Molecular and Cellular Biology
Core concepts
Life’s molecules are built from monomers: carbohydrates (sugars), lipids (fatty acids and glycerol), proteins (amino acids), and nucleic acids (nucleotides). Cells are prokaryotic (no membrane-bound organelles, circular DNA) or eukaryotic (organelles, linear nuclear DNA). Key organelles: nucleus (DNA), ribosomes (protein synthesis), mitochondria (aerobic ATP), chloroplasts (photosynthesis), ER and Golgi (processing and transport), lysosomes (digestion), and the plasma membrane (phospholipid bilayer with proteins). Transport ranges from passive diffusion and osmosis to facilitated diffusion and active transport (requires ATP), plus endocytosis and exocytosis. Enzymes are catalysts that lower activation energy; they are affected by temperature, pH, and substrate concentration and can be competitively or allosterically inhibited. ATP couples energy-releasing and energy-requiring reactions. Photosynthesis converts light energy to chemical energy; cellular respiration (glycolysis, Krebs cycle, electron transport chain) harvests it. The central dogma-DNA to RNA to protein-involves replication, transcription, and translation.
Traps
Osmosis moves water toward higher solute concentration. In eukaryotes, transcription occurs in the nucleus and translation on ribosomes; replication is nuclear. Enzymes do not change the overall energy yield. Plant cells have cell walls and chloroplasts; animal cells have centrioles.
Worked example
A cell with 30% solute inside is placed in a 10% solute solution. Water enters, so an animal cell swells and may lyse; a plant cell becomes turgid. Rule of thumb: water follows solute.