Biology (HESI A2)
What the Biology domain covers
The HESI A2 Biology section tests foundational cell and molecular biology rather than advanced physiology. Expect items on biomolecules, organelle functions, membrane transport, cellular respiration and photosynthesis, DNA/RNA and protein synthesis, mitosis versus meiosis, and Mendelian genetics. Most questions are recall or one-step application, so precise definitions and the ability to read a Punnett square or follow a codon relationship matter more than long derivations.
Core rules and concepts
- Biomolecules: carbohydrates are primary quick energy; lipids form membranes and store energy; proteins are amino-acid polymers that include enzymes; nucleic acids are nucleotide polymers that store and transmit genetic information.
- Enzymes lower activation energy, are substrate-specific, and are not consumed. Temperature and pH outside the optimum denature them by disrupting shape.
- Organelles: nucleus stores DNA; mitochondria carry out aerobic respiration; ribosomes make proteins; rough ER moves proteins; smooth ER makes lipids; Golgi modifies and packages; lysosomes digest; chloroplasts perform photosynthesis.
- Transport: diffusion moves solute high-to-low; osmosis moves water across a selectively permeable membrane; facilitated diffusion uses channels; active transport uses ATP; endocytosis and exocytosis move material in vesicles.
- Respiration: glycolysis in the cytoplasm yields 2 ATP net and pyruvate; the Krebs cycle occurs in mitochondria; the electron transport chain produces most ATP. Anaerobic fermentation yields far less ATP.
- Photosynthesis: light reactions in thylakoids make ATP and NADPH; the Calvin cycle in the stroma fixes CO2.
- DNA/RNA: DNA is double-stranded with A-T and C-G; RNA is single-stranded and uses uracil instead of thymine. Transcription makes mRNA from DNA; translation at ribosomes uses tRNA anticodons to add amino acids.
- Division: mitosis gives two identical diploid cells; meiosis gives four haploid gametes and includes crossing over and independent assortment.
Worked example
Cross two heterozygous tall pea plants: Tt x Tt. Each parent passes T or t with equal probability. The four Punnett boxes are TT, Tt, Tt, tt. Phenotypes: three tall (TT or Tt) and one short (tt), a 3:1 ratio. Genotypes: 1 TT : 2 Tt : 1 tt. The reasoning is that the recessive phenotype appears only when both alleles are recessive, so P(tt) = 1/2 x 1/2 = 1/4.
Traps that cost marks
Transcription is DNA to mRNA; translation is mRNA to protein. Uracil belongs to RNA, not DNA. Meiosis produces four haploid cells, not two diploid cells. Most ATP comes from the electron transport chain, not glycolysis. Enzymes are catalysts, not reactants. Osmosis refers to water movement only. A dominant allele is not necessarily the more common allele in a population; dominance describes expression in a heterozygote.