MCAT Practice questions and answers with solutions 2025

Why are triacylglycerols used in the human body for energy storage? A) They are highly hydrated, and therefore can store lots of energy. B) They always have short fatty acid chains, for easy access by metabolic enzymes. C) The carbon atoms of the fatty acid chains are highly reduced, and therefore yield more energy upon oxidation. D) Polysaccharides, which would actually be a better energy storage form, would dissolve in the body. - ANSWER C Triacylglycerols are highly hydrophobic and therefore not highly hydrated (which would add extra weight from the water of hydration, taking away from the energy density of these molecules), eliminating choice (A). The fatty acid chains produce twice as much energy as polysaccharides during oxidation because they are highly reduced. The fatty acid chains vary in length and saturation. Action potentials travel down neural axons that resist the flow of current. In considering a theoretical circuit, one normally assumes that the wire has no resistivity. In the real world, however, wire resistivity is an important consideration. Which of the following is a way to increase resistance? A) Lengthen the wire B) Widen the wire C) Lower the temperature D) Change from tungsten to copper wire - ANSWER A This question tests your knowledge of resistance in wires. An electrical wire's resistance is directly proportional to its length and inversely proportional to its cross-sectional area. Think of it as gravel in a pipe—the wider the pipe and the shorter the pipe, the easier it is for water to go through it. B is incorrect because when a wire's cross-sectional area is increased, its resistivity is decreased. C is incorrect because lowering temperature decreases resistivity. Think superconductors. D is incorrect because tungsten has higher resistivity than copper. Tungsten is used for lightbulb filaments because of its higher resistivity. Sexually reproducing species have a selective advantage over asexually reproducing species, because sexual reproduction: A) is more energy efficient. B) creates novel genetic recombinations.