Biology: From Molecules to Ecosystems
Core concepts to master
Cell theory and the differences between prokaryotic and eukaryotic cells are foundational. Know the organelles and their functions, especially the mitochondrion (aerobic respiration), chloroplast (photosynthesis), ribosome (protein synthesis), and the nucleus. Understand the central dogma: DNA is transcribed to mRNA, which is translated to protein at the ribosome. Genetics requires fluency with Mendelian ratios, genotype versus phenotype, dominant and recessive alleles, and the use of Punnett squares. Population genetics adds allele frequencies and natural selection. Physiology questions test homeostasis, transport across membranes (diffusion, osmosis, active transport), and the roles of enzymes, whose activity depends on temperature and pH.
Traps to watch
Many candidates confuse mitosis with meiosis: mitosis produces two identical diploid cells, meiosis produces four genetically distinct haploid gametes. Distinguish genotype from phenotype and remember that a recessive trait appears only when both alleles are recessive. Do not assume the dominant allele is the more common one in a population. In osmosis questions, water moves toward the higher solute concentration, not toward the higher water concentration. Read carefully whether a question asks about DNA, mRNA, or protein.
Worked example
Cross two heterozygous pea plants (Tt x Tt) for tallness, where T is dominant. The Punnett square gives TT, Tt, Tt, tt. Genotypic ratio is 1:2:1; phenotypic ratio is 3 tall to 1 short. So the probability of a short offspring is 1/4, or 25 percent. Rule of thumb: always set up the square before answering, and label which allele is dominant.