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HESI A2 Biology practice questions

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These are original practice items written to the HESI A2 blueprint (4 options, no penalty for wrong answers), not actual exam questions. Try each one before you open the answer.

Practice by topic

Topics come from the tags on our own items, so this list reflects what the bank actually covers in Biology rather than a generic syllabus.

Biology: Cell Structure 71 in the bank Biology: Genetics 55 in the bank Biology 38 in the bank

Biology sample questions

Question 1

In a species of flower, red petal color (R) is incompletely dominant over white (r). A heterozygous plant (Rr) produces pink flowers. If a pink-flowered plant is crossed with a white-flowered plant, what percentage of the offspring will have pink flowers?

  1. A 25%
  2. B 50%
  3. C 0%
  4. D 100%
Show the answer and explanation

Correct answer: B

Cross Rr (pink) × rr (white). The pink parent passes R or r with equal chance, and the white parent always passes r, so the offspring are 50% Rr (pink) and 50% rr (white).

Why the other options are wrong:
A: 25% is the share of red (RR) or white (rr) offspring in an Rr × Rr cross; this cross makes half Rr.
C: 0% pink would require that no offspring receive R, as in rr × rr; the pink parent carries R.
D: 100% pink would require every offspring to be Rr, as in RR × rr; here half the offspring are rr.

Question 2

During which phase of the cell cycle does DNA replication occur?

  1. A G1 phase
  2. B S phase
  3. C M phase
  4. D G2 phase
Show the answer and explanation

Correct answer: B

The S (synthesis) phase of interphase is when DNA is replicated in preparation for cell division.

Why the other options are wrong:
A: G1 phase is a growth phase where the cell increases in size and produces proteins; no DNA replication occurs.
C: M phase (mitosis) is when the cell divides; DNA replication is already complete.
D: G2 phase is another growth phase after DNA replication, focused on preparing for mitosis.

Question 3

Which organelle is primarily responsible for producing ATP through oxidative phosphorylation?

  1. A Mitochondrion
  2. B Golgi apparatus
  3. C Ribosome
  4. D Nucleus
Show the answer and explanation

Correct answer: A

Mitochondria are known as the powerhouses of the cell because they generate most of the cell's ATP via oxidative phosphorylation in the inner membrane.

Why the other options are wrong:
B: The Golgi apparatus modifies, sorts, and packages proteins, but does not produce ATP.
C: Ribosomes synthesize proteins, not ATP.
D: The nucleus stores genetic material and directs cell activities, but does not produce ATP.

Question 4

During a strenuous workout, muscle cells may switch to anaerobic respiration. Which product accumulates in the muscles and contributes to fatigue?

  1. A Acetyl-CoA
  2. B Ethanol
  3. C Lactic acid
  4. D Pyruvate
Show the answer and explanation

Correct answer: C

In animals, anaerobic respiration (fermentation) converts pyruvate to lactic acid, which accumulates and lowers pH, contributing to muscle fatigue and soreness.

Why the other options are wrong:
A: Acetyl-CoA is an intermediate in aerobic respiration that enters the Krebs cycle; it does not accumulate under anaerobic conditions.
B: Ethanol is produced by yeast and some bacteria during fermentation, not in human muscle cells.
D: Pyruvate is the end product of glycolysis; under anaerobic conditions it is converted to lactic acid, so pyruvate itself does not accumulate.

Question 5

Which organelle is primarily responsible for producing ATP through aerobic respiration?

  1. A Lysosome
  2. B Ribosome
  3. C Golgi apparatus
  4. D Mitochondrion
Show the answer and explanation

Correct answer: D

Mitochondria are known as the powerhouses of the cell because they generate most of the cell's ATP via aerobic respiration. Ribosomes synthesize proteins, the Golgi apparatus modifies and packages proteins, and lysosomes digest waste materials.

Why the other options are wrong:
A: Lysosomes break down cellular waste and foreign materials; they do not generate ATP.
B: Ribosomes are involved in protein synthesis, not ATP production.
C: The Golgi apparatus modifies, sorts, and packages proteins for secretion, but does not produce ATP.

Question 6

During which stage of cellular respiration is the majority of ATP produced?

  1. A Electron transport chain
  2. B Glycolysis
  3. C Krebs cycle
  4. D Fermentation
Show the answer and explanation

Correct answer: A

The electron transport chain (ETC) produces approximately 34 ATP molecules per glucose molecule, which is the majority of ATP generated during aerobic respiration. Glycolysis yields a net of 2 ATP, the Krebs cycle yields 2 ATP, and fermentation yields only 2 ATP per glucose and occurs without oxygen.

Why the other options are wrong:
B: Glycolysis produces only 2 ATP per glucose, which is a small fraction of the total ATP yield.
C: The Krebs cycle produces 2 ATP per glucose (via substrate-level phosphorylation), not the majority.
D: Fermentation is an anaerobic process that produces only 2 ATP per glucose, far less than the ETC.

Question 7

A nurse is explaining cell organelles to a patient. Which organelle is primarily responsible for producing ATP through oxidative phosphorylation?

  1. A Lysosome
  2. B Mitochondrion
  3. C Ribosome
  4. D Golgi apparatus
Show the answer and explanation

Correct answer: B

The mitochondrion is known as the powerhouse of the cell because it generates ATP via oxidative phosphorylation in its inner membrane.

Why the other options are wrong:
A: Lysosomes function in digestion and waste removal, not energy production.
C: Ribosomes are involved in protein synthesis, not ATP production.
D: The Golgi apparatus modifies, sorts, and packages proteins, but does not produce ATP.

Question 8

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

  1. A Lipids
  2. B Carbohydrates
  3. C Proteins
  4. D Nucleic acids
Show the answer and explanation

Correct answer: A

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

Why the other options are wrong:
B: Carbohydrates are digested by amylases, unrelated to fat digestion.
C: Proteins are broken down by proteases, not directly affected by fat digestion issues.
D: Nucleic acids are digested by nucleases, not involved in fat breakdown.

Question 9

In the current three-domain system of classification, which domain includes organisms whose cells contain a nucleus and other membrane-bound organelles?

  1. A Archaea
  2. B Eukarya
  3. C Bacteria
  4. D Protista
Show the answer and explanation

Correct answer: B

Domain Eukarya includes all organisms with eukaryotic cells—cells that have a true nucleus and membrane-bound organelles. This domain encompasses kingdoms Animalia, Plantae, Fungi, and Protista.

Why the other options are wrong:
A: Archaea are also prokaryotes, though they differ from bacteria in cell wall composition and other biochemical features.
C: Bacteria are prokaryotes; they lack a nucleus and membrane-bound organelles.
D: Protista is a kingdom within Eukarya, not a domain; the three domains are Bacteria, Archaea, and Eukarya.

Question 10

In a food chain, primary producers contain 10,000 kilocalories of energy. Assuming the typical 10 percent rule for energy transfer between trophic levels, approximately how much energy is available to secondary consumers, two levels above the producers?

  1. A 10 kilocalories
  2. B 10,000 kilocalories
  3. C 1,000 kilocalories
  4. D 100 kilocalories
Show the answer and explanation

Correct answer: D

About 10% of the energy passes to each higher level: producers have 10,000 kcal, primary consumers about 1,000 kcal, and secondary consumers about 100 kcal.

Why the other options are wrong:
A: 10 kcal is the energy one level higher still, at the tertiary consumers.
B: 10,000 kcal is the energy in the producers themselves, before any transfer.
C: 1,000 kcal reaches the primary consumers, only one level above the producers.

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