BIO 101 Lecture Notes (Weeks 1-3) PDF

WEEKS 1-3 ORIGIN AND EVOLUTION OF LIFE The planet earth came into existence sometime between 4 and 5 billion years ago. Life evolved on planet earth about 3.5 billion years ago. Since then, approximately 15 million different species of organisms have evolved. But only about two mil...

WEEKS 1-3 ORIGIN AND EVOLUTION OF LIFE The planet earth came into existence sometime between 4 and 5 billion years ago. Life evolved on planet earth about 3.5 billion years ago. Since then, approximately 15 million different species of organisms have evolved. But only about two million have been identified so far. The study of such a wide variety of organisms becomes convenient only when they are grouped according to similarities and differences, named, and their evolutionary relationships established. Origin of Life Introduction: Study of living organisms such as plants, animals and human etc is the active area of life science. Now, the question is how you will define “LIFE”. Life is defined as “the ability of an organism to reproduce, grow, produce energy through chemical reactions to utilize the outside materials”. But scientists and philosophers have tried to understand two important questions related to life: 1. How life originated on earth? 2. How different kinds of organisms are formed in the world? So first the question is how earth formed and how its internal structure supports the life? evidences suggest that earth and other planets in solar system came to existence around 4.5-5 billion years ago. Earth originally had two components: solid mass lithosphere and the surrounding gaseous envelope atmosphere. Once the temperature of primitive earth cooled down below 1000oC, the liquid components are known as hydrosphere. The formed earth consists of three parts as given in Figure 1. These parts are as follows: 1. Bryosphere: it is the central core of the earth. It is filled with molten magma with large quantity of iron and nickel. Baryosphere has two zones: inner core region (~800 miles radius) and outer core region (~1400miles radius). 2. Pyrosphere: it is the middle part of the earth, also known as mantle. It is ~1800 miles in thickness and mainly consists of silica, manganese and magnesium. 3. Lithosphere: it is the outermost region of the earth, also known as crust. It is 20-25 miles in thickness and mainly has silica and aluminum. Page 1 of 27 Figure 1: Schematic cross section through the present-day Earth outlining differences in composition (left) and rheology (right) between layers. Not to scale (Source: Palin et al., 2020). Conditions that support life on Earth There are multiple conditions on earth that support life on earth. These are as follows: Primitive earth with little or no oxygen: The original earth had a reducing environment due to presence of hydrogen and hydrogen compounds with water (such as CH4) and ammonia (NH3). Due to gravitational forces, these gases remain within the atmosphere of primitive earth. The reducing environment of primitive earth will help to synthesize organic compounds from interaction of inorganic substances. Inorganic raw material for origin of life: Inorganic material in the earth interact to form organic material required to produce life. Energy source. The energy source on primitive earth came from the following sources: Page 2 of 27  ✓ Solar radiation ✓ Electric discharge ✓ Volcanic eruptions ✓ Heat ✓ Cosmic Rays ✓ Radioactive Decays Infinite time: As per estimate it took almost 1 billion years from the formation of earth to appearance of life. Such a long time is needed for chemical reactions to occur without the help of enzyme. HOW LIFE ORIGINATED ON EARTH Six major theories are proposed to explain the origin of life on earth. These theories are as follows: 1. THEORY OF SPECIAL CREATIONS: The theory of special creation proposed that life on earth is created by a supernatural power, the GOD. According to the Christian belief, God has created the universe, planet, animal, plant and human in six natural days. Similar beliefs are also been proposed by other religion as well. There are believes in the theory of special creation. These points are as follows: A. All living organisms were created same day [NO DIFFERENCE IN THEIR APPEARANCE]. B. They were created in the present form [NO EVOLUTION]. C. Their bodies and organs are fully developed to meet the requirement to run the life [NO ADAPTATION] Objections to the theory of special creation: It was purely based on religious belief. There were no experimental evidences to support the assumptions. The age of different fossils proves that living organism appear on earth in different time frame. 2. THEORY OF SPONTANEOUS GENERATIONS: The theory of spontaneous generation or abiogenesis assumes that non-living material in a spontaneous manner give rise to life. There are several observations supporting this theory, which are as follows: Hair of horse tail dipped in the water gives rise to horsehair worm, Gordius. Fly larvae develops on rotten meat. In ancient Egypt, it was believed that frog, snake, crocodiles in the mud of Nile River warmed with sun. Van Helmont claimed that he can produce mice from the dirty shirt and handful of wheat grains kept in dark cupboard in three weeks. Evidences against the theory of spontaneous generation: Theory of spontaneous generation was criticized by Lazzaro Spallanzani, Francisco Redi and Louis Pasteur. These great scientists performed well designed scientific experiments to disprove the theory of spontaneous generations. a. Redi’s experiment: Francisco Redi did conclusive and well-designed experiment to disapprove the theory of spontaneous generation. He placed meat and fishes in 3 separate jars. Jar No. 1 was left open, No. 2 was covered with gauze and the third one was covered with paper. The meat/fishes decayed in all Page 3 of 27 three jars and attracted flies. In Jar No. 1, flies entered and layed eggs which eventually gave birth new larvae. Whereas in Jar No. 2, flies couldn’t be able to enter and no larvae was found inside the jar. But flies laid eggs on gauze that produced larvae. This has conclusively proved that organisms arise from the pre-existed organism rather than non-living matter. Figure 2: Miller Experiment to show the synthesis of organic compounds. b. Spallanzani’s Experiment: The experimental setup by Spallanzani is as shown below. In the designed experiment to test the validity of theory of spontaneous generation. In this experiment, Spallanzani has prepared animal or vegetable broth and boiled them for several hours and then either remained open or sealed immediately. This broth remained free from microorganism growth. He concluded that high temperature boiling had killed all microorganisms and in the absence of microorganism life could not appear. The broth left open or exposing of sealed broth shows growth of microorganisms. Page 4 of 27 Figure 3: Diagram showing Spallanzani's experimental setup and results. Note: (Top) An uncovered flask containing clear broth is sterilized by boiling. After a period of time (hours/days) the flask is re-examined and found to be turbid. This turbidity is microbial growth in the uncovered flask. Microbes from the air were able to land in the sterile broth and begin growing and dividing to produce a microbial population/community that appears turbid. (Bottom) A covered flask containing clear broth is sterilized by boiling. After a period of time (hours/days) the flask is re-examined and found to be clear. This clear appearance in the sealed flask is an indication that medium remained sterile. Since the sterile medium was sealed, microbes could not land in the medium and begin growing and dividing. As a result, the medium remained sterile. c. Louis Pasteur Experiment: In another conclusive experiment, Pasteur had designed experiment in a flask with “S” shaped curve tube (Figure 4 below). He took hay infusion in the flask and boiled for several minutes. After cooling, the steam condensed into the lower part of tube and act as barrier to stop the entry of microorganisms. No life appeared in the flask for several months. Analysis of condensed water indicates appearance of microorganism in the neck of the tube. Breaking of “S” tube allowed the growth of microorganisms in the flask. Page 5 of 27 Figure 4: Pasteur Experiment on spontaneous generation 3. THEORY OF CATASTROPHISM: This is the extension of the theory of special creation. This theory assumes that life originated by the creation and it is followed by catastrophe due to geographical disturbances. Each catastrophe destroyed the life completely whereas each creation forms life different from the previous one. Hence, each round of catastrophe/creation is responsible for evolvement of different types of organisms on earth. The critics of the current theory is same as previous one. No scientific experiment to support the hypothesis and mostly be based on imaginary concepts. 4. THEORY OF COSMOZOIC: This theory was put forward by Richter and strongly supported by Arrhenius. The theory assumes that life was present in the form of resistant spores and appeared on earth from another planet. Since the condition of earth was supporting the life, these spores grew and evolved into different organisms. This theory was also known as “theory of panspermia or spore theory”. The theory initially got the support from the fact that fossils of microorganism were found in meteorites in 1961. But no mechanism is known about the transfer of spores from other planet or whether these spores could survive the journey in space. The absence of life forms on any planet except earth and no details about the spores, its origin and mechanism of crossing interplanetary space and reaching earth. In addition, this theory doesn’t add much into the fundamental details about origin of life. No scientific experiment was given to support the theory. As a result, the hypothesis didn’t receive much attention. 5. THEORY OF ETERNITY OF LIFE: This theory assumes that life had no beginning or end. It believes that life has ever been in existence and it will continue to be so ever. It further believes that there is no question of origin of life as it has no beginning or end. The theory is also known as steady state theory. The main objection against the proposed theory is that it could not be able to explain; evidences support that initially earth forms and then life appeared on it. Where life exist before the formation of earth? 6. MODERN THEORY: The modern theory is also known as “chemical theory” or theory of primary abiogenesis. In the modern theory, the hypothesis of abiogenesis was proposed with a condition that the non-living materials can give rise to life in the condition of primitive earth. The condition of the primitive earth is different from the present conditions which do not permit abiogenesis. The idea of chemical theory was put forward by two scientist, A.I. Oparin and J.B.S Haldane. It has made following assumptions: 1. Spontaneous generation of life under the present environment is not possible. 2. Earth’s atmosphere ~1 billion years is very different from the current conditions. Page 6 of 27 3. Primitive earth’s atmosphere was reducing in nature. 4. Under these conditions, the chemical molecules (inorganic molecules) react with each other through a series of reactions to form organic substances and other complex biomolecules. 5. The solar energy and UV radiation provided the energy for the chemical reactions. Experimental Evidences supporting chemical theory: The hypothesis proposed by Haldane didn’t find much support without scientific experimentation. To conclusively support the chemical theory, Miller and Urey conducted experiment in mimicking primitive earth environment. The experimental design used for the experiment is given in the Figure 2 above. The experimental setup consists of a glass flask, a condenser, and a liquid flask interconnected with tubes and a source of electric spark to provide energy (Figure 2 above). He introduced a mixture of methane (CH4), ammonia (NH3), and hydrogen (H2) in the ratio of 2:2:1 and water (H2O) vapor at 8000oC. He allowed to circulate the mixture into this closed glass apparatus for 18 days continuously. He provided energy in the form of spark by supplying electricity of 75,000 volts through two electrodes. The electric sparks mimicks lighting in the primitive earth atmosphere. While passing the mixture, gases were passes through a liquid flask to simulate the volcano. The mixture was collected from the stop cock and analyzed using chromatographic and calorimetric techniques. The analysis of mixture indicates the presence of amino acids such as glycine, alanine, aspartic acid, nitrogen base adenine and simple sugar ribose. In addition, he found hydrogen cyanide (HCN), formaldehyde (HCHO) and other active intermediate compounds such as acetylene and cyanoacetylene. The chemical reactions which might explain the formation of these compounds are as follows: i. Formation of HCN, HCHO etc: CO2 → CO + [O] (atomic oxygen)--------------------------------------[Eq 1] CH4 + 2[O] → CH2O (or HCHO) + H2O ------------------------------[Eq 2] CO + NH3 → HCN + H2O -----------------------------------------------[Eq 3] CH4 + NH3 → HCN + 3H2 -----------------------------------------------[Eq 4] ii. Formation of Glycine: The formaldehyde, ammonia, and HCN then react to form glycine. CH2O + HCN + NH3 → NH2-CH2-CN (Aminoacetonitrile)+ H2O ------------------------[Eq 5] NH2-CH2-CN + 2H2O → NH3 + NH2-CH2-COOH (Glycine) ------------------[Eq 6] Note: a. Vitamin B12, a natural chemical containing cyanide, is beneficial to your body because it prevents anemia (iron-poor blood). The cyanide binds in vitamin B12 so that it does not serve as a source of cyanide exposure and cannot harm you. b. Formaldehyde is an aqueous solution that can be useful as a disinfectant as it kills most bacteria and fungi (including their spores). It is used as an additive in vaccine manufacturing to inactivate toxins and pathogens. c. Glycine is a building block for making proteins in the body. Glycine is also involved in transmitting chemical signals in the brain, so there's interest in using it for schizophrenia and improving memory. A typical diet contains about 2 grams of glycine daily. STEPS PROPOSED IN THE ORIGIN OF LIFE According to the chemical theory of origin of life, a series of chemical synthesis give rise to life. As per the hypothesis, origin of life has four major steps: Page 7 of 27 1) Formation of Inorganic molecules: The high temperature of primitive earth didn’t allow the condensation of atoms to form inorganic molecules. As temperature of earth goes down, condensation of different atoms give rise to simpler molecules. The elements most abundant on the primitive earth are hydrogen, oxygen, nitrogen and carbon. The reaction of these molecules give rise to the different gases such as hydrogen, nitrogen, ammonia, methane, carbon dioxide and water vapor. The molecules present on initial earth is given in the Figure 5. The energy for these reactions was provided by sunlight, lighting or volcanic eruptions. Figure 5: Different molecules prevalent on primitive earth. (2) Spontaneous formation of monomeric organic compounds: The simple molecules interact with each to form simple monomeric organic compounds. These molecules were sugar, fatty acids, glycerol, amino acids and organic bases (purine/pyrimidine). The reactions between the inorganic to give simple organic molecules occurs in reducing environment inside ocean. The inorganic molecules were condensed in the form of rain as temperature of earth lower down. Hence, both inorganic compounds and simple organic compounds were present in the primordial ocean. Page 8 of 27 Figure 6: Different simple organic molecules prevalent on primitive earth. (3) Spontaneous formation of complex organic compounds: The small, simpler organic compounds react to form complex organic compounds. The simple amino acids react to form polypeptides, sugar reacts to form large sugar molecules, fatty acid and glycerol combined together to give fat (Figure 7). Heat of the sun is utilized for providing energy for these reactions. Figure 7: Different complex organic molecules prevalent on primitive earth. Page 9 of 27 (4) Spontaneous formation of molecular aggregates: large organic molecules came together to form large colloidal aggregates known as coacervates. A layer of water molecules forms around the protein molecules presents in coacervates. The membrane present around the molecules protect the molecule and bring high local concentration to enhance the chemical reactions. The colloidal aggregates absorb protein and other molecules from the ocean. This results in growth in coacervate as well as internal complexity (Figure 8). As coacervate divides into multiple small ones. Figure 8: Formation of aggregates. These coacervates are the initial species present in the ocean to start the formation of primary cells. This process is accomplished in two steps: Note: Coacervates (large colloidal particles that precipitate out in aqueous medium) are the first pre- cells which gradually transformed into living cells, according to theory of chemical evolution. Formation of eobionts or protocells: The coacervates has the ability to take up new molecules to replace the degraded molecules and maintain the size. Thus, coacervates has the basic property of living system but it doesn’t have complex molecules such as enzyme etc (Figure 9). The process of acquiring new molecules was not regulated. Later, nucleic acid is entrapped within the coacervates and process of division became precise and controlled. This form of coacervates with nucleic acid is known as eobionts or protocells. Formation of first cells: Protein molecules and appearance of enzymes has enhanced the synthesis of several of biomolecules in eobionts. RNA and DNA developed and these molecules has taken over the protein synthesis. Interaction of lipid and protein allowed the formation of biomembrane which has provided selectivity in the primitive cell for intake or exclusion of material. It allowed the appearance of membrane bound protocell and that has eventually given first cell on earth. The mutation in DNA and selection of fast-growing cell give rise to the appearance of first primordial cell. The first cellular form on earth appeared ~2000 million years ago. Page 10 of 27 Figure 9: Coacervates Formation Figure 10: Coacervate formation and division to form protocell Page 11 of 27  EVOLUTION What is Evolution? The formation of complex organisms through ‘gradual change’ from simple ancestral types over the course of geological time is termed Evolution or Organic Evolution. According to the Theory of Organic Evolution i. The various present-day organisms were not created in the same form in which they exist today, but have gradually evolved from much simple ancestral forms from a common ancestor. ii. The characteristics of organisms had been changing in the past; they are changing even today, and will continue to do so in the future as well. This is due to the fact that the environment in which organisms live also changes and organisms need to adapt to the changed environment in order to survive. iii. Several living organisms of the past have become extinct. iv. The origin of the various forms (species) found on earth has been a gradual and extremely slow process, requiring hundreds or even thousands of years. However, the evolution of black peppered moth or polyploid varieties of some crops or pesticide resistant mosquitoes happened in much shorter periods of time. Chemical Evolution: The term evolution refers to change from one form to another. Change in living organism with time is known as organic or biological evolution. The process of evolution can be understood from the fact that unicellular organism appears first, simple multicellular and later development of complex multicellular organisms such as seed plants and vertebrate animals. The fishes were the initial early vertebrate and it gradually change to form amphibians. These amphibians have produced reptiles and that has evolved further to give birds and mammals. These hierarchical linking of different species is considered by ladder of chain by Aristotle as given in Figure 11. In the same series, mammals have evolved to human involving ape-like primates by acquiring changes over the course of time. Page 12 of 27 Figure 11: Aristotle’s ladder of Nature. Evidences of organic evolution The evidences supporting organic evolution are derived from a number of fields of Biology. Those discussed here are: i. Morphological evidences ii. Embryological evidences iii. Palaeontological evidences iv. Molecular evidences 1. Evidences from Morphology Though organisms of different species and groups are quite different from each other, they still retain certain common features. Morphological evidences for evolution are derived from - (a) Homologous (b) analogous organs (c)Vestigial organs (d) Connecting links Page 13 of 27 (a) Homologous Organs: Homologous organs are the organs which are similar in structure and origin but may look very different and perform different functions. For example: Forelimbs of vertebrates are a good example of homologous organs. They are built on the same fundamental plan yet they appear different and perform different functions. (b)Analogous organs: The structures which are functionally similar but structurally different are called analogous organs. For example: The wing of an insect, and that of a bird or bat or pterodactyl are examples of analogous organs. The function of the wing is the same (for flying) but the insect wing has no structural resemblance with that of the vertebrates. (c) Vestigial Organs: Vestigial organ is any small degenerate or imperfectly developed (non-functional) organ or part which may have been complete and functional in some ancestor. Wisdom tooth Appendix Body hair in male Tail vertebra Figure 12: Some examples of vestigial organs in human body The only rational explanation for the presence of these non-functional organs is that they have been inherited from ancestors in which they were functional. (d) Connecting Links: The animals or plants which possess characters of two different groups of organisms are known as connecting links. The connecting links establish continuity in the series of organisms by proving that one group has evolved from the other. A good example is that of a fossil bird Archaeopteryx, which was a connecting link between reptiles and birds. This bird had a beak with teeth and a long tail (with bones) like the lizards. It had feathers on the wings and on the body like the birds. Figure 12: An extinct bird – Archeopteryx 2. Evidences from Embryology Embryology is the study of development of an organism Page 14 of 27 The aspects of embryology which support the doctrine of organic evolution are: ✓ similar stages of early development (morula, blastula or gastrula) in all the animals; ✓ the embryos of all vertebrates are similar in shape and structure in their early stages. ✓ All the vertebrates start their life from a single cell, the zygote. ✓ All of them during their life history, pass through two-layered blastula and three-layered gastrula stage and then through fish like stage with gill-slits. All the different aspects of embryology strongly support the fact that the different classes of vertebrates had common ancestors. 3. Evidences from Paleontology Paleontology is the study of fossils. Fossils are the remains or traces of animal and plant life of the past, found embedded in rock either as petrified hard parts or as moulds, casts or tracks. The fossils of the earliest era in the geological time scale were those of bacteria, then invertebrates and then successively of fishes, amphibians, reptiles and lastly of birds and mammals and among mammals’ primitive fossils of humans are the most recent. 4. Molecular Evidence of Evolution ✓ All organisms have cell as the basic unit of life. The cell is made of biomolecules common to all organisms. ✓ Ribosomes, the cellular organelles are of universal occurrence in organisms. ✓ DNA is the hereditary material of all organisms, except for some viruses. ✓ ATP is the molecule which stores and releases energy for biological processes. ✓ The same 22 amino acids form the constituents of proteins of almost all organisms. ✓ The genetic code is universal (exceptions are very few). ✓ The central dogma which deals with the transfer of genetic information in cells is the same. ✓ The basic steps of transcription and translation for protein synthesis are similar in all organisms. ✓ The sequence of nucleotides such as that for the promoter gene (TATA box) is common to all organisms. However, organisms sharing same chemical characteristics show closer evolutionary relationships. For example (i) human blood proteins are most similar to those of the chimpanzee among all apes, or (ii) only plants and some algae have chlorophyll so they are more closely related. Similarity in chemical constituents between organisms is termed molecular homology or biochemical homology and are used in recent times, to establish evolutionary relationships and form the basis of systematics. WEEK 3 CELL THEORY Biological cell theory explains the idea of organismal constitution, structure, and function. It describes how we all start our lives with a cell that has been contributed by our parents’ existing cells. In its full integrity, a cell is an independently functioning and self-sustaining entity that arises from a pre-existing cell and gives rise to new cells in its lifetime. In the field of biology, we define cell theory as a scientific explanation or scientific theory of how living organisms are built. This theory emphasizes the fact that all living beings (living organisms) are made Page 15 of 27 up of a basic life unit called “cell” and each of the body’s cells has started its life journey from a pre- existing cell only! All cells have the same basic chemical composition. This theory was put together in the mid-19th century by two scientists named Matthias Schleiden and Theodor Schwann. The theory was later improved by Rudolf Virchow who explained that all cells have arisen from some pre-existing cells. Both speculated that cells originate due to spontaneous generation. There are three main principles of cell theory (cellular theory) as discussed below: Three Parts of Cell Theory The three principles of cell theory are discussed as the 3 parts of cells theory here. The cell theory states: 1. All living organisms are constituted of cells. Explanation: Schleiden (a botanist) and Schwann (a physiologist), both separately concluded that cells are the basic structural as well as functional units of all living organisms. While Schleiden specifically studied plant tissues and proposed that all plants are composed of cells, Schwann extended this idea to animal cells, suggesting that all animal tissues are also made up of cells. Their combined observations led to the realization that cells are the building blocks of life. 2. The cell is the basic unit of life in living organisms; speaking both structurally as well as functionally. Explanation: Both Schleiden and Schwann recognized that cells are not just static units but are also essential for organismal functions. Cells carry out specific functions and activities necessary for life and the overall structure and function of an organism depend on the interactions and activities of its cells. Energy flow occurs within all cells. 3. All cells have arisen at some point in time from preexisting cells. Explanation: Both scientists observed that cells did not arise spontaneously but instead originated from the division of pre-existing cells. This concept, known as cell division or cell reproduction, is a fundamental process in biology and is central to the growth, development, and reproduction of all living organisms. Page 16 of 27 Figure 14: The cell is the most fundamental unit of a living body and forms the foundation of the hierarchy; cells, tissue, organ, organ system, and organism (Image Credit: Bodell and downloaded from the internet 13th July 2024) Contributions To Cell Theory Many scientists have contributed to the development and understanding of cell theory the way we know it today. Additionally, scientific knowledge is subject to ongoing research and discoveries, and research groups across the globe are continuously bettering our existing understanding of classical cell theory. Table 1: Details of scientists who contributed to the development of different ideas surrounding cell theory. Scientist Year of Contribution Description Contribution Robert Hooke 1665 Observed cork cells under a microscope, coined the term “cells” and initiated the study of cell structure. Antonie van 1674 First to observe living cells including bacteria and Leeuwenhoek protists, using improved microscopes. (discovered bacteria) Page 17 of 27  Matthias Schleiden 1838 Proposed that all plants are composed of cells, hence contributing to the formulation of the cell theory. (His theory of free cell formation was rejected.) Theodor Schwann 1839 Extended the cell theory to animals by stating that all animal tissues are also made up of cells. Rudolf Virchow 1855 Formulated the idea that cells arise from pre-existing cells, completing the cell theory’s principles. Carl Zeiss and Late 1800s Developed the modern compound microscope with Ernst Abbe improved optics and revolutionized cell study. Camillo Golgi 1873 Discovered the “Golgi apparatus” – an organelle involved in cellular transport and secretion. Santiago Ramón y Late 1800s – Pioneered neuron theory that showed that the nervous Cajal Early 1900s system consists of discrete individual cells (neurons). Robert Brown 1831 Identified the cell nucleus (an essential organelle housing genetic material in eukaryotic cells). James Watson and 1953 Proposed the double-helix structure of DNA (which Francis Crick unlocked the genetic information within cells) Lynn Margulis 1967 Proposed the “endosymbiotic theory” which suggests that certain organelles, like mitochondria, originated from symbiotic bacteria engulfed by eukaryotic cells. Rita Levi- 1986 Discovered nerve growth factor (NGF) showing that cells Montalcini produce specific molecules to regulate cell growth and differentiation. Shinya Yamanaka 2012 Discovered cellular reprogramming which proved that and John Gurdon specialized cells can be converted back into stem cells, eventually opening up new possibilities in regenerative medicine. Data Source: Akanksha Saxena of Biology Online and downloaded on the 13th July 2024 from the internet VIRUSES AND THE CELL THEORY: UNRAVELLING THE ENIGMATIC NATURE OF VIRAL LIFE Despite “not fitting” the traditional criteria of living organisms according to the cell theory, viruses are captivating entities that continue to intrigue scientists and molecular biologists. Their unique abilities to interact with host cells and influence evolution make them invaluable subjects of research with potential Page 18 of 27 applications ranging from medicine to cutting-edge biotechnologies. As we delve deeper into the mysteries of viral life, we uncover fascinating insights that expand our understanding of the intricacies of the biological world. Some of the notable points of viruses that challenge cell theory are: 1. Acellular Entities o Viruses challenge the conventional cell theory as they “lack the basic cellular structures” found in living organisms. o The absence of organelles, cell membranes, and independent metabolism raises questions about their classification as living organisms. 2. Genetic Hijackers: o Despite their acellular nature, viruses exhibit a remarkable ability to hijack host cells’ machinery for replication. o Viruses carry genetic material (DNA or RNA) that serves as the blueprint for their reproduction and assembly. 3. Obligate Intracellular Parasites o Viruses cannot reproduce or carry out essential life processes independently. o They are “obligate intracellular parasites” entirely reliant on host cells for their survival and propagation. 4. Viral Invasion o Viruses have evolved specific mechanisms to enter host cells through binding to surface receptors. o Once inside, they co-opt the host cell’s resources, redirecting them to produce new viral particles. 5. Controversial Living Status o The debate continues regarding whether viruses should be classified as living or non- living entities. o Some scientists view viruses as “biological entities” that blur the boundaries between living and non-living matter, thus maybe defying cell theory (in case they are living matter)! CELL What is a Cell? A cell is the structural and fundamental unit of life. The study of cells from its basic structure to the functions of every cell organelle is called Cell Biology. Robert Hooke was the first Biologist who discovered cells. All organisms are made up of cells. They may be made up of a single cell (unicellular), or many cells (multicellular). Mycoplasmas are the smallest known cells. Cells are the building blocks of all living beings. They provide structure to the body and convert the nutrients taken from the food into energy. Cells are complex and their components perform various functions in an organism. They are of different shapes and sizes, pretty much like bricks of the buildings. Our body is made up of cells of different shapes and sizes. Page 19 of 27 Cells are the lowest level of organisation in every life form. From organism to organism, the count of cells may vary. Humans have more number of cells compared to that of bacteria. Cells comprise several cell organelles that perform specialised functions to carry out life processes. Every organelle has a specific structure. The hereditary material of the organisms is also present in the cells. Characteristics of Cells The following are the various essential characteristics of cells: Cells provide structure and support to the body of an organism. The cell interior is organised into different individual organelles surrounded by a separate membrane. The nucleus (major organelle) holds genetic information necessary for reproduction and cell growth. Every cell has one nucleus and membrane-bound organelles in the cytoplasm. Mitochondria, a double membrane-bound organelle is mainly responsible for the energy transactions vital for the survival of the cell. Lysosomes digest unwanted materials in the cell. Endoplasmic reticulum plays a significant role in the internal organisation of the cell by synthesising selective molecules and processing, directing and sorting them to their appropriate locations. Components of a cell The cell structure comprises individual components with specific functions essential to carry out life's processes. These components include- cell wall, cell membrane, cytoplasm, nucleus and cell organelles. Cell Types Prokaryotes: Prokaryotes are generally small cells that are enclosed by the plasma membrane. Depending on the species, they have a distinctive cell wall, varying in composition. Prokaryotes do not contain a nucleus and membrane-bound organelles. They possess circular or linear DNA. The chromosomal area and the cytoplasm are found in the protoplasm of prokaryotes. Prokaryotes are divided into two domains: archaea and bacteria. Eukaryotes: Eukaryotic cells have nucleus, organelles, and a plasma membrane surrounding them. Eukaryotic cells evolved particular organelles, parts of the cell that have a specific function. These Page 20 of 27 organelles include the endoplasmic reticulum, which sorts and bundles proteins; the mitochondria, which provide energy; and chloroplasts, found in plants and produce food from sunlight and carbon dioxide. Figure 15: Prokaryotic vs Eukaryotic cells Similarities between prokaryotic and eukaryotic cells Page 21 of 27 Figure 16: Similarities between Prokaryotic vs Eukaryotic cells i) Cell membrane: ✓ Both eukaryotic and prokaryotic cells contain lipid bilayers. ✓ These lipid bilayers are primarily a configuration of proteins and phospholipids. ✓ These lipids act as a selection barrier between the outer and inner environments of the bilayer cell. ii) Genetic material: ✓ Eukaryotic and prokaryotic cells use deoxyribonucleic DNA or acids as the basis for their genetic information. ✓ This genetic material is needed to regulate and report cell function through the formation of RNA by transcription, which then leads to the formation of proteins through translation. iii) Ribosomes: ✓ Ribosomes facilitate RNA translation and protein synthesis. ✓ This feature is required for the function of both prokaryotic and eukaryotic cells. iv) Cytoplasm: ✓ Cell biochemical reactions take place through the cytoplasm, Page 22 of 27  ✓ The primary ingredient in this reaction is cytosol. Table 2: Difference between Prokaryotic and Eukaryotic Cells Prokaryotes Eukaryotes Type of Cell Always unicellular Unicellular and multi-cellular Cell size Ranges in size from 0.2 μm – 2.0 μm in Size ranges from 10 μm – 100 μm in diameter diameter Cell wall Usually present; chemically complex in When present, chemically simple in nature nature Nucleus Absent. Instead, they have a nucleoid Present region in the cell Ribosomes Present. Smaller in size and spherical in Present. Comparatively larger in size shape and linear in shape DNA arrangement Circular Linear Mitochondria Absent Present Cytoplasm Present, but cell organelles absent Present, cell organelles present Endoplasmic Absent Present reticulum Plasmids Present Very rarely found in eukaryotes Ribosome Small ribosomes Large ribosomes Lysosome Lysosomes and centrosomes are absent Lysosomes and centrosomes are present Cell division Through binary fission Through mitosis and meiosis Flagella The flagella are smaller in size The flagella are larger in size Reproduction Asexual Both asexual and sexual Page 23 of 27  Example Bacteria and Archaea Plant and Animal cells Single-celled Organisms: Cell theory can be well understood by looking at single-celled species. Modern microscopes made it simple to observe and study cell theory mechanisms. Placing a drop of pond water under a microscope provides an excellent opportunity to observe the application of the cell theory. Plants cells Plant cells are the basic unit of life in organisms of the kingdom Plantae. They are eukaryotic cells, which have a true nucleus along with specialized structures called organelles that carry out different functions. Plant cells have special organelles called chloroplasts, which create sugars via photosynthesis. Figure 17: Structure of a plant cell Animals Cells An animal cell is a type of eukaryotic cell that lacks a cell wall and has a true, membrane-bound nucleus along with other cellular organelles”. Animal cells range in size from a few microscopic microns to a few millimetres. The largest known animal cell is the ostrich egg, which can stretch over 5.1 inches across and Page 24 of 27 weighs about 1.4 kilograms. This is in stark contrast to the neuron in the human body, which is just 100 microns across. The shape of animal cells also varies, with some being flat, others oval or rod-shaped. There are also more intriguing shapes such as curved, spherical, concave and rectangular. Most of the cells are microscopic in size and can only be seen under the microscope. As stated before, animal cells are eukaryotic cells with a membrane-bound nucleus. Furthermore, these cells exhibit the presence of DNA inside the nucleus. They also comprise other membrane-bound organelles and cellular structures which carry out specific functions necessary for a cell to function properly. Every cell in the human body is a different type of cell. According to cell theory, each of these cells originated from the zygote, a single cell produced upon the fertilisation of an egg by a sperm. Then, this cell multiplies, divides, and starts differentiating into the various types of cells that make up the human body. A fully developed organism is eventually formed. Figure 18: Structure of an animal cell Page 25 of 27  Similarities between plant and animal cells i. A membrane-bound nucleus, which contains the DNA. ii. A plasma membrane or cell membrane. iii. Cytoplasm in which other organelles are suspended. iv. Nucleolus. v. Endoplasmic reticulum (ER) vi. Ribosomes. vii. Mitochondrion. viii. Golgi bodies or Golgi apparatus. Table 2: Difference between Plant Cell and Animal Cell Basis of Plant Cell Animal Cell Comparison They are eukaryotic cells having a true It is a type of eukaryotic cell that lacks nucleus along with specialized structures a cell wall and has a true, membrane- Meaning known as organelles which perform bound nucleus along with other cellular specific functions. organelles. Fixed-size which is generally larger. The Irregular and small in size. Ranges Size normal range is from 10-100 micrometres. from 10-30 micrometres. They have a fixed and rigid shape – Shape They have round and irregular shapes. generally rectangular or cubical. Has a cell wall composed of the cell Cell Wall Does not have a cell wall. membrane and cellulose. Surrounded By Rigid wall cell and plasma membrane Flexible and thin plasma membrane Plastids Present Absent Chloroplast Present in plant cells Not present in animal cells Page 26 of 27 Nucleus Lies on one side Lies in the cell wall’s centre Cilia Absent Generally present Centrioles Absent Present Mitochondria Present in a small number Present in a large number Glyxoxysomes May be present Absent Lysosomes Absent Present Generally large and provide structural Vacuoles are small in size. Many Vacuoles support. One huge vacuole. vacuoles. Plant cells can synthesize their own Essential Animal cells cannot synthesize their nutrients such as amino acids, vitamins Nutrients own nutrients. coenzymes that are required by the plant. Present as complex carbohydrates and Reserve Food Present as starch glycogen Page 27 of 27