22 Biology Practice Questions & Answers

Every Biology practice question from the HESI A2 Practice Test, with the correct answer and a short explanation.

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1. In the taxonomic hierarchy, which level is broader than order but narrower than phylum?
- A.Family
- B.Kingdom
- C.Genus
- D.Class✓ Answer
The levels run from broadest to most specific as Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species, so class sits directly between phylum and order. Family and genus are narrower than order, and kingdom is broader than phylum.
Ref QB-7XDY53Source: Elsevier HESI A2 — Biology: classification (taxonomic hierarchy)Report a problem with this question
2. Which two domains of life are made up entirely of prokaryotic organisms?
- A.Bacteria and Archaea✓ Answer
- B.Archaea and Eukarya
- C.Eukarya and Protista
- D.Bacteria and Eukarya
Bacteria and Archaea consist of cells with no nucleus and no membrane-bound organelles, which is the definition of prokaryotic. Eukarya includes every organism whose cells have a nucleus, and protists are eukaryotes placed inside Eukarya, not a separate domain.
Ref QB-REXFW4Source: Elsevier HESI A2 — Biology: classification (three domains; prokaryote vs eukaryote)Report a problem with this question
3. Sweating cools the body because evaporating water carries away a large amount of heat. Which property of water explains this?
- A.Low specific heat value
- B.Low density when frozen
- C.High capillary adhesion
- D.High heat of vaporization✓ Answer
Hydrogen bonds must be broken for a water molecule to escape as vapor, so each gram that evaporates absorbs a lot of heat from the skin. Adhesion explains capillary action, low ice density explains why ice floats, and water's specific heat is actually high, not low.
Ref QB-8XRAB4Source: Elsevier HESI A2 — Biology: water (heat of vaporization and evaporative cooling)Report a problem with this question
4. In liquid water, a hydrogen bond forms between which atoms?
- A.Hydrogen of one molecule and hydrogen of another
- B.Oxygen of one molecule and oxygen of another
- C.Oxygen of one molecule and hydrogen of another✓ Answer
- D.Oxygen and hydrogen within the same molecule
Water is polar: its oxygen carries a partial negative charge and its hydrogens carry partial positive charges, so the oxygen of one molecule attracts a hydrogen of a neighboring molecule. The oxygen and hydrogens inside a single molecule are held by covalent bonds, and like partial charges (hydrogen to hydrogen or oxygen to oxygen) repel rather than attract.
Ref QB-NEVFDZSource: Elsevier HESI A2 — Biology: water (polarity and hydrogen bonding)Report a problem with this question
5. Animals store excess glucose in the liver and skeletal muscles in the form of which polysaccharide?
- A.Chitin
- B.Glycogen✓ Answer
- C.Starch
- D.Cellulose
Glycogen is a highly branched chain of glucose that animals build for short-term energy storage and break down when blood glucose falls. Starch is the storage form in plants, cellulose forms plant cell walls, and chitin is a structural carbohydrate in fungal walls and arthropod exoskeletons.
Ref QB-5XPATYSource: Elsevier HESI A2 — Biology: biological molecules (carbohydrates — glycogen, starch, cellulose)Report a problem with this question
6. When two glucose molecules are joined by dehydration synthesis to form the disaccharide maltose, what else happens?
- A.A water molecule is released✓ Answer
- B.A water molecule is used up
- C.A peptide bond is formed
- D.An ATP molecule is released
Dehydration synthesis links monomers by removing an -OH from one and an -H from the other, which leave together as one water molecule. Using up water describes the reverse reaction, hydrolysis; peptide bonds join amino acids, not sugars; and building a polymer consumes energy rather than releasing ATP.
Ref QB-PE8TJ2Source: Elsevier HESI A2 — Biology: biological molecules (dehydration synthesis vs hydrolysis)Report a problem with this question
7. A nucleotide, the monomer of DNA and RNA, is made of which three parts?
- A.Glycerol and three fatty acid chains
- B.A sugar, a phosphate, and a base✓ Answer
- C.A sugar, a fatty acid, and a base
- D.An amino, a carboxyl, and an R group
Every nucleotide has a five-carbon sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base; sugars and phosphates link to form the backbone. Glycerol plus three fatty acids is a triglyceride (lipid), and the amino, carboxyl, and R groups describe an amino acid.
Ref QB-6X2NYZSource: Elsevier HESI A2 — Biology: biological molecules (nucleic acids — nucleotide structure)Report a problem with this question
8. A human enzyme is heated to 70°C (158°F), far above its optimum temperature, and permanently stops working. What is the best explanation?
- A.Its active site lost its shape✓ Answer
- B.Its substrate supply ran out
- C.It was used up by the reaction
- D.A competitive inhibitor bound it
High heat breaks the weak bonds that hold a protein's three-dimensional shape, so the enzyme denatures and its active site no longer fits the substrate. Enzymes are catalysts and are not consumed, an inhibitor is a separate molecule rather than an effect of heat, and running out of substrate would not cause permanent loss of activity.
Ref QB-UEZFE8Source: Elsevier HESI A2 — Biology: metabolism and enzymes (temperature and denaturation)Report a problem with this question
9. A drug shaped like an enzyme's normal substrate binds to the active site and keeps the substrate out. Adding much more substrate restores the reaction rate. What is this effect called?
- A.Noncompetitive inhibition
- B.Competitive inhibition✓ Answer
- C.Allosteric activation
- D.Thermal denaturation
A competitive inhibitor competes with the substrate for the same active site, so flooding the enzyme with substrate outcompetes it and the rate recovers. A noncompetitive inhibitor binds a different site and changes the enzyme's shape, so extra substrate does not help; denaturation is caused by heat or pH, not a substrate look-alike.
Ref QB-BXTAU7Source: Elsevier HESI A2 — Biology: metabolism and enzymes (competitive vs noncompetitive inhibition)Report a problem with this question
10. Liver cells that detoxify drugs and alcohol and also synthesize lipids contain an unusually large amount of which organelle?
- A.Golgi body
- B.Lysosome
- C.Smooth ER✓ Answer
- D.Rough ER
The smooth endoplasmic reticulum lacks ribosomes and specializes in lipid synthesis and detoxification. The rough ER is studded with ribosomes and processes proteins, the Golgi modifies and packages products for shipping, and lysosomes digest worn-out parts and debris.
Ref QB-5PQ4DCSource: Elsevier HESI A2 — Biology: cells and organelles (smooth vs rough endoplasmic reticulum)Report a problem with this question
11. Which pair of structures is found in plant cells but not in animal cells?
- A.Cell wall and chloroplasts✓ Answer
- B.Mitochondria and ribosomes
- C.Centrioles and lysosomes
- D.Nucleus and cell membrane
Plant cells have a cellulose cell wall and chloroplasts for photosynthesis (plus a large central vacuole), which animal cells lack. Both cell types have mitochondria, ribosomes, a nucleus, and a cell membrane, and centrioles are characteristic of animal cells rather than plant cells.
Ref QB-N6W9YCSource: Elsevier HESI A2 — Biology: cells and organelles (plant vs animal cells)Report a problem with this question
12. A plant cell is placed in distilled water, which is hypotonic to the cell. What happens?
- A.Water enters; the cell wall bursts
- B.Water leaves; the cell plasmolyzes
- C.Water enters; the cell becomes turgid✓ Answer
- D.No net movement; size stays the same
In a hypotonic solution water moves by osmosis into the cell, toward the higher solute concentration, and the central vacuole swells and presses against the wall, producing turgor pressure. The rigid cellulose wall keeps the cell from bursting, unlike an animal cell that would lyse; plasmolysis happens in a hypertonic solution, and no net movement occurs only when the solution is isotonic.
Ref QB-6P3FJDSource: Elsevier HESI A2 — Biology: cells (osmosis and tonicity — turgor in plant cells)Report a problem with this question
13. In a eukaryotic cell, where does the electron transport chain of cellular respiration take place?
- A.Matrix of the mitochondrion
- B.Cytoplasm outside organelles
- C.Outer mitochondrial membrane
- D.Inner mitochondrial membrane✓ Answer
The protein complexes of the electron transport chain are embedded in the inner mitochondrial membrane, folded into cristae to increase surface area, where most of the cell's ATP is made. The Krebs cycle runs in the matrix, glycolysis runs in the cytoplasm, and the outer membrane does not carry the chain.
Ref QB-P69M5ESource: Elsevier HESI A2 — Biology: cellular respiration (electron transport chain location)Report a problem with this question
14. In aerobic cellular respiration, which molecule is the final electron acceptor at the end of the electron transport chain?
- A.Pyruvate
- B.Oxygen✓ Answer
- C.Glucose
- D.NAD+
Oxygen accepts the electrons at the end of the chain and combines with hydrogen ions to form water; without it, the chain backs up and aerobic ATP production stops. NAD+ is an electron carrier that delivers electrons to the chain rather than the final acceptor, while pyruvate and glucose are fuels broken down earlier.
Ref QB-3PZE2ASource: Elsevier HESI A2 — Biology: cellular respiration (oxygen as final electron acceptor)Report a problem with this question
15. During a hard sprint, a runner's muscle cells run short of oxygen and switch to fermentation. Which product do these muscle cells make?
- A.Citric acid
- B.Lactic acid✓ Answer
- C.Acetic acid
- D.Ethyl alcohol
Human muscle cells carry out lactic acid fermentation, converting pyruvate to lactic acid (lactate) so that NAD+ is regenerated and glycolysis can keep making its 2 ATP. Ethyl alcohol plus CO2 is the product of alcoholic fermentation in yeast, and citric acid is an intermediate of the aerobic Krebs cycle.
Ref QB-K67MMASource: Elsevier HESI A2 — Biology: cellular respiration (anaerobic respiration — lactic acid fermentation)Report a problem with this question
16. The oxygen gas that a plant releases during photosynthesis comes from the splitting of which molecule?
- A.Carbon dioxide
- B.Glucose molecules
- C.Chlorophyll a
- D.Water molecules✓ Answer
In the light-dependent reactions in the thylakoid membranes, water is split (photolysis) to replace electrons lost by chlorophyll, and its oxygen atoms are released as O2. The oxygen atoms from carbon dioxide end up in glucose during the Calvin cycle, glucose is a product rather than a reactant, and chlorophyll absorbs light but is not broken down.
Ref QB-4PCS7BSource: Elsevier HESI A2 — Biology: photosynthesis (light-dependent reactions — photolysis of water)Report a problem with this question
17. Where does the Calvin cycle (light-independent reactions) occur, and what does it use to build sugar from CO2?
- A.In the thylakoids, splitting water
- B.In the matrix, using pyruvate and O2
- C.In the stroma, using ATP and NADPH✓ Answer
- D.In the stroma, using O2 and glucose
The Calvin cycle runs in the stroma, the fluid around the thylakoids, and spends the ATP and NADPH made by the light-dependent reactions to fix CO2 into sugar. Splitting water happens in the thylakoids during the light-dependent reactions, O2 and glucose are outputs rather than inputs, and the mitochondrial matrix belongs to cellular respiration.
Ref QB-M6JXSBSource: Elsevier HESI A2 — Biology: photosynthesis (Calvin cycle in the stroma)Report a problem with this question
18. During which phase of mitosis are sister chromatids pulled apart toward opposite poles of the cell?
- A.Anaphase✓ Answer
- B.Metaphase
- C.Telophase
- D.Prophase
In anaphase the centromeres split and spindle fibers shorten, pulling sister chromatids to opposite poles. Chromosomes condense and the spindle forms in prophase, they line up at the cell's middle in metaphase, and nuclear envelopes re-form around the separated sets in telophase.
Ref QB-ZNASP7Source: Elsevier HESI A2 — Biology: cell division (phases of mitosis — PMAT)Report a problem with this question
19. A human skin cell divides by mitosis. Which statement describes each of the two daughter cells?
- A.46 chromosomes, genetically unique
- B.46 chromosomes, genetically identical✓ Answer
- C.23 chromosomes, genetically identical
- D.23 chromosomes, genetically unique
Mitosis copies the DNA once and divides once, so each daughter cell is diploid (2n = 46 in humans) and genetically identical to the parent cell, which suits growth and tissue repair. Halving the number to 23 and producing genetically unique cells are results of meiosis, which makes gametes.
Ref QB-H6GX98Source: Elsevier HESI A2 — Biology: cell division (mitosis vs meiosis outcomes; human 2n = 46)Report a problem with this question
20. One strand of a DNA molecule has the base sequence A-T-G-C-C-A. Pairing base by base, what is the sequence of the complementary DNA strand?
- A.U-A-C-G-G-U
- B.G-C-A-T-T-G
- C.A-T-G-C-C-A
- D.T-A-C-G-G-T✓ Answer
In DNA, adenine always pairs with thymine and guanine with cytosine, so A-T-G-C-C-A pairs with T-A-C-G-G-T. Uracil appears only in RNA, repeating the original sequence ignores base pairing, and swapping A with G and T with C confuses which bases are complementary.
Ref QB-YWRM7HSource: Elsevier HESI A2 — Biology: genetics and DNA (complementary base pairing A-T, G-C)Report a problem with this question
21. A red-flowered plant is crossed with a white-flowered plant of the same species, and every offspring has pink flowers. Which inheritance pattern does this show?
- A.Incomplete dominance✓ Answer
- B.Sex-linked inheritance
- C.Complete dominance
- D.Codominant inheritance
In incomplete dominance neither allele fully masks the other, so the heterozygote shows an intermediate blend such as pink. In codominance both traits appear fully and side by side (red and white patches, like type AB blood); complete dominance would give all red or all white offspring; and flower color here is not tied to a sex chromosome.
Ref QB-FEKFMGSource: Elsevier HESI A2 — Biology: genetics (non-Mendelian inheritance — incomplete dominance vs codominance)Report a problem with this question
22. Hemophilia A is an X-linked recessive trait. A carrier mother (X^H X^h) and a father with normal clotting (X^H Y) have a son. What is the probability that this son has hemophilia?
- A.0%
- B.100%
- C.50%✓ Answer
- D.25%
A son always receives the Y chromosome from his father and one X from his mother, and the carrier mother passes X^h half the time, so each son has a 50% chance of being X^h Y and affected. The 25% figure is the chance for any child of either sex, and the father's normal X protects only daughters, not sons.
Ref QB-ZW4YCJSource: Elsevier HESI A2 — Biology: genetics (sex-linked inheritance — X-linked recessive Punnett square)Report a problem with this question

1. In the taxonomic hierarchy, which level is broader than order but narrower than phylum?

2. Which two domains of life are made up entirely of prokaryotic organisms?

3. Sweating cools the body because evaporating water carries away a large amount of heat. Which property of water explains this?

4. In liquid water, a hydrogen bond forms between which atoms?

5. Animals store excess glucose in the liver and skeletal muscles in the form of which polysaccharide?

6. When two glucose molecules are joined by dehydration synthesis to form the disaccharide maltose, what else happens?

7. A nucleotide, the monomer of DNA and RNA, is made of which three parts?

8. A human enzyme is heated to 70°C (158°F), far above its optimum temperature, and permanently stops working. What is the best explanation?

9. A drug shaped like an enzyme's normal substrate binds to the active site and keeps the substrate out. Adding much more substrate restores the reaction rate. What is this effect called?

10. Liver cells that detoxify drugs and alcohol and also synthesize lipids contain an unusually large amount of which organelle?

11. Which pair of structures is found in plant cells but not in animal cells?

12. A plant cell is placed in distilled water, which is hypotonic to the cell. What happens?

13. In a eukaryotic cell, where does the electron transport chain of cellular respiration take place?

14. In aerobic cellular respiration, which molecule is the final electron acceptor at the end of the electron transport chain?

15. During a hard sprint, a runner's muscle cells run short of oxygen and switch to fermentation. Which product do these muscle cells make?

16. The oxygen gas that a plant releases during photosynthesis comes from the splitting of which molecule?

17. Where does the Calvin cycle (light-independent reactions) occur, and what does it use to build sugar from CO2?

18. During which phase of mitosis are sister chromatids pulled apart toward opposite poles of the cell?

19. A human skin cell divides by mitosis. Which statement describes each of the two daughter cells?

20. One strand of a DNA molecule has the base sequence A-T-G-C-C-A. Pairing base by base, what is the sequence of the complementary DNA strand?

21. A red-flowered plant is crossed with a white-flowered plant of the same species, and every offspring has pink flowers. Which inheritance pattern does this show?

22. Hemophilia A is an X-linked recessive trait. A carrier mother (X^H X^h) and a father with normal clotting (X^H Y) have a son. What is the probability that this son has hemophilia?

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