Study guide
This chapter is educational content only and does not guarantee any exam outcome. It covers the chemistry and structures that make life possible: the macromolecules, enzymes, pH systems, membranes, and organelles that the AP Biology framework groups under Units 1 and 2. Because these topics anchor the exam's data-analysis questions, pay close attention to how enzyme and osmosis experiments are set up and interpreted, not just to vocabulary.
Macromolecules and Their Building Blocks
Living things are built mainly from four classes of macromolecules, each assembled from monomers through dehydration synthesis (which releases a water molecule per bond formed) and broken apart by hydrolysis (which consumes a water molecule per bond broken). Carbohydrates, built from monosaccharides like glucose, function in short-term energy storage (starch in plants, glycogen in animals) and structural support (cellulose in plant cell walls, chitin in fungal walls and arthropod exoskeletons). Lipids are largely hydrophobic and include triglycerides for long-term energy storage, phospholipids that form the backbone of cell membranes because they have a polar phosphate head and nonpolar fatty acid tails, and steroids such as cholesterol that regulate membrane fluidity. Proteins, built from twenty amino acids linked by peptide bonds, have four levels of structure: primary (the amino acid sequence), secondary (alpha helices and beta pleated sheets from hydrogen bonding along the backbone), tertiary (the overall three-dimensional shape from interactions among side chains, or R groups), and quaternary (the arrangement of multiple polypeptide subunits, as in hemoglobin). A protein's shape determines its function, so a change in even one amino acid can disrupt folding and cause a functional loss, as researchers see in sickle-cell hemoglobin. Nucleic acids, DNA and RNA, are built from nucleotides (a sugar, a phosphate, and a nitrogenous base) and store and transmit hereditary information. A student named Priya studying for her exam finds it useful to remember that all four macromolecule classes share the same basic logic: small repeating monomers, linked and unlinked by adding or removing water, produce enormously varied polymers suited to storage, structure, or catalysis.
Enzymes, Reaction Rates, and Interpreting Kinetics Data
Enzymes are proteins (or, in some cases, RNA molecules called ribozymes) that lower the activation energy of a reaction without being consumed, allowing metabolic reactions to proceed fast enough to sustain life. Each enzyme has an active site whose shape is complementary to its substrate; the induced-fit model describes how the active site changes shape slightly as it binds substrate, improving the fit. Enzyme activity is sensitive to temperature and pH, each having an optimum beyond which the enzyme denatures, or loses its functional shape, so activity drops sharply outside that range. Consider an invented experiment: a student named Marcus measures the rate of hydrogen peroxide breakdown by the enzyme catalase at five temperatures, recording oxygen gas produced in 60 seconds at 10, 20, 30, 40, and 50 degrees Celsius as 2, 5, 9, 4, and 1 milliliters respectively. The rate rises from 10 to 30 degrees, consistent with more frequent effective collisions between enzyme and substrate as molecules move faster, then falls sharply after 30 degrees as rising temperature begins denaturing the enzyme faster than it boosts collision rate, revealing an optimum near 30 degrees Celsius rather than an ever-increasing rate. Enzyme inhibition also appears often in data questions: competitive inhibitors resemble the substrate and bind the active site, so their effect can be overcome by adding more substrate, while noncompetitive (allosteric) inhibitors bind elsewhere and change the enzyme's shape, so adding more substrate does not restore the original rate. On a Michaelis-Menten style graph of reaction rate versus substrate concentration, competitive inhibition raises the apparent Km (more substrate needed to reach half-maximal velocity) while maximum velocity, Vmax, stays the same if enough substrate is added; noncompetitive inhibition lowers Vmax while Km stays roughly unchanged. Reading which parameter shifts tells you which inhibition type a dataset represents.
pH, Buffers, and Water's Properties
Water's polarity and hydrogen bonding give it properties essential to life: cohesion (water molecules sticking to each other, enabling surface tension and the upward pull of water columns in plant xylem), adhesion (water sticking to other polar surfaces), a high specific heat (water resists temperature change, buffering organisms against rapid swings), and its behavior as a versatile solvent for polar and ionic substances. The pH scale measures hydrogen ion concentration on a logarithmic scale from 0 to 14, with each whole-number drop representing a tenfold increase in H+ concentration; most biological systems function within a narrow pH range near neutral, and even small shifts can denature proteins or disrupt enzyme function. Buffers resist changes in pH by absorbing excess H+ or OH- ions; the bicarbonate buffer system in blood, for example, shifts between carbonic acid and bicarbonate ion to keep blood pH stable despite metabolic acid production. In an experimental design question, a student might add small increments of acid to a buffered solution and an unbuffered control, plotting pH against milliliters of acid added; the buffered solution should show a long, nearly flat region (the buffering range) where pH changes little, followed by a steep rise once the buffer's capacity is exceeded, while the unbuffered control shows a steep, immediate pH change from the first drop. Recognizing that flat region as evidence of active buffering, and identifying where the buffer is overwhelmed, is a common way this concept appears on the exam.
Membrane Structure, Transport, and Water Potential
The plasma membrane is described by the fluid mosaic model: a phospholipid bilayer embedded with proteins that can drift laterally, giving the membrane both structure and flexibility. Small nonpolar molecules (oxygen, carbon dioxide) diffuse directly across the bilayer, while polar or charged substances (ions, glucose) require transport proteins: channel proteins for facilitated diffusion of ions, and carrier proteins for facilitated diffusion of larger polar molecules. Passive transport (diffusion, facilitated diffusion, osmosis) requires no cellular energy because molecules move down their concentration gradient; active transport, such as the sodium-potassium pump, requires ATP to move substances against their gradient. Osmosis, the diffusion of water across a selectively permeable membrane, moves water toward the region of lower water potential, which combines both solute concentration (solute potential, always negative or zero) and physical pressure (pressure potential). Water-potential problems are a staple data item: imagine a plant physiology lab where potato cores are placed in sucrose solutions of 0.0, 0.2, 0.4, 0.6, and 0.8 molar, and percent mass change after 24 hours is measured as +12%, +4%, -2%, -9%, and -15%. Plotting percent mass change against molarity produces a line crossing zero near 0.3 molar; that crossing point is the solution concentration at which the potato tissue's solute potential roughly equals the external solution's, meaning no net water movement occurs there. Solutions more dilute than that point are hypotonic to the tissue (cells gain water and swell), and more concentrated solutions are hypertonic (cells lose water and shrink), which for plant cells is seen as plasmolysis, the pulling away of the cell membrane from the wall.
Cell Organelles and Prokaryotic vs. Eukaryotic Organization
Prokaryotic cells (bacteria and archaea) lack a nucleus and membrane-bound organelles; their DNA is a single circular chromosome located in a nucleoid region, and they are generally smaller and simpler than eukaryotic cells. Eukaryotic cells compartmentalize functions into membrane-bound organelles: the nucleus houses linear chromosomes and controls gene expression; mitochondria, bounded by a double membrane with inner folds called cristae, generate ATP through cellular respiration; chloroplasts, found in plants and algae, capture light energy for photosynthesis; the endoplasmic reticulum (rough, studded with ribosomes, for protein synthesis and processing; smooth, for lipid synthesis and detoxification) and Golgi apparatus modify, sort, and package proteins and lipids for secretion or delivery elsewhere in the cell; and lysosomes contain hydrolytic enzymes for breaking down waste and worn-out organelles. The endosymbiotic theory explains that mitochondria and chloroplasts likely originated as free-living prokaryotes engulfed by an ancestral eukaryotic cell, a claim supported by evidence including their own circular DNA, double membranes, and ribosomes resembling those of bacteria rather than the eukaryotic cytoplasm. Surface area to volume ratio constrains cell size: as a cell grows, its volume (and metabolic demand) increases faster than its surface area (which governs exchange with the environment), which is why most cells stay small or fold their membranes (as in microvilli or cristae) to increase exchange surface without an equivalent volume increase.
Key terms
- Dehydration synthesis
- — A reaction that links monomers into a polymer by removing a water molecule for each bond formed.
- Hydrolysis
- — A reaction that breaks a polymer into monomers by adding a water molecule across each bond broken.
- Active site
- — The region of an enzyme where substrate binds and catalysis occurs, shaped to fit the substrate via the induced-fit model.
- Competitive inhibitor
- — A molecule that resembles the substrate and binds the active site, raising the apparent Km but not changing Vmax if substrate is increased enough.
- Noncompetitive inhibitor
- — A molecule that binds an enzyme at a site other than the active site, lowering Vmax without changing Km.
- Water potential
- — A measure combining solute concentration and pressure that predicts the direction water will move across a membrane; water moves toward lower (more negative) water potential.
- Plasmolysis
- — The shrinking of a plant cell's cytoplasm and pulling away of the membrane from the cell wall when placed in a hypertonic solution.
- Fluid mosaic model
- — The model describing the plasma membrane as a flexible bilayer of phospholipids embedded with proteins that can move laterally.
- Facilitated diffusion
- — Passive transport of polar or charged substances across a membrane through channel or carrier proteins, without energy input.
- Endosymbiotic theory
- — The theory that mitochondria and chloroplasts descended from free-living prokaryotes engulfed by an ancestral eukaryotic cell.
- Surface area to volume ratio
- — The relationship limiting cell size, since volume (metabolic demand) grows faster than surface area (exchange capacity) as a cell enlarges.
Exam tips
- On enzyme graphs, first identify what changed on the x-axis (temperature, pH, or substrate concentration) before deciding whether the pattern reflects denaturation, an optimum, or inhibition.
- For inhibitor data, check whether adding more substrate restores the original maximum rate; if yes, suspect competitive inhibition, if no, suspect noncompetitive inhibition.
- In water-potential or osmosis lab questions, find where a mass-change or volume-change line crosses zero — that intercept marks the solution concentration matching the tissue's internal solute potential.
- Do not confuse hypertonic/hypotonic (properties of a solution relative to a cell) with the resulting process (plasmolysis, lysis) — questions often test both directions.
- When asked to support endosymbiotic theory, cite specific evidence (double membrane, own circular DNA, similarity to bacterial ribosomes) rather than restating the conclusion.