During a unit on evolution, a student says, "Evolution is just a theory, so scientists are still guessing about whether it happened." Which teacher response BEST addresses the student's misconception about the nature of science?

Explain that a theory is a well-tested explanation that unifies many lines of evidence.

Explain that evolution will become a scientific law once enough additional evidence supports it.

Explain that theories are hypotheses that scientists have not yet been able to test.

Explain that evolution is accepted mainly because most scientists have voted for it.

Which observation provides the STRONGEST evidence that a mass extinction, rather than ongoing background extinction, occurred at a particular rock boundary?

Many unrelated marine and land groups vanish in one geologically brief interval.

One genus of trilobites disappears and is replaced by a closely related genus.

Species numbers in one habitat decline slowly over tens of millions of years.

Fossils just above the boundary are larger on average than fossils below it.

Students observing pill bugs (isopods) in a classroom terrarium propose several hypotheses. Which hypothesis is BEST suited for testing in a controlled classroom experiment?

More pill bugs will settle on the moist side of a choice chamber than the dry side.

Pill bugs feel happier and calmer in dark places than they do in bright places.

Pill bugs prefer dark places because they were designed to hide from predators.

Pill bugs are important to their ecosystems because they help recycle nutrients.

Simplified mammal fossil record Interval Mammal families known Body mass of largest mammals (kg) 10 million years before the boundary 25 ≈ 10 At the extinction boundary 18 ≈ 5 10 million years after the boundary 78 ≈ 500 The mammal fossil record changed as shown (table) across the mass extinction that ended the age of nonavian dinosaurs. Which explanation BEST accounts for the pattern after the boundary?

The asteroid impact caused mutations that directly produced many new mammal families

Mammals outcompeted the dinosaurs, which caused the dinosaurs to go extinct at the boundary

Fossilization became more likely after the boundary, so earlier mammal diversity was hidden

Praxis Biology (5236)

Free Praxis Biology 5236 Practice Questions

10 sample questions for the Praxis Biology (5236), with the correct answer and an explanation for each. No account needed. Try each one before you open the answer.

Sections covered on the exam: Nature and Impact of Science and Engineering, Cell Biology: Cell Structure and Function, Genetics and Evolution, Diversity of Life and Organismal Biology, Ecology: Organisms and Environments.

Question 1 · Nature and Impact of Science and Engineering
During a unit on evolution, a student says, "Evolution is just a theory, so scientists are still guessing about whether it happened." Which teacher response BEST addresses the student's misconception about the nature of science?
- A.Explain that evolution will become a scientific law once enough additional evidence supports it.
- B.Explain that a theory is a well-tested explanation that unifies many lines of evidence.
- C.Explain that theories are hypotheses that scientists have not yet been able to test.
- D.Explain that evolution is accepted mainly because most scientists have voted for it.
Show answer and explanation
Answer: B. Explain that a theory is a well-tested explanation that unifies many lines of evidence.
In science, a theory is a broad, well-substantiated explanation supported by many independent lines of evidence (fossils, comparative anatomy, molecular data, observed selection), not a guess. Telling students a theory "becomes a law" reinforces a common misconception: laws describe patterns, while theories explain them, so one never graduates into the other. Acceptance rests on evidence, not on a vote.
Question 2 · Cell Biology: Cell Structure and Function
A bacterial toxin is applied to a sheet of intestinal epithelial cells. Afterward, a dye placed on the lumen side seeps between neighboring cells into the tissue, yet the cells remain firmly attached to one another and small molecules still pass directly from the cytoplasm of one cell to the next. Which structure did the toxin MOST likely disrupt?
- A.Gap junctions
- B.Desmosomes
- C.Adherens junctions
- D.Tight junctions
Show answer and explanation
Answer: D. Tight junctions
Tight junctions seal adjacent plasma membranes and block leakage of fluid through the spaces between epithelial cells, so their loss lets dye seep between cells. Firm adhesion shows desmosomes are intact, and continued cytoplasm-to-cytoplasm passage shows gap junctions still work. Adherens junctions, like desmosomes, anchor cells to one another rather than sealing the spaces between them.
Question 3 · Genetics and Evolution
In the cross AaBbCc × AaBbcc, the three genes assort independently and each shows complete dominance. What fraction of the offspring will show the dominant phenotype for all three traits?
- A.9/32
- B.27/64
- C.9/64
- D.3/16
Show answer and explanation
Answer: A. 9/32
Treat each gene separately and multiply: Aa × Aa gives 3/4 A_, Bb × Bb gives 3/4 B_, and Cc × cc gives 1/2 C_, so 3/4 × 3/4 × 1/2 = 9/32. The value 27/64 comes from wrongly treating the third gene as Cc × Cc, and 9/64 comes from using 1/4 instead of 1/2 for the Cc × cc cross.
Question 4 · Genetics and Evolution
Which observation provides the STRONGEST evidence that a mass extinction, rather than ongoing background extinction, occurred at a particular rock boundary?
- A.One genus of trilobites disappears and is replaced by a closely related genus.
- B.Species numbers in one habitat decline slowly over tens of millions of years.
- C.Many unrelated marine and land groups vanish in one geologically brief interval.
- D.Fossils just above the boundary are larger on average than fossils below it.
Show answer and explanation
Answer: C. Many unrelated marine and land groups vanish in one geologically brief interval.
A mass extinction is defined by the loss of a large fraction of species across many unrelated taxa and environments in a short span of geologic time. The turnover of a single genus and a slow decline in one habitat fit normal background extinction, and a change in body size says nothing about how many lineages were lost.
Question 5 · Diversity of Life and Organismal Biology
An angiosperm species has a diploid chromosome number of 14. After double fertilization, how many chromosomes are in each cell of the embryo and of the endosperm, respectively?
- A.14 and 14
- B.7 and 21
- C.14 and 21
- D.21 and 14
Show answer and explanation
Answer: C. 14 and 21
One sperm (n = 7) fertilizes the egg (n = 7), forming a 2n = 14 zygote that develops into the embryo. The second sperm fuses with the two polar nuclei of the central cell (n + n), forming triploid endosperm with 3n = 21. Answering 14 for the endosperm ignores the second polar nucleus.
Question 6 · Nature and Impact of Science and Engineering
Students observing pill bugs (isopods) in a classroom terrarium propose several hypotheses. Which hypothesis is BEST suited for testing in a controlled classroom experiment?
- A.Pill bugs feel happier and calmer in dark places than they do in bright places.
- B.Pill bugs prefer dark places because they were designed to hide from predators.
- C.More pill bugs will settle on the moist side of a choice chamber than the dry side.
- D.Pill bugs are important to their ecosystems because they help recycle nutrients.
Show answer and explanation
Answer: C. More pill bugs will settle on the moist side of a choice chamber than the dry side.
A good hypothesis is testable and falsifiable : it names a measurable outcome (number of pill bugs on each side) and a manipulated variable (moisture). "Happier" cannot be measured directly, "designed" invokes a purpose that cannot be tested experimentally, and ecosystem importance is a general statement rather than a prediction a classroom experiment can check.
Question 7 · Cell Biology: Cell Structure and Function
Organelle volume (% of cytoplasm)
Cell Rough ER Smooth ER Mitochondria Lysosomes
W 22 2 8 1
X 3 35 12 1
Y 2 4 40 1
Z 4 3 9 15
The table gives the percentage of cytoplasmic volume occupied by selected organelles in four cell types. Which cell is MOST likely an adrenal cortex cell that synthesizes steroid hormones?
- A.Cell W
- B.Cell Y
- C.Cell X
- D.Cell Z
Show answer and explanation
Answer: C. Cell X
Steroid hormones are lipids synthesized largely by enzymes of the smooth ER, so a steroid-secreting cell has an unusually extensive smooth ER, as in cell X (35%). Cell W, with abundant rough ER, fits a protein-secreting cell such as a pancreatic acinar cell; cell Y, rich in mitochondria, fits a muscle cell; cell Z, rich in lysosomes, fits a phagocyte such as a macrophage.
Question 8 · Genetics and Evolution
A child has type O blood and her mother has type A blood. Which man could NOT be the child's biological father?
- A.A man with type O blood
- B.A man with type AB blood
- C.A man with type A blood
- D.A man with type B blood
Show answer and explanation
Answer: B. A man with type AB blood
The ABO gene has three alleles (Iᴬ and Iᴮ are codominant, and both are dominant to i); a type O child is ii, so she received an i allele from each parent. A type AB man is IᴬIᴮ and carries no i allele, so he cannot be the father. Men with type A or B could be heterozygous (Iᴬi or Iᴮi), and a type O man is ii.

Organelle volume (% of cytoplasm)
Cell	Rough ER	Smooth ER	Mitochondria	Lysosomes
W	22	2	8	1
X	3	35	12	1
Y	2	4	40	1
Z	4	3	9	15

Question 9 · Genetics and Evolution

Simplified mammal fossil record
Interval	Mammal families known	Body mass of largest mammals (kg)
10 million years before the boundary	25	≈ 10
At the extinction boundary	18	≈ 5
10 million years after the boundary	78	≈ 500

The mammal fossil record changed as shown (table) across the mass extinction that ended the age of nonavian dinosaurs. Which explanation BEST accounts for the pattern after the boundary?

A.The asteroid impact caused mutations that directly produced many new mammal families
B.Extinctions freed ecological niches, and surviving mammals underwent adaptive radiation
C.Mammals outcompeted the dinosaurs, which caused the dinosaurs to go extinct at the boundary
D.Fossilization became more likely after the boundary, so earlier mammal diversity was hidden

Show answer and explanation

Answer: B. Extinctions freed ecological niches, and surviving mammals underwent adaptive radiation

Mammal families fell slightly at the boundary but more than quadrupled (18 → 78) within 10 million years, and maximum body size rose about a hundredfold as lineages diversified into roles previously filled by dinosaurs. Rapid diversification into newly vacant niches after a mass extinction is adaptive radiation . Mammals had coexisted with dinosaurs for over 100 million years without displacing them, and mutations arise randomly rather than being produced to fill niches.

Question 10 · Diversity of Life and Organismal Biology
A nail is driven into the trunk of a 3-meter-tall young tree at 1.5 m above the ground. Ten years later the tree is 9 m tall. About how high above the ground is the nail now?
- A.4.5 m
- B.7.5 m
- C.1.5 m
- D.9 m
Show answer and explanation
Answer: C. 1.5 m
Stems lengthen only at apical meristems at the shoot tips, so tissue already formed at 1.5 m does not move upward; the trunk thickens through the vascular cambium, a lateral meristem, but does not elongate. Answering 4.5 m assumes the whole trunk stretched uniformly (3 m → 9 m is a tripling), and 7.5 m assumes the nail rose by the full 6 m of growth.

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