Molecular and Cellular Biology
Molecules of life
Four macromolecule families: carbohydrates (quick energy, structural cellulose), lipids (fats, phospholipids, steroids - nonpolar and energy-dense), proteins (amino acids; enzymes, structure, transport), and nucleic acids (DNA/RNA; nucleotides). Enzymes are catalysts that lower activation energy but do not change the net energy of a reaction and are not consumed; they are specific and sensitive to temperature and pH.
Cells and membranes
Prokaryotes lack a nucleus and membrane-bound organelles but have ribosomes, DNA, and a plasma membrane; eukaryotes have nuclei and organelles. Endosymbiotic theory explains mitochondria and chloroplasts (own DNA, double membranes, bacteria-like ribosomes). Surface area-to-volume ratio limits cell size.
Membrane transport: simple diffusion and osmosis are passive; facilitated diffusion uses proteins but no ATP; active transport uses ATP (e.g., Na+/K+ pump). Osmosis moves water toward the side with higher solute concentration - a common trap is predicting solute movement instead.
Energy
Photosynthesis (chloroplasts) converts light energy into chemical energy in glucose; cellular respiration (mitochondria) harvests ATP from glucose. They are not simple reverses: respiration occurs in light or dark, and the pathways and enzymes differ. Glycolysis is anaerobic and cytosolic; the Krebs cycle and electron transport chain are aerobic and mitochondrial.
Worked example
A cell with 2% solute is placed in a 5% solution. The solution is hypertonic, so water leaves the cell and it shrinks (crenation). Rule of thumb: water follows solute - always compare solute concentrations on the two sides.