Molecules and membranes 2 — polymers, directionality, membrane structure and water potential

The first two units of AP Biology, Chemistry of Life and Cells, are sampled heavily in the multiple-choice section, and the questions nearly always test structure and function: how a molecule's parts determine what it does, and what changes when one part changes. This material is a second set of practice on those units, focused on the ideas the first set does not cover — how polymers are built, why nucleic acids have direction, how proteins fold, how a membrane is organized, and where mitochondria came from.

The molecular questions start with dehydration synthesis: how many water molecules are released when twelve amino acids are joined into a chain. They continue with directionality, which trips up many students: nucleotides are added only to the 3′ end, and the two strands of DNA are antiparallel, so writing a complementary strand correctly means pairing the bases and then reversing their order. The protein questions separate the four levels of structure — the backbone hydrogen bonds of secondary structure, the R-group interactions of tertiary structure, the association of several polypeptides in quaternary structure — and ask what happens when a mutation places a charged R group in the tightly packed, hydrophobic core of an enzyme.

The membrane questions apply the same logic to lipids and proteins. You locate the hydrophobic R groups of a protein that spans the bilayer, explain why unsaturated fatty acids with kinked tails make a membrane more fluid, and identify the components of the fluid mosaic model: phospholipids with embedded proteins, cholesterol, glycoproteins and glycolipids. The water questions predict the direction of water movement between a cell and a solution from their water potentials, and explain why a plant cell in pure water becomes turgid rather than bursting.

The last question turns to the origin of cell compartments: which features of mitochondria — a double membrane, their own circular DNA, ribosomes similar to bacterial ones — support their descent from free-living prokaryotes engulfed by an ancestral cell.

The material offers a quiz and a flashcard deck. The quiz explains every answer in structural terms, so you can reason through an unfamiliar molecule on the exam. The flashcards review dehydration synthesis and hydrolysis, the water count for a polymer, DNA polarity and synthesis direction, the four levels of protein structure, unsaturated fatty acids, the fluid mosaic model, water potential and the endosymbiotic theory. For calculations with water potential and solute potential, pair it with the equations material in this category.

• Relate dehydration synthesis and hydrolysis to building and breaking polymers
• Write the complementary strand of DNA with correct 5′ and 3′ orientation
• Distinguish the four levels of protein structure and predict the effect of a change in R groups
• Describe the fluid mosaic model and the effect of fatty-acid saturation on fluidity
• Predict water movement from water potential and explain the evidence for endosymbiosis

Practice material written by Zestly, based on the College Board AP Biology course framework (topics 1.3–1.7, 2.3–2.7 and 2.10).