Cell Structure and Living Organisms

Fundamental Characteristics of Living Organisms

• Nutrition: All living organisms require nutrients to survive. Green plan. manufacture organic nutrients from inorganic compounds using photosynthesis, whereas animals ingest and digest organic matter derived from other living organisms.

• Respiration: Organisms release usable metabolic energy through the biochemical breakdown of food substrates inside living cells.

• Excretion: Organisms eliminate the toxic chemical waste products produced by internal metabolic reactions.

• Response to Stimuli (Sensitivity): Organisms possess the detect and respond to alterations in their external and internal surroundings.

• Movement: Organisms demonstrate directional physical motion, executed by the contraction of muscles in animals and slow growth movements (tropisms) in plants.

• Control of Internal Conditions (Homeostasis): Living things regulate and preserve a constant internal physiological environment.

• Reproduction: Organisms produce viable offspring to ensure the generational continuity of the species.

• Growth and Development: Organisms undergo a permanent increase in dry mass, physical dimensions, and structural complexity by utilizing materials acquired from nutrition.

Eukaryotic Cell Structure and Ultrastructure

• Eukaryotic Definition: Eukaryotic organisms possess cells containing a membrane-bound nucleus and discrete membrane-bound organelles (such as mitochondria and chloroplasts). Animals, plants, fungi, and protoctists are eukaryotic.

• Cytoplasm: The living substance filling the interior of the cell, displaying a physical consistency resembling a sloppy jelly (intermediate between a solid and a liquid). It is a complex suspension containing organelles and dissolved molecules where vital chemical reactions occur.

• Organelles and Microscopy: Sub-cellular structures within the cytoplasm are known as organelles. While larger organelles are detectable under standard light microscopes, their detailed internal ultrastructure requires the high magnification and resolving power of an electron microscope.

• Nucleus: The largest organelle in eukaryotic cells, serving to direct and coordinate all metabolic and cellular activities. It contains chromosomes ($46$ in human somatic cells) composed of deoxyribonucleic acid (DNA) carrying genes. Genes dictate cellular function by determining the specific sequence of proteins produced.

• Ribosomes: Tiny, non-membrane-bound sites of protein synthesis scattered in their thousands throughout the cytoplasm. Instructions transcribed from nuclear DNA are dispatched to cytoplasmic ribosomes for peptide assembly. Ribosomes are too small to be seen using an optical light microscope.

• Enzymes: Biocatalytic proteins synthesized by cells to regulate and accelerate intracellular chemical reactions within the cytoplasm.

• Cell Surface Membrane: A thin, flexible outer boundary separating the internal cytoplasm from the external environment. It is partially permeable and selectively permeable, exercising active control over the specific chemical substances permitted to enter or exit.

• Mitochondria (singular: Mitochondrion): Cylindrical or spherical organelles residing within the cytoplasm that execute aerobic cellular respiration, releasing biologically usable energy. Highly active tissues with high metabolic energy demands (such as skeletal muscle cells, motor neurones, and epithelial cells of the kidney tubule) contain elevated densities of mitochondria.

• Structures Specific to Plant Cells:

  • Cell Wall: A non-living structural layer enveloping the cell surface membrane, composed primarily of tough cellulose fibers. It maintains cell rigidity, prevents osmotic lysis, and imparts structural support to the whole plant through turgor pressure. It is porous and freely permeable to water and dissolved solutes.

  • Permanent Vacuole: A large, persistent fluid-filled space situated centrally in mature plant cells, enclosed by a membrane. It contains cell sap, which acts as a storage reservoir for water, mineral ions, and dissolved sugars. (Animal cells occasionally exhibit vacuoles, but they are consistently small, transient structures).

  • Chloroplasts: Large, chlorophyll-rich organelles restricted to the green tissues of plants (such as leaf mesophyll and young stems). Chlorophyll traps light energy to power photosynthesis. Chloroplasts are absent in non-photosynthetic organs, including floral petals, underground roots, and woody stems.

• Linear Dimensional Scale: One micrometre ($1\,\mu\text{m}$) equals $\frac{1}{1000}\,\text{mm}$ ($10^{-6}\,\text{m}$). Typical animal cells range between $10\,\mu\text{m}$ and $50\,\mu\text{m}$ in diameter.

Kingdom Plantae

• General Organization: Multicellular eukaryotic organisms whose cellular walls are constructed of cellulose.

• Autotrophic Mode of Nutrition: Contain green chloroplasts containing chlorophyll to carry out photosynthesis, transforming simple inorganic compounds (water and carbon dioxide) into complex organic molecules using light energy:     $6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

• Carbohydrate Reserves and Transport:

  • Starch: An insoluble complex storage carbohydrate synthesized directly from photosynthetic sugars and retained within plant tissues.

  • Sucrose: A transport disaccharide sugar translocated through vascular tissues across plant organs, frequently stored in fruits, seeds, and root structures.

• Flowering and Non-Flowering Divisions: Includes advanced flowering plants as well as simpler, non-flowering taxonomic groups such as mosses and ferns.

• Flowering Plant Examples:

  • Pea Plant (Pisum sativum): Features chloroplast-dense leaves and stems, with white flowers adapted for insect-mediated pollination.

  • Maize (Zea mays): A monoecious, wind-pollinated cereal crop possessing separate male flowers producing airborne pollen and female flowers that mature into seeds following successful fertilization.

Kingdom Animalia

• General Characteristics: Multicellular eukaryotes whose cells completely lack chloroplasts, photosynthetic pigments, and rigid cell walls.

• Heterotrophic Mode of Nutrition: Incapable of producing organic food through autotrophic pathways; animals ingest and assimilate nutrients by feeding on other living organisms (plants or other animals).

• Cell Flexibility and Locomotion: The absence of a rigid cell wall allows animal cells to alter their external geometry, permitting tissue movement, cellular motility, and coordinated locomotion.

• Neurological Coordination: Motility and responses to stimuli are coordinated by an integrated, specialized nervous system.

• Carbohydrate Storage: Excess carbohydrate is stored intracellularly in the form of the complex polysaccharide glycogen.

• Vertebrates and Invertebrates:

  • Vertebrates: Animals possessing a vertebral column or backbone. Composed of five principal taxonomic classes: fish, amphibians, reptiles, birds, and mammals (including Homo sapiens).

  • Invertebrates: Animals lacking a vertebral column or backbone. Comprises sponges, molluscs, worms, starfish, crustaceans, and insects.

  • Insects: Constitute the largest single sub-group within Kingdom Animalia, accounting for approximately $60\%$ of all classified animal species (e.g., the housefly and the blood-feeding mosquito).

Kingdom Fungi

• Cell Wall Composition and Evolutionary Distinction: Historically grouped with plants, fungi are distinct organisms with cell walls composed of chitin (the tough nitrogenous polysaccharide that also constitutes the insect exoskeleton), never cellulose. Fungi never produce chloroplasts and are incapable of photosynthesis.

• Multicellular vs. Unicellular Forms:

  • Multicellular Fungi: Encompasses macro-fungi (mushrooms and toadstools) and filamentous moulds (such as Mucor).

  • Unicellular Fungi: Represented by microscopic yeasts. Yeast cells inhabit terrestrial surfaces, fruit skins, soil, water, and airborne dust particles. Dehydrated commercial yeast powders used in baking contain millions of individual, viable yeast cells measuring approximately $1\,\mu\text{m}$ across.

• Mycelial and Hyphal Morphology in Moulds:

  • Hyphae (singular: Hypha): Narrow, branching thread-like filaments surrounded by an outer wall of chitin. The internal continuous cytoplasm contains multiple nuclei, meaning hyphae are not partitioned into discrete, independent cells.

  • Mycelium: The tangled vegetative meshwork formed by extensively branching hyphae penetrating and colonizing a nutritional substrate.

  • Fruiting Bodies and Spores: Above-ground reproductive structures (such as mushrooms, toadstools, or the vertical aerial stalks with dark terminal spore cases seen in the pin mould Mucor). These structures synthesize and liberate airborne reproductive spores to colonize fresh food sources.

• Mechanism of Saprotrophic Nutrition:

  • Fungi ingest dead or decaying organic matter through saprotrophic nutrition.

  • Hyphae invade the organic substrate (such as bread, soil, decaying leaves, or rotting fruit) and secrete digestive enzymes outwards into the environment.

  • These secreted biocatalysts are designated extracellular enzymes.

  • Extracellular digestion hydrolyzes large, insoluble organic macromolecules into small, soluble nutrient molecules (such as glucose and amino acids).

  • The hyphae subsequently absorb these soluble digestion products across their cell surface membranes.

Kingdom Protoctista

• Classification Criteria: Informally termed the "dustbin kingdom" because it functions as an assemblage of eukaryotic organisms that cannot be classified within the plant, animal, or fungal kingdoms.

• Cellular Organization: Predominantly microscopic single-celled organisms, though select algal forms exhibit complex multicellularity.

• Protozoa: Unicellular protoctists sharing structural and physiological parallels with animal cells (e.g., Amoeba, a freshwater organism living in pond water that lacks a cell wall and ingests food).

• Algae: Protoctists possessing chloroplasts and carrying out autotrophic photosynthesis. Range from microscopic single-celled organisms like Chlorella (a freshwater unicellular alga) to giant, macroscopic multicellular marine forms such as seaweeds.

• Ambiguous Genera (Euglena): Illustrates the intermediate nature of protoctists by combining plant-like traits (chloroplasts containing chlorophyll for autotrophic photosynthesis) with animal-like traits (absence of a rigid cellulose cell wall and presence of an active flagellum used for swimming).

• Pathogenic Protoctists: Several protoctist species are parasites that cause human disease, most notably Plasmodium, the protozoan parasite responsible for malaria.

Kingdom Prokaryotae (Bacteria)

• Prokaryotic Organization: Prokaryotic organisms ("before nucleus") are fundamentally simpler than eukaryotes. They lack a true nuclear membrane, a distinct nucleus, and all membrane-bound organelles (mitochondria, chloroplasts, and permanent vacuoles are absent).

• Dimensions: Bacteria are small, single-celled organisms measuring between $1\,\mu\text{m}$ and $5\,\mu\text{m}$ in length. In comparison, eukaryotic animal cells measure $10\,\mu\text{m}$ to $50\,\mu\text{m}$ in diameter; a bacterial cell's total volume is thousands of times smaller than that of a typical animal cell.

• Morphological Types: Bacteria occur in three structural geometries: spheres (cocci), rods (bacilli), and spirals (spirilla). Cells may present as solitary individuals, linked pairs, linear chains, or clusters.

• Cell Envelope and Wall Architecture:

  • Cell Wall: A rigid exterior layer providing mechanical stability, composed of peptidoglycan (a complex polymer matrix of carbohydrates and cross-linked polypeptides), never cellulose.

  • Capsule or Slime Layer: A protective outer mucous envelope found in select bacterial species exterior to the peptidoglycan wall, conferring resistance against desiccation and phagocytosis.

  • Cell Surface Membrane: Lies immediately beneath the peptidoglycan cell wall, enclosing the central cytoplasm.

• Genetic Material and Plasmids:

  • Nucleoid: Because bacteria lack a nucleus, the primary genetic material consists of a single circular chromosome of double-stranded DNA floating unbound within the central cytoplasm.

  • Plasmids: Small, discrete circular rings of double-stranded DNA distinct from the primary chromosome. Plasmids carry non-essential accessory genes and are present in approximately three-quarters ($75\%$) of all known bacterial species. Plasmids serve as vectors in genetic engineering and biotechnology.

• Locomotion: Many bacteria swim through aqueous media via the rapid, corkscrew-like rotation of helical protein filaments called flagella (singular: flagellum). Many non-flagellated bacterial species are completely non-motile.

• Metabolic Modes and Human Significance:

  • Decomposition: The majority of bacteria are heterotrophs that consume living or dead organic matter. Alongside fungi, they act as primary ecological decomposers, recycling essential nutrients within soil and aquatic biomes.

  • Photosynthesis: Certain bacterial species possess specialized bacterial chlorophyll within their cytoplasm, allowing them to carry out photosynthesis.

  • Food Production: Lactobacillus bulgaricus is a rod-shaped bacterium utilized commercially to ferment the lactose in milk into lactic acid during yoghurt manufacturing.

  • Bacterial Pathogens: Many species are pathogenic; for example, the spherical bacterium Pneumococcus causes pneumonia in humans.

  • Physiological Processes: Despite their primitive morphology, bacteria carry out all fundamental life processes, including aerobic/anaerobic respiration, feeding, waste excretion, cellular growth, reproduction (via binary fission), and behavioral responses to chemical gradients (chemotaxis).

Viruses and Non-Cellular Entities

• Acellular Status: Viruses are non-cellular entities. Because they lack a cytoplasm, organelles, and a nucleus, biologists classify them on the borderline between living organisms and non-living chemicals.

• Absence of Basal Life Functions: Viruses do not feed, respire, excrete, move, grow, or respond to ambient external stimuli. Their sole biological activity is reproduction, which occurs exclusively through obligate parasitic infection of living host cells.

• Physical Dimensions: Viruses are smaller than bacteria, ranging from $0.01\,\mu\text{m}$ to $0.1\,\mu\text{m}$ in diameter ($10\,\text{nm}$ to $100\,\text{nm}$).

• Structural Architecture:

  • Core Genetic Material: Contains a minimal set of genes consisting of either DNA or ribonucleic acid (RNA), but never both.

  • Capsid (Protein Coat): A protective proteinaceous shell enclosing the core genetic material.

  • Viral Envelope: Present in select viral particles (such as the influenza virus). This surrounding lipid bilayer is derived directly from the host cell's surface membrane during viral egress.

• Parasitic Replication Cycle:

  • A viral particle binds to and penetrates a compatible living host cell (infecting animals, plants, or bacteria).

  • The virus commandeers the host cell's transcription, translation, and replication machinery to assemble new viral genomes and protein coats.

  • The host cell undergoes lysis (dies), bursting open to discharge hundreds of mature progeny virions that infect adjacent host cells.

• Human Viral Pathologies:

  • Causes acute infections including influenza (flu), the common cold, measles, mumps, polio, and rubella ("German measles").

  • Human Immunodeficiency Virus (HIV) and AIDS: HIV selectively attacks and dismantles the human immune system. Acquired Immune Deficiency Syndrome (AIDS) is not a single disease, but a syndrome—a defined constellation of debilitating symptoms resulting from progressive immune destruction. Immunocompromised individuals become vulnerable to opportunistic infections (such as Mycobacterium tuberculosis) and unusual cancers.

• Plant Viral Pathologies (Tobacco Mosaic Virus - TMV): TMV infects susceptible plant hosts, interrupting chloroplast synthesis in the leaves. This causes stunted growth and characteristic chlorotic, mottled patches across the leaf lamina.

Pathogens Across Taxonomic Groups

• Definition: A pathogen is any biological agent or microorganism capable of causing disease within a host organism.

• Pathogenic Diversity Across Groups:

  • Viruses: Entirely parasitic and inherently pathogenic (e.g., influenza virus, HIV, TMV).

  • Bacteria: Encompasses widespread human pathogens (e.g., Pneumococcus, the causative agent of bacterial pneumonia).

  • Protoctists: Diverse pathogenic species (e.g., Plasmodium, transmitted by mosquito vectors to cause malaria).

  • Fungi: Select mould and yeast species cause diseases in plants and animals (e.g., the dermatophytic fungal mould responsible for the human skin infection athlete's foot).

Review Questions, Worked Problems, and Exercises

• Enzyme Thermal Kinetics (Volcanic Spring Microorganism):

  • Question: What is the optimum temperature of this enzyme extracted from volcanic mineral springs?

  • Analysis: The rate of reaction curve rises progressively from $10\,^\circ\text{C}$, peaks between $70\,^\circ\text{C}$ and $75\,^\circ\text{C}$, and drops to zero at $90\,^\circ\text{C}$. The optimum temperature is approximately $72\,^\circ\text{C}$ to $75\,^\circ\text{C}$.

  • Question: Explain why the activity of the enzyme is greater at $60\,^\circ\text{C}$ than at $30\,^\circ\text{C}$.

  • Explanation: Elevated thermal energy increases the kinetic energy of both enzyme and substrate molecules. Molecules travel faster, increasing the frequency of successful collisions between substrates and enzyme active sites per unit time, which accelerates the formation of enzyme-substrate complexes.

  • Question: The optimum temperature of enzymes in the human body is about $37\,^\circ\text{C}$. Explain why this enzyme is different.

  • Explanation: The microorganism lives in hot mineral springs near a volcano. Its enzymes have evolved structural adaptations that resist thermal denaturation at high temperatures, allowing metabolic reactions to continue in extreme heat.

  • Question: What happens to the enzyme at $90\,^\circ\text{C}$?

  • Explanation: The enzyme undergoes denaturation. Excessive thermal vibration disrupts the hydrogen bonds and secondary/tertiary interactions stabilizing the active site. The active site loses its specific shape and can no longer bind the substrate, causing the reaction rate to drop to zero.

• Transport Mechanisms (Diffusion vs. Active Transport):

  • Diffusion: The passive net movement of particles down a concentration gradient (from a region of higher concentration to a region of lower concentration). It requires no metabolic energy ($\text{ATP}$) from respiration.

  • Active Transport: The movement of ions or molecules across a cellular membrane against a concentration gradient (from a region of lower concentration to a region of higher concentration). It requires carrier proteins embedded within the membrane and cellular energy derived from the hydrolysis of $\text{ATP}$.

• Specialized Cellular Adaptations:

  • Motor Neurone Adaptations: Possesses an elongated axon to transmit electrical impulses rapidly across long physiological distances. Enclosed within a lipid-rich myelin sheath that insulates the axon and speeds up nerve transmission. The cell body has branching dendrites to receive synaptic inputs, and terminal synaptic endings release neurotransmitters onto effector muscle fibers.

  • Palisade Leaf Mesophyll Adaptations: Packed with high densities of chloroplasts to maximize light absorption for photosynthesis. Its elongated, columnar shape allows tight vertical packing near the upper leaf epidermis, maximizing sunlight capture while minimizing light scattering.

• Human Kidney Tubule Epithelial Transport:

  • Question: What is the function of the mitochondria in kidney tubule epithelial cells?

  • Explanation: Mitochondria carry out aerobic respiration, producing adenosine triphosphate ($\text{ATP}$) to power cellular work.

  • Question: The tubule cell contains a large number of mitochondria needed to transport glucose across the membrane into the blood at boundary 'A'. Suggest and explain the method used.

  • Explanation: Active transport. Moving glucose against a steep concentration gradient into the blood requires metabolic energy ($\text{ATP}$), which is generated by the abundant mitochondria located near membrane 'A'.

  • Question: Mitochondria are not needed to transport glucose into the cell from the tubule lumen at boundary 'B'. Name and explain the process occurring at 'B'.

  • Explanation: Facilitated diffusion (or passive diffusion). Glucose moves down its concentration gradient from high luminal concentration to lower intracellular concentration. This passive process does not require cellular energy ($\text{ATP}$), so mitochondria are not required at boundary 'B'.

  • Question: The surface membrane of the tubule cell at 'B' is greatly folded. Suggest how this adaptation helps the cell carry out its function.

  • Explanation: Folding forms microvilli, which substantially increases the total surface area of the cell membrane exposed to the luminal filtrate. A higher surface area increases the number of available transport proteins, maximizing the rate of glucose absorption.

• Conceptual Multiple-Choice Evaluations:

  • Question: Which of the following is not a characteristic of plants? (Options: Cells contain chloroplasts; Cell wall made of cellulose; Cells store carbohydrate as glycogen; Multicellular).

  • Evaluation: Storing carbohydrate as glycogen is unique to animals and fungi; plants store carbohydrate as insoluble starch.

  • Question: Fungi carry out saprotrophic nutrition. What is the meaning of this term?

  • Evaluation: Extracellular digestion of dead organic matter (Option A). Extracellular digestive enzymes are secreted onto the substrate, and the soluble products are absorbed.

  • Question: Which of the following organisms are prokaryotic: 1. viruses, 2. bacteria, 3. yeasts?

  • Evaluation: Bacteria only (Option B). Viruses are non-cellular biological entities, and yeasts are unicellular eukaryotic fungi.

  • Question: Which of the following diseases is not caused by a virus: influenza, measles, malaria, AIDS?

  • Evaluation: Malaria (Option C). Malaria is caused by the unicellular protoctist parasite Plasmodium.

• Kingdom Assignments and Organismal Classifications:

  • Mushroom: Kingdom Fungi.

  • Chlorella: Kingdom Protoctista.

  • Moss: Kingdom Plantae.

  • Lactobacillus: Kingdom Prokaryotae (Bacteria).

  • Euglena: Classified within Kingdom Protoctista because it combines traits from multiple kingdoms: it possesses photosynthetic chloroplasts like a plant, lacks a cellulose cell wall, and swims using a motile flagellum like an animal.

• Viral Biology Analysis:

  • Is a virus a living organism? No. Biologists consider viruses non-living biological entities because they lack cellular organization and do not carry out basal metabolic processes (respiration, nutrition, excretion, movement, growth, and sensitivity). Their replication is entirely dependent on host cell machinery.

  • Explain the statement "viruses are all parasites": Viruses cannot replicate independently. They are obligate intracellular parasites that must infect a living host cell, commandeer its genetic and enzymatic machinery, and use host resources to produce new viral particles, often destroying the host cell in the process.

  • Invertebrate Definition: An animal lacking an internal vertebral column or backbone.