Chapter 2: Cell: The Building Block of Life - Vocabulary Flashcards

Origin of Life and the Cellular Basis

• Environmental Origins of Life

  • Life is widely accepted by the scientific community to have originated in water.

  • Some researchers hypothesize that life began in small water pools with fluctuating environmental conditions, such as hot springs, rather than in vast oceans.

  • Puga Valley Hot Springs: Located in Ladakh, India, these hot springs maintain temperatures near the boiling point of water despite being in an extremely cold climate.

  • Conditions in hot springs like Puga Valley resemble early Earth's environment from approximately $3.5\,\text{billion years}$ ago.

  • Thermophiles: Heat-loving, unicellular bacteria that inhabit these extreme hot spring environments.

• Role of Mineral Deposits in Early Life

  • Researchers from the Birbal Sahni Institute of Palaeosciences in Lucknow studied the Puga Valley hot springs and observed rapid calcium carbonate deposition around them.

  • These calcium carbonate deposits served two critical functions:

    1. Protected early organic molecules from extreme environmental conditions and harmful radiation.

    2. Assisted in forming the first protective membrane, creating the fundamental boundary defining a cell.

• Unicellular vs. Multicellular Organisms

  • The cell represents the basic organizational level of life.

  • Unicellular Organisms: Consist of a single cell that carries out all vital functions (e.g., bacteria, yeast).

  • Multicellular Organisms: Composed of millions to trillions of cells working in a coordinated manner (e.g., plants, fish, birds, humans).

• Structural Hierarchy of Life

  • $\text{Cells} \rightarrow \text{Tissues} \rightarrow \text{Organs} \rightarrow \text{Organ Systems} \rightarrow \text{Organism}$

  • Tissues: Formed by groups of similar cells performing specific, common functions.

  • Organs: Formed by the structured organization of different tissues.

  • Organ Systems: Formed by multiple organs working cooperatively.

    • Example: The respiratory system is comprised of nasal pores, nasal cavity, trachea, and lungs.

  • Despite complex higher-level organization, the cell remains the fundamental structural and functional unit in all living organisms.

How to Study Cells and Microscopic Scale

• Resolution and Human Vision

  • Limit of Resolution: The ability of the human eye to distinguish two close objects as separate and distinct entities.

  • When viewed from the human eye's near point of approximately $25\,\text{cm}$, two points separated by at least $0.1\,\text{mm}$ can be distinguished as distinct.

  • If the distance between two points is less than $0.1\,\text{mm}$, they appear as a single merged point.

  • The limit of resolution of the unaided human eye is $0.1\,\text{mm}$.

• Scale of Biological and Physical Objects

  • $0.1\,\text{nm}$: Atoms

  • $1\,\text{nm}$: Small molecules

  • $10\,\text{nm}$: Lipids, Proteins

  • $100\,\text{nm}$: Ribosomes, Viruses

  • $1\,\mu\text{m}$: Smallest bacteria, Mitochondrion

  • $10\,\mu\text{m}$: Most bacteria, Nucleus

  • $100\,\mu\text{m}$: Most plant and animal cells, Amoeba, Fish egg

  • $1\,\text{mm}$: Chicken egg

  • $1\,\text{cm}$: Great Indian Bustard egg

  • $0.1\,\text{m}$ to $1\,\text{m}$: Human height

  • $10\,\text{m}$: Neem tree

  • $100\,\text{m}$ to $1\,\text{km}$: Rocket

  • Visibility Ranges:

    • Unaided Eye: $0.1\,\text{mm}$ to over $1\,\text{km}$.

    • Light Microscope: $100\,\text{nm}$ to $1\,\text{mm}$.

    • Electron Microscope: $0.1\,\text{nm}$ to $100\,\mu\text{m}$.

• History and Instruments of Microscopy

  • Robert Hooke (1665): First observed cells while examining a thin slice of cork using a self-designed microscope capable of approximately $200\text{X} \text{ to } 300\text{X}$ magnification. He described tiny, box-like compartments and named them 'cells'.

  • Light Microscope: Uses visible light and multiple glass lenses to magnify objects in school laboratories.

    • Components: Eyepiece, Body tube, Coarse adjustment knob, Fine adjustment knob, Handle, Base, Stage, Mirror, Objective lenses (e.g., $10\text{X}$, $40\text{X}$).

    • Key Features Improved Over Time:

      1. Resolution: The measure of image clarity.

      2. Contrast: The difference in brightness between distinct parts of an object.

      3. Magnification: The degree to which an object is enlarged.

  • Electron Microscope: Uses a high-energy beam of electrons instead of visible light to produce ultra-high-magnification images.

    • Reveals detailed cellular structures at the nanometre ($\text{nm}$) scale ($1\,\text{nm} = 10^{-9}\,\text{m}$ or $0.000001\,\text{mm}$).

    • Example: Scanning Electron Microscope (SEM) micro-photographs show detailed stomatal structures on the lower surface of a Colocasia leaf.

• Activity 2.1: Estimating Cell Size and Magnification

  1. Place a transparent ruler with millimetre ($\text{mm}$) markings on the microscope stage.

  2. Focus using adjustment knobs and measure the field-of-view diameter in $\text{mm}$ through the eyepiece.

  3. Convert the field-of-view diameter from $\text{mm}$ to micrometres ($\mu\text{m}$):        $1\,\text{mm} = 1000\,\mu\text{m}$        $\text{If diameter} = 5\,\text{mm}\text{, then } 5 \times 1000 = 5000\,\mu\text{m}$

  4. Replace the ruler with a slide of onion peel.

  5. Count the number of cells lined up straight across the field-of-view diameter.

  6. Calculate estimated cell size using the formula:        $\text{Estimated size of cell} = \frac{\text{Diameter of visible field in }\mu\text{m}}{\text{Number of cells along the diameter}}$        $\text{Example: } \frac{5000\,\mu\text{m}}{25\,\text{cells}} = 200\,\mu\text{m per cell}$

  7. Total Magnification Calculation:        $\text{Total Magnification} = \text{Eyepiece Power} \times \text{Objective Lens Power}$        $\text{Example: } 10\text{X (Eyepiece)} \times 10\text{X (Objective)} = 100\text{X Total Magnification}$        An onion cell of actual size $200\,\mu\text{m}$ appears $100$ times larger under $100\text{X}$ total magnification.

Structure of the Cell Membrane and Transport Mechanisms

• Cell Membrane (Plasma Membrane)

  • Thin, protective outer boundary that defines the individuality of every living cell.

  • Thickness: Approximately $7 \text{ to } 10\,\text{nm}$ ($1\,\text{nm} = 0.000001\,\text{mm}$).

  • Selective Permeability: Regulates entry and exit by allowing specific substances to pass while blocking others.

  • Fluid Mosaic Model:

    • Lipid Bilayer: Double layer of lipid (fat) molecules organized with hydrophilic (water-attracting) heads pointing outwards and hydrophobic (water-repelling) tails pointing inwards.

    • Embedded Proteins: Various proteins are embedded within or span across the lipid bilayer, acting as gatekeepers for transport.

    • Fluidity: Lipid and protein molecules can rotate, move laterally, and flip within the membrane layer.

    • Mosaic Pattern: Named 'mosaic' because embedded components form a pattern similar to mosaic tiles.

• Activity 2.2: Demonstration of Potato Osmosis

  1. Cut a fresh potato into two equal-sized pieces using a kitchen knife.

  2. Weigh both pieces on a weighing balance and record their initial weights.

  3. Place one potato piece into Beaker A filled with plain water.

  4. Place the second potato piece into Beaker B filled with a $20\%$ salt or sugar solution.

  5. Leave both beakers undisturbed for roughly one hour.

  6. Remove, measure, and record the final weights of both potato pieces to determine weight changes.

  7. Observations:

     • Beaker A (Plain water): Potato piece swells and increases in weight.

     • Beaker B ($20\%$ Solution): Potato piece shrinks and decreases in weight.

  8. Inference: The plasma membrane allows water molecules to pass through but restricts sugar or salt solute molecules.

• Principles of Cellular Transport

  • Concentration Gradient: A difference in the concentration of particles between two regions.

  • Diffusion: Net movement of solute or solvent particles from a region of higher concentration to a region of lower concentration down a concentration gradient (occurs without requiring a membrane).

  • Osmosis: Diffusion of water molecules across a selectively permeable membrane from an area of higher water concentration (dilute solution) to an area of lower water concentration (concentrated solution) until equilibrium is reached.

    • Application: Plant roots absorb water directly from soil via osmosis.

• Effects of Solution Tonicity on Cells

  • Isotonic Solution:         $\text{Solute concentration of extracellular medium} = \text{Solute concentration of intracellular medium}$

    • No net change in cellular volume or weight.

  • Hypotonic Solution:         $\text{Solute concentration of extracellular medium} < \text{Solute concentration of intracellular medium}$

    • Water enters the cell via endosmosis, causing the cell to swell.

  • Hypertonic Solution:         $\text{Solute concentration of extracellular medium} > \text{Solute concentration of intracellular medium}$

    • Water exits the cell via exosmosis, causing the cell to shrink.

• What If Scenario: Soaked Mung Bean Seeds

  • If mung bean seeds soaked in water for $12\,\text{hours}$ are placed into a concentrated salt/sugar solution, water leaves the seed cells via exosmosis, causing them to shrink and lose weight.

Cell Wall and Comparative Cellular Structure

• Structure and Function of the Cell Wall

  • Additional rigid outer boundary surrounding the cell membrane in plants, fungi, and bacteria.

  • Composition: Plant cell wall is primarily made of cellulose, a complex carbohydrate constructed from repeating glucose units.

    • Dietary role: Cellulose serves as dietary fiber (roughage) assisting human digestion.

  • Permeability: Fully permeable to water and dissolved mineral solutes.

  • Functional Significance:

    • Provides structural support, rigidity, and mechanical strength.

    • Helps plants withstand severe environmental conditions (high wind speeds, rain).

    • Maintains plant upright posture and keeps leaves and flowers firm.

    • Prevents plant cells from bursting when exposed to extreme hypotonic conditions.

• Activity 2.3: Comparative Study of Plant and Animal Cells

  1. Prepare a temporary mount of thin onion peel (Allium cepa) or Cradle lily (Rhoeo) leaf peel stained with safranin.

  2. Prepare a temporary mount of human cheek epithelial cells collected via cotton swab/toothpick, stained with methylene blue.

  3. Examine under a light microscope.

  4. Observations:

     • Plant Cells (Onion/Rhoeo): Distinct, regular box-shaped boundaries arranged systematically.

     • Animal Cells (Cheek): Irregularly shaped, flexible boundaries without fixed geometry.

  5. Plasmoylsis Investigation:

     • Apply a $20\%$ sugar solution to both Rhoeo and cheek cell slides and observe after $30\,\text{minutes}$.

     • Plant Cell Response: Outer cell wall boundary maintains its shape; inner cytoplasm and plasma membrane shrink away from the wall (plasmolysis), enlarging the space between the membrane and cell wall.

     • Animal Cell Response: Lacking a rigid wall, cheek cells shrink completely in size.

• Pause and Ponder Insights

  • Need for Cell Wall: Non-motile plants require rigid cell walls to maintain shape and resist abiotic pressures; motile animals require flexible cell membranes to enable tissue movement and locomotion.

  • Consequences of Flexible Plant Cell Walls: Plants would collapse under gravity, lose structural uprightness, and plant cells would risk bursting in hypotonic soil environments.

  • Experimental Controls in Potato Activity: Equal sizing and initial weight records isolate water flux as the sole independent variable determining mass changes.

Prokaryotic and Eukaryotic Cell Organization

• Core Components of Cells

  1. Plasma Membrane: Selectively permeable outer boundary.

  2. Cytoplasm: Semi-fluid, jelly-like matrix filling the cell interior.

  3. Nuclear Region: Central region holding genetic materials.

• Prokaryotic vs. Eukaryotic Structural Classification

  • Prokaryotic Cells (pro = primitive; karyon = nucleus):

    • Lack a well-defined nuclear membrane.

    • Genetic material is concentrated in an unenclosed region called a nucleoid.

    • Lack membrane-bound internal organelles.

    • Metabolic activities occur directly inside the cytoplasm.

  • Eukaryotic Cells (eu = true; karyon = nucleus):

    • Contain a well-defined nucleus bound by a double nuclear membrane.

    • Contain multiple specialized membrane-bound organelles.

• Structural Comparison of Cell Types (Table 2.1 & Table 2.2)

  • Cell Membrane: Present in Bacterial, Plant, and Animal cells.

  • Cell Wall: Present in Bacterial and Plant cells; Absent in Animal cells.

  • Cytoplasm: Present in Bacterial, Plant, and Animal cells.

  • Well-Defined Nucleus: Absent in Bacterial cells; Present in Plant and Animal cells.

  • Primitive Nucleus (Nucleoid): Present in Bacterial cells; Absent in Plant and Animal cells.

  • Membrane-Bound Organelles: Absent in Bacterial cells; Present in Plant and Animal cells.

  • Cell Diameter:

    • Prokaryotic cell: $1 \text{ to } 10\,\mu\text{m}$

    • Eukaryotic cell: $10 \text{ to } 100\,\mu\text{m}$

  • Organism Complexity:

    • Prokaryotic cell: Exclusively unicellular.

    • Eukaryotic cell: Unicellular or multicellular.

• Acellular Infectious Agents (Ready to Go Beyond)

  • Microscopic non-cellular entities that cannot be visualized under basic light microscopes:

    • Viruses: Genetic material enclosed within a protective protein coat (capsid).

    • Viroids: Infectious naked genetic material lacking a protein coat.

    • Prions: Infectious misfolded proteins completely lacking genetic material.

• Cytoskeleton and Cell Inclusions (Ready to Go Beyond)

  • Cytoskeleton: Fine network of microscopic protein fibers in eukaryotic cytoplasm that provides structural frame, preserves cellular shape, and directs intracellular transport (visible via electron microscopy).

  • Cell Inclusions: Non-living metabolic storage products suspended in cytoplasm, such as starch granules in plant cells or crystalline deposits of calcium oxalate and silica.

Detailed Structure and Function of Eukaryotic Cell Organelles

• Nucleus: House of Coded Instructions

  • Enclosed by a double-layered nuclear membrane containing nuclear pores that regulate molecular transport between nucleus and cytoplasm.

  • Nucleolus: Dense spherical structure inside the nucleus responsible for synthesizing ribosomal subunits, which exit into the cytoplasm to assemble functional ribosomes.

  • Chromatin & Chromosomes:

    • In non-dividing cells, genetic material exists as an entangled, thread-like mass called chromatin.

    • During cell division, chromatin condenses into distinct rod-shaped structures called chromosomes.

    • Chromosomes consist of DNA (Deoxyribonucleic acid) and structural proteins.

    • Genes: Functional sub-segments of DNA that contain hereditary instructions passed from parents to offspring.

  • Prokaryotic Nuclear Equivalent: Single circular DNA molecule associated with specific proteins located in the nucleoid.

  • Specialized Human RBCs: Mature human Red Blood Cells (RBCs) lack a nucleus (enucleate) to maximize interior volume for oxygen-carrying hemoglobin. Lacking genetic material, RBCs cannot repair or divide, yielding a short lifespan of approximately $120\,\text{days}$.

• Ribosomes: Protein Factories

  • Tiny, non-membrane-bound structures distributed freely within cytoplasm or bound to the endoplasmic reticulum.

  • Primary site of cellular protein synthesis.

• Endoplasmic Reticulum (ER): Manufacturing Factory

  • Extensive membrane-network of tubes and sacs continuous with the outer nuclear envelope.

  • Rough Endoplasmic Reticulum (RER): Surface studded with ribosomes, giving a rough appearance under electron microscopes. Involved in synthesizing and secreting proteins (e.g., digestive enzymes in pancreatic cells).

  • Smooth Endoplasmic Reticulum (SER): Lacks surface ribosomes. Involved in lipid (fat) synthesis, steroid hormone production, and toxin storage/metabolism.

• Golgi Apparatus: Packaging and Shipping Centre

  • Composed of stacks of membrane-bound flattened sacs (cisternae) and vesicles.

  • Functionally processes, modifies, sorts, and packages proteins and lipids received from the ER into specialized vesicles for intracellular target routing, secretion, or lysosome assembly.

  • Discovery (Camillo Golgi, 1898): Observed a thread-like reticular network in barn owl nerve cells using specialized staining techniques. Confirmed decades later through electron microscopy.

• Lysosomes: Clean-Up System

  • Membrane-bound vesicles containing powerful digestive enzymes.

  • Break down metabolic cellular waste, foreign debris, and damaged organelles, recycling molecular breakdown products into the cytoplasm for reuse.

  • Sperm Cell Application: Human sperm heads contain specialized lysosomal enzymes that digest the protective outer layers of an egg cell during fertilization.

• Mitochondria: Powerhouses of the Cell

  • Organelle responsible for producing cellular energy via cellular respiration.

  • Double Membrane Structure:

    • Outer Membrane: Smooth and porous.

    • Inner Membrane: Folded inward into finger-like projections called cristae, maximizing surface area for chemical respiratory reactions.

  • Glucose breakdown products are oxidized in mitochondria to yield ATP (Adenosine Triphosphate), the universal intracellular energy currency.

  • Contain their own independent circular DNA and ribosomes, allowing them to synthesize some of their own structural proteins.

• Plastids: Food Synthesis and Storage Centres

  • Double-membrane organelles present exclusively in plant cells and photosynthetic protists.

  • Chloroplasts:

    • Site of photosynthesis.

    • Contain green photosynthetic pigment called chlorophyll.

    • Filled with semi-fluid matrix called stroma enclosing disc-shaped membranous structures (thylakoids).

    • Chlorophyll traps solar light energy to synthesize carbohydrates, stored in stroma as starch granules.

  • Chromoplasts (chroma = colour):

    • Contain non-green pigments (yellow, orange, red).

    • Impart vibrant colors to flower petals and ripe fruits, attracting animal pollinators and facilitating seed dispersal.

  • Leucoplasts (leukos = white):

    • Non-pigment, colourless plastids dedicated to nutrient storage.

    • Store starches, lipids, or proteins (e.g., starch storage in potato tubers and taro/Colocasia roots).

  • Evolutionary Origin: Plastids and mitochondria contain distinct DNA and ribosomes similar to bacteria, pointing to an endosymbiotic evolutionary history with ancient single-celled organisms.

• Vacuoles: Storage and Turgidity Support

  • Plant Cell Vacuole: Mature plant cells feature a single, massive central vacuole bounded by a single membrane, containing liquid cell sap (water, minerals, sugars, metabolic waste).

    • Exerts turgor pressure against the cell wall to maintain cellular firmness; water deficits deplete vacuolar volume, causing wilting.

  • Animal Cell Vacuoles: Small, transient vacuoles used for temporary storage.

• Pause and Ponder Insights

  • White Flowers: Lack chromoplast pigments, containing colourless leucoplasts that reflect all visible wavelengths.

Cell Division, Growth, and Reproduction

• Synthetic Life Creation Insight

  • Researchers synthesized the artificial DNA genome of Mycoplasma mycoides and transplanted it into an emptied cell membrane envelope of Mycoplasma capricolum (devoid of its original genome).

  • The synthetic DNA assumed total functional control, orchestrating protein synthesis and cell division. This proved that DNA governs all structural and operational activities of a cell.

• Activity 2.5: Observing Cell Division in Onion Root Tips

  1. Place an onion bulb on a water-filled glass jar with its basal root ring submerged for $5 \text{ to } 6\,\text{days}$.

  2. Cut $2 \text{ to } 3\,\text{cm}$ off the newly grown root tips.

  3. Transfer root tips into freshly prepared aceto-alcohol fixative (Glacial acetic acid : Ethanol in a $1:3$ ratio) for $24\,\text{hours}$.

  4. Transfer fixed roots to $70\%$ ethanol for long-term preservation.

  5. Wash preserved root tips in water, place on a slide, and add dilute Hydrochloric acid ($HCl$) for $10 \text{ to } 15\,\text{minutes}$ to soften tissue matrix.

  6. Rinse off $HCl$, stain with aceto-carmine for $5 \text{ to } 10\,\text{minutes}$, and warm carefully over a spirit lamp flame.

  7. Excise the terminal tip, cover with a coverslip, and squash firmly with the thumb to spread a single-cell layer.

  8. Observation: Microscopic examination reveals continuous active cell division stages.

• Cell Replacement Scale

  • Hundreds of billions of cells are replaced daily in the human body, representing approximately $1\%$ of total body cells.

• Types of Eukaryotic Cell Division

  • Mitosis (Equational Division):

    • Primary mode of somatic cell division for tissue growth, repair, cellular replacement, and asexual reproduction.

    • A single parent cell divides once to yield two genetically identical daughter cells.

    • Maintains the original chromosome number and identical DNA sequences in both daughter cells.

  • Meiosis (Reduction Division):

    • Specialized division restricted to germline cells in reproductive organs to generate gametes for sexual reproduction.

    • Sites of Meiosis: Testes (sperm) and ovaries (eggs) in animals; Anthers (pollen/sperm) and ovaries (ovules) in plants.

    • Involves two sequential cellular divisions producing four daughter cells (gametes).

    • Division 1: Homologous chromosomes separate, halving chromosome numbers.

    • Division 2: Sister chromatids separate (similar to mitosis).

    • Yields gametes containing half the original parental chromosome number ($n$).

    • Restores diploid chromosome number ($2n$) upon gametic fusion during fertilization.

    • Introduces genetic variation and biological diversity.

• Prominent Scientist: Arun Kumar Sharma

  • Renowned Indian cytogeneticist and botanist.

  • Pioneered novel lab staining and preparation techniques for studying plant chromosome structure, evolution, and development.

  • Honoured with the Shanti Swarup Bhatnagar Award and the Padma Bhushan.

• Pause and Ponder Insights

  • Multiple Small Mitochondria vs. Single Giant Mitochondrion: Multiple tiny mitochondria exponentially increase inner-membrane surface area-to-volume ratios, maximizing regional ATP diffusion and localized energy supply.

  • Skin Cells Undergoing Meiosis: If skin cells divided via meiosis, daughter cells would possess halved chromosome counts ($n$), resulting in genetically unstable, non-viable tissue incapable of repairing skin wounds.

Cell Culture, Cell Theory, and Cell Death

• Cell Culture Technology (Bridging Science and Society)

  • The practice of isolating cells from organisms and growing them in vitro within artificial nutrient-rich media.

  • Requires strict sterile conditions, precise temperature regulation, moisture control, and optimum pH levels.

  • Applications: Essential for investigating fundamental cellular physiology, producing vaccines, therapeutic drugs, functional biochemicals, and lab-grown food.

• Pathological Consequences of Cell Division Errors

  • Errors in Mitosis: Uncontrolled division leads to hyper-proliferation, chromosome number abnormalities (aneuploidy), and tumor formation.

  • Errors in Meiosis: Yields chromosomal numerical anomalies in gametes, leading to developmental birth defects, reduced fertility, or early pregnancy loss.

• Formulation of Classical Cell Theory

  • Matthias Schleiden (1838): German botanist who concluded that all plant structures are composed of cells.

  • Theodor Schwann (1839): German zoologist who concluded that all animal tissues are composed of cells.

  • Rudolf Virchow (1855): German pathologist who added Omnis cellula e cellula, establishing that new cells originate exclusively from pre-existing cells.

  • Three Fundamental Tenets of Cell Theory:

    1. All living organisms are composed of one or more cells.

    2. The cell is the fundamental unit of structure and function in living beings.

    3. All cells arise from pre-existing cells.

• Plant Totipotency and Tissue Culture

  • Gottlieb Haberlandt (1902): Austrian botanist who proposed that any fully differentiated living plant cell retains the genetic potential to regenerate into an entire functional plant under appropriate nutrient and environmental conditions.

  • Totipotency: The inherent biological capacity of a single cell to divide and differentiate into all specialized cell types of an organism.

  • Haberlandt's concept laid the foundational basis for Plant Tissue Culture Technology.

• Regulation of Cell Death and Malignancy

  • Contact Inhibition: Normal animal cells cease division upon physical contact with neighboring cells.

  • Cancer Mechanics: Cancer cells lose contact inhibition control, dividing continuously to form abnormal cell masses termed tumours.

    • Benign Tumours: Non-invasive localized cell masses.

    • Malignant Tumours: Invasive cancerous masses that destroy adjacent tissues and metastasize to distant body sites.

  • Programmed Cell Death (PCD) / Apoptosis:

    • A genetically regulated, active suicide mechanism for selective cellular destruction.

    • Essential for tissue remodeling, quality control, and immune clearance.

    • Developmental Example: During human embryonic hand development, PCD eliminates interdigital web tissue to form distinct, free fingers.