The HESI A2, formally known as the Health Education Systems Incorporated Admissions Assessment, is a standardized entrance examination used by nursing and allied health programs across the United States to evaluate applicants before admission. It measures academic readiness across multiple subject areas including reading comprehension, vocabulary, grammar, mathematics, biology, chemistry, anatomy and physiology, and physics. Programs use your scores to predict whether you have the foundational knowledge necessary to succeed in a rigorous healthcare curriculum, making this exam one of the most consequential assessments in your nursing school application process.
Biology and chemistry carry particular weight within the HESI A2 because they form the scientific foundation upon which all clinical nursing knowledge is built. A nurse who does not understand basic cell biology cannot fully grasp how infections spread or how medications work at the cellular level. A nurse without chemistry knowledge cannot interpret laboratory values, understand acid-base balance, or appreciate how drugs interact within the body. Performing well on these two sections demonstrates to admissions committees that you possess the scientific literacy required to absorb the complex material covered in nursing school coursework.
How the Exam Is Structured and Scored for Applicants
The HESI A2 is a computer-based examination administered at testing centers or proctored online environments depending on the institution requiring it. Each subject area is scored separately on a scale from zero to one hundred, with most nursing programs requiring a minimum score of 75 to 80 in each section, though some competitive programs set their thresholds considerably higher. Your individual section scores and a cumulative composite score are both reported, giving admissions committees a detailed picture of your academic strengths and weaknesses.
One important feature of the HESI A2 is that different nursing programs require different combinations of subject sections. Some programs require all available sections while others require only a specific subset relevant to their curriculum. Before beginning your preparation, confirm exactly which sections your target programs require so you invest your study time appropriately. For programs requiring biology and chemistry specifically, these two sections deserve focused, sustained preparation because their content is technical, specific, and not easily absorbed through passive review alone.
Cell Biology Foundations That Appear Repeatedly on the Exam
Cell biology is one of the most consistently tested topics within the HESI A2 biology section, and a thorough command of both cell structure and cell function is essential for achieving a competitive score. You must know the structure and function of every major organelle including the nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and the cell membrane. Each organelle has a specific role in cellular function, and exam questions frequently describe a cellular malfunction and ask you to identify which organelle is responsible.
The distinction between prokaryotic and eukaryotic cells is another foundational concept that appears regularly. Prokaryotic cells, which include bacteria, lack a membrane-bound nucleus and most membrane-bound organelles, while eukaryotic cells, which include all plant, animal, and fungal cells, contain a true nucleus and a full complement of organelles. Questions may also address the differences between plant and animal cells specifically, including the presence of a cell wall, chloroplasts, and a large central vacuole in plant cells that animal cells do not possess. Knowing these distinctions with precision rather than vague familiarity is what separates adequate scores from excellent ones.
Cell Division Processes and Their Biological Significance
The HESI A2 biology section tests your knowledge of both mitosis and meiosis, and a clear conceptual grasp of how these two processes differ in purpose, mechanism, and outcome is necessary for answering related questions correctly. Mitosis is the process by which a single cell divides to produce two genetically identical daughter cells, and it is the mechanism behind growth, tissue repair, and asexual reproduction. The phases of mitosis proceed in order through prophase, metaphase, anaphase, and telophase, followed by cytokinesis, which is the physical division of the cytoplasm.
Meiosis produces four genetically unique daughter cells each containing half the chromosomal number of the original cell, and it is the process responsible for the production of gametes including eggs and sperm. Meiosis proceeds through two successive divisions called meiosis one and meiosis two, with crossing over occurring during prophase one to introduce genetic variation. Exam questions about cell division often describe a specific phase or event and ask you to identify it, or present a scenario describing an error in division and ask you to identify the likely consequence. Knowing the phases in sequence and what distinguishes each phase from the others gives you a reliable framework for answering these questions accurately.
Genetics and Heredity Concepts Tested on the Biology Section
Genetics questions on the HESI A2 biology section cover the principles established by Gregor Mendel and their application to predicting inheritance patterns. You must understand the difference between dominant and recessive alleles, homozygous and heterozygous genotypes, and phenotype versus genotype. Punnett squares are a core tool for solving genetics problems, and the exam may present a cross between two organisms and ask you to predict the probability of specific offspring genotypes or phenotypes.
Beyond basic Mendelian genetics, the exam may test your knowledge of codominance, incomplete dominance, and sex-linked traits. Codominance occurs when both alleles are fully expressed in the phenotype simultaneously, as seen in AB blood type. Incomplete dominance produces a blended phenotype rather than full expression of either allele, as seen when red and white flowered plants produce pink offspring. Sex-linked traits are carried on the X chromosome, which explains why conditions like color blindness and hemophilia appear far more frequently in males than females. Understanding why this sex-linked pattern occurs mechanistically rather than simply memorizing it gives you the ability to reason through novel scenarios on the exam.
DNA Structure, Replication, and Protein Synthesis Essentials
DNA structure is a high-yield topic for the HESI A2 biology section that encompasses the double helix model, base pairing rules, and the relationship between DNA sequence and protein production. The four nitrogenous bases of DNA are adenine, thymine, guanine, and cytosine, with adenine always pairing with thymine and guanine always pairing with cytosine. RNA substitutes uracil for thymine, which is a frequently tested distinction. The sugar-phosphate backbone runs antiparallel between the two strands, meaning one strand runs in the five prime to three prime direction while the complementary strand runs in the opposite direction.
Protein synthesis involves two sequential processes called transcription and translation. During transcription, the DNA sequence of a gene is copied into a messenger RNA strand inside the nucleus. During translation, the messenger RNA sequence is read by ribosomes in the cytoplasm, and transfer RNA molecules bring the appropriate amino acids to be assembled into a polypeptide chain according to the codon sequence. Each codon is a three-base sequence on the messenger RNA that corresponds to a specific amino acid or a stop signal. Questions about protein synthesis often ask you to identify the product of a specific codon sequence or to trace the flow of genetic information from DNA through RNA to protein.
Basic Chemistry Concepts and Atomic Structure Knowledge
The HESI A2 chemistry section begins with atomic structure, and a solid command of this foundational material is necessary before any of the higher-level chemistry topics make complete sense. An atom consists of a nucleus containing protons and neutrons surrounded by electrons arranged in energy levels or shells. The atomic number of an element equals the number of protons in its nucleus and defines the element’s identity. The mass number equals the sum of protons and neutrons, and isotopes are atoms of the same element with different numbers of neutrons and therefore different mass numbers.
Electron configuration is tested because it determines an element’s chemical behavior and reactivity. Electrons fill energy levels from the innermost shell outward, with the first shell holding a maximum of two electrons, the second shell holding eight, and subsequent shells following specific rules. The outermost electrons, called valence electrons, are the ones involved in chemical bonding, and the number of valence electrons an atom has directly determines what kinds of bonds it can form and with how many other atoms. Elements in the same column of the periodic table share the same number of valence electrons, which is why they exhibit similar chemical properties.
Chemical Bonding Types and Their Properties Explained
Chemical bonding is a central chemistry topic that the HESI A2 tests across several question types. Ionic bonds form when one atom transfers one or more electrons to another atom, creating oppositely charged ions that attract each other electrostatically. This type of bonding typically occurs between metals and nonmetals and produces compounds that are solid at room temperature, have high melting points, and dissolve readily in water to produce conducting solutions. Table salt, sodium chloride, is the classic example of an ionic compound.
Covalent bonds form when two atoms share electrons rather than transferring them, and they typically occur between nonmetal atoms. Covalent bonds can be single, double, or triple depending on how many electron pairs are shared, and molecules held together by covalent bonds have properties quite different from ionic compounds. Hydrogen bonds are a weaker type of intermolecular attraction rather than a true chemical bond, occurring between a hydrogen atom bonded to a highly electronegative atom and another electronegative atom nearby. Hydrogen bonding is critically important in biological systems because it stabilizes the DNA double helix and determines many of water’s unique properties including its relatively high boiling point.
Acids, Bases, and the pH Scale in Biological Contexts
The chemistry of acids and bases is directly relevant to nursing practice because pH balance is a central concern in patient care, making this topic both a chemistry exam question and a clinical foundation worth genuinely understanding. An acid is a substance that donates hydrogen ions in solution, lowering the pH of the solution. A base is a substance that accepts hydrogen ions or donates hydroxide ions, raising the pH. The pH scale runs from zero to fourteen, with seven representing neutral, values below seven representing acidic conditions, and values above seven representing basic or alkaline conditions.
The relationship between pH and hydrogen ion concentration is logarithmic rather than linear, meaning a solution with a pH of five has ten times more hydrogen ions than a solution with a pH of six, and one hundred times more than a solution with a pH of seven. This logarithmic relationship is frequently misunderstood by candidates who treat pH differences as linear, leading to errors on quantitative questions. Buffer systems are mixtures of weak acids and their conjugate bases that resist pH changes, and the human body relies on several buffer systems including the bicarbonate buffer system to maintain blood pH within the narrow range of 7.35 to 7.45 that is compatible with normal cellular function.
Chemical Reactions and Reaction Types You Must Recognize
The HESI A2 chemistry section tests your ability to recognize and classify different types of chemical reactions. Synthesis reactions involve two or more reactants combining to form a single product, written in the general form A plus B produces AB. Decomposition reactions are the reverse, where a single compound breaks down into two or more simpler products. Single replacement reactions occur when one element replaces another within a compound, and double replacement reactions involve two ionic compounds exchanging ions to produce two new compounds. Combustion reactions involve a substance reacting with oxygen to produce carbon dioxide and water along with energy.
Balancing chemical equations is a skill the exam assesses because balanced equations reflect the law of conservation of mass, which states that atoms are neither created nor destroyed in a chemical reaction. To balance an equation, you adjust the coefficients placed in front of each compound until the number of atoms of each element is equal on both sides of the equation. You may only change coefficients, never subscripts, since changing subscripts would change the identity of the compounds involved. Practicing equation balancing with increasingly complex examples builds the pattern recognition that allows you to balance equations quickly and confidently during the exam.
Organic Chemistry Basics Relevant to Biological Systems
Organic chemistry, which is the study of carbon-containing compounds, is tested on the HESI A2 at a foundational level that focuses on the four major classes of biological macromolecules. Carbohydrates are made of carbon, hydrogen, and oxygen in an approximate one to two to one ratio and serve as the primary energy source for cellular functions. Simple sugars called monosaccharides, including glucose and fructose, are the basic building blocks that link together to form disaccharides and polysaccharides.
Lipids include fats, oils, phospholipids, and steroids, and they serve functions including long-term energy storage, cell membrane construction, and hormone production. Proteins are polymers of amino acids linked by peptide bonds, and they perform structural, enzymatic, transport, and regulatory functions throughout the body. Nucleic acids, which include DNA and RNA, are polymers of nucleotide monomers and carry genetic information. The exam may ask you to identify which macromolecule class a given molecule belongs to based on its components, or to match a macromolecule to its primary biological function.
Enzyme Function and Biochemical Reaction Principles
Enzymes are biological catalysts that accelerate chemical reactions within living organisms by lowering the activation energy required for a reaction to proceed. They are almost always proteins, and their function depends entirely on their three-dimensional shape, particularly the shape of the active site where the substrate binds. The lock-and-key model describes the highly specific relationship between an enzyme and its substrate, while the induced fit model refines this concept by recognizing that both the enzyme and substrate change shape slightly upon binding.
Enzyme activity is affected by several factors that the HESI A2 tests directly. Temperature increases generally increase reaction rate up to an optimal point, beyond which the enzyme denatures and loses its shape and function. pH similarly affects enzyme activity, with each enzyme having an optimal pH range outside of which it functions less effectively or not at all. Inhibitors are molecules that reduce enzyme activity either by binding to the active site competitively or by binding elsewhere on the enzyme and changing its shape through allosteric inhibition. Understanding these factors and being able to interpret graphs showing enzyme activity under different conditions is a tested skill.
Effective Study Strategies for Technical Science Content
Studying biology and chemistry for the HESI A2 requires active engagement with material rather than passive reading. Simply reading a chapter about cell division or chemical bonding without testing yourself on the content produces a false sense of familiarity that evaporates under exam conditions. Active study methods including flashcard drilling, practice question completion, teaching concepts aloud to yourself, and drawing diagrams from memory produce significantly better long-term retention than reading and highlighting alone.
Spaced repetition is the most evidence-supported method for retaining technical scientific vocabulary and concepts over time. Rather than reviewing all your biology flashcards every day, spaced repetition systems present each card at increasing intervals based on how confidently you know it, concentrating your review time on the material you find most difficult. Digital flashcard platforms that implement spaced repetition algorithms allow you to study efficiently even in short sessions of fifteen to twenty minutes, making them ideal for candidates balancing exam preparation with work and other commitments.
Practice Test Approaches That Reveal Real Knowledge Gaps
Taking practice tests under timed, realistic conditions is the single most reliable way to identify the specific biology and chemistry topics where your preparation is insufficient. Practice tests reveal not just what you do not know but also how you perform under time pressure, which is a distinct skill from knowing the material in an untimed environment. Many candidates discover during practice testing that they know the content adequately but consistently run out of time, which points to a pacing problem that requires its own specific solution.
After each practice test, analyze your results by topic area rather than simply noting your overall percentage score. If you missed four out of five genetics questions but answered all chemical bonding questions correctly, your review time should go toward genetics rather than chemistry regardless of which subject feels less comfortable to study. This data-driven approach to identifying and closing knowledge gaps is far more efficient than reviewing content randomly or following a generic study guide schedule without reference to your actual performance data.
Building a Realistic and Sustainable Study Schedule
A realistic HESI A2 study schedule for biology and chemistry should span four to eight weeks depending on your existing science background and the amount of time you can dedicate daily. Candidates with recent college biology and chemistry coursework may need less time to activate and refine existing knowledge, while candidates who have not studied science formally in several years will need more time to rebuild foundational understanding before tackling exam-level questions. Honest self-assessment at the start of your preparation period prevents both overconfidence and unnecessary anxiety.
Divide your study sessions so that each science topic receives dedicated attention across multiple sessions rather than being covered once and abandoned. A weekly schedule might allocate Monday and Thursday to cell biology and genetics, Tuesday and Friday to chemistry concepts, and Wednesday to mixed practice questions across both subjects. Weekends can serve as longer review and practice test sessions. This distributed approach prevents the burnout that comes from studying a single subject for too many consecutive hours and ensures that material covered early in your preparation period is revisited before exam day rather than forgotten.
Conclusion
Earning strong scores on the HESI A2 biology and chemistry sections is achievable for any motivated candidate who prepares with the right combination of content knowledge, strategic study habits, and consistent practice. These two sections test material that is genuinely interconnected, with cellular biology depending on chemistry for its deepest explanations and chemistry concepts gaining meaning and relevance when connected to their biological applications. Studying these subjects in parallel rather than in complete isolation helps you build the integrated scientific understanding that both the exam and your future nursing coursework require.
Return to the foundational concepts whenever a higher-level topic feels confusing. If enzyme kinetics seems opaque, revisit protein structure. If genetics problems feel overwhelming, review cell division first. Complex biological and chemical concepts are almost always built on simpler foundations, and shoring up those foundations removes the confusion that makes advanced topics unnecessarily difficult. This recursive approach to studying, where you regularly loop back to review earlier material as you encounter new topics that depend on it, is how genuine scientific understanding develops rather than superficial memorization.
Vocabulary is a significant component of both the biology and chemistry sections, and technical terms must be known precisely rather than approximately. A candidate who confuses mitosis with meiosis or conflates ionic and covalent bonds will miss straightforward questions that their overall knowledge level should make answerable. Building a personal glossary of key terms as you study each topic area gives you a targeted vocabulary review resource that is far more efficient than re-reading entire chapters when you need to refresh your memory on specific definitions.
Connect what you are studying to the clinical world you are preparing to enter. When you study pH and buffers, think about respiratory acidosis and metabolic alkalosis. When you study enzyme inhibition, think about how certain medications work by inhibiting specific enzymes in metabolic pathways. When you study DNA replication, think about how chemotherapy drugs interfere with cell division to slow tumor growth. These clinical connections make abstract science content memorable and meaningful, and they simultaneously begin building the clinical reasoning foundation that nursing school will require you to develop further.
On exam day, trust the preparation you have invested. Read each question carefully, eliminate obviously incorrect answer choices first to improve your odds if you are uncertain, and avoid changing answers without a clear and specific reason to do so. Your first instinct after thorough preparation is usually reliable, and second-guessing without new information introduces errors rather than correcting them. Approach the biology and chemistry sections with the confidence that comes from having genuinely studied the material, practiced under realistic conditions, and closed the specific knowledge gaps your practice tests revealed. The HESI A2 is a challenging but entirely passable examination, and the scientific knowledge you build in preparing for it will serve you throughout your entire nursing career.
