organized in a methodical pattern in upper level to make it most efﬁcient.e.g: plant leaf and human eyes. (ii) Metabolism The sum of all chemical activities taking place in an organism is its metabolism. It includes catabolic reactions and anabolic reactions. (iii) Growth and development All organisms begin their life as a single cell. During growth an irreversible increase in dry mass occurs, which is characterized only by the living. Irreversible changes that occur during the life span of an organism is development.Growth and development are two consequent processes that happen in the life span of organisms. (iv) Irritability and coordination Irritability is the ability to respond to stimuli from both internal and external environment. Movement of organisms occurs as a result of irritability and coordination. In animals Resource Book G.C.E. (Advanced Level) Biology this happens as a result of coordinated efforts of nervous, hormonal, muscular and skeletal systems (v) Adaptation Adaptation is a peculiarity of structure, physiology or behavior that promotes the likelihood of an organism’s survival and reproduction in a particular environment. E.g: Sunken stomata in Xerophytes, Viviparity in some mangroves, Splayed out foot in camels. (vi) Reproduction Ability to produce offspring for continuous existence of species (vii) Heredity & Evolution Organisms have genes that pass from one generation to the next and control speciﬁc physiological, morphological and behavioral characters of organisms. Ability of organisms to change over time as a result of genetic modiﬁcation is evolution Many non living entities may have one or more of these characteristics but not all of them e.g., crystals grow, waves move but only living organisms display all these characteristics simultaneously or at some point during their life cycle. By considering this it can be said that these are occurring in single celled organisms as well as highly complex organisms such as humans and Anthophytes (ﬂowering plants). Hierarchiel levels of organization of living things The cell is the basic structural and functional unit of life. Some organisms are unicellular while others are multicellular.Cell is composed of several organells which are formed by different organic molecules.Then hierarchieal levels of organization of living things can be constructed by using relevant examples at each level. Molecules,Organells, Cells, Tissues, Organs, Organ systems, Organisms, Populations, Communities, Ecosystems, Biosphere G.C.E. (Advanced Level) Biology Resource Book 02 Chemical and cellular basis of life Elemental composition of living matter T here are about ninety two elements naturally occur in earth’s crust. Of which, about 20-25% elements are essential to continue healthy life and reproduction. (about 25- elements are essential for humans and about 17 for plants). Oxygen (O), Carbon (C), Hydrogen (H), and Nitrogen (N) make up 96% of living matter. Calcium (Ca), Phosphorous (P), potassium (K) and sulphur (S)- make up most of the remaining 4% of the mass of the organism. In humans, C, H, O, N- accounts for 96.3% of the body mass and Ca, P, K, S, Na, Cl, Mg and trace elements accounts for the remaining 3.7%. (e.g. B (Boron) , Co (Cobalt), Cu (Copper), Cr (Chromium), F (Fluorine), I (Iodine), Fe (Iron), Mo (Molybdenum), Mn (Manganese), Se (Selenium), Si (Silicon), Sn (Tin), V (Vanadium), Zn (Zinc) Physical and chemical properties of water important for life Water is a vital inorganic molecule; life could not exist on this planet without water. It is important due to following reasons, 1. Vital chemical constituent of living cell 2. Provides a biological medium for all organisms Most of above properties are based on the chemical structure of water molecule. Physical and chemical properties of water molecule provide the ability to render the vitality. Water molecule is a small, polar and angular molecule. Resource Book G.C.E. (Advanced Level) Biology δ+ partial positive δ-partial negative Fig 2.1: Chemical structure of the water molecule Polarity is an uneven charge distribution within a molecule. In water molecule, oxygen atom is slightly negative and hydrogen atom is slightly positive. Weak attractions between the slightly polar hydrogen atom of one water molecule and the slightly polar oxygen atom of adjacent water molecuole are known as hydrogen bonds.These hydrogen bonds play a major role in maintaining all the properties of water. Fig 2.2: Hydrogen bonding in water The properties of water arise due to attractions of different water molecules. When the water is in liquid form its H bonds are very fragile. H bonds form, break and reform with great frequency. Four major properties of water to maintain life on earth Cohesive behavior Ability to moderate temperature Expansion upon freezing Versatility as a solvent Properties of water related to functions G.C.E. (Advanced Level) Biology Resource Book 1. Cohesive behavior Attraction between water molecules due to hydrogen bonding is known as cohesion. Attraction between water molecules and other substances are known as adhesion. Both of the above properties of water allow it to act as a transport medium. Due to cohesion between water molecules, water and dissolved substances such as minerals and nutrients transport through vascular tissues, xylem and phloem against gravity. Adhesion between water molecules and cell walls also helps in conduction of water and dissolved substances. Water has a high surface tension. This ability is given to water molecules, due to cohesion between the water molecules. Therefore, in an aquatic system, upper surface water molecules are attracted by lower surface molecules and it forms a water ﬁlm. Small insects e.g. water skaters can walk on the surface of a pond. 2. Ability to moderate temperature Water can absorb or release a relatively high amount of heat energy by a slight change in its own temperature. Due to the high speciﬁc heat, water will function as thermal buffer in living system and aquatic bodies during the temperature ﬂuctuations on earth. Due to the high heat of vaporization, with the minimum loss of water an organism can release much heat energy. Therefore, body surface of an organism maintained as cool surface. e.g. Prevent from overheating. Evaporation of sweat from human skin helps to maintain the body temperature at constant level. Transpiration in plants keeps the plant body surface as a cool surface and prevent from becoming too warm in the sunlight. 3. Expansion upon freezing Generally, in an increase in temperature of any substances, reduces their density and on the other hand, in a decrease in temperature increases their density. When the temperature of water falls below 4 ˚C, it begins to freeze and forms a crystalline lattice called ice cubes. Therefore water has the maximum density at 4˚C.Hence, ice ﬂoats on the surface of water bodies. It is an important property of water in polar regions, where, organisms in aquatic bodies can survive during the winter. Resource Book G.C.E. (Advanced Level) Biology 4. Versatility as a solvent This ability is given to water due to their polarity. Polar molecules (e.g. Glucose), non polar ionic (e.g. NaCl), both polar and ionic (e.g. lysozymes) can dissolve in water, because water molecules surround each of the solute molecules and form hydrogen bonds with them. Solubility depends on polarity and not in their ionic nature. Chemical Nature and Functions of Main Organic Compounds of Organisms Carbohydrates Most abundant group of organic compound on earth is carbohydrates. Major elemental composition is C, H, and O. Hydrates of carbon contain the same proportion of H: O which equals to 2:1 as in water.General formula is Cx(H2O)y. Three major groups of carbohydrates are monosaccharides, disaccharides and polysaccharides. Generally carbohydrates include sugars (monosaccharides and disaccharides) and polysaccharides. Monosaccharides The simplest form of carbohydrates having general molecular formula as (CH2O)n are monosaccharide. Where C varies from 3-7. All monosaccharide are reducing sugars, water soluble and occur in crystalline form. According to the number of carbon atoms, they are named as; 3C- Triose e.g. Glyceraldehydes (Phosphoglyceraldehyde is a derivative of Triose) 4C- Tetroses.g. Erythrose (rare in nature) 5C- Pentoses.g. Ribose, Deoxyribose, Ribulose (RUBP is a derivative of ribulose) 6C- Hexoses e.g. Glucose, Fructose, Galactose According to the type of carbonyl (Keto, aldo)group, they are classiﬁed as; a. Aldoses-glucose, galactose b. Ketoses-fructose G.C.E. (Advanced Level) Biology Resource Book Aldose C O Aldehyde CH2OH group O C H H H C OH OH HO C H OH H C OH H C OH CH2OH Fig 2.3: Solid form of glucose Fig 2.4: Aqueous form of Glucose molecule Ketose C1H2OH O H Keto group CH2OH C 2 C O C H OH OH CH2OH H C3 OH C C H C4 OH OH H H C5 OH C6H2OH Fig 2.5: Solid form of fructose Fig 2.6: Aqueous form of fructose In aqueous media some monosaccharides are in ring form (No need to memorize the chemical structures) Resource Book G.C.E. (Advanced Level) Biology Disaccharides They are sugars formed by joining two monosaccharides by a glycosidic bond. Fig 2.7: Formation of sucrose Fig 2.8: Formation of maltose (no need to memorize the chemical structures) Glycosidic bond is formed by removal of a water molecule from two adjacent monosaccharides by a condensation reaction. Water molecule is formed from OH group of one monosaccharide molecule and H from adjoining monosaccharide molecule. Glucose + Glucose Condensation Maltose + H20 Glucose + fructose Condensation Sucrose + H20 Condensation Glucose + Galactose Lactose + H20 Maltose and lactose are reducing sugars and sucrose is a non reducing sugar. Polysaccharides They are macromolecules and biopolymers. Polysaccharides are made up of few hundred to a few thousand monosaccharide subunits They are non crystalline, water insoluble, and not considered as sugars. Some polysaccharides act as storage components where others contribute to the structure of living organisms. Based on their function they are categorized as storage polysaccharides and structural polysaccharides. Storage- Starch, Glycogen Structural- Cellulose, Hemicellulose, Pectin Based on their architecture they are categorized as Linear forms- Cellulose, Amylose Branched forms- Glycogen, Amylopectin, Hemicellulose G.C.E. (Advanced Level) Biology Resource Book Table 2.1: Major polysaccharides, their monomers and functions Polysaccharide Monomer Functions Starch Glucose Stored in plants Glycogen Glucose Stored in animals and fungi Cellulose Glucose Component of Cell wall Inuline Fructose Stored in tubers of Dhalia Pectin Galacturonic acid Component of Middle lamella of plant cell wall Hemicellulose Pentose Component of Plant cell walls Chitin (nitrogen Glucosamine Component of containing Fungal cell walls polysaccharide) and exoskeleton of Arthropods Functions of carbohydrates Monosaccharides Energy source Building blocks of disaccharides and polysaccharides (disaccharides such as maltose, sucrose and polysaccharides such as starch, glycogen) Components of nucleotides (DNA, RNA) Disaccharides Storage sugar in milk- Lactose Translocation in phloem –Sucrose Storage sugar in sugarcane- Sucrose Polysaccharides a.) Storage polysaccharides- starch stores glucose as energy source in plants and chlorophytes glycogen stores glucose as energy source in animals and fungi inulin stores fructose as energy source in Dahlia tubers Resource Book G.C.E. (Advanced Level) Biology b.) structural polysaccharides- Cellulose in the cell walls of plants and chlorophytes Pectin in the middle lamella of plant tissues. Hemicellulose in cell walls of plants. Peptidoglycan in the cell walls of prokaryotes. Chitin in the cell walls of fungi and in exoskeleton in Arthropods. Lipids Diverse group of hydrophobic molecules Large biological molecules but not considered as polymers or macromolecules. Consist of C, H, O and H:O ratio is not 2:1. Comparatively more H are present. Biologically important types of lipids: Fats, Phospholipids and Steroids. Fats Fats are made up of glycerol and fatty acids; Glycerol belongs to alcohol group having 3 carbons where each of them bear single hydroxyl group. Fatty acids are hydrocarbon chains with long (16-18) carbon skeleton with a carboxyl group at its one terminal. Fatty acid molecules bind to each hydroxyl group of glycerol by ester bond. Resulting fat molecules are called as triacylglycerol. Fig 2.9: Formation of Triacylglycerol Hydrocarbon chains of fatty acids contribute to the hydrophobic nature of the fats. Based on the nature of hydrocarbon chains of fatty acids, they are categorized as a) Saturated fats- fats are made up of saturated fatty acids: fatty acids with hydrocarbons having no any double bonds. Usually animal fats come under this category. They are mostly solid at room temperature. e.g: butter b) Unsaturated fats- fats are made up of unsaturated fatty acids- fatty acids with hydrocarbons having one or more double bonds. Usually plant fats come under G.C.E. (Advanced Level) Biology Resource Book this category. They are mostly liquid in room temperature. e.g: vegetable oils. Unsaturated fats may classify based on the nature of their double bonds. a) Cis Unsaturated fat b) Trans Unsaturated fat Consumption of excess saturated fats and trans unsaturated fats contribute arthrosclerosis. Phospholipids Phospholipids are major components of the cell membranes. They are composed of two fatty acids and one phosphate group attached to one glycerol molecule. The phosphate group gives the negative electrical charge to the phospholipid molecule. Typically an additional polar molecule or small charged molecule is also linked to the phosphate group e.g. choline. The two ends of the phospholipids show different behavior. The hydrocarbon tails are hydrophobic while phosphate group and its attachment (head) are hydrophilic. Choline Phosphate Glycerol Fatty acids (a) Strucural formula (b) Space - ﬁlling model (no need to memorize the structure) Fig 2.10: structure of the phospholipid molecule Functions of Lipids food reserve as energy source (triglycerides such as fats and oils) maintain the ﬂuidity of plasma membrane (phospholipids, cholestrol) act as signaling molecules (eg. Hormones) that travel through the body found as components of animal cell membrane (cholesterol) Resource Book G.C.E. (Advanced Level) Biology Protein Proteins are made up of amino acids. Twenty different amino acids are involved in the formation of proteins. Elemental composition is C, H,O,N and S. At the centre of the amino acid is an asymmetric carbon atom except in glycine. Each amino acid is composed of an amino group, a carboxyl group, a hydrogen atom and a variable group symbolized by R, which is an alkyl group. In the case of glycine R is replaced by H atom. The R group also called the ‘side chain’ differs with each amino acid where as the other groups are in the ‘ back bone’ (including the H atom). Alkyl Group Amine Carboxyl Group Group Fig 2.11: Structure of an Amino acid molecule Amino acids may have one or more carboxyl groups and amino groups. Amino group has alkaline nature and carboxyl group has acidic nature. When both characteristics are found in one molecule they are known as amphoteric molecules. Therefore, amino acids are amphoteric. Two Amino acids undergo condensation reaction by removing a water molecule from both and result a bond known as peptide bond; Fig 2.12: Formation of peptide bond G.C.E. (Advanced Level) Biology Resource Book Protein is composed of one or more polypeptide chains which are composed of amino acids. Levels of protein structures There are four levels of structure which play important roles in their functions; a) Primary b) Secondary c) Tertiary d) Quaternary a) Primary structure The unique sequence of linearly arranged amino acids linked by peptide bonds is the primary structure of proteins. b). Secondary structure The primary structure of a single polypeptide chain coils and folds, as a result of intra molecular hydrogen bonds between the oxygen atoms and the hydrogen atoms attached to the nitrogen atoms, of the same poly peptide chain backbone, to form the secondary structure, which is either β pleated or alpha helical. Alpha helix- e.g.Keratin. β pleated sheet e.g.spider’s silk ﬁber Fig 2.13: beta pleated sheet and alpha helix of secondary structures of proteins Resource Book G.C.E. (Advanced Level) Biology b) Tertiary structure Usually the secondary polypeptide chain bends and folds extensively forming a precise compact unique, functional and three-dimensional shape resulting from following interactions between the side chain/ R-group of amino acids; H bonds Disulphide bonds Ionic bonds Van der Waals interactions/ Hydrophobic interactions e.g.most of the enzymes, myoglobin, albumin c) Quaternary structure Aggregation of two or more polypeptide chains involve in the formation of one functional protein. Separate chains are called protein subunits which were held together by inter and intra-molecular interactions. e.g.Haemoglobin, Collagen Heme Iron β subunit α subunit α subunit β subunit Hemoglobin Fig 2.14: structure of the hemoglobin molecule Denaturation of proteins Denaturation of protein is the loss of speciﬁc chemical three dimensional shape due to the alteration of weak chemical bonds and interactions. Agents affecting the denaturation 1. High temperature and high energy radiation 2. Strong acids, alkaline and high concentrations of salts 3. Heavy metals 4. Organic solvents and detergents G.C.E. (Advanced Level) Biology Resource Book Functions of the proteins Table 2.2 Functions of Proteins Type of protein Example Functions Catalytic protein Pepsin, Amylase Catalyze biochemical reaction Structural protein Keratin, Prevent desiccation Collagen Provide strength and support Storag e Ovalbumin Storage protein in egg Casein Storage protein in milk Transport Haemoglobin Transport O2 and CO2 Serum albumin Transport fatty acids Hormones Insulin Regulate blood glucose Glucagon level Contractile/ Motor Actin/Myosin Contraction of muscle ﬁbres Defensive Immunoglobins Eliminate foreign bodies Nucleic acids Nucleic acids are Polymers exist as polynucleotides made up of monomers called nucleotides. They contain C, H, O, N and P. Nucleic acids are macromolecules, biopolymers. There are two types of Nucleic acids: DNA (Deoxyribo nucleic acids) and RNA (Ribonucleic acids). Structure of nucleotides Nucleotides have 3 components; namely pentose sugar, nitrogenous base and a phosphate group Phosphae Group Nitrogenous Base Pentose Sugar Fig 2.15: Structure of nucleotides ( no need to memorize chemical structures) A nucleotide without a phosphate group is called a nucleoside.e.g. Adenosine, Guanosine Resource Book G.C.E. (Advanced Level) Biology Pentose sugar Pentose sugars are two types; namely Deoxy ribose and ribose (in deoxyribose one oxygen atom is less than in ribose) Nitrogenous bases There are two major groups of nitrogenous bases: 1. Purines- larger in size with two rings 2. Pyrimidines- smaller in size with a single ring In purines there are two types; namely Adenine, Guanine. In pyrimidens there are three types, Thyamine, Uracil and Cytocine. Bases are commonly represented by letters A, G, T, U and C respectively. Phosphate group It gives the nucleic acids the acidic nature. Formation of nucleic acids Millions of nucleotides join by phospho-di-ester bond to form polynucleotide chains by condensation between the –OH group of the phosphate of one nucleotide with the –OH attached to 3rd carbon of pentose sugar of the other. These bonds results in a backbone with a repeating pattern of sugar-phosphate units. Nucleic acids are linear polymers of nucleotides. There are two kinds of nucleic acids depending on the type of the sugar molecules involved. If the sugar molecule in the nucleotide is deoxyribose,the nucleic acid is (DNA). If the pentose sugar is ribose, then the nucleic acid is RNA. DNA contains Adenine, Thymine, Guanine and Cytosine and RNA contains Adenine, Guanine, Cytosine and Uracil. Structure of DNA molecule (Watson and Crick model) DNA molecules have two anti-parallel polynucleotide chains that spiral around an imaginary axis, forming a double helix. The two sugar-phosphate backbones run in opposite directions from each other, and the arrangement is referred to as anti-parallel. The sugar phosphate backbones are on the outside of the helix, and the nitrogenous bases are paired in the interior of the helix. The two strands are held together by hydrogen bonds between the paired nitrogen bases. Base pair rule Always a purine base, pairs with a speciﬁc,pyrimidine base, A=T (2 hydrogen bonds) G≡C (3 hydrogen bonds) G.C.E. (Advanced Level) Biology Resource Book Hence two chains (strands) are said to be complementary to each other. These pairs are known as complementary base pairs.In this original double helical structure, one complete turn consists of ten base pairs as shown in the diagram. Fig2.16: The structure of the DNA and RNA molecules ( no need to memorize chemical structures) Functions of DNA Store and transmit genetic information from one generation to the next generation Store the genetic information for protein synthesis Structure of RNA This is normally a single stranded nucleic acid composed of ribo-nucleotides containing bases, Uracil (U), Cytosine (C ), Guanine (G), Adenine (A). Complementary base pairing between two RNA molecules or within the same molecule may occur in some. Complementary base pairing facilitates three dimensional shapes essential for their functioning. Adenine binds with Uracil with two hydrogen bonds and Guanine binds with Cytosine with three hydrogen bonds. There are three types of RNA present in cells, 1. Messenger RNA (mRNA) 2. Transfer RNA (tRNA) 3. Ribosomal RNA (rRNA) Resource Book G.C.E. (Advanced Level) Biology 1. Messenger RNA Messenger RNA is a linear molecule and is the least abundant type of RNA in a cells comparatively. There are two functions; Copies the genetic information stored in DNA molecule as a sequence of nitrogenous bases Transports genetic information from nucleoplasam to the site of protein synthesis (ribosome) through nucleopores 2. Transfer RNA (tRNA) Smallest RNA molecule. Linear, but forms three- looped structure as shown in the diagram. Fig 2.17: Structure of the tRNA molecule Function - transportation of amino acids to the site of protein synthesis. 3. Ribosomal RNA It is the most abundant type of RNA. rRNA has a complex irregular structure. It provides the site where polypeptide chains are assembled. Differences between DNA and RNA 1. DNA is double stranded molecule while RNA is a single stranded molecule. 2. DNA consists of A, T, G and C and absence of U, while RNA consists of A,U, G and C and absence of T 3. Sugar molecule in RNA is ribose, while in DNA it is deoxyribose. G.C.E. (Advanced Level) Biology Resource Book Nucleotides other than those found in nucleic acids ATP, NAD+, NADP+, FAD and their functions Functions of ATP Universal energy carrier Functions of NAD+ Act as a coenzyme Act as an electron carrier Function as an oxidizing agent during respiration Functions of NADP+ Act as coenzymes Act as an electron carrier NADP+ act as a reducing agent in photosynthesis Functions of FAD Act as a coenzyme Act as an electron carrier Contribution of microscope to the expansion of knowledge on cells and cellular organization Advancement of the cytology is mostly based on the microscopy. The discovery and early study of cells progressed with the invention of microscope. Light microscope Visible light is passed through the specimen and then through glass lenses. The lenses refract the light in such a way that the image of the specimen is magniﬁed as it is projected into the eye.The simplest microscope is a single lens. The compound light microscope Compound light microscopes are commonly used in school laboratories and it is used in medical laboratories as a diagnostic tool. Resolution power and magniﬁcation are important parameters which can be seen in a microscope. Magniﬁcation is ratio of an object’s image size to its actual size. Usually the maximum magniﬁcation of light microscope is 1000 times the actual size of the specimen) Resolution power is minimum distance between two points that can be distinguished as separate points (resolution power of light microscope is 0.2µm). It is a measure of Resource Book G.C.E. (Advanced Level) Biology the clarity of the image. Magniﬁcation is limited due to the resolution. Light from an object (specimen on the slide) passes ﬁrst through objective lens. Then produce a magniﬁed image. Above image then acts as an object for the second lens (the eye piece lens) which further magniﬁes it. The total magniﬁcation is hence the product of the magniﬁcation of each lens. Magniﬁcation of Magniﬁcation of Total magniﬁcation = X objective lens objective lens e.g-.If magniﬁcation of Objective lens = ×40, eyepiece =×15 Total is =15 × 40= ×600 time magniﬁed The Electron Microscope The limitation imposed upon the resolution power of the light microscope by the wavelength of light. The resolution power is inversely proportional to the wavelength. Due to this, scientists considered the use of other forms of radiations with comparatively shorter wavelengths. As a result, electron microscopes were developed. In electron microscopy, a beam of electrons is focused through the specimen or on to its surface. This means, that in theory, the electron microscope should be able to magnify objects up to 1×108 times. In practice, it magniﬁes just over 5×105 times. Electron microscopes have revealed many organelles and other sub cellular structures those were impossible to resolve with the light microscopes. There are two types of electron microscopes. 1. Transmission electron microscopes (TEM) 2. Scanning electron microscopes(SEM) Transmission electron microscopes It is used to study the internal structures of cells.In this microscope, a beam of electrons is passed through a thin, especially prepared slice of material. A very thin specimen is used. Specimens stained with heavy metals which attach more to certain cellular structures than other areas. Image reﬂects the pattern of electrons passed through the specimen, displays on a screen. While electrons pass through the specimen, more electrons may get displayed in regions where structures were densely stained. G.C.E. (Advanced Level) Biology Resource Book Scanning electron microscopes In this instrument, a ﬁne beam of electrons is reﬂected from the surface of specimen. Specimen is mostly coated with gold prior to observation. Here the specimen scatters many electrons whereas others are absorbed. This instrument is ideal to observe the surface view in three dimensional appearances. Table 2.3: Differences between light and electron microscope Light Microscope Electron microscope Glass lenses are used to focus the Powerful magnets are used to light rays focus beam of electrons Image is directly detected by naked Not directrly detected by naked eye eye, micrographs are used Living and non living objects can Only non-living objects are be observed observed Actual color of the object can be Actual color cannot be observed. observed Images are developed Dyes used to stain the object Heavy metals are used to stain the object Historical background of the cell and analyses the structure and functions of the sub cellular units Cell theory All organisms are composed of cells. Recall the hierarchy of life, the levels of organization mentioned earlier. The basic unit which can be called “living” is the cell, which may form a single celled organism (e.g.Chlamydomonas, Yeast) or a multi-cellular plant or animal. The cell is the basic structural and functional unit of life. The level of organization of matter represented by a cell shows all the characteristics of life. Any stage below level of a cell cannot be considered living, whether it is a single celled organism or multi-cellular plant or an animal. Robert Hooke (1665) examined a cork using simple microscope and gave the term “CELL” to describe the basic units. Anton Van Leeuwenhook (1650), a contemporary of Robert Hooke, was the ﬁrst to describe and record living single celled organisms, Euglena & bacteria Matthias Schleiden (1831), a botanist, studying plant tissues concluded that all plants are made up of cells. Theodore Schwann a zoologist (1839) concluded that animal tissues are also made up of cells. Resource Book G.C.E. (Advanced Level) Biology Rudolf Virchow (1855) showed that all cells arise from pre-existing cells by cell division, Schleiden, Schwann and Virchow presented the ‘Cell Theory’ which included the following. 1.All organisms are composed of one or more cells. 2.The basic structural and functional unit of organisms is the cell. 3.All cells arise from pre-existing cells. Organization of cells There are two kinds of cellular organization - Prokaryotic and Eukaryotic All cells share certain basic features. They are; All cells are bounded by a plasma membrane which is a selective barrier Within the cell have, a semiﬂuid, jelly like substance which is called cytosol. Subcellular components are suspended within the cytosol. They carry DNA as genetic materials. Ribosomes are found in all cells Table 2.4: The differences between Prokaryotic cells and Eukaryotic cells Feature Prokaryote Eukaryote organism Bacteria, Archaebacteria Protists,Fungi, plants, animals Cell size Average diameter 1-5µm 10µm-100µm diameter Form Mainly unicellular Mainly multicellular (except most of protista and some fungi are unicellular) Evolutionary 3.5 billion years ago 1.8 billion years ago ,evolved from origin prokaryotes Cell division Binary ﬁssion, Mitosis, meiosis, or both; no mitosis and meiosis Genetic material DNA is circular and lies DNA is linear and contained in a nucleus. free in the cytoplasm. DNA is associated with proteins This region is called nucleoid, DNA is naked and not associated with proteins Type of ribosomes 70s ribosome (smaller) Both 70s (Mitochondria and Chloroplast) and 80s ribosomes (larger) present (may attach to endoplasmic reticulum) G.C.E. (Advanced Level) Biology Resource Book Organelles Few organelles, none Many organelles, membrane bounded are surrounded by organelles present. Great diversity of membrane organelles. Internal membranes e.g. nucleus, mitochondria, chloroplasts scarces; if present bounded by two membranes. usually associated e.g. Lysosomes, Vacuole, bounded by with respiration, single membrane. photosynthesis and N2 ﬁxation. Cell walls Peptidoglycan Cell walls of green plants and fungi present in Bacteria are rigid and contain polysaccharides; and cyanobacteria, cellulose in plant cell walls and chitin in polysaccharide and fungal walls (none in animal cells) protein present in Archae bacteria Flagella Simple, lacking Complex, with ‘9+2’ arrangement of microtubules; microtubules; intracellular (surrounded by extracellular (not cell surface membrane) 200nm diameter enclosed by cell surface membrane) 20nm diameter Respiration Mostly by mesosomes Mitochondria for aerobic respiration Photosynthesis No chloroplasts; takes Chloroplasts containing membranes place on membranes which are usually stacked into lamellae or which show no stacking grana Nitrogen ﬁxation Some have the ability None have the ability Bacteria, Cyanobacteria and Achaea are prokaryotic cells. All the other organisms have eukaryotic cells. Fig 2.18: Structure of an animal cell Resource Book G.C.E. (Advanced Level) Biology Fig 2.19: Structure of plant cell Structures and functions of organelles and other subcellular components Plasma membrane: Plasmamembrane is the outer limit of cytoplasm. All cellular membranes resemble the ultra structure of plasma membrane. In 1972, Singer and Nicolson put forward the ﬂuid mosaic model of cell membrane. It is mainly composed of; Phospholipids (most abundant type of lipid in plasma membrane) Protein The Plasma membrane has the following features. It is about 7nm thick. It is mainly made up of a phospholipid bilayer. Phospholipids are amphipathic molecules. The hydrophilic heads of the phospholipids face outwards into the aqueous environment of both inside and outside of the cell. The hydrophobic hydrocarbon tails face inwards and create a hydrophobic interior. Plasmamembrane is compared to the ﬂuid mosaic model. Since phospholipid molecules are moveable, they provide the ﬂuid nature to the membrane. Protein molecules embedded randomly contribute to its mosaic nature. Some of the protein molecules penetrate all the way through the membrane, called transmembrane proteins and some others penetrate only part of the way into the membrane. These are called integral proteins. G.C.E. (Advanced Level) Biology Resource Book Most of the integral proteins are transmembrane proteins which have hydrophilic channels. These act as pores through which ions and certain polar molecules can pass. Some proteins are not embedded in the lipid bilayer at all, and are loosely bound to the inner surface of the membrane, called peripheral proteins. Some proteins and lipids have short branching carbohydrate chains like antennae, forming glycoprotein and glycolipids, respectively. Animal’s cell membrane may contain few cholesterol molecules randomly integrated into the lipid bilayer. These cholesterol molecules provide ﬂexibility and stability to the membrane by reducing membrane ﬂuidity at moderate temperatures and prevent membrane solidiﬁcation at low temperatures. The two sides of the membrane may differ in composition and function. Fig 2.20: Structure of the plasma membrane Functions The plasma membrane surrounds the cytoplasm of living cell physically separating the intracellular components from the extracellular environment. Plasma membrane is selectively permeable and able to regulate the exchange of material needed for survival. Proteins embedded in the plasma membrane identify the cell, enabling nearby cells to communicate with each other (involved in cell recognition). Some protein molecules act as receptor molecules for interacting with speciﬁc biochemical, such as hormones, neurotransmitters and immune proteins. Some proteins in the cell membrane attach to some cytoskeletal ﬁbers and help Resource Book G.C.E. (Advanced Level) Biology to maintain the shape of the cell. Some proteins in the membrane act as enzymes. (e.g.Microvillus on epithelial cell lining of some parts of the gut contains digestive enzymes in their cell surface membrane.) Subcellular components There are many sub-cellular components in the cell. Some of them are organelles, which are bound by membranes and suspended in the cytosol of eukaryotic cell to perform specialized functions. Nucleus Most prominent organelle, consist most of the genes, having an average diameter of and enclosed by a double membrane cover called nuclear envelope. Nuclear envelope- composed of two membranes, inner and outer membranes, separated by a space of 20-40 nm. Nuclear envelope is perforated by nuclear pores which has pore complex to regulate the entry and exit of substances. It has nuclear lamina, made up of protein ﬁlaments which line the interior side of the nuclear envelope. Nuclear matrix is made up of protein ﬁlaments and extended throughout the interior of the nucleus. Chromatin and nucleolus are embedded in the nuclear matrix. Nucleolus- appears as darkly stained granules with ﬁbers adjoining part of the chromatin. Chromatin –appears as a diffused mass in electron micrographs of non dividing cells. It is a complex of DNA and proteins. During nuclear divisions, chromatin condenses, tightly coils and form threads, called chromosomes. Each species has a constant number of chromosomes. (e.g. typical human cell has 46 chromosomes). Functions Control all cellular activities. Synthesize DNA to produce new nuclei for cell divisions. Synthesize rRNAs and ribosomal subunits required for protein synthesis, through nucleolus. Synthesize mRNA and tRNA according to the information present on the DNA. Store and transport genetic information. G.C.E. (Advanced Level) Biology Resource Book Ribosomes These are subcellular components which carryout protein synthesis. They consist of two subunits; larger subunit and smaller subunit. They are composed of rRNA and protein. Ribosomes are found in two types; 70S and 80S. 70S ribosomes are found freely on the cytoplasam of prokaryotes, matrix of mitochondria and stroma of chloroplasts. 80S ribosomes are found only in eukaryotes. Based on the nature of presence, 80S ribosomes are categorized as two types; free ribosomes and bound ribosomes. Free ribosomes: freely available as group in cytoplasam. Bound ribosomes are attached to the membrane surface of rough endoplasmic reticulum. Functions Protein synthesis Endoplasmic reticulum It is a network of internal membranes forming ﬂattened or tubular sacs separating cytosol from ER lumen. It is continuous with the outer membrane of nuclear envelope. There are two types of ER; Rough ER and Smooth ER Rough ER Rough ER consists of ﬂattened sacs, and ribosomes bound to surface. Proteins synthesized by ribosomes move into lumen of ER. Functions Transport protein synthesized by ribosomes Synthesizing glycoproteins Produce transport vesicles Facilitate the growth of own membrane by adding phospholipids proteins and carbohydrates. Therefore called as membrane factory Smooth ER Smooth ER is a network of tubular sacs without ribosomes. Membrane bound enzymes are present. Functions It synthesizes lipids including oils, steroids and phospholipids. Metabolism of carbohydrates. Produce transport vesicles to transport within cell. Involves in detoxiﬁcation. Stores Ca2+ ions. Resource Book G.C.E. (Advanced Level) Biology Fig 2.21: structure of the endoplasmic reticulum Golgi apparatus Fig 2.22: Structure of the golgi apparatus G.C.E. (Advanced Level) Biology Resource Book Golgi apparatus is a stacks of ﬂattened sacs or Cisternae. Inner and outer surfaces can be identiﬁed as cis face and transface respectively. Cis face is located near the E.R to receive vessicles from E.R. Trans face give rise to secretory vessicles which budded off and travel other side. Golgi complex is abundant in secretory cells. Functions Collecting, packaging and distribution of materials Manufacturing cellulose and non cellulose cell wall components such as pectin Produce lysosomes Lysosomes They are single membrane bounded vesicles contributing to digestive activity. They contain hydrolytic enzymes which catalyze breakdown of carbohydrates, proteins, lipids and nucleic acids. Functions Digest food particles received by phagocytosis Transport residue material out of cell by exocytosis. Digest worn out organelles Autolysis causing cell death. Peroxisome They are single membrane bounded vesicles with oxidizing enzymes.They are present in both plants and animals. Enzymes in peroxysome catalyze the breakdown of H2O2. Functions Detoxiﬁcation of peroxides Photorespiration in plants Specialized peroxysomes called glyoxysomes are found in fat storing tissues in plants. Glyoxysomes converts fatty acids into sugar. Mitochondria It is one of the most common organelles in eukaryotic cells. It is an elongated organelle with two enclosing membranes. Outer membrane is smooth but the inner membrane is convoluted to form cristae. Cristae increase the surface area and they contain stalk particles. The gap/space in between inner and outer membranes of the mitochondrion is called intermembrane space. The inner most part of the organelle is known as mitochondrial matrix, which consists of 70 s ribosomes circular DNA Resource Book G.C.E. (Advanced Level) Biology molecule (mitochondrial DNA), phosphate granules and enzymes. The matrix carries enzymes for the reactions in Krebs cycle (in cellular respiration). Further, cristae composed of proteins and enzymes essential for electron transport chain and oxidative phosphorylation. Functions Synthesize ATP in aerobic respiration Involve in Photorespiration Fig 2.23: Structure of the mitochondria Chloroplast It is a biconvex lens shaped organelle with two membranes which is found in plants and some protists. The outer and inner membranes are smooth and are separated by a very narrow intermembrane space. Inside the chloroplast there is another membrane system. This membrane produces ﬂattened and interconnected sacks called thylakoids. Thylakoids contain complexes called photosystems which are made up of photosynthetic pigments. Thylakoids stacked to form a granum. The grana are interconnected by inter granal lamellae. The ﬂuid outside the thylakoid is stroma which contain circular DNA (chloroplast DNA), 70s ribosomes, many enzymes, starch granules and lipid droplets. G.C.E. (Advanced Level) Biology Resource Book Fig 2.24: Structure of the chloroplast Functions Photosynthesis Cytoskeleton Cytoskeleton is the supporting structure of the cell and maintains its shape. It is more important for animal cells which lack cell walls. Cytoskeleton is made out of microtubules and protein ﬁlaments. Additionally, it is Dynamic hence, has the ability to break and reform as needed. There are three types of components in the Cytoskeleton as follows; Microtubules Actin ﬁlaments or Microﬁlaments, Intermediate ﬁlaments Resource Book G.C.E. (Advanced Level) Biology Table 2.5: Differences between Microtubules, Microﬁlaments and intermediate ﬁlaments Property Microtubules Microﬁlaments Intermediate ﬁlaments (Tubulin (Actin ﬁlaments) polymers) Structure Hollow tubes; Two intertwined Fibrous proteins wall consists of strands of actin, supercoiled into thicker 13 columns of each strand is a cables tubulin molecules polymer of actin subunitrs Protein subunits Tubulin Actin One of several different proteins (e.g. Keratin), depending on the cell type. Main functions Maintenance of Maintenance of Maintaining of cell cell shape Cell cell shape (tension- shape (tension-bearing motility (as in bearing elements) elements) cilia or ﬂagella) Changes in cell Anchorage of nucleus Chromosome shape Muscle and certain other movements in cell contraction organelles.Formation division Cytoplasmic of nuclear lamina Organelle streamingin movements plant cells Cell motility (as in pseudopodia) Cell division in animal cells (cleavage furrow formation) Functions Provide strength to the cytoplasm Anchorage organelles and cytosolic enzymes of the cell Movement of cytoplasm, cytoplasmic streaming, positioned organelles and move chromosomes when necessary. Maintain the shape of the cell (mainly in animal cells) G.C.E. (Advanced Level) Biology Resource Book Cilia and Flagella Cilia and ﬂagella share a common structure. Flagella are long elongated structures and Cilia are short cellular projections that are often organized in rows. Cilia are more numerous than ﬂagella on the cell surface. They are made of microtubules, with a 9+2 structure (Nine doublets of microtubules are arranged in a ring, with two single microtubules in its center). They are covered by plasma membrane and bound to a basal body which anchors the cilium or ﬂagellum to the cell. The Basal body has 9 + 0 arrangement (no microtubules in its center) Functions Act as locomotor appendages Can move ﬂuid over the surface of the tissue Cilia lining in oviducts help move an egg toward the uterus Fig 2.25: Structure of the Cilium Centrioles Centriole is made up of cylindrically arranged microtubules which are non membrane bounded subcellular component present only in animal cells. Each centriole composed of nine sets of triplet microtubules arranged in a ring (9+0). A pair of centrioles which arranged perpendicular to each other are located in a region called centrosome near the nucleus. Functions Produce aster and spindle in cell division Resource Book G.C.E. (Advanced Level) Biology Central Vacuole Central vacuole is a large structure, bound by tonoplast, ﬁlled with liquid called cell sap found in plant cells. The composition of sap differs from cytosol and it contains water, ions such as Potassium and Chloride and sometimes water soluble colored pigments such as anthocyanin. Functions Stores water and other materials such as sugars, ions and pigments. Maintains water balance of the cell Gives turgidity and support to cell. Produce colours in some plants with sap pigments Stores soluble substances needed for cellular activities. Extracellular components 1. Cell wall Cell wall is an extracellular structure of plant cells. Animal cells do not have cell walls. However, prokaryotes, fungi and some protists also have a thin and ﬂexible cell wall.The chemical composition of the wall greatly varies from species to species and even from one cell type to another even in the same plant. Nevertheless in Plants, cell wall is generally made up of cellulose, pectin, hemicellulose, lignin and suberin (in some plant cells only). Plants generate two types of cell walls: primary and secondary walls. Young cells ﬁrst secrete primary cell wall: it is the wall laid down during plant cell division. Just outside the primary wall there is a thin layer (middle lamella) which is rich in sticky polysaccharides called pectins (magnesium and calcium pectate).Middle lamella glues adjacent cells together. Due to the deposition of hardening substances on the primary wall a secondary cell wall is generated secondarily. Primary cell wall is permeable, relatively thin, ﬂexible, composed mainly of cellulose ﬁbers which are laid unevenly running through the extracellular matrix (middle lamella)Water can move freely through the free spaces of cell wall Secondary cell wall lies between plasma membrane and primary cell wall. It contains several layers of hard materials, forming a rigid structure. In addition to cellulose, impermeable substances such as lignin and suberine are also incorporated in to the secondary wall. Lignin cement anchors cellulose ﬁbers together providing hard and rigid matrix, giving the cell wall an extra support. G.C.E. (Advanced Level) Biology Resource Book Cell wall has pits through which cytoplasm of adjoining cells join through plasmodesmata. Functions Protection and support Allows development of turgidity when water enters the cell Prevents bursting during turgidity Limits and control cell growth Component of appoplast pathway Maintaining cell shape hold the plant up against the force of gravity 2. Cell junctions Cell junctions are structures at which neighbouring plasma membranes are joined. They are also interact and communicate via sites of direct physical contacts. Functions Connects the internal chemical environment of adjacent cells. Cell junctions are structures at which cytoplasm of adjoining cells are joined. There are three types of cell junctions in animal cells Tight junctions – connect the plasma membranes of adjacent cells tightly bound by speciﬁc proteins forming continous seals around the cells. Prevent leakages of extracellular ﬂuids through intercellular space. e.g. skin epithelium Desmosomes/Anchor junctions – mechanically attach the cytoskeletons of adjoining cells by intermediate ﬁlaments for strong binding.e.g. muscle tissue Gap junctions /Communicating junctions – provide cytoplasmic channels from one cell to an adjescent cell. Gap junctions consists of special membrane proteins that surround the pore through which ions, sugars amino acids may pass. They allow signal and material exchange between adjacent cells through direct connections. e.g.heart muscles, animal embryo. Plasmodesmata Microscopic channels which runs through plant cell walls. They are cytoplasmic living connections between cytoplasm of adjoining cells. These are membrane lined channels ﬁlled with cytoplasm. Resource Book G.C.E. (Advanced Level) Biology Extracellular matrix of animal cells Although animal cells lack cell walls they do have elaborate extracellular matrix (ECM). Main components of the ECM are glycoproteins and other carbohydrates containing molecules secreted by the cells. Most abundant glycoprotein in the ECM of most animal cell is collagen which forms strong ﬁbres outside the cell. The collagen ﬁbres are embedded in a network woven out of proteoglycan secreted by cells. Functions Forms a protective layer over the cell surface Linking extra cellular matrix and cytoskeleton. Inﬂuences the cell behavior by Involving in the mechanical and chemical signaling. The cell cycle and the process of cell division The sequence of events that takes place in the cell from the end of one cell division to the end of the next cell division is referred to as cell cycle. At the end of the cell division, two genetically identical daughter cells resembling the parent cell are produced in mitosis. Fig 2.26: The cell cycle G.C.E. (Advanced Level) Biology Resource Book Eukaryotic cell cycle Mitosis Eukaryotic cell cycle may divided into two major phases. Interphase Mitotic phase/ M-phase Interphase is the longer phase of cell division. It covers about 90% of the cell cycle. Interphase could be divided into three phases; G1 phase (ﬁrst gap phase) S phase (synthetic phase) G2 phase (second gap phase) G1 phase In this phase synthesis of proteins and production of cellular organelles leading to cell growth occur. Proteins essential for S phase are produced during this phase. S phase DNA replication occurs and synthesis of histone proteins takes place. DNA wind around histone beads and form chromatin. G2 phase Cells continue to grow through protein synthesis as well as cellular organelles. Proteins essential for mitotic phase will be synthesized. Duplication of centrosomes takes place. There are cell cycle-controlling checkpoints available at G1, G2 and M phases to ensure that the cell is ready for moving into upcoming phases of cell division. Some cells receive a go-head signal at the G1 check point, it will usually complete the G1, S, G2 and M phases and divide. If it does not receive a go head signal at that point it may exit the cycle, entering into a non dividing stage called the Go phase. The most cells of the human body are actually in the Go phase. e.g. nerve cells and muscle cells. Mitotic phase/ M phase M phase covers only about 10% of cell cycle. This includes mitosis and cytokinesis. Mitosis Mitosis is referred to the nuclear division which gives rise to two genetically identical daughter nuclei from a mother nucleus. This may get divided into ﬁve stages; prophase, prometaphase, metaphase, anaphase and telophase in order to ease the learning of activities of cell cycle. Resource Book G.C.E. (Advanced Level) Biology 1. Prophase Chromatin ﬁbers get condensed by shortening and thickening and transformed into chromosomes. As a result chromosomes will be visible through light microscope. Nucleoli get disappeared and chromosomes appear with two sister chromatids attached at the centromere. Chromosomal arms of sister chromatids attached by special proteins called cohesion. The formation of mitotic spindles begins. Spindle includes the centrosomes, the spindle microtubules and the aster. Centrosomes move toward opposite poles of the cell due to the lengthening of microtubules between them. 2. Prometaphase The nuclear envelope fragments. Chromosomes get even more condensed. A special protein called kinetochore attaches the sister chromatids of each chromosome at their centromere. Some of the microtubules that attach to the kinetochore of the chromosomes move the chromosomes back and forth. Microtubules which are not attached to the kinetochore interact with those from the opposite poles. 3. Metaphase Centrosomes reach the opposite poles. The chromosomes have arrived to a place called metaphase plate which is located in equal distance from each pole. The centromeres of all chromosomes are located in the metaphase plate. At the end of this phase, each chromosome of the cell get attached to the kinetochore microtubule at their centromere and aligned at the metaphase plate. 4. Anaphase Sister chromatids are separated at the centromere. Microtubules attached to kinetochore get shorten and pull sister chromatids towards the opposite poles. Cell elongates as the non kinetochore microtubules are lengthen. By the end of anaphase equal and complete set of chromosomes found at each pole of the cell. 5. Telophase Nuclear envelope reforms around each set of chromosomes at opposite poles. Nucleoli reappears. Spindle microtubules get deplolymerized. Chromosomes unwind and become less condense to form chromatin. Two genetically identical daughter nuclei are formed. Cytokinesis The division of the cytoplasam starts at the end of the telophase. Therefore at the end of the mitosis two genetically identical daughter cells are produced. In animal cells- a cleavage furrow forms. This produces two genetically identical daughter cells. G.C.E. (Advanced Level) Biology Resource Book In plant cells- cell plate forms as a result of vesicle produced by golgi apparatus. This divides the cytoplasm in to two and generates two genetically identical daughter cells to the parent cell. Signiﬁcances of mitosis 1. Maintains the genetic stability 2. Growth and development 3. Cell repair, replacement and regeneration 4. Asexual reproduction G2 of interphase Prophase Prometaphase Metaphase Anaphase Telophase and cytokinesis Fig 2.27 : The phases of mitotic cell cycle Resource Book G.C.E. (Advanced Level) Biology Meiosis Sexually reproducing organisms undergo different type of cell division called meiosis. Meiosis Meiosis is a type of nuclear division which gives rise to four haploid, genetically non identical daughter nuclei, from a diploid mother nucleus. Meiosis involves two consecutive nuclear divisions, Meiosis I and Meiosis II. Meiosis I is a reduction division and Meiosis II is similar to mitosis, each stage consists of four sub-phases: prophase, metaphase, anaphase, and telophase. Before meiosis one cell is in interphase, during S phase of the interphase DNA replication occur. Meiosis I 1. Prophase I Cell enters to the prophase from interphase.Chromosomes begin to condense. Nucleolus begins to disappear. Next the formation of zipper like structure called the synaptonemal complex by a speciﬁc proteins holds two homolg tightly together. The pairing and physical connection of homologous chromosomes is called synapsis. During synapsis part of the DNA molecule of non-sister chromatids paired homologous chromosomes break, exchange and rejoin at corresponding point. This process is called crossing over. These points of crossing over become visible as chiasmata after the synaptonemal complex dissembles and the homologous chromosomes slightly apart from each other Nuclear envelop breaks. Centrosomes move towards opposite poles forming spindle in animal cells. The kinetochore of each homologue attach to microtubule from one pole or the other. The homologous pair then moves toward the metaphase plate. 2. Metaphase I The pair of homologous chromosomes get arranged on the metaphase plate with one chromosome of each pair faces each pole. Both chromatids of a homologue are attached to kinetochore microtubules from one pole and those of the other homolog are attached to kinetochore microtubules from the opposite pole. Homologous chromosome arrange randomly at metaphase plate. G.C.E. (Advanced Level) Biology Resource Book 3. Anaphase I Kinetochore microtubules of the spindle get shorten. Homologous pair separates and one chromosome of each pair moves towards the opposite pole. Sister chromatids of each chromosome remain attached at the centromere and move as a single unit towards the same pole. 4. Telophase I One complete haploid set of chromosomes accumulate at each pole. Nuclear envelope reforms around each set of chromosomes.Nucleoli reappear. Spindle disintegrates. Chromosomes decondensed into chromatin.Genetically non identical, haploid, two daughter nuclei are formed within one cell. Cytokinesis Usually occurs simultaneously with telophase I. Genetically non identical, haploid, two daughter cells are formed. In animal cells, cleavage furrow is formed. In plant cells a cell plate is formed. No DNA replication occurs between meiosis I and meiosis II Meiosis II 1. Prophase II Centrosomes start producing spindle apparatus (spindle ﬁbers, aster centrosome). Chromatin ﬁbers condense and produce chromosomes with two sister chromatids. Nuclear envelope breaks down into fragments. Nucleolus disappears. During the late prophase II centromere of the chromosomes are moved to the metaphase II plate. 2. Metaphase II All Chromosomes get attached to the microtubules at their centromere and aligned on the metaphase plate. Kinetochores of sister chromatids are attached to microtubules extending from both poles. Due to the crossing over in meiosis I, the two sister chromatids of each chromosome are not genetically identical. Meiosis II usually takes place in the perpendicular direction of Meiosis I. Therefore, metaphase plate of meiosis II is perpendicular to the metaphase plate of meiosis I. 3. Anaphase II Due to the breakdown of proteins attaching sister chromatids, they are separated at centromere. As a result of shortening of microtubules , sister chromatids of each chromosome move towards opposite poles. Resource Book G.C.E. (Advanced Level) Biology 4. Telophase II Nuclear envelope and nucleolus reform. Chromosomes decondense into chromatin. Spindle disassembles. Genetically non identical,haploid, four daughter nuclei are formed from one parent cell. Cytokinesis Cytokinesis occurs as in mitosis. Genetically non identical, haploid, four daughter cells are formed. These four daughter cells are not even identical to their parent cell. Centrosomes or centrioles are not available in plant cells. However, spindle is formed during cell division from accumulated microtubule complex. Meiosis I Meiosis I Prophase I Metaphase I G.C.E. (Advanced Level) Biology Resource Book Meiosis I Meiosis I Anaphase I Telophase I and cytokinesis Meiosis II Meiosis II Prophase II Metaphase II Resource Book G.C.E. (Advanced Level) Biology Meiosis II Meiosis II Anaphase II Telophase II and cytokinesis Fig 2.28: The phases of meiosis Signiﬁcance of meiosis Maintains the constant number of chromosomes through generations in sexually reproducing species. Producse new genetic variations leading to evolution. Genetic variation occurs due to crossing over ,recombination and independent assortment. Tumor, cancer and galls Cell division is driven by external and internal factors. They may be chemical or physical factors Cancer cells do not respond to normally to the body’s control mechanism They divide excessively and invade other tissues. If unchecked they can kill the organism. G.C.E. (Advanced Level) Biology Resource Book Cancer cells do not consider the normal signals that regulate the cell cycle. They do not need growth factors. They may make required growth factors themselves or giving signals to continue cell cycle without growth factors. Another possibly is an abnormal cell cycle control system. The problem begins when a single cell in a tissue undergoes transformation, the process converts a normal cell to abnormal cell. If the body immune system can not recognize and destroy it, it may leads to proliferation of cells and formation of a tumor. If the abnormal cells remain at the original site, the lump is called benign tumor. Most benign tumors do not cause serious problems and can be completely removed by a surgery. A malignant tumor becomes invasive and attack one or more organs. An individual with a malignant tumor is said to have a cancer. A few tumor cells may separate from the original tumor, enter blood vessels or lymph vessels and travel to other parts of the body. They may proliferate and form a new tumor. This spread of cancer cells to locations distant from their original site is called metastasis. Galls in plants This occurs due to uncontrolled mitotic division of plant cell. The plant cell division is controlled by maintaining a proper balance between plant growth regulators such as auxins and cytokinins. When this balance is lost plant cells produce undifferentiated mass of cells. Galls are the bumbs and growths that develop on different parts of plants after being invaded by some very unique organisms. Galls have range of causes, including viruses, fungi, bacteria, insects and mites. Usually the gall causers in some way attack or penetrate the plants growing tissues and causes the host to reorganize its cells and to develop an abnormal growth. The energy relationships in metabolic processes Sum of all biochemical reactions of living being is known as the metabolism and it consists of all catabolic and anabolic reactions. Catabolism is breaking down of complex molecules into simple molecules by releasing free energy. Therefore it is an exergonic reaction. Anabolism is making complex molecules from the simple molecules by absorbing free energy. Hence it is an endergonic reaction. Biochemical reactions involved in usage of energy released by catabolic reactions in Resource Book G.C.E. (Advanced Level) Biology living system are called as anabolic reactions. ATP acts as the energy carrier in all living organism including the simplest bacteria. Therefore the ATP is known as the universal currency of energy transactions. Energy can be deﬁned as the capacity to do work. All living organisms require energy for their living process in many ways. Such processes are; Synthesis of substances Active transport across plasma membrane Transmission of nerve impulses Muscle contraction Beating of cilia and ﬂagella Bioluminescence Electrical discharges. Overall idea of the energy relations of living system on biosphere is composed of following steps. Energy ﬂows into biological systems from the environment through solar radiation. (Primary energy source is the Sun) Light energy is captured in the cells having photosynthetic pigments (chlorophyll) by the process of photosynthesis and stored as chemical energy in the organic compounds such as carbohydrates Captured energy in organic food is transformed into chemical energy in ATP by a process called cellular respiration. The energy stored in ATP is utilized in various energy requiring processes. ATP (Adenosine Tri Phosphate) ATP is a nucleotide, consisting of, Ribose- sugar Adenine - nitreogenous base A chain of three phosphate groups. During the hydrolysis of ATP, ADP and Pi are produced. As a result, a very high energy is released. This is because the reactants (ATP and water) contain more energy in comparison to products (ADP and Pi). Therefore it yields energy and is an exergonic reaction. When ATP is hydrolyzed, the free energy yield of each of the two end phosphate groups is -30.5kJ/mol. G.C.E. (Advanced Level) Biology Resource Book Fig 2.29: Chemical structure of ATP molecule (need not be memorized) Most biological reactions use the energy released during breaking of the terminal phosphate bond. ATP is mobile. Therefore it can carry energy to anywhere in the cell, for any energy consuming reaction. ATP can be produced within living cells within a short period of time, using ADP, inorganic phosphate (Pi) and energy. Production of ATP within cells is called phosphorylation. According to the energy source phosphorylation is divided as; i. Photophosphorylation – synthesis of ATP using solar energy in photosynthesis ii. Substrate phosphorylation – synthesis of ATP using energy released by the breaking down of complex molecules into simple ones. iii. Oxidative phosphorylation – synthesis of ATP using energy released as a result of oxidation of molecules. In living cells energy in ATP is transformed in to various energy forms which are used for different functions. Resource Book G.C.E. (Advanced Level) Biology electricity The role of Enzymes in regulating metabolic reactions An enzyme is a macromolecule, which acts as a biological catalyst. Enzymes are produced in living cells/ General characteristics of an enzyme: 1. Most of the enzymes are globular proteins. 2. Enzymes are biological catalysts. They lower the activation energy of the reaction they catalyze (increases the rate of reaction). 3. Most enzymes are heat liable/ sensitive 4. Their presence does not alter the nature or properties of the end products ` of any reaction. 5. Enzymes are highly speciﬁc to the substrate (substrate speciﬁc) 6. Most enzyme catalyzed reactions are reversible. 7. The rate of enzyme activity is affected by pH, temperature and substrate concentrations. 8. They are not being used up during the reaction. 9

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