HESI A2 Biology Practice Test

25 practice questions

Choose an answer, then check it under Show answer.

1. Question 1 of 25Medium

   In a population of birds, individuals with medium-sized beaks survive better than those with very small or very large beaks. Which type of natural selection does this scenario illustrate?

   1. AA. Sexual selection
   2. BB. Directional selection
   3. CC. Stabilizing selection
   4. DD. Disruptive selection
   Show answer to question 1

   Answer: C. Stabilizing selection

   Stabilizing selection occurs when environmental pressures favor intermediate phenotypes over extreme variations at either end of the spectrum. Directional selection occurs when one extreme phenotype is favored, shifting the population distribution toward that single extreme.

2. Question 2 of 25Medium

   Which of the following best describes the relationship between genotype and phenotype?

   1. AA. The phenotype is the genetic makeup, while the genotype is the physical expression.
   2. BB. The genotype is the genetic makeup, while the phenotype is the physical expression.
   3. CC. The genotype and phenotype are always identical in all organisms.
   4. DD. The genotype is the physical expression, while the phenotype is the genetic makeup.
   Show answer to question 2

   Answer: B. The genotype is the genetic makeup, while the phenotype is the physical expression.

   Genotype refers to the specific combination of alleles an organism carries, whereas phenotype refers to the observable physical or physiological characteristics resulting from those alleles and environmental factors. They are not always identical in cases of incomplete dominance or environmental influence.

3. Question 3 of 25Easy

   Which of the following best describes the movement of water across a semipermeable membrane?

   1. AA. Active transport
   2. BB. Osmosis
   3. CC. Diffusion
   4. DD. Phagocytosis
   Show answer to question 3

   Answer: B. Osmosis

   Osmosis is the specific term for the diffusion of water across a semipermeable membrane. While it is a form of diffusion, it specifically refers to the solvent movement in biological systems.

4. Question 4 of 25Medium

   In a specific species of flower, red color (R) is dominant over white color (r). If a purebred white flower is crossed with a heterozygous red flower, what percentage of the offspring will be white?

   1. AA. 100%
   2. BB. 0%
   3. CC. 50%
   4. DD. 25%
   Show answer to question 4

   Answer: C. 50%

   A cross between rr and Rr results in 50% Rr (red) and 50% rr (white) offspring. Because the red parent is heterozygous, it can pass on the recessive allele to half of the progeny.

5. Question 5 of 25Easy

   Which enzyme is responsible for unwinding the DNA double helix by breaking hydrogen bonds between complementary nitrogenous bases?

   1. AA. DNA polymerase
   2. BB. Primase
   3. CC. Ligase
   4. DD. Helicase
   Show answer to question 5

   Answer: D. Helicase

   Helicase is the enzyme that separates the two strands of DNA by disrupting the hydrogen bonds. DNA polymerase is responsible for synthesizing the new strand, while ligase joins fragments together.

6. Question 6 of 25Medium

   Which metabolic pathway is common to both aerobic and anaerobic respiration?

   1. AA. Beta-oxidation
   2. BB. Glycolysis
   3. CC. The Krebs cycle
   4. DD. The Calvin cycle
   Show answer to question 6

   Answer: B. Glycolysis

   Glycolysis is the initial, universal metabolic pathway that breaks down glucose into pyruvate, occurring in the absence or presence of oxygen. The Krebs cycle requires oxygen indirectly to function as part of the aerobic process.

7. Question 7 of 25Easy

   Which enzyme is primarily responsible for synthesizing pre-mRNA from a DNA template during transcription?

   1. AA. Helicase
   2. BB. DNA ligase
   3. CC. RNA polymerase
   4. DD. DNA polymerase
   Show answer to question 7

   Answer: C. RNA polymerase

   RNA polymerase binds to a gene's promoter region and synthesizes a complementary RNA transcript using the DNA template strand. DNA polymerase is primarily responsible for synthesizing DNA during replication, not transcribing RNA.

8. Question 8 of 25Medium

   Which statement best describes the role of ribosomal RNA in protein synthesis?

   1. AA. It acts as a structural and catalytic component of the ribosome to form peptide bonds.
   2. BB. It carries the three-nucleotide anticodon sequence that specifies each incoming amino acid.
   3. CC. It attaches a polyadenine tail to the three-prime terminal end of nascent messenger RNA.
   4. DD. It transports mature transcripts across the nuclear envelope into the eukaryotic cytosol.
   Show answer to question 8

   Answer: A. It acts as a structural and catalytic component of the ribosome to form peptide bonds.

   Ribosomal RNA combines with proteins to form ribosomal subunits and contains the peptidyl transferase center that catalyzes peptide bond formation. Carrying anticodons to match mRNA codons is the role of transfer RNA, not ribosomal RNA.

9. Question 9 of 25Medium

   In a certain species of flower, petal color shows codominance. A red-flowered plant (RR) is crossed with a white-flowered plant (WW). What will be the phenotype of the offspring?

   1. AA. 75% red, 25% white
   2. BB. All offspring have red and white spotted petals
   3. CC. All offspring have pink petals
   4. DD. 50% red, 50% white
   Show answer to question 9

   Answer: B. All offspring have red and white spotted petals

   Codominance means both alleles are fully expressed in the heterozygote. The cross RR × WW produces all RW offspring, which will show both red and white traits simultaneously (e.g., spots or patches).

   Why the other options are wrong

   • A75% red, 25% white would occur from a monohybrid cross of two heterozygotes under complete dominance, not codominance.
   • CPink petals result from incomplete dominance, not codominance.
   • D50% red, 50% white would occur from a test cross (Rr × rr) under complete dominance, not from RR × WW.

10. Question 10 of 25Medium

    Which biological process directly removes carbon dioxide from the atmosphere and incorporates it into organic compounds?

    1. AA. Decomposition
    2. BB. Combustion
    3. CC. Cellular respiration
    4. DD. Photosynthesis
    Show answer to question 10

    Answer: D. Photosynthesis

    Photosynthesis takes in atmospheric carbon dioxide and combines it with water to produce glucose and oxygen, sequestering inorganic carbon into biomass. Cellular respiration does the reverse by breaking down organic sugars and releasing carbon dioxide back into the atmosphere.

11. Question 11 of 25Medium

    What is the primary function of cholesterol within the animal cell membrane?

    1. AA. Transporting ions
    2. BB. Regulating fluidity
    3. CC. Acting as an enzyme
    4. DD. Storing genetic information
    Show answer to question 11

    Answer: B. Regulating fluidity

    Cholesterol molecules are embedded within the phospholipid bilayer to help regulate membrane fluidity and stability across different temperatures. It does not serve as a primary transporter or a storage site for genetic material.

12. Question 12 of 25Easy

    Which taxonomic rank represents the most inclusive and broadest level of biological classification?

    1. AA. Kingdom
    2. BB. Domain
    3. CC. Phylum
    4. DD. Class
    Show answer to question 12

    Answer: B. Domain

    Domain is the highest and most inclusive taxonomic category in modern biological classification, encompassing all kingdoms. Kingdom was traditionally the highest category before the three-domain system was adopted, making it less inclusive than domain.

13. Question 13 of 25Medium

    Which pigment in plant chloroplasts directly captures light energy for use in the light-dependent reactions of photosynthesis?

    1. AA. Carotenoid
    2. BB. Phycobilin
    3. CC. Chlorophyll b
    4. DD. Chlorophyll a
    Show answer to question 13

    Answer: D. Chlorophyll a

    Chlorophyll a is the primary photosynthetic pigment; it directly absorbs light (mainly red and blue wavelengths) and transfers the energy to the reaction centers of photosystems. Chlorophyll b and carotenoids are accessory pigments that broaden the absorption spectrum and pass energy to chlorophyll a. Phycobilins are found in cyanobacteria and red algae, not in chloroplasts of land plants.

    Why the other options are wrong

    • ACarotenoids are accessory pigments that also protect against photodamage, but they are not the primary light-capturing pigment.
    • BPhycobilins are accessory pigments in certain aquatic organisms; they are not present in typical land plant chloroplasts.
    • CChlorophyll b is an accessory pigment; it absorbs light and transfers energy to chlorophyll a, but does not directly initiate the light reactions.

14. Question 14 of 25Medium

    Which mechanism allows populations of organisms to change over many generations to better suit their environment?

    1. AA. Spontaneous generation
    2. BB. Homeostasis
    3. CC. Evolution
    4. DD. Acclimatization
    Show answer to question 14

    Answer: C. Evolution

    Evolution is the process by which the inherited characteristics of a population change over successive generations. Acclimatization refers to individual physiological changes within a single lifetime, not permanent genetic changes in a population.

15. Question 15 of 25Medium

    Which of the following organelles is responsible for the detoxification of drugs and the synthesis of lipids?

    1. AA. Rough endoplasmic reticulum
    2. BB. Peroxisome
    3. CC. Nucleolus
    4. DD. Smooth endoplasmic reticulum
    Show answer to question 15

    Answer: D. Smooth endoplasmic reticulum

    The smooth endoplasmic reticulum is specialized for lipid synthesis and the detoxification of metabolic byproducts or drugs. The rough endoplasmic reticulum is primarily associated with protein synthesis due to attached ribosomes.

16. Question 16 of 25Easy

    Which of the following represents the correct sequence of biological organization from simplest to most complex?

    1. AA. Tissue, cell, organ, organ system, organism
    2. BB. Cell, organ, tissue, organ system, organism
    3. CC. Organism, organ system, organ, tissue, cell
    4. DD. Cell, tissue, organ, organ system, organism
    Show answer to question 16

    Answer: D. Cell, tissue, organ, organ system, organism

    Biological organization increases in complexity starting from the cell, which groups into tissues, then organs, then organ systems, and finally the whole organism. An incorrect sequence like cell to organ skips the tissue level.

17. Question 17 of 25Hard

    Where is ATP synthase located in eukaryotic cells during aerobic respiration?

    1. AA. Outer mitochondrial membrane
    2. BB. Mitochondrial matrix
    3. CC. Inner mitochondrial membrane
    4. DD. Cytoplasmic membrane
    Show answer to question 17

    Answer: C. Inner mitochondrial membrane

    ATP synthase is embedded in the inner mitochondrial membrane. It uses the protomotive force (H+ gradient) across that membrane to catalyze ATP formation. The outer membrane is permeable and not involved in ATP synthesis. The matrix contains enzymes of Krebs cycle but not ATP synthase. The cytoplasmic membrane is not part of mitochondrion.

    Why the other options are wrong

    • AThe outer membrane is permeable and does not host ATP synthase.
    • BThe matrix is the site of Krebs cycle reactions but lacks ATP synthase; the enzyme is in the membrane.
    • DATP synthase is not on the plasma membrane; it is in mitochondrial inner membrane.

18. Question 18 of 25Hard

    A scientist conducts an experiment and finds that the data does not support the original hypothesis. What is the most appropriate next step?

    1. AA. Revise the hypothesis and test again
    2. BB. Change the data to fit the hypothesis
    3. CC. Discard the results and start over
    4. DD. Conclude that the experiment was invalid
    Show answer to question 18

    Answer: A. Revise the hypothesis and test again

    When a hypothesis is not supported by experimental data, it should be refined or replaced, and the process should be repeated. A failed hypothesis does not mean the experiment was invalid or that the data should be discarded.

19. Question 19 of 25Hard

    Which factor would most likely cause a population to stabilize at a lower carrying capacity?

    1. AA. An increase in toxic waste buildup
    2. BB. A decrease in intra-species competition
    3. CC. An increase in available water
    4. DD. A decrease in natural predators
    Show answer to question 19

    Answer: A. An increase in toxic waste buildup

    The accumulation of toxic waste or a decrease in essential resources lowers the carrying capacity of an environment. Increasing water or decreasing predators would typically allow for a higher carrying capacity.

20. Question 20 of 25Easy

    How is the genetic material arranged in a typical prokaryotic cell?

    1. AA. Organized into a circular strand in the nucleoid
    2. BB. Wrapped tightly around histone proteins in pairs
    3. CC. Contained within a double-membrane envelope
    4. DD. Segregated inside a specialized nucleolus
    Show answer to question 20

    Answer: A. Organized into a circular strand in the nucleoid

    Prokaryotes lack a membrane-enclosed nucleus, and their single circular chromosome is concentrated in an irregularly shaped cytoplasmic area called the nucleoid. Eukaryotic genomes, by contrast, consist of multiple linear chromosomes complexed with histone proteins within a true nucleus.

21. Question 21 of 25Easy

    A patient with a digestive disorder has difficulty breaking down dietary fats. Which macromolecule is most directly affected by this condition?

    1. AA. Lipids
    2. BB. Carbohydrates
    3. CC. Proteins
    4. DD. Nucleic acids
    Show answer to question 21

    Answer: A. Lipids

    Dietary fats are primarily lipids. Conditions such as pancreatic insufficiency reduce lipase production, impairing lipid digestion.

    Why the other options are wrong

    • BCarbohydrates are digested by amylases, unrelated to fat digestion.
    • CProteins are broken down by proteases, not directly affected by fat digestion issues.
    • DNucleic acids are digested by nucleases, not involved in fat breakdown.

22. Question 22 of 25Hard

    A newly discovered organism is unicellular, has a cell wall made of chitin, and reproduces by budding. To which kingdom does it most likely belong?

    1. AA. Fungi
    2. BB. Protista
    3. CC. Plantae
    4. DD. Animalia
    Show answer to question 22

    Answer: A. Fungi

    Fungi are characterized by chitin in their cell walls, and many (e.g., yeasts) are unicellular and reproduce by budding. This combination of traits is diagnostic for the kingdom Fungi.

    Why the other options are wrong

    • BProtista is a diverse group; some have cell walls (e.g., algae have cellulose), but chitin is not typical, and budding is not a common reproductive mode in protists.
    • CPlantae have cell walls made of cellulose, not chitin, and are primarily multicellular photosynthetic organisms.
    • DAnimalia lack cell walls entirely and do not reproduce by budding (most are multicellular).

23. Question 23 of 25Hard

    A DNA sample is found to contain 20% cytosine. Based on Chargaff's rules, what percentage of the sample is composed of adenine?

    1. AA. 20%
    2. BB. 60%
    3. CC. 30%
    4. DD. 40%
    Show answer to question 23

    Answer: C. 30%

    According to Chargaff's rules, the amount of cytosine equals guanine, so there is 40% total for C and G. The remaining 60% must be divided equally between adenine and thymine, resulting in 30% adenine.

24. Question 24 of 25Hard

    A cell is observed to have a high density of rough endoplasmic reticulum and numerous Golgi apparatus. What is the most likely function of this cell?

    1. AA. Secretion of digestive enzymes
    2. BB. Rapid ATP production
    3. CC. Detoxification of drugs
    4. DD. Storage of lipids
    Show answer to question 24

    Answer: A. Secretion of digestive enzymes

    Cells that secrete large amounts of proteins (such as digestive enzymes) have abundant rough ER for protein synthesis and Golgi apparatus for processing and packaging. ATP production is associated with mitochondria; lipid storage occurs in adipocytes; detoxification is primarily carried out by smooth ER in liver cells.

    Why the other options are wrong

    • BRapid ATP production would require many mitochondria, not abundant rough ER and Golgi.
    • CDrug detoxification is mainly performed by smooth ER in hepatocytes, not by rough ER and Golgi.
    • DLipid storage is characteristic of adipose cells, which have large lipid droplets but not extensive rough ER.

25. Question 25 of 25Hard

    If a toxin becomes more concentrated in the tissues of organisms as it moves up the food chain, this process is known as:

    1. AA. Trophic cascading
    2. BB. Eutrophication
    3. CC. Bioaccumulation
    4. DD. Biomagnification
    Show answer to question 25

    Answer: D. Biomagnification

    Biomagnification refers to the increasing concentration of a substance, such as a heavy metal, in the tissues of organisms at higher trophic levels. Bioaccumulation refers to the buildup within a single organism over time, rather than through the food chain levels.

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