Chemical Basis of Biology — Hard Practice Quiz

A Biology cheat sheet for Chemical Basis of Biology — every key formula with its symbols defined — plus a hard-level practice quiz to test recall.

Practice quiz

1. An atom of element X has $15$ protons, $16$ neutrons, and $15$ electrons. If this atom forms an ionic bond by losing $3$ electrons, and subsequently undergoes a process where its nucleus gains $2$ neutrons, what is the correct description of the final species?

   • An ion of a different element with $17$ protons and $18$ neutrons.
   • An isotope of element X with $15$ protons, $18$ neutrons, and a charge of $3+$.
   • A neutral atom of element X with $15$ protons and $18$ neutrons.
   • An ion of element X with $15$ protons, $16$ neutrons, and a charge of $3+$.

   Answer: An isotope of element X with $15$ protons, $18$ neutrons, and a charge of $3+$.

2. Consider two hypothetical elements, A and B. Element A readily forms an ionic bond with element B, while element C forms a covalent bond with element D. If element A typically loses electrons and element C typically shares electrons to achieve stability, which statement accurately describes the resulting compounds and their fundamental units?

   • The compound of A and B is a molecule, while the compound of C and D is an ionic lattice.
   • The compound of A and B is formed by electron transfer, and its smallest unit is often a formula unit; the compound of C and D is formed by electron sharing, and its smallest unit is a molecule.
   • Both compounds are molecules, but only the A-B compound involves electron transfer.
   • Both compounds are formed by electron sharing, but only the C-D compound forms a distinct molecule.

   Answer: The compound of A and B is formed by electron transfer, and its smallest unit is often a formula unit; the compound of C and D is formed by electron sharing, and its smallest unit is a molecule.

3. A chemist prepares a solution by dissolving a substance that releases hydroxide ions ($OH^-$) into pure water, resulting in a $pH$ of $10$. If a buffer solution is then added, and subsequently, a strong acid is introduced, what is the most likely outcome regarding the $pH$ of the solution?

   • The $pH$ will immediately drop to below $7$ due to the strong acid.
   • The $pH$ will remain exactly $10$ because of the buffer.
   • The $pH$ will decrease, but the buffer will significantly resist a sharp, sudden change, keeping it above $7$ for some time.
   • The $pH$ will increase further as the buffer neutralizes the base.

   Answer: The $pH$ will decrease, but the buffer will significantly resist a sharp, sudden change, keeping it above $7$ for some time.

4. A complex biological structure is found to be composed primarily of repeating units that contain a pentose sugar, a phosphate group, and a nitrogenous base. This structure is crucial for storing genetic information. Which of the following macromolecules is being described, and what is its monomer?

   • Protein; Amino Acid.
   • Nucleic Acid; Nucleotide.
   • Carbohydrate; Monosaccharide.
   • Lipid; Fatty Acid.

   Answer: Nucleic Acid; Nucleotide.

5. Water exhibits both cohesion and adhesion. If a plant relies on these properties to transport water from its roots to its leaves, and a scientist discovers a new liquid that has significantly higher cohesive forces but negligible adhesive forces compared to water, how would this new liquid likely behave in a narrow capillary tube compared to water?

   • It would rise higher in the capillary tube than water due to stronger cohesive forces.
   • It would not rise significantly in the capillary tube, and might even show a convex meniscus, unlike water's concave meniscus.
   • It would rise to the same height as water, as cohesion is the dominant factor in capillary action.
   • It would evaporate more quickly due to the lack of adhesive forces.

   Answer: It would not rise significantly in the capillary tube, and might even show a convex meniscus, unlike water's concave meniscus.

6. An element with $11$ protons and $12$ neutrons forms a stable compound with another element that has $17$ protons and $18$ neutrons. Describe the type of bond formed and the resulting charge on the first element's ion within the compound.

   • Covalent bond; $1-$ charge.
   • Ionic bond; $1+$ charge.
   • Covalent bond; $1+$ charge.
   • Ionic bond; $2+$ charge.

   Answer: Ionic bond; $1+$ charge.

7. Two isotopes of the same element, $X$ and $Y$, exist. Isotope $X$ has an atomic mass of $35$ and contains $18$ neutrons. Isotope $Y$ has an atomic mass of $37$. How many protons and neutrons are present in the nucleus of isotope $Y$?

   • $17$ protons, $18$ neutrons.
   • $18$ protons, $19$ neutrons.
   • $17$ protons, $20$ neutrons.
   • $20$ protons, $17$ neutrons.

   Answer: $17$ protons, $20$ neutrons.

8. A specific protein acts as an enzyme, catalyzing a crucial metabolic reaction. If a mutation occurs in the gene encoding this protein, leading to a change in the sequence of a few amino acids, what is the most probable consequence for the enzyme's function and the overall metabolic pathway?

   • The enzyme's function will remain unchanged, as only a few amino acids were altered.
   • The enzyme's three-dimensional structure may be altered, potentially reducing or eliminating its catalytic activity, thereby disrupting the metabolic pathway.
   • The enzyme will become a different type of macromolecule, such as a lipid, due to the change in its building blocks.
   • The enzyme will begin to catalyze a completely different reaction, leading to a new metabolic pathway.

   Answer: The enzyme's three-dimensional structure may be altered, potentially reducing or eliminating its catalytic activity, thereby disrupting the metabolic pathway.

9. A scientist isolates a novel biological macromolecule from an extremophile organism. Upon analysis, it is determined that this macromolecule is composed of repeating units, each containing a five-carbon sugar, a phosphate group, and a nitrogenous base. However, unlike typical DNA, this macromolecule contains uracil instead of thymine. Which of the following statements correctly identifies this macromolecule and its primary function?

   • It is a protein, primarily involved in structural support.
   • It is a nucleic acid (RNA), primarily involved in carrying genetic information from DNA and protein synthesis.
   • It is a carbohydrate, serving as an energy storage molecule.
   • It is a nucleic acid (DNA), primarily responsible for long-term genetic storage.

   Answer: It is a nucleic acid (RNA), primarily involved in carrying genetic information from DNA and protein synthesis.

10. Consider a substance formed by the chemical combination of two or more elements in definite proportions, where the smallest unit of this substance displays all its properties. If this substance is then dissolved in water to form a homogeneous mixture, what terms best describe the initial substance and the resulting mixture, respectively?

    • Element; Suspension.
    • Compound/Molecule; Solution.
    • Isotope; Acid.
    • Atom; Buffer.

    Answer: Compound/Molecule; Solution.