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Biology Core

Cell, Biomolecules & Cell Cycle

Biology Unit 3
25 min read
IAT Core Framework
High Yield

1. Core Concept

The cell is the fundamental structural and functional unit of life. Life processes are driven by Biomolecules (carbon compounds) and maintained through a tightly regulated Cell Cycle that ensures the continuity of genetic information.

2. Cell: The Unit of Life

Prokaryotic vs. Eukaryotic Cells

  • Prokaryotes (Bacteria): Lacks a membrane-bound nucleus. Contains Mesosomes (infoldings of the cell membrane for respiration and cell wall secretion) and Plasmids (circular extra-chromosomal DNA providing antibiotic resistance). Ribosomes are 70S.
  • Eukaryotes (Protists, Fungi, Plants, Animals): Exhibit extensive compartmentalization. Ribosomes are 80S (except inside mitochondria and chloroplasts).

Cell Organelles & Functions

  • Cell Membrane: Fluid Mosaic Model (Singer & Nicolson). Lipids (phospholipids) are arranged in a bilayer; proteins are integral or peripheral.
  • Endomembrane System:
    • ER: RER (Protein synthesis; has ribosomes) and SER (Lipid/Steroid synthesis; detoxification).
    • Golgi Apparatus: Modifies, sorts, and packages proteins/lipids. Important site for the formation of Glycoproteins and Glycolipids.
    • Lysosomes: "Suicidal bags"; contain hydrolytic enzymes strictly active at acidic pH.
    • Vacuoles: In plants, occupies 90% volume; membrane is called Tonoplast (transports ions against concentration gradient).
  • Mitochondria & Chloroplasts: Semi-autonomous (have their own circular DNA and 70S ribosomes). Mitochondria are the site of aerobic respiration (ATP production). Chloroplasts are the site of photosynthesis.
  • Cytoskeleton: Network of filamentous proteinaceous structures (Microtubules, Microfilaments). Provides mechanical support and motility.
  • Cilia/Flagella: 9+2 arrangement of microtubules.
  • Centrioles: 9+0 arrangement; form the basal body of cilia/flagella and spindle fibers.
  • Nucleus: Contains the Nucleolus (non-membrane bound; site for active rRNA synthesis) and Chromatin (DNA + basic Histone proteins). Chromosomes are classified based on centromere position: Metacentric (middle), Sub-metacentric, Acrocentric, Telocentric (end).

3. Biomolecules

Carbohydrates

Linked strictly by Glycosidic bonds.

  • Storage: Starch (Plants), Glycogen (Animals).
  • Structural: Cellulose (Plant cell wall - the most abundant organic polymer).

Proteins

Polymers of Amino Acids linked by Peptide bonds.

  • Primary structure: Linear sequence (determines function).
  • Tertiary structure: 3D folding (necessary for biological activity/enzymes).
  • Key Proteins: Collagen (most abundant in animal world), RuBisCO (most abundant in whole biosphere).

Lipids

Generally water-insoluble. Fats, oils, phospholipids (in membranes). Arachidonic acid has exactly 20 carbons.

Nucleic Acids

Polymers of Nucleotides (Sugar + Nitrogenous Base + Phosphate) linked by Phosphodiester bonds.

DNA: Double helix (Watson & Crick). A=T (2 H-bonds), G≡C (3 H-bonds).

Enzymes (Biocatalysts)

  • They function by lowering the Activation Energy of a reaction.
  • Influenced by: Temperature, pH, and substrate concentration ([S]).
  • Competitive Inhibition: Inhibitor chemically mimics the substrate. Km increases (affinity decreases), but Vmax remains the same.

4. Cell Cycle and Cell Division

Interphase (95% of cycle duration)

  • G1 (Gap 1): Cell grows continuously; metabolically active.
  • S (Synthesis): DNA replication (amount of DNA doubles 2C → 4C, but chromosome number strictly remains the same). Centrioles double in the cytoplasm.
  • G2 (Gap 2): Protein synthesis for mitosis continues.

M Phase (Mitosis - Equational Division)

  • Prophase: Chromosomes condense; nuclear envelope and organelles disappear.
  • Metaphase: Chromosomes align tightly at the equatorial plate. Best stage to study morphology.
  • Anaphase: Centromeres split; sister chromatids move to opposite poles.
  • Telophase: Reverse of prophase.

Meiosis (Reductional Division)

Meiosis I: Reduces chromosome number (2n → n).

Prophase I (Crucial Stages):

  1. Leptotene: Chromosome condensation.
  2. Zygotene: Synapsis (pairing) of homologs; Synaptonemal complex forms.
  3. Pachytene: Crossing over occurs (mediated by the enzyme Recombinase).
  4. Diplotene: Dissolution of complex; Chiasmata (X-shaped structures) become visible.
  5. Diakinesis: Terminalization of chiasmata.

Meiosis II: Mechanically resembles Mitosis.

5. Common Mistakes

  • S Phase Trap: DNA amount doubles (2C → 4C), but chromosome number remains exactly the same. Don't double the chromosomes!
  • Ribosomes: Remember Prokaryotes = 70S; Eukaryotes = 80S (in cytoplasm) BUT 70S (inside organelles like mitochondria).
  • Enzyme Inhibition: In competitive inhibition, Vmax is NOT changed. You can overcome the inhibition entirely just by adding more substrate.
  • Plant vs Animal Cytokinesis: Plants use a Cell Plate (grows centrifugal/outwards); Animals use a Furrow (grows centripetal/inwards).

6. IAT Exam Focus Points

  • Meiosis Prophase I Stages: Sequence and specific events (Synapsis → Crossing Over → Chiasmata) are extremely high yield.
  • Organelle Recognition: Focus on unique functional features (e.g., mesosomes in bacteria, tonoplast in plants, 9+2 vs 9+0 microtubule arrangements).
  • Biomolecule Bonds: Be ready to rapidly match the bond type with the molecule (Protein: Peptide, Polysaccharide: Glycosidic, DNA: Phosphodiester).
  • Km and Vmax: Graphical questions analyzing enzyme kinetics and inhibition shifts.
  • Ploidy Changes: Calculating chromosome/chromatid numbers at different specific stages of Mitosis and Meiosis. (e.g., if a cell has 2n = 16, how many chromatids are present in Anaphase I? Answer: 32).

7. Practice Mock Test

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Cell, Biomolecules & Cell Cycle

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