Cells_Basic_Unit_of_Life_Notes

Introduction to Life and Biochemistry

  • Life = complex, dynamic, information-based phenomenon that is:
    • Highly organized from biomolecules → organelles → tissues → organs → whole organism
    • Self-sustaining through constant energy flow & regulated chemical reactions
  • All living entities are carbon-based (organic); carbon skeletons enable vast structural diversity.
  • Every living system can be viewed as a vast network of chemical pathways → subject of Biochemistry.
  • Core message: “Life is cellular.” No process of life occurs outside the context of cells.

Characteristics of Life (must know!)

  • Cellular Organization
    • Organisms comprised of one cell (unicellular) or many cells (multicellular).
  • Homeostasis
    • Ability to maintain relatively constant internal environment (e.g., body temp ≈ 37\,^\circ\text{C}, blood pH ≈ 7.35–7.45, water balance).
  • Metabolism
    • Sum of all biochemical reactions; includes catabolism (breakdown, energy release) & anabolism (biosynthesis, energy use).
  • Reproduction
    • Production of offspring; may be asexual (binary fission, budding) or sexual (gamete fusion).

What Is Biochemistry?

  • Formal definition: Science that explores chemical processes in & related to living organisms.
  • Multidisciplinary links:
    • Medicine (drug design, diagnostics)
    • Environmental science (bioremediation pathways)
    • Agriculture (crop bio-fortification, pest-resistant plants)
    • Nutrition (metabolic fate of nutrients)
  • Investigates molecular basis of disease, heredity, evolution, bio-energetics, etc.

Features Shared by All Living Organisms

  • Cell = fundamental unit of life → smallest entity exhibiting all hallmarks of life.
  • Living processes = regulated biochemical reactions; perturbation → disease/death.
  • Universality of four major classes of biomolecules:
    • Proteins (catalysis, structure, signaling)
    • Nucleic acids (genetic information, some catalysis)
    • Carbohydrates (energy, recognition, structure)
    • Lipids (membranes, energy storage, signaling)

Types of Cells

  • Prokaryotic Cells
    • Small (≈ 0.2–2\,\mu\text{m}), structurally simple, always unicellular.
    • Domains: Bacteria & Archaea.
  • Eukaryotic Cells
    • Larger (≈ 10–100\,\mu\text{m}), internal membrane-bound organelles including a nucleus.
    • Kingdoms: Protists, Plants, Animals, Fungi (plants & some protists possess chloroplasts).

Comparative Snapshot: Prokaryotes vs. Eukaryotes

  • Nucleus
    • Prokaryotes: none (DNA in nucleoid).
    • Eukaryotes: membrane-bound nucleus.
  • Membranes
    • Plasma membrane present in both; internal membranes (ER, Golgi, mitochondria, etc.) only in eukaryotes.
  • Energy Organelles
    • Prokaryotes: oxidative enzymes on plasma membrane; no mitochondria.
    • Eukaryotes: mitochondria; chloroplasts in green plants/algae.
  • Size & Complexity
    • Prokaryotes simpler, fewer compartments; eukaryotes compartmentalized, specialized.

Prokaryotic Cell Structure & Function

  • Cell Wall
    • Rigid layer, determines shape, prevents osmotic lysis; peptidoglycan in bacteria, pseudo-murein in some archaea.
  • Plasma Membrane
    • Phospholipid bilayer; selective permeability barrier, site of respiration/photosynthesis enzymes.
  • Cytoplasm
    • Aqueous matrix; houses nucleoid (circular DNA), ribosomes.
  • Pili & Flagella
    • Pili: adhesion, conjugation (DNA transfer).
    • Flagella: motility via rotary motion.

General Architecture of Eukaryotic Cells

  • Plasma (Cell) Membrane
    • Fluid lipid bilayer + proteins; provides shape, mechanical strength, controls solute traffic; defines compartments.
  • Cytosol
    • Viscous, water-based matrix; ~70\% water, rich in ions, metabolites, enzymes; site for glycolysis, biosynthetic reactions, storage granules.
  • Cytoplasm = cytosol + suspended organelles.

Information Center

  • Nucleus
    • Houses linear chromosomes (DNA + histones); governs gene expression & metabolic regulation.
  • Nucleoplasm
    • Semi-fluid medium where DNA replication & transcription occur.
  • Nuclear Envelope
    • Double membrane with nuclear pores (selective gates for mRNA, proteins).
  • Nucleolus
    • Dense region for rRNA synthesis & ribosomal subunit assembly.

Membrane Systems

  • Rough Endoplasmic Reticulum (RER)
    • Flattened sacs studded with ribosomes; synthesizes secretory & membrane proteins; initial glycosylation.
  • Smooth Endoplasmic Reticulum (SER)
    • Tubular; lipid, phospholipid & steroid synthesis; detoxification (adds —OH to non-polar toxins → ↑ solubility → excretion).

Protein Handling & Trafficking

  • Ribosomes
    • \sim 25\,\text{nm} particles of rRNA + proteins; free (cytosolic proteins) or bound (RER).
    • Translate mRNA → polypeptide using peptidyl transferase activity.
  • Golgi Apparatus
    • Stacked cisternae; modifies (e.g., glycosylates), sorts & packages proteins/lipids into vesicles for lysosomes, plasma membrane, or secretion.

Special Vesicular Organelles

  • Lysosomes
    • Acidic (pH \approx 5) sacs; contain hydrolytic enzymes; degrade macromolecules, recycle organelles (autophagy), perform endocytotic digestion.
  • Peroxisomes
    • Contain oxidases & catalase; detoxify \text{H}2\text{O}2, metabolize long-chain fatty acids, aid in photorespiration (plants).

Energy Organelles

  • Mitochondria
    • Double membrane; inner membrane forms cristae with electron transport chain.
    • Site of oxidative phosphorylation: \text{ADP} + P_i \rightarrow \text{ATP} via chemiosmosis.
    • Possess own circular DNA & ribosomes (endosymbiotic origin).
  • Chloroplasts (plants/algae)
    • Thylakoid membranes with chlorophyll; perform photosynthesis: 6\text{CO}2 + 6\text{H}2\text{O} + \text{light} \rightarrow C6H{12}O6 + 6\text{O}2.

Structural & Storage Elements (Plants)

  • Cell Wall
    • Cellulose matrix; provides rigidity, prevents excessive uptake of water.
  • Central Vacuole
    • Large, fluid-filled; stores ions, waste, pigments; maintains turgor pressure.

Master Table – Organelles & Key Functions (quick reference)

  • Nucleus → genome repository, DNA/RNA synthesis.
  • Mitochondrion → aerobic respiration, ATP production.
  • Chloroplast → photosynthesis (plants).
  • ER (Rough) → protein synthesis/modification; ER (Smooth) → lipid synthesis, detox.
  • Golgi → protein/lipid processing & trafficking.
  • Lysosomes → hydrolytic digestion, recycling.
  • Peroxisomes → ROS detox, fatty acid oxidation.
  • Plasma Membrane → boundary, selective transport, signaling.
  • Cell Wall (plants) → protection, shape.
  • Central Vacuole (plants) → storage, turgor.

Illustrative Examples & Scenarios

  • Fever disrupts homeostasis; enzymes denature above 40\,^\circ\text{C}, illustrating narrow operating range maintained by feedback loops.
  • Antibiotics (e.g., penicillin) target prokaryotic cell wall synthesis; minimal effect on human cells lacking cell walls → therapeutic specificity.
  • Statins inhibit hepatic SER enzyme HMG-CoA reductase, lowering cholesterol → demonstrates SER’s biochemical role.

Connections to Previous & Future Topics

  • Knowledge of organelles sets foundation for:
    • Enzyme kinetics & metabolic pathways (glycolysis, TCA cycle) located in cytosol / mitochondria.
    • Molecular genetics (replication, transcription, translation) rooted in nucleus, cytosol, ribosomes.
    • Cell signaling & cancer biology (membrane receptors, nuclear oncogenes).

Ethical / Philosophical / Practical Implications

  • Genetic engineering (CRISPR edits within nucleus) raises bioethics: germline modification vs therapy.
  • Organelle-specific drug delivery (liposomes, mitochondrial targeting peptides) aims for precision medicine while minimizing side effects.
  • Biotechnology harnesses prokaryotes for sustainable manufacturing (biofuels, biodegradable plastics) but requires ecological oversight.

Key Numerical & Chemical References (memorize where appropriate)

  • Approx. water content of cytosol: \sim 70\%.
  • Human cell diameter: 10–30\,\mu\text{m}; bacterial cell: 0.5–5\,\mu\text{m}.
  • Intracellular pH: 7.2 (cytosol), 5 (lysosome).
  • ATP hydrolysis free-energy change: \Delta G^{\circ\,'}_{ATP} \approx -30.5\,\text{kJ}\,\text{mol}^{-1}.

Study Tips

  • Draw & label an animal, plant, and prokaryotic cell → visualize compartment functions.
  • Practice comparing organelles to factory analogies: nucleus = headquarters, RER = assembly line, Golgi = shipping/labeling, lysosome = recycling center.
  • Link each characteristic of life to a real physiological example.