Chapter 2: Cell: The Building Block of Life
2.0 Introduction to Cells
- Origin of Life: Widely accepted to have originated in water, possibly small water pools with changing conditions like hot springs.
- Early Earth Conditions: Hot springs in places like Puga Valley, Ladakh, mimic early Earth conditions (3.5 billion years ago), hosting thermophiles.
- Calcium Carbonate Deposits: Protected early organic molecules and helped form the first protective membrane.
- Cell Definition: The basic level at which life exists, forming all living organisms.
- Unicellular Organisms: Consist of one cell (e.g., bacteria, yeast).
- Multicellular Organisms: Made of millions of cells working together (e.g., plants, fish, humans).
- Levels of Organization: Cells → Tissues → Organs → Organ Systems.
- Cell as Fundamental Unit: Remains the fundamental unit of structure and function even when organized into tissues and organs.
2.1 How to Study Cells?
2.1.1 Limit of Resolution of the Human Eye
- Resolution: Ability to see two very close objects as separate and distinct.
- Human Eye Limit: Approximately 0.1 mm when viewed from 25 cm; objects closer than this appear as a single point.
- Cell Size: Cells are usually too small to be seen by the unaided eye.
2.1.2 Microscopes
- Magnification: Achieved using convex lenses or combinations (objective and eyepiece) to make objects appear larger.
- Robert Hooke (1665): First person to observe cells (box-like compartments in cork) using a self-designed microscope (capable of about 200–300× magnification).
- Light Microscopes: Used in school labs, employ different objective lenses (e.g., 10×, 40×) for magnification and resolution under visible light.
- Electron Microscopes: Powerful instruments that use a beam of electrons instead of light, providing highly magnified images at the nanometre scale.
- Microscope Improvements: Enhanced over years in resolution (clarity), contrast (brightness difference), and magnification.
2.1.3 Activity 2.1: Estimating Cell Size
- Procedure: Measure field of view diameter, count cells across it, then calculate: Estimated size = (Diameter of visible field in µm) / (Number of cells).
- Unit Conversion: 1 millimetre (mm) = 1000 micrometre (µm).
- Example Calculation: If field diameter is 5000 µm and 25 cells are seen, then cell size = 5000 µm / 25 = 200 µm.
- Total Magnification: Magnifying power of eyepiece × objective lens (e.g., 10× × 10× = 100×).
2.2 Structure of a Cell
2.2.1 Cell MembraneâThe Universal Feature of a Cell
- Definition: Thin boundary surrounding a cell, protecting its contents and defining its individuality. Also called the plasma membrane.
- Selectively Permeable: Allows some substances to pass through while blocking others.
2.2.1.1 Activity 2.2: Potato Osmosis Experiment
- Observation (Plain Water): Potato piece swells; weight increases due to water moving into cells.
- Observation (20% Salt/Sugar Solution): Potato piece shrinks; weight decreases due to water moving out of cells.
- Inference: Cell membrane allows water movement but not sugar/salt molecules.
- Osmosis: Diffusion of water across a selectively permeable membrane from higher to lower water concentration.
- Diffusion: Net movement of particles from higher to lower concentration (occurs even without a membrane).
2.2.1.2 Effect of Solutions of Different Concentrations on a Cell
- Isotonic Solution: Extracellular solute concentration = Intracellular solute concentration; no net water movement.
- Hypotonic Solution: Extracellular solute concentration < Intracellular solute concentration; water moves into the cell, causing it to swell.
- Hypertonic Solution: Extracellular solute concentration > Intracellular solute concentration; water moves out of the cell, causing it to shrink.
2.2.1.3 Structure of the Cell Membrane (Fluid-Mosaic Model)
- Thickness: Extremely thin, about 7 to 10 nanometres (nm).
- Composition: Made up of lipids (fats) and proteins.
- Lipid Bilayer: Two layers of special fat molecules with water-attracting (hydrophilic) heads outwards and water-repelling (hydrophobic) tails inwards.
- Proteins: Embedded within the lipid bilayer, acting as gatekeepers for substance passage.
- Fluidity: Molecules can move sideways, flip, and rotate within the membrane.
- Mosaic Arrangement: Molecules are arranged like tiles in a mosaic.
2.2.2 Cell WallâThe Outer Covering of Cells
- Presence: Additional layer around the cell membrane in plant, fungi, and bacteria cells.
- Function in Plants: Provides rigid structure to withstand environmental stresses (wind, rain), helps leaves/flowers remain firm, maintains shape, and keeps plants upright.
- Permeability: Permeable to water and some dissolved minerals.
- Osmosis Role: Along with cell membrane's selective permeability, helps plant roots absorb water/nutrients.
- Shape Maintenance: In concentrated solutions, plant cells lose water but maintain shape due to the rigid cell wall, preventing complete shrinkage (inner content pulls away from cell wall).
- Animal Cells: Do not have a cell wall, so they lose water and shrink significantly in concentrated solutions, allowing cellular flexibility.
- Composition: Primarily made of cellulose, a carbohydrate formed by many glucose units.
- Cellulose in Diet: Acts as roughage, aiding digestion.
- Microorganisms: Fungi and bacteria also have a cell wall for protection and structural support.
2.2.2.1 Activity 2.3: Observing Plant vs. Animal Cells
- Plant Cells (Onion/Rhoeo): Box-shaped and regularly arranged.
- Animal Cells (Cheek Cells): Irregularly arranged.
- Observation in Sugar Solution: Plant cell boundaries remain, but inner content shrinks; cheek cells shrink considerably.
2.3 The Cell InteriorâA Coordinated Working System
2.3.1 Core Components of a Cell
- Plasma Membrane: Selectively permeable outer boundary.
- Cytoplasm: Semi-fluid, jelly-like substance filling the cell.
- Nucleus: Prominent control center.
- Organelles: Sub-cellular components within the cytoplasm, mostly visible with an electron microscope.
2.3.2 Prokaryotic vs. Eukaryotic Cells
- Prokaryotic Cells: Primitive nucleus (pro-karyon), lack a well-defined nucleus and membrane-bound organelles. Genetic material in a region called the nucleoid. Cellular activities occur directly in the cytoplasm. (e.g., bacteria)
- Eukaryotic Cells: True nucleus (eu-karyon), have a well-defined nucleus and several membrane-bound organelles. (e.g., plant and animal cells)
2.3.2.1 Comparison Summary
- Primitive Nucleus: Present in prokaryotic, Absent in eukaryotic.
- Cell Diameter: Prokaryotic: 1 to 10 µm; Eukaryotic: 10 to 100 µm.
- Number of Cells: Prokaryotic: Usually unicellular; Eukaryotic: Can be unicellular or multicellular.
- Membrane-bound Organelles: Absent in prokaryotic, Present in eukaryotic.
- Membrane-bound Nucleus: Absent in prokaryotic, Present in eukaryotic.
- Cytoskeleton: Network of fine fibres in eukaryotic cells providing structural support, maintaining shape, and enabling movement/internal transport.
- Cell Inclusions: Cytoplasm may store substances like starch (plant cells) or crystals of calcium oxalate/silica.
2.3.3 Role of Organelles in Eukaryotic Cells
- Function: Carry out various life processes independently and simultaneously.
- Activities: Help build new materials, remove waste, and provide energy.
- Coordinated System: Organelles work together like a tiny living factory, each with a specific job.
2.3.4 NucleusâHouse of Coded Instructions
- Nuclear Membrane: Double-layered covering with pores for material transfer between nucleus and cytoplasm.
- Nucleolus: Dense round body in the nucleus, site of ribosomal subunit synthesis.
- Chromosomes: Rod-shaped structures visible during cell division. Contain information for inheritance in the form of DNA (Deoxyribonucleic acid) molecules.
- Composition: Chromosomes are composed of DNA and specific proteins.
- Genes: Functional segments of DNA containing genetic information.
- Chromatin Material: In non-dividing cells, DNA is present as an entangled mass of thread-like structures. Organizes into chromosomes before cell division.
- RBCs (Red Blood Cells): Mature RBCs in humans are enucleate (no nucleus), providing more space for haemoglobin to transport oxygen. Short lifespan (approx. 120 days).
- Prokaryotic Nucleoid: Contains a single circular DNA molecule associated with specific proteins, but lacks a well-defined membrane-bound nucleus.
2.3.5 RibosomesâThe Protein Factories
- Location: Tiny structures present freely in the cytoplasm or attached to the endoplasmic reticulum.
- Function: Sites of protein synthesis.
2.3.6 Endoplasmic Reticulum (ER)âManufacturing Factory
- Structure: Large organelle, network spread within the cytoplasm, continuous with the outer nuclear envelope.
- Key Role: Synthesis and transport of proteins, fats (lipids), and some hormones.
- Types:
- Rough Endoplasmic Reticulum (RER): Appears rough due to attached ribosomes. Involved in protein synthesis and secretion (e.g., gland cells).
- Smooth Endoplasmic Reticulum (SER): Lacks ribosomes, thus smooth. Involved in synthesis and storage of fats and hormones.
2.3.7 Golgi ApparatusâThe Packaging and Shipping Centres
- Structure: Stacks of flattened, sac-like structures (cisternae). Functionally linked to the ER, cell membrane, and other organelles.
- Function: Acts as the cell’s post office; modifies, sorts, and packages proteins and/or lipids into vesicles for transport, secretion, or lysosome formation.
- Discovery: First observed in 1898 by Camillo Golgi.
2.3.8 LysosomesâThe Clean-up System
- Function: Break down unwanted proteins, carbohydrates, fats, and damaged cell parts, keeping the cell clean and healthy.
- Structure: Single membrane-bound sacs filled with enzymes.
- Reusability: Breakdown products are released into the cytoplasm and may be reused.
- Sperm Cells: Human sperm contain lysosomal enzymes to break down the outer layer of an egg during fertilization.
2.3.9 MitochondriaâThe Powerhouse of the Cell
- Function: Supply energy for most cellular activities.
- Membranes: Surrounded by two membranes.
- Outer Membrane: Smooth and porous.
- Inner Membrane: Folded into finger-like projections called cristae.
- Cristae: Increase surface area for chemical reactions and energy production.
- Cellular Respiration: Glucose and other molecules are broken down to release energy.
- ATP (Adenosine Triphosphate): Energy released is stored as ATP, which acts as the energy currency for cellular activities.
2.3.10 PlastidsâCentre for Food Synthesis and Storage
- Presence: Special organelles in plant cells for food synthesis and storage.
- Types:
- Chloroplasts: Double-membrane-bound organelles containing chlorophyll (green pigment). Site of photosynthesis (absorbs sunlight).
- Stroma: Semi-fluid substance inside chloroplasts, containing disc-shaped membrane structures with chlorophyll. Sugars synthesized are stored here as starch granules.
- Chromoplasts (chroma = colour): Contain pigments other than chlorophyll (yellow, orange, red). Source of bright colors in flowers and fruits, attracting pollinators and seed dispersers.
- Leucoplasts (leukos = white): Colorless plastids that store food materials like starch, oils or proteins (e.g., in potato and taro cells).
- Evolutionary History: Mitochondria and plastids have their own DNA and ribosomes, suggesting an evolutionary history with single-celled organisms.
2.3.11 VacuolesâOrganelles for Storage and Support
- Plant Cells: Usually one large central vacuole, surrounded by a single selectively permeable membrane.
- Cell Sap: Watery fluid filling the vacuole, storing water, minerals, sugars, and waste material.
- Turgor Pressure: Stores large amounts of water, maintaining pressure inside the cell, which keeps plant cells firm.
- Wilting: When a plant lacks water, vacuoles lose water, cells become less firm, and the plant wilts.
- Animal Cells: Vacuoles are sometimes present, but are not as large as in plants, and help in temporary storage.
2.4 How Do Normal Cells Grow and Divide?
2.4.1 Cell Growth and Replacement
- Healing and Growth: Cells grow and divide to replace old, dead, or damaged cells (e.g., skin cuts, hair growth).
- Cell Size Limit: Cells grow only up to a certain size; growth occurs primarily through cell division.
2.4.2 Activity 2.5: Observing Cell Division in Onion Root Tips
- Process: Growing root tips contain continuously dividing cells, exhibiting different structures corresponding to various stages of cell division.
2.4.3 Cell Division (Overview)
- Definition: Process by which new cells are formed from pre-existing cells.
- Purpose: Allows organisms to grow, repair damaged tissues, and reproduce.
- Regulation: Eukaryotic cells divide in a controlled, orderly manner via the cell cycle.
2.4.3.1 Mitosis
- Common Type: Most common type of cell division, increasing cell number.
- Outcome: Produces two genetically identical daughter cells from one parent cell.
- Genetic Information: Each new cell receives the same DNA and number of chromosomes as the parent cell.
- Importance: Essential for normal growth, repair, maintenance, and asexual reproduction.
2.4.3.2 Meiosis
- Location: Occurs only in cells of reproductive organs to produce gametes.
- Outcome: Two-step process forming four daughter cells (gametes).
- Chromosome Reduction: First division reduces chromosome number by half. Second division is similar to mitosis.
- Genetic Diversity: Creates variations and diversity among living organisms.
- In Animals: Occurs in testes (males) to produce sperm and ovaries (females) to produce eggs.
- In Plants: Occurs in anthers (male) to form pollen grains and ovaries (female) to produce egg cells.
- Fertilization: Combines gametes from two individuals, restoring the original chromosome number.
- Cell Culture: Growing plant and animal cells outside the body in special conditions (nutrient-rich medium, controlled temperature, pH, moisture, sterile conditions). Crucial for research, biochemical production, medicines, vaccines.
- Errors in Cell Division:
- Mitosis Errors: Lead to uncontrolled cell divisions (tumors, abnormal chromosome numbers).
- Meiosis Errors: Result in genetic disorders, developmental problems, physical features, early pregnancy loss, or reduced fertility.
2.5 Cell TheoryâThe Unifying Principle of Biology
2.5.1 Historical Context
- Matthias Schleiden (1838): German botanist, reported all plants are made of cells.
- Theodor Schwann (1839): German zoologist, found all animals are made of cells.
- Rudolf Virchow (1855): German scientist, stated new cells are formed only from pre-existing cells.
2.5.2 Classical Cell Theory Principles
- Principle 1: All living organisms are made up of one or more cells.
- Principle 2: The cell is the basic unit of structure and function in living beings.
- Principle 3: All cells arise from pre-existing cells.
- Significance: Unifies all biology, from bacteria to humans, explaining life's continuity through cell division.
2.5.3 Cell Lifespan and Growth Control
- Controlled Growth: Cells grow, divide in a controlled way, perform functions, and eventually die when no longer needed.
- Replacement: Dead cells are replaced by new cells with the same function; every cell has a definite lifespan.
- Contact Inhibition (Animal Cells): Cell division usually stops when cells come into contact with neighboring cells.
- Cancer Cells: Lose contact inhibition, dividing uncontrollably and leading to tumor formation.
- Plant Cells: Due to rigid cell walls, do not show contact inhibition and follow a different growth pattern.
- Programmed Cell Death (PCD): Genetically regulated process for selective cell destruction, essential for normal development (e.g., forming fingers by eliminating cells between digits), quality control, and immune function.
- Totipotency (Plant Cells): Proposed by Gottlieb Haberlandt (1902), the ability of any living plant cell to develop into a complete plant under suitable conditions; foundation for Plant Tissue Culture Technology.
2.6 At a Glance (Summary)
- Cell: Basic structural and functional unit of all living organisms.
- Prokaryotic Cells: No well-defined nucleus; genetic material in nucleoid; lack membrane-bound organelles.
- Eukaryotic Cells: Larger, more complex; well-defined nucleus and membrane-bound organelles.
- Cell Boundaries: All cells surrounded by a cell membrane. Plants, fungi, bacteria also have a cell wall.
- Nucleus (Eukaryotic): Contains chromosomes (DNA + proteins) carrying genetic information.
- Cytoplasm (Eukaryotic): Contains various cell organelles, each with a specific function.
- Important Organelles: Nucleus, endoplasmic reticulum, mitochondria, Golgi apparatus, ribosomes, lysosomes.
- Plant-Specific Organelles: Plastids (chloroplasts, leucoplasts, chromoplasts).
- Mitosis: Produces two daughter cells identical to the parent cell.
- Meiosis: Two-step division producing four daughter cells, each with half the number of chromosomes.
- Cell Growth Control: Normal cells grow and die naturally; cancer cells divide uncontrollably, forming tumors.