HESI A2 Biology Practice Questions: 30 Questions with Answers [2026]

Master HESI A2 Biology with Targeted Practice

The HESI A2 Biology section tests your knowledge of fundamental life science concepts that form the foundation of nursing education. With 30 questions on the actual exam, this section covers a broad range of topics — but certain areas are tested far more frequently than others.

This practice set contains 30 exam-style questions organized by the five most commonly tested biology topics on the HESI A2. Each question includes a detailed explanation that goes beyond the correct answer to help you build deeper understanding.

How to Get the Most from This Practice Set

Time yourself: Aim for about 1 minute per question (30 minutes total) to practice pacing
Don't peek at answers: Write down your answers first, then check the explanations
Study the wrong answers too: Understanding why each incorrect option is wrong strengthens your knowledge
Track your results by topic: Identify which areas need more review

Topic 1: Cell Structure & Function (Questions 1–8)

Question 1

Which organelle is responsible for producing ATP, the cell's primary energy currency?

A) Golgi apparatus
B) Endoplasmic reticulum
C) Mitochondria
D) Lysosome

Answer: C) Mitochondria

Explanation: Mitochondria are known as the "powerhouse of the cell" because they produce ATP through cellular respiration. They have a double membrane — the inner membrane is folded into cristae to increase surface area for ATP production. Cells with high energy demands (like muscle cells) contain more mitochondria.

Question 2

What is the primary function of ribosomes?

A) DNA replication
B) Protein synthesis
C) Lipid production
D) Cell division

Answer: B) Protein synthesis

Explanation: Ribosomes are the sites of protein synthesis, where messenger RNA (mRNA) is translated into amino acid chains that fold into functional proteins. Ribosomes can be free (floating in cytoplasm, making proteins for internal use) or bound (attached to rough ER, making proteins for export or membrane insertion).

Question 3

The cell membrane is best described as:

A) A rigid protein wall
B) A selectively permeable phospholipid bilayer
C) A single layer of carbohydrates
D) A solid barrier that nothing can cross

Answer: B) A selectively permeable phospholipid bilayer

Explanation: The cell membrane consists of two layers of phospholipids with embedded proteins. It is selectively permeable, meaning it controls what enters and exits the cell. The fluid mosaic model describes how proteins and lipids move fluidly within the membrane. Cholesterol molecules help maintain membrane flexibility.

Question 4

Which organelle is responsible for modifying, packaging, and shipping proteins?

A) Nucleus
B) Mitochondria
C) Golgi apparatus
D) Smooth endoplasmic reticulum

Answer: C) Golgi apparatus

Explanation: The Golgi apparatus (also called Golgi body or complex) receives proteins from the rough endoplasmic reticulum, modifies them (adding sugars, lipids, or phosphate groups), sorts them, and packages them into vesicles for transport. Think of it as the cell's "post office" or "shipping department."

Question 5

What would happen to a red blood cell placed in a hypertonic solution?

A) It would swell and burst (lyse)
B) It would shrink (crenate)
C) It would remain unchanged
D) It would divide

Answer: B) It would shrink (crenate)

Explanation: In a hypertonic solution (higher solute concentration outside the cell), water moves OUT of the cell via osmosis, causing it to shrink. In a hypotonic solution, water moves IN, causing swelling (and possible lysis). In an isotonic solution, water moves equally in both directions — no net change.

Question 6

Which of the following is NOT found in a prokaryotic cell?

A) Cell membrane
B) Ribosomes
C) Membrane-bound nucleus
D) DNA

Answer: C) Membrane-bound nucleus

Explanation: Prokaryotic cells (bacteria and archaea) lack a membrane-bound nucleus — their DNA floats freely in the cytoplasm in a region called the nucleoid. They also lack other membrane-bound organelles (mitochondria, ER, Golgi). Eukaryotic cells (plants, animals, fungi) have a true nucleus and membrane-bound organelles.

Question 7

Lysosomes function primarily in:

A) Energy production
B) Protein synthesis
C) Intracellular digestion
D) DNA replication

Answer: C) Intracellular digestion

Explanation: Lysosomes are membrane-bound organelles containing digestive enzymes (hydrolases) that break down worn-out organelles, cellular debris, bacteria, and foreign materials. They maintain an acidic pH (~5) for optimal enzyme function. When lysosomes malfunction, undigested material accumulates — this causes lysosomal storage diseases like Tay-Sachs disease.

Question 8

Active transport differs from passive transport because active transport:

A) Moves substances from high to low concentration
B) Does not require a membrane
C) Requires energy (ATP) to move substances against their concentration gradient
D) Only moves water molecules

Answer: C) Requires energy (ATP) to move substances against their concentration gradient

Explanation: Active transport uses ATP to move molecules from areas of LOW concentration to HIGH concentration (against the gradient). Passive transport (diffusion, osmosis, facilitated diffusion) moves molecules DOWN the concentration gradient without energy. The sodium-potassium pump (Na⁺/K⁺-ATPase) is the most important example of active transport.

Topic 2: Genetics & DNA (Questions 9–16)

Question 9

DNA is composed of nucleotides, each containing:

A) A sugar, a phosphate group, and an amino acid
B) A sugar, a phosphate group, and a nitrogenous base
C) A lipid, a phosphate group, and a nitrogenous base
D) A sugar, a carboxyl group, and a nitrogenous base

Answer: B) A sugar, a phosphate group, and a nitrogenous base

Explanation: Each DNA nucleotide has three components: a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (adenine, thymine, guanine, or cytosine). The bases pair specifically: A-T (2 hydrogen bonds) and G-C (3 hydrogen bonds). In RNA, thymine is replaced by uracil.

Question 10

In a DNA molecule, adenine always pairs with:

A) Cytosine
B) Guanine
C) Thymine
D) Uracil

Answer: C) Thymine

Explanation: DNA base pairing follows Chargaff's rules: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C). Remember: A-T (both have a "T" sound) and G-C. In RNA, adenine pairs with uracil instead of thymine.

Question 11

What is the correct sequence of the central dogma of molecular biology?

A) RNA → DNA → Protein
B) Protein → RNA → DNA
C) DNA → RNA → Protein
D) DNA → Protein → RNA

Answer: C) DNA → RNA → Protein

Explanation: The central dogma describes the flow of genetic information: DNA is transcribed into mRNA (transcription, occurs in the nucleus), then mRNA is translated into a protein (translation, occurs at ribosomes). This is the fundamental principle of molecular biology.

Question 12

A parent with genotype Bb crosses with another parent with genotype Bb. What percentage of offspring are expected to show the dominant phenotype?

A) 25%
B) 50%
C) 75%
D) 100%

Answer: C) 75%

Explanation: Using a Punnett square for Bb × Bb: BB (25%), Bb (25%), Bb (25%), bb (25%). The dominant phenotype is shown by BB and Bb genotypes = 75%. Only bb (25%) shows the recessive phenotype. This is the classic 3:1 phenotypic ratio of a monohybrid cross.

Question 13

Mitosis results in:

A) Four genetically unique daughter cells
B) Two genetically identical daughter cells
C) One cell with double the DNA
D) Four genetically identical daughter cells

Answer: B) Two genetically identical daughter cells

Explanation: Mitosis produces two genetically identical diploid daughter cells. It is used for growth, repair, and replacement of cells. The phases are: Prophase → Metaphase → Anaphase → Telophase (remember: PMAT). Meiosis, by contrast, produces four genetically unique haploid cells (gametes).

Question 14

Which type of RNA carries the genetic code from the nucleus to the ribosome?

A) tRNA (transfer RNA)
B) rRNA (ribosomal RNA)
C) mRNA (messenger RNA)
D) snRNA (small nuclear RNA)

Answer: C) mRNA (messenger RNA)

Explanation: mRNA carries the genetic instructions from DNA in the nucleus to the ribosome in the cytoplasm. tRNA brings amino acids to the ribosome during translation. rRNA is a structural component of the ribosome itself. Each type of RNA plays a distinct role in protein synthesis.

Question 15

A human somatic (body) cell contains how many chromosomes?

A) 23
B) 46
C) 44
D) 92

Answer: B) 46

Explanation: Human somatic cells are diploid (2n) with 46 chromosomes — 22 pairs of autosomes and 1 pair of sex chromosomes. Gametes (sperm and egg) are haploid (n) with 23 chromosomes. When a sperm fertilizes an egg, the resulting zygote has the full 46 chromosomes.

Question 16

A mutation that changes a single nucleotide base in DNA is called a:

A) Frameshift mutation
B) Point mutation
C) Chromosomal mutation
D) Polyploidy

Answer: B) Point mutation

Explanation: A point mutation (also called a substitution) changes a single nucleotide base. It can be silent (no change in protein), missense (different amino acid), or nonsense (creates a premature stop codon). Frameshift mutations (insertions or deletions) shift the entire reading frame, usually causing more severe effects.

Topic 3: Metabolism & Energy (Questions 17–22)

Question 17

The overall equation for cellular respiration is:

A) 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
B) C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP
C) 2C₂H₅OH + 2CO₂ → C₆H₁₂O₆
D) ATP → ADP + P + Energy

Answer: B) C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP

Explanation: Cellular respiration breaks down glucose (C₆H₁₂O₆) using oxygen to produce carbon dioxide, water, and ATP (energy). This is essentially the reverse of photosynthesis. The three stages are: glycolysis (cytoplasm), Krebs cycle (mitochondrial matrix), and the electron transport chain (inner mitochondrial membrane). One glucose molecule produces approximately 36–38 ATP.

Question 18

Photosynthesis takes place primarily in which organelle?

A) Mitochondria
B) Nucleus
C) Chloroplast
D) Vacuole

Answer: C) Chloroplast

Explanation: Chloroplasts contain chlorophyll, the green pigment that captures light energy. Photosynthesis occurs in two stages: the light reactions (in thylakoid membranes) and the Calvin cycle (in the stroma). The overall equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂.

Question 19

Glycolysis occurs in which part of the cell?

A) Mitochondria
B) Nucleus
C) Cytoplasm
D) Endoplasmic reticulum

Answer: C) Cytoplasm

Explanation: Glycolysis is the first step of cellular respiration and occurs in the cytoplasm. It splits one glucose molecule (6 carbons) into two pyruvate molecules (3 carbons each), producing a net gain of 2 ATP and 2 NADH. Glycolysis does not require oxygen — it occurs in both aerobic and anaerobic conditions.

Question 20

Enzymes speed up chemical reactions by:

A) Increasing the temperature
B) Lowering the activation energy
C) Adding more reactants
D) Changing the products

Answer: B) Lowering the activation energy

Explanation: Enzymes are biological catalysts (usually proteins) that speed up reactions by lowering the activation energy — the minimum energy needed to start a reaction. Enzymes are specific to their substrates (lock-and-key model) and are not consumed in the reaction. Factors affecting enzyme activity include temperature, pH, and substrate concentration.

Question 21

Anaerobic respiration in human muscle cells produces:

A) Ethanol and CO₂
B) Lactic acid
C) Acetic acid
D) Glucose

Answer: B) Lactic acid

Explanation: When oxygen is limited (e.g., during intense exercise), human muscle cells perform lactic acid fermentation. Pyruvate from glycolysis is converted to lactic acid, regenerating NAD⁺ to keep glycolysis running. This produces only 2 ATP per glucose (vs. 36–38 with aerobic respiration). Lactic acid buildup causes muscle soreness and fatigue. Yeast performs alcoholic fermentation, producing ethanol and CO₂ instead.

Question 22

ATP is best described as:

A) A structural protein
B) The cell's energy currency
C) A type of lipid
D) A genetic material

Answer: B) The cell's energy currency

Explanation: Adenosine triphosphate (ATP) stores and transfers energy within cells. When ATP loses a phosphate group (ATP → ADP + P), energy is released for cellular work (muscle contraction, active transport, biosynthesis). ATP is continuously recycled — the average human body produces and uses approximately its own body weight in ATP every day.

Topic 4: Biological Molecules (Questions 23–27)

Question 23

Which biological molecule serves as the primary short-term energy source for cells?

A) Proteins
B) Nucleic acids
C) Carbohydrates
D) Lipids

Answer: C) Carbohydrates

Explanation: Carbohydrates (sugars and starches) are the body's preferred short-term energy source. Glucose is the primary fuel for cellular respiration. Lipids serve as long-term energy storage. Proteins are used for structure and function (enzymes, antibodies). Nucleic acids store genetic information. Remember: Carbs = quick energy, Lipids = stored energy.

Question 24

Proteins are polymers made of which monomers?

A) Nucleotides
B) Fatty acids
C) Monosaccharides
D) Amino acids

Answer: D) Amino acids

Explanation: Proteins are built from chains of amino acids linked by peptide bonds. There are 20 different amino acids; their sequence determines the protein's shape and function. Protein functions include enzymes (catalysts), antibodies (immune defense), hemoglobin (oxygen transport), and collagen (structural support). The four levels of protein structure are primary, secondary, tertiary, and quaternary.

Question 25

Which type of lipid forms the basis of cell membranes?

A) Triglycerides
B) Steroids
C) Phospholipids
D) Waxes

Answer: C) Phospholipids

Explanation: Phospholipids have a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails. They spontaneously arrange into a bilayer in water, forming the cell membrane. Triglycerides store energy, steroids (like cholesterol) serve as hormones and membrane components, and waxes provide waterproof coatings.

Question 26

What is the function of enzymes in biological reactions?

A) They provide energy for reactions
B) They are consumed during reactions
C) They act as biological catalysts
D) They change the products of reactions

Answer: C) They act as biological catalysts

Explanation: Enzymes are protein catalysts that speed up biological reactions without being consumed. Each enzyme has an active site that fits a specific substrate (lock-and-key or induced-fit model). Enzymes can be affected by denaturation (loss of shape due to extreme temperature or pH changes), which permanently inactivates them.

Question 27

Which molecule stores genetic information and is found in the nucleus?

A) Carbohydrates
B) Lipids
C) Proteins
D) DNA

Answer: D) DNA

Explanation: Deoxyribonucleic acid (DNA) is a nucleic acid that stores the genetic instructions for building and maintaining all living organisms. DNA is found primarily in the nucleus (nuclear DNA) but also in mitochondria (mitochondrial DNA). It has a double-helix structure discovered by Watson and Crick in 1953.

Topic 5: Ecology & Body Systems Integration (Questions 28–30)

Question 28

In a food chain, organisms that produce their own food through photosynthesis are called:

A) Consumers
B) Decomposers
C) Producers (autotrophs)
D) Heterotrophs

Answer: C) Producers (autotrophs)

Explanation: Producers (autotrophs) make their own food — primarily through photosynthesis (plants, algae) or chemosynthesis (some bacteria). They form the base of every food chain. Consumers (heterotrophs) eat other organisms: primary consumers eat producers, secondary consumers eat primary consumers, and so on. Decomposers break down dead organic matter.

Question 29

Homeostasis is best defined as:

A) The process of cell division
B) The maintenance of a stable internal environment
C) The breakdown of food for energy
D) The transport of oxygen in the blood

Answer: B) The maintenance of a stable internal environment

Explanation: Homeostasis is the body's ability to maintain stable internal conditions (temperature, pH, blood glucose, fluid balance) despite changes in the external environment. It relies on negative feedback loops — when a variable deviates from its set point, the body activates mechanisms to return it to normal. Example: When body temperature rises, sweating cools the body back down.

Question 30

Which of the following is an example of a negative feedback mechanism?

A) Blood clotting after an injury
B) Uterine contractions during labor
C) Insulin release when blood sugar rises
D) Fever increasing during infection

Answer: C) Insulin release when blood sugar rises

Explanation: When blood glucose rises (after eating), the pancreas releases insulin, which signals cells to absorb glucose, lowering blood sugar back to normal. This is negative feedback — the response opposes the initial change. Options A, B, and D are examples of positive feedback, where the response amplifies the initial change until the process is complete.

Score Interpretation

Calculate your score and evaluate your readiness:

27–30 correct (90–100%): Outstanding! You have a strong biology foundation. Review any missed questions and move on to full-length practice exams.
24–26 correct (80–89%): Solid performance. Focus your review on the specific topics where you missed questions.
21–23 correct (70–79%): Getting there, but needs more work. Revisit the explanations for missed questions and study those topic areas in depth.
Below 21 (under 70%): Significant review needed. Create a focused biology study plan covering cell structure, genetics, and metabolism before retesting.

Biology Study Tips for the HESI A2

Draw it out: Sketch cell diagrams, DNA replication, and metabolic pathways — visual learning is powerful for biology
Make comparison charts: Mitosis vs. meiosis, DNA vs. RNA, prokaryotes vs. eukaryotes — the HESI loves comparison questions
Focus on functions, not just names: Know what each organelle DOES, not just what it's called
Connect to nursing: Understanding cell biology and genetics directly applies to pharmacology, pathophysiology, and patient care in nursing school
Use the process of elimination: Even if you're unsure, eliminating 1–2 wrong answers significantly improves your odds