ACT Biology Practice Questions with Answers

Biology questions on the ACT Science section account for approximately 25% to 30% of the total science content you will encounter on test day. Success in this area requires more than just memorizing facts; it demands the ability to interpret experimental data, understand biological models, and apply scientific reasoning to unfamiliar scenarios. By using targeted ACT Reading Practice Practice Questions alongside your science prep, you can improve your speed in dissecting the complex passages that accompany these biology problems.

Concept Explanation

ACT Biology focuses on three main categories: cell biology, genetics, and organismal biology/ecology. Unlike a high school biology exam that might ask for the definition of a ribosome, the ACT will present you with a study on protein synthesis and ask you to predict what happens if a specific organelle is inhibited. You must be comfortable with the ACT Prep standards for interpreting graphs, identifying independent and dependent variables, and recognizing conflicting viewpoints between scientists. Key themes often include:

• Cellular Processes: Photosynthesis, cellular respiration, and mitosis/meiosis.
• Genetics: DNA replication, Punnett squares, and protein synthesis (transcription and translation).
• Ecology: Food webs, population dynamics, and nutrient cycles (nitrogen and carbon).

Because the ACT Science section is heavily data-driven, you should focus on how biological systems respond to changes in their environment. For instance, if a graph shows enzyme activity peaking at $37^{\circ} \text{C}$, you should be able to infer that the enzyme denatures at higher temperatures. Integrating tools like an AI Question Generator can help you simulate these specific data-interpretation tasks.

Solved Examples

1. Genetics Example: In a species of pea plants, tallness ($T$) is dominant to shortness ($t$). If a heterozygous tall plant is crossed with a short plant, what is the probability of the offspring being short?
   1. Identify the genotypes: Heterozygous tall is $Tt$; short is $tt$.
   2. Set up a Punnett square: The possible combinations are $Tt, Tt, tt, \text{and } tt$.
   3. Calculate the ratio: Two out of four offspring are $tt$.
   4. Final Answer: 50% probability.
2. Cellular Respiration Example: A scientist measures the oxygen consumption of yeast in a closed flask. Over 10 minutes, the oxygen level drops from 21% to 15%. What process is occurring?
   1. Observe the change: Oxygen is being consumed (decreasing).
   2. Recall biological pathways: Aerobic respiration uses oxygen to break down glucose for energy.
   3. Analyze the byproduct: As oxygen is consumed, carbon dioxide is typically released.
   4. Final Answer: Aerobic cellular respiration.
3. Ecology Example: A study shows that as the population of wolves (predators) in a park increases, the population of elk (prey) decreases. However, after two years, the wolf population also begins to decline. Why?
   1. Identify the relationship: This is a classic predator-prey cycle.
   2. Analyze the cause: The decline in elk means there is less food available for the wolves.
   3. Apply ecological principles: Carrying capacity and resource availability limit population growth.
   4. Final Answer: The wolf population declined due to a decrease in their primary food source.

Practice Questions

1. A researcher observes that a specific plant species grows significantly faster when treated with a nitrogen-rich fertilizer compared to a phosphorus-rich fertilizer. What is the independent variable in this experiment?
2. In a DNA molecule, if 30% of the bases are Adenine ($A$), what percentage of the bases are Cytosine ($C$)?
3. During which phase of the cell cycle does DNA replication occur?

4. If a cell containing 46 chromosomes undergoes mitosis, how many chromosomes will each daughter cell contain?
5. Which organelle is responsible for converting light energy into chemical energy in plant cells?
6. A scientist finds that a certain enzyme works best at a pH of 2.0. In which part of the human body is this enzyme most likely active?
7. In a food chain consisting of grass \u2192 grasshopper \u2192 frog \u2192 snake, which organism represents the secondary consumer?
8. What is the primary difference between prokaryotic and eukaryotic cells regarding their genetic material?
9. If a segment of mRNA has the sequence 5'-AUG-CCG-UAA-3', what was the original DNA template strand sequence?
10. Which type of natural selection favors individuals at both extremes of a phenotypic range?

Answers & Explanations

1. Answer: The type of fertilizer. The independent variable is the factor the researcher changes or manipulates to observe an effect. In this case, it is the nutrient composition of the fertilizer.
2. Answer: 20%. According to Chargaff's rules, the amount of Adenine ($A$) equals Thymine ($T$), and Cytosine ($C$) equals Guanine ($G$). If $A = 30\%$, then $T = 30\%$, totaling 60%. The remaining 40% must be split equally between $C$ and $G$, so $C = 20\%$.
3. Answer: S phase (Synthesis phase). During the S phase of interphase, the cell replicates its DNA to ensure that each daughter cell receives a complete set of genetic instructions.
4. Answer: 46. Mitosis is a process of nuclear division that results in two genetically identical daughter cells with the same number of chromosomes as the parent cell.
5. Answer: Chloroplast. Chloroplasts contain chlorophyll, which captures sunlight to drive the process of photosynthesis, producing glucose.
6. Answer: The stomach. The stomach is a highly acidic environment with a pH typically around 2.0, which is necessary for enzymes like pepsin to function.
7. Answer: The frog. In this food chain, grass is the producer, the grasshopper is the primary consumer, and the frog is the secondary consumer because it eats the primary consumer.
8. Answer: Presence of a nucleus. Eukaryotic cells house their DNA within a membrane-bound nucleus, whereas prokaryotic cells have their DNA in a nucleoid region without a surrounding membrane.
9. Answer: 3'-TAC-GGC-ATT-5'. mRNA is synthesized using the DNA template strand through complementary base pairing ($A$ with $U/T$, $C$ with $G$). Note that DNA uses Thymine ($T$) while mRNA uses Uracil ($U$).
10. Answer: Disruptive selection. Disruptive selection favors the extremes of a trait (e.g., very small and very large) over the intermediate forms, potentially leading to speciation.

1. Which molecule serves as the direct energy source for most cellular activities?

• AGlucose
• BATP
• CDNA
• DRNA

Frequently Asked Questions

How much biology is on the ACT Science section?

Biology typically accounts for about 25% of the Science section, appearing in 1 or 2 of the 6-7 passages. You will likely see questions on genetics, ecology, or cell biology mixed with data interpretation tasks.

Do I need to memorize the Krebs cycle for the ACT?

No, you do not need to memorize every step of complex metabolic pathways like the Krebs cycle or Calvin cycle. The ACT will provide the necessary diagrams or descriptions, and you must be able to interpret how those processes function based on the provided data.

What is the best way to study for ACT Biology?

Focus on practicing data interpretation and scientific reasoning rather than rote memorization. Use resources like AI Flashcard Generator to master high-frequency terms, but spend most of your time analyzing graphs and tables from official practice tests.

Are Punnett squares common on the ACT?

Yes, basic Mendelian genetics and Punnett squares are frequently tested. You should understand dominant and recessive traits, as well as how to calculate the probability of specific genotypes and phenotypes in offspring.

Can I use a calculator on the ACT Science section?

No, calculators are not permitted on the Science section of the ACT. Any math required will be basic arithmetic, such as calculating percentages or identifying trends in numerical data, so practice doing these calculations by hand.