Molecules to Organisms: Structures and Processes
Core concepts
Life is built from four macromolecule families: carbohydrates (quick energy and structure), lipids (long-term energy storage, membranes, signaling), proteins (enzymes, transport, structure, signaling), and nucleic acids (information storage and transmission). Cells are the basic unit of structure and function. Prokaryotic cells lack a membrane-bound nucleus; eukaryotic cells compartmentalize processes into organelles. The plasma membrane is selectively permeable, so substances move by diffusion, facilitated diffusion, osmosis, and active transport. Osmosis is the diffusion of water across a membrane toward the side with higher solute concentration. Photosynthesis captures light energy to make glucose and releases oxygen; cellular respiration breaks down glucose to release usable ATP. Enzymes are protein catalysts that lower activation energy and are not consumed. Homeostasis maintains stable internal conditions through feedback, and mitosis produces genetically identical cells for growth and repair.
Traps to watch
- Active transport requires energy and can move substances against a concentration gradient; diffusion does not.
- Enzymes are reusable; high temperature or wrong pH denatures them by changing shape, not by "using them up."
- Plants both photosynthesize and respire; oxygen production does not mean respiration has stopped.
- Mitochondria are the site of aerobic respiration; chloroplasts are the site of photosynthesis.
- Water moves toward the higher solute concentration, not toward pure water.
Worked example
A cell with a 1% solute concentration is placed in a 5% solute solution. The outside has the higher solute concentration, so water leaves the cell by osmosis and the cell shrinks. Rule of thumb: water follows solute.