Biology: Systems, Pathways, and Evidence
Core concepts
MDCAT biology rewards a systems view. Master cell ultrastructure and organelle functions, membrane transport, enzymes and metabolic regulation, cellular respiration and photosynthesis, cell division, Mendelian and molecular genetics, evolution, and human physiology. Link each process to its location: glycolysis in the cytosol, the Krebs cycle in the mitochondrial matrix, oxidative phosphorylation on the inner membrane, light reactions in the thylakoid membrane, and the Calvin cycle in the stroma. For physiology, trace cause and effect: a change in blood osmolarity triggers ADH release, which alters collecting-duct permeability and urine concentration.
Exam traps
Questions often swap similar terms: prokaryotic versus eukaryotic ribosomes, competitive versus noncompetitive inhibition, mitosis versus meiosis, and genotype versus phenotype. Watch for absolute words such as 'always' and 'never' in genetics and evolution, and translate any described diagram into a labeled pathway before answering. In enzyme questions, distinguish denaturation (shape loss, usually irreversible) from allosteric regulation (reversible). In genetics, do not assume independent assortment when genes are linked.
Worked example
A competitive inhibitor is added to an enzyme-catalyzed reaction. What happens to Km and Vmax? The inhibitor competes with substrate at the active site, so more substrate is needed to reach half-maximal velocity: Km increases. At saturating substrate, the inhibitor is outcompeted, so Vmax is unchanged.
Rule of thumb
Name the compartment, the substrate, and the product before choosing an answer.