Cells as the Basis of Life – Bullet Summary

Cell Theory

  • Cells are fundamental: smallest life units, make up all living things, arise from pre-existing cells.

Cellular Organisation

  • Unicellular: single cell performs all functions (e.g. bacteria).
  • Multicellular: many specialised cells with division of labour.
  • Domains: Prokaryotes (Bacteria, Archaea); Eukaryotes (Animals, Plants, Fungi, Protists).

Prokaryotes vs Eukaryotes

  • Prokaryotes: no membrane-bound organelles, DNA in nucleoid, size 1−5 μm1{-}5\,\mu m.
  • Eukaryotes: nucleus & organelles, size 10−100 μm10{-}100\,\mu m.
  • Shared features: plasma membrane, cytoplasm, DNA, ribosomes.

Key Organelles & Functions

  • Plasma membrane – regulates transport (fluid mosaic of phospholipids, proteins, cholesterol).
  • Cell wall – structural support (plant: cellulose, fungi: chitin, bacteria: peptidoglycan).
  • Cytoplasm – metabolic medium.
  • Nucleus – houses DNA; nuclear pores export RNA/proteins.
  • Ribosomes – protein synthesis (free or on rough ER).
  • Rough ER – protein folding/modification; Smooth ER – lipid synthesis.
  • Golgi – packages & ships proteins/lipids.
  • Vacuole – storage; large central vacuole in plants.
  • Lysosome – animal-cell digestion/recycling.
  • Mitochondrion – cellular respiration ⇒ ATP.
  • Chloroplast – photosynthesis.

Microscopy Essentials

  • Light microscope: max ≈1000×\approx 1000\times, resolution ∼200 nm\sim 200\,nm, colour, live cells.
  • Electron microscope: uses electrons; higher magnification & resolution; grayscale; fixed cells.
    • SEM: surface 3-D; TEM: internal ultrastructure.
  • Total magnification =eyepiece×objective=\text{eyepiece}\times\text{objective}.
  • Field of View =Field NumberTotal Magnification=\dfrac{\text{Field Number}}{\text{Total Magnification}}.

Surface Area : Volume (SA:V)

  • For a cube: SA=6L2SA=6L^2, V=L3V=L^3, SA:V=6LSA:V=\dfrac{6}{L}.
  • Smaller cells (higher SA:V) exchange materials faster; explains cell size limits & shapes.

Membrane Structure – Fluid Mosaic

  • Phospholipid bilayer: hydrophilic heads, hydrophobic tails.
  • Proteins: receptors, transporters.
  • Cholesterol: modulates fluidity.

Transport Across Membranes

  • Passive (no ATP):
    • Diffusion: solutes down concentration gradient.
    • Osmosis: water from high to low water potential.
  • Active (ATP required): pumps (e.g. Na+/K+\text{Na}^+/\text{K}^+), endocytosis, exocytosis.
  • Rate influenced by gradient steepness, SA:V, molecule properties.

Cellular Requirements

  • Energy: light ⇒ photosynthesis (chloroplasts); glucose + O2_2 ⇒ respiration (mitochondria) → ATPATP.
  • Matter: O<em>2O<em>2, CO</em>2CO</em>2, H2OH_2O, ions, amino acids, fatty acids, metals.
  • Waste removal: diffusion, exocytosis.

Enzymes

  • Biological catalysts (proteins) lower activation energy; unchanged after reaction.
  • Active site specificity: lock-and-key or induced-fit.
  • Factors affecting activity:
    • Temperature: optimum ∼35−40∘C\sim 35{-}40^\circ\text{C} (humans); heat denatures, cold slows.
    • pH: each enzyme has optimal range; extremes denature.
    • Substrate concentration: rate rises to saturation point.
    • Enzyme concentration: proportional until substrate becomes limiting.
  • Denaturation: breaking hydrogen bonds changes active site.
  • Inhibition: competitive inhibitors occupy active site.
  • Cofactors/coenzymes (e.g. metals, vitamins) often required for activity.