Biology HESI A2 2024

Correct Answer: C

Rationale: The correct answer is C: thymine. In DNA, adenine always pairs with thymine through two hydrogen bonds. This is a fundamental principle known as Chargaff's rule, where A pairs with T and C pairs with G. Thymine is a pyrimidine base, complementing adenine, which is a purine base. Therefore, thymine is the most suitable choice for adenine in a DNA strand. Choices A, B, and D are incorrect because adenine does not pair with cytosine (A), uracil (B), or guanine (D) in DNA.

Correct Answer: D

Rationale: The correct answer is D: Carbohydrates. Glucose is a type of simple sugar and a carbohydrate. Carbohydrates are composed of carbon, hydrogen, and oxygen in a 1:2:1 ratio, which is the molecular formula of glucose (C6H12O6). Proteins (choice A) are composed of amino acids, lipids (choice B) are composed of fatty acids and glycerol, and nucleic acids (choice C) are composed of nucleotides. Therefore, only carbohydrates, specifically glucose, contain the specific molecular structure that characterizes glucose.

Correct Answer: A

Rationale: The correct answer is A because chlorophyll appears green due to its absorption spectrum. It absorbs light most efficiently in the blue and red regions of the spectrum, while reflecting or transmitting green light. This selective absorption of light wavelengths results in chlorophyll appearing green to our eyes. Choice B is incorrect because chlorophyll does not primarily absorb green light. Choice C is incorrect as chlorophyll actually does absorb green light, just not as efficiently as blue and red light. Choice D is incorrect because chlorophyll primarily absorbs red and blue light, not red light alone.

Correct Answer: C

Rationale: The purpose of phloem tissue in plant stems is to conduct food throughout the plant. Phloem is responsible for transporting sugars, amino acids, and other organic compounds produced through photosynthesis from the leaves to other parts of the plant for growth and energy. This process is known as translocation. Choice A is incorrect because water is transported by xylem tissue, not phloem. Choice B is incorrect as phloem does not conduct light energy. Choice D is incorrect as carbon dioxide is absorbed by the plant through stomata and transported through the plant via diffusion, not through phloem tissue.

Correct Answer: C

Rationale: The correct answer is C: Producers > secondary consumers. This is true because producers, like plants, have the highest energy content in an ecosystem as they convert sunlight into energy through photosynthesis. Secondary consumers, such as carnivores, obtain energy by consuming primary consumers or other secondary consumers, resulting in a lower energy content compared to producers. A: Decomposers < secondary consumers - Incorrect. Decomposers break down organic matter, releasing nutrients, and have a lower energy content compared to secondary consumers. B: Primary consumers < producers - Incorrect. Primary consumers, like herbivores, consume producers directly, so they have a lower energy content than producers. D: Secondary consumers < primary consumers - Incorrect. Secondary consumers typically have a lower energy content than primary consumers as they obtain energy by consuming them.