SACE Biology - Cells as the Basis of Life Study Notes

Cell Theory and Membrane Structure

• The Cell Theory states:

  • All organisms consist of one or more cells and cell products.

  • All cells arise from pre-existing cells.

• All cells possess a cell membrane (plasma membrane) approximately $8\,nm$ thick.

• Fluid Mosaic Model:

  • Phospholipid molecules arranged in a fluid bilayer.

  • Proteins (integral and peripheral) are embedded in the bilayer, creating a "mosaic."

  • Integral proteins include transport proteins (channels, pumps), receptors, and enzymes.

• Membrane Fluidity:

  • Unsaturated fatty acids create "kinks" that increase fluidity.

  • Cholesterol generally increases fluidity at the core but provides rigidity at the face.

Prokaryotic and Eukaryotic Cells

• Prokaryotes:

  • Size: $1 - 10\,\mu m$ diameter.

  • No true nucleus or membrane-bound organelles.

  • DNA is circular (nucleoid region) or as plasmids.

  • Possess a cell wall made of peptidoglycan.

  • Divide via binary fission.

• Eukaryotes:

  • Size: $10 - 100\,\mu m$ diameter.

  • Contain a nucleus and membrane-bound organelles (mitochondria, chloroplasts, etc.).

  • DNA is linear and associated with histone proteins.

  • Cell walls present in plants (cellulose) and fungi (chitin); absent in animals.

  • Divide via mitosis.

Organelle Structure and Function

• Nucleus: Contains chromosomes and the nucleolus ($RNA$ synthesis); regulates cell activity.

• Mitochondrion: Double-membrane structure with inner folds (cristae); site of aerobic respiration to produce $ATP$.

• Chloroplast: Contains grana (thylakoid stacks) for light absorption and stroma for enzyme activity; site of photosynthesis.

• Ribosome: Site of protein synthesis; found free in cytoplasm or on the Rough Endoplasmic Reticulum (RER).

• Golgi Body: Packages and secretes proteins into vesicles.

• Lysosome: Membrane-bound vesicle containing digestive enzymes.

• Cytoskeleton: Protein fibres maintaining cell shape and organelle anchorage.

Cell Metabolism and Energy

• Autotrophs: Convert simple inorganic molecules ($CO_2$, $H_2O$) into complex organic molecules (glucose).

• Heterotrophs: Ingest ready-made complex organic molecules for nutrition.

• Photosynthesis: Conversion of light energy into chemical energy in chloroplasts.

• Aerobic Respiration: Releases energy from glucose using oxygen; yields $36\,ATP$.

• Fermentation (Anaerobic): Occurs in cytoplasm without oxygen; yields $2\,ATP$.

  • In plants/yeast: Produces ethanol and $CO_2$.

  • In animals: Produces lactic acid.

• $ATP$ (Adenosine Tri-Phosphate): The immediate energy source for cells; energy is released when converted to $ADP$ and $Pi$ (inorganic phosphate).

Movement of Materials

• Surface-Area-to-Volume Ratio: As cell size increases, the ratio decreases, reducing the rate of exchange.

• Passive Transport (No energy required):

  • Diffusion: Random movement of particles down a concentration gradient.

  • Facilitated Diffusion: Movement via protein channels or carrier molecules.

  • Osmosis: Diffusion of water from a dilute to a concentrated solution.

• Active Transport: Requires $ATP$ to move substances against a concentration gradient (low to high).

• Bulk Transport:

  • Endocytosis: Inward transport via vesicles (Phagocytosis for solids; Pinocytosis for liquids).

  • Exocytosis: Export of materials via vesicles fusing with the plasma membrane.

The Cell Cycle and Division

• Binary Fission: Rapid asexual reproduction in prokaryotes; DNA attaches to the cell membrane.

• Mitosis: Eukaryotic division for growth and repair; produces two genetically identical diploid ($2n$) daughter cells.

  • Phases: Prophase, Metaphase, Anaphase, Telophase.

• Meiosis: Production of four genetically different haploid ($n$) gametes; involves crossing over and independent assortment for genetic variation.

• Cell Cycle Checkpoints:

  • $G_1$: Checks for nutrients, growth factors, and DNA damage.

  • $G_2$: Checks for cell size and successful DNA replication.

  • Metaphase: Checks for chromosome spindle attachment.

Regulation and Interference

• Metabolic Pathways: Regulated by specific enzymes at each step; energy is lost as heat at each transition.

• Enzyme Factors: Activity influenced by temperature and pH; extremes cause denaturation (alteration of the active site).

• Chemical Inhibitors:

  • Competitive: Compete with substrate for the active site.

  • Non-competitive: Bind to an allosteric site, changing the enzyme's shape.

• Cancer: Uncontrolled mitosis caused by mutations in oncogenes due to carcinogens (e.g., UV radiation, tobacco smoke, X-rays).

Cell Culture Techniques

• Purpose: Growing isolated cells in vitro in a nutrient medium under sterile conditions.

• Steps: Dissection, tissue disruption (mechanical or enzymatic), placement in culture medium, and incubation at optimum temperature.

• Hatflick Limit: Cultured animal cells typically only grow for 30-40 generations.

• Applications:

  • Research: Testing chemical toxicity or mutagenic potential.

  • Medicine: Growing skin for burns, vaccine production, and stem cell research.

  • Agriculture: Propagating endangered species and disease-resistant plants.