The HESI A2 Biology section tests your understanding of fundamental life science concepts — from the structure of a cell to how traits are inherited. With only 30 questions and 25 minutes, you need to know the material cold. This guide covers every major topic so you can walk into the exam confident.
What the HESI A2 Biology Section Covers
Expect questions on these core areas:
- Cell structure and function
- Cell division (mitosis and meiosis)
- Genetics and heredity
- DNA and RNA
- Photosynthesis and cellular respiration
- Biological molecules (macromolecules)
- Classification of organisms
- Ecology basics
Cell Structure & Function
Cells are the foundation of all living things. You need to know the key organelles and their functions:
Must-Know Organelles
| Organelle | Function | Memory Tip |
|---|---|---|
| Nucleus | Contains DNA, controls cell activities | "Brain" of the cell |
| Mitochondria | Produces ATP (energy) | "Powerhouse" of the cell |
| Ribosomes | Protein synthesis | Found on rough ER or free in cytoplasm |
| Endoplasmic Reticulum (ER) | Rough ER: protein processing; Smooth ER: lipid synthesis | "Highway" for transport |
| Golgi Apparatus | Packages and ships proteins | "Post office" of the cell |
| Lysosomes | Digests waste and foreign material | "Recycling center" |
| Cell Membrane | Controls what enters/exits the cell | Selectively permeable "gatekeeper" |
| Cytoplasm | Jelly-like fluid holding organelles | The "filling" inside the cell |
Plant vs. Animal Cells
- Plant cells have: cell wall, chloroplasts, large central vacuole
- Animal cells have: centrioles, small vacuoles, no cell wall
- Both have: nucleus, mitochondria, ribosomes, ER, Golgi, cell membrane
Cell Transport
- Passive transport (no energy required): diffusion, osmosis, facilitated diffusion — moves molecules from high to low concentration
- Active transport (energy required): sodium-potassium pump, endocytosis, exocytosis — moves molecules against the concentration gradient
Osmosis key terms:
- Hypertonic — more solute outside the cell → cell shrinks (crenation)
- Hypotonic — less solute outside the cell → cell swells (may lyse)
- Isotonic — equal concentration → cell stays the same
Cell Division
Mitosis (Somatic Cell Division)
Mitosis produces 2 identical daughter cells for growth and repair. Remember the phases with PMAT:
- Prophase — chromosomes condense, nuclear membrane breaks down
- Metaphase — chromosomes line up at the cell's equator
- Anaphase — sister chromatids are pulled to opposite poles
- Telophase — nuclear membranes reform, cytoplasm divides (cytokinesis)
Meiosis (Reproductive Cell Division)
Meiosis produces 4 genetically unique daughter cells (gametes — sperm and eggs) with half the chromosome number (haploid).
- Goes through two rounds of division (Meiosis I and Meiosis II)
- Crossing over in Prophase I creates genetic variation
- Reduces chromosome number from 46 (diploid) to 23 (haploid) in humans
Mitosis vs. Meiosis Quick Reference
| Feature | Mitosis | Meiosis |
|---|---|---|
| Result | 2 identical cells | 4 unique cells |
| Chromosome # | Same as parent (diploid) | Half (haploid) |
| Purpose | Growth & repair | Produce gametes |
| Divisions | 1 | 2 |
| Genetic variation | No | Yes (crossing over) |
DNA & RNA
Understanding nucleic acids is essential for the HESI A2:
DNA (Deoxyribonucleic Acid)
- Double-stranded helix
- Sugar: deoxyribose
- Bases: Adenine–Thymine, Cytosine–Guanine
- Located in the nucleus
- Function: stores genetic information
RNA (Ribonucleic Acid)
- Single-stranded
- Sugar: ribose
- Bases: A–Uracil (replaces Thymine), C–G
- Three types: mRNA (messenger), tRNA (transfer), rRNA (ribosomal)
Protein Synthesis
- Transcription (nucleus): DNA → mRNA. The DNA template strand is read and an mRNA copy is created.
- Translation (ribosome): mRNA → Protein. mRNA codons are read by ribosomes; tRNA brings matching amino acids to build the protein chain.
Memory aid: TransCRIPTion creates a sCRIPT (mRNA). TransLATion uses that script to build something (protein).
Genetics & Heredity
Key Vocabulary
- Gene — segment of DNA that codes for a trait
- Allele — a version of a gene (e.g., brown eyes vs. blue eyes)
- Genotype — the genetic makeup (BB, Bb, bb)
- Phenotype — the physical expression (brown eyes)
- Homozygous — two identical alleles (BB or bb)
- Heterozygous — two different alleles (Bb)
- Dominant — expressed when at least one copy is present (B)
- Recessive — only expressed when homozygous (bb)
Punnett Squares
You may need to solve simple Punnett square problems. Example:
Cross: Bb × Bb (both parents heterozygous for brown eyes)
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
Result: 75% brown eyes (BB or Bb), 25% blue eyes (bb). Genotype ratio: 1 BB : 2 Bb : 1 bb.
Biological Molecules (Macromolecules)
| Macromolecule | Monomer | Function | Examples |
|---|---|---|---|
| Carbohydrates | Monosaccharides | Quick energy, structure | Glucose, starch, cellulose |
| Proteins | Amino acids | Enzymes, structure, transport | Hemoglobin, keratin, insulin |
| Lipids | Fatty acids + glycerol | Long-term energy, insulation, membranes | Fats, oils, phospholipids |
| Nucleic Acids | Nucleotides | Store/transmit genetic info | DNA, RNA |
Enzymes are proteins that speed up chemical reactions. They are specific to their substrate (lock-and-key model) and are affected by temperature, pH, and concentration.
Photosynthesis & Cellular Respiration
Photosynthesis (in chloroplasts)
Equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
Plants convert carbon dioxide and water into glucose and oxygen using sunlight.
Cellular Respiration (in mitochondria)
Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP (energy)
Cells break down glucose to produce ATP. Three stages:
- Glycolysis (cytoplasm) — glucose → 2 pyruvate, produces 2 ATP
- Krebs Cycle (mitochondrial matrix) — pyruvate is further broken down, produces 2 ATP + electron carriers
- Electron Transport Chain (inner mitochondrial membrane) — produces ~34 ATP
Key relationship: Photosynthesis and cellular respiration are essentially reverse reactions. The products of one are the reactants of the other.
Classification of Living Things
Remember the taxonomic hierarchy with: "Dear King Philip Came Over For Good Spaghetti"
- Domain
- Kingdom
- Phylum
- Class
- Order
- Family
- Genus
- Species
Practice Questions
1. Which organelle is responsible for producing ATP?
Answer: Mitochondria
2. How many chromosomes do human gametes contain?
Answer: 23 (haploid)
3. What are the base pairing rules for DNA?
Answer: Adenine–Thymine, Cytosine–Guanine
4. If both parents are heterozygous (Bb), what percentage of offspring will show the recessive phenotype?
Answer: 25%
5. Where does the Krebs Cycle take place?
Answer: Mitochondrial matrix
Study Plan for HESI A2 Biology
- Days 1–2: Cell structure, organelles, plant vs. animal cells, cell transport
- Days 3–4: Cell division — mitosis vs. meiosis
- Days 5–6: DNA, RNA, protein synthesis
- Days 7–8: Genetics, Punnett squares, heredity
- Days 9–10: Macromolecules, enzymes, photosynthesis, cellular respiration
- Days 11–12: Full practice tests under timed conditions
Pair this guide with our Biology flashcards for quick review, and test yourself with our practice exams that include dedicated Biology sections.