🏛️ Cell Biology & Cytology Notes
Complete Notes for All Exams

1. Cytology Basics & Historical Discoveries 🔬

Cytology is the branch of life science dealing with the structural and functional study of cells. The cell is the fundamental structural, functional, and biological unit of all living organisms.

High-Probability Timeline of Discoveries

• Robert Hooke (1665): Observed dead cork cells under a primitive microscope; coined the term "cell" (from Latin cella, meaning "small room"). Acclaimed as the Father of Cytology.
• Antonie van Leeuwenhoek (1674): First to observe and describe a living cell (bacteria and protozoa in pond water).
• Robert Brown (1831): Discovered and named the Nucleus within plant cells.
• Matthias Schleiden & Theodor Schwann (1838–1839): Formulated the Cell Theory, establishing that all living organisms are composed of one or more cells, and the cell is the basic unit of structure and organization.
• Rudolf Virchow (1855): Added the legendary dictum "Omnis cellula e cellula" to the Cell Theory, demonstrating that new cells arise exclusively from pre-existing cells via division.
• Friedrich Miescher (1869): Isolated "nuclein" (now known as DNA) from the nuclei of human white blood cells.
• George E. Palade (1955): Discovered Ribosomes using electron microscopy (termed "Palade particles").
• Christian De Duve (1955 / 1965): Discovered Lysosomes (1955) and Peroxisomes (1965).
• Singer & Nicolson (1972): Proposed the Fluid Mosaic Model to explain the structure and dynamics of the cell membrane.

Extremes of Cell Sizes and Shapes

• Smallest Cell (World): Mycoplasma gallisepticum (a bacterium), measuring only 0.1 to 0.3 µm.
• Largest Cell (World): The egg yolk of an Ostrich.
• Longest Cell (Human): Neuron (nerve cell), which can span over a meter to transmit electrical impulses.
• Smallest Cell (Human): Sperm (male gamete) / Red Blood Cells (~7 µm) / Platelets.
• Largest Cell (Human): Ovum (female gamete/egg cell).

Plasma Membrane

Smooth ER

Nucleus

Mitochondrion

Rough ER

Centrosome

Lysosome

Golgi Body

2. Cell Classification, Boundaries & Transport 🦠

Prokaryotic vs. Eukaryotic Structural Contrast

Cell Outer Boundaries

• Cell Wall: A non-living, rigid, and freely permeable outermost protective layer. Strictly absent in animal cells.
  • Plants: Primarily composed of Cellulose.
  • Fungi: Composed of Chitin.
  • Bacteria: Composed of Peptidoglycan (Murein).
  • Algae: Composed of Cellulose and Pectin.
• Cell/Plasma Membrane: A living, thin, elastic, and selectively permeable barrier present in all cells.
  • Chemical Composition: Comprises approximately 52% Proteins, 40% Lipids (phospholipids & cholesterol), and 8% Carbohydrates.

Glycoprotein

Hydrophilic Head

Hydrophobic Tail

Integral Protein (Channel)

Cellular Transport Mechanics & Tonicity

• Passive Transport: Movement along a concentration gradient without expenditure of ATP.
  • Diffusion: Movement of solute particles from high to low concentration (Gases > Liquids > Solids).
  • Osmosis: Movement of water/solvent molecules through a semi-permeable membrane from higher solvent concentration to lower.
• Active Transport: Movement against a concentration gradient requiring ATP.
  • Endocytosis: Engulfing materials. Includes Phagocytosis ("cell eating", e.g., Amoeba) and Pinocytosis ("cell drinking").
  • Exocytosis: Expelling waste or hormones via vesicle fusion (colloquially termed "cell vomiting").

3. Cell Organelles & Their Specialized Functions ⚙️

• Mitochondria ("Powerhouse of the Cell"): Sausage-shaped, double-membraned organelle. The inner membrane is folded into finger-like projections called cristae to maximize the surface area for ATP synthesis via respiration.
  Apoptosis: Mitochondria are the key orchestrators of programmed cell death.
• Semi-Autonomous Organelles

  Mitochondria and Chloroplasts possess their own circular DNA and 70S ribosomes. This allows them to duplicate and synthesize some of their own proteins independently.

• Lysosomes ("Suicidal Bags" / "Atom Bomb"): Membrane-bound spherical sacs containing around 40 types of acidic hydrolytic (digestive) enzymes. They digest worn-out organelles and foreign pathogens. If the cell is heavily damaged, lysosomes burst and digest the host cell itself. Strictly absent in mammalian RBCs.
• Endoplasmic Reticulum (ER): Highway for cellular transport between the nucleus and the cytoplasm.
  • Rough ER (RER): Covered with ribosomes; synthesizes and processes proteins.
  • Smooth ER (SER): Lacks ribosomes; synthesizes lipids, fats, and steroids. In the liver, the SER detoxifies drugs and poisons via the Cytochrome P450 enzyme system.
• Golgi Apparatus ("Post Office of the Cell"): A stack of flattened membrane sacs called cisternae. Modifies, packages, and dispatches proteins and lipids.
  • Polarity: It has a Cis face (convex, receiving side facing the nucleus) and a Trans face (concave, shipping side facing the cell membrane).
  • Plant Version: Termed Dictyosomes in plant cells.
• Ribosomes ("Protein Factory"): Non-membranous particles composed of ribosomal RNA (rRNA) and proteins. They translate mRNA into polypeptide chains.
• Vacuoles: Storage sacs enclosed by a membrane called the Tonoplast.
  • Plants: Contain a single, massive, and permanent central vacuole occupying 50% to 90% of the cell volume. Stores cell sap and maintains turgidity.
  • Animals: Multiple, extremely small, and temporary vacuoles.
• Centrosome & Centrioles: A non-membranous organelle found only in animal cells. It consists of two centrioles aligned perpendicularly (made of microtubules in a 9+0 triplet pattern). Organizes spindle fibers during cell division.
• Peroxisomes: Contain oxidative enzymes (such as Catalase). Execute beta-oxidation of fatty acids and neutralize toxic Hydrogen Peroxide (H₂O₂) into water and oxygen.
• Plastids (Plant Cells Only): Double-membraned organelles categorized into three types:
  1. Chloroplasts ("Kitchen of the Cell"): Contain green chlorophyll pigments.
     Grana (stacks of thylakoids) is the site of the **light reactions** of photosynthesis. Stroma (fluid matrix) is the site of the **dark reactions**.
     Metallic core: Every chlorophyll molecule contains a single, essential atom of Magnesium (Mg²⁺) at its center.
  2. Chromoplasts: Store colored pigments, providing color to flowers and fruits:
     • Lycopene: Bright red color in **Tomatoes**.
     • Capsanthin: Red color in **Chillies**.
     • Xanthophyll: Yellow color.
     • Anthocyanin: Blue, purple, or dark red colors.
  3. Leucoplasts: Colorless plastids for nutrient storage:
     • Amyloplasts: Store starch/carbohydrates.
     • Elaioplasts: Store lipids/fats/oils.
     • Aleuroplasts: Store proteins.

Outer Membrane

Inner Membrane

Cristae (Folds)

Matrix

Mitochondrial DNA

4. Genetics, DNA Replication & Cell Division 🧬

Nucleic Acids: DNA vs. RNA

• DNA (Deoxyribonucleic Acid): Double-stranded antiparallel helix.
  • Bases: Purines (Adenine A, Guanine G) and Pyrimidines (Cytosine C, Thymine T).
  • Pairing: Adenine pairs with Thymine (A = T, 2 Hydrogen bonds); Guanine pairs with Cytosine (G ≡ C, 3 Hydrogen bonds).
• RNA (Ribonucleic Acid): Single-stranded nucleic acid containing ribose sugar and Uracil (U) in place of Thymine.
  • mRNA (messenger): Transports the genetic blueprint from nucleus to ribosomes.
  • tRNA (transfer): Brings amino acids to ribosomes during translation.
  • rRNA (ribosomal): Forms the structural core of ribosomes.

Chromosome Number Rankings (Somatic Cells)

Chromosomes consist of DNA wrapped around alkaline histone proteins. Human somatic cells contain 46 chromosomes (23 pairs): 22 pairs of Autosomes and 1 pair of Allosomes (sex chromosomes; XX for female, XY for male).

DNA Replication Machinery

DNA replication is semi-conservative. The key enzymes involved are:

• Helicase: "Unzips" the double-stranded DNA helix by breaking hydrogen bonds.
• DNA Primase: Lays down a short RNA primer to initiate synthesis.
• DNA Polymerase: Adds complementary nucleotides in a 5' to 3' direction. Doubles the DNA content.
• DNA Ligase: "Molecular glue" that joins Okazaki fragments on the lagging strand.
• Topoisomerase: Prevents DNA supercoiling ahead of the replication fork.

The Cell Cycle & Cell Divisions

The cell cycle consists of Interphase (resting/preparation phase, ~95% of cycle time) and M-phase (division phase).

• Interphase Sub-phases:
  • G1 Phase (Gap 1): Rapid cell growth and protein synthesis.
  • S Phase (Synthesis): DNA replication takes place. DNA content doubles (2C to 4C), but chromosome number (2n) remains same.
  • G2 Phase (Gap 2): Final preparation for division.
  • G0 Phase (Quiescent): Cell cycle exit; cells remain active but do not divide (e.g., neurons).

Mitosis vs. Meiosis:

5. Quick Revision Table 📊

6. Rapid Fire Practice Check 🎯