AP®BiologyCollege BoardExam QuestionsUnit 1: Chemistry of LifeElements of Life

Elements of Life (College Board AP® Biology)

Exam Questions

5 mins5 questions
11 mark

Water molecules are composed of the atoms hydrogen and oxygen, which are held together by covalent and hydrogen bonds. Which of the following models best demonstrates the arrangement of hydrogen bonds between water molecules? (Hydrogen bonds are depicted by dashed lines.)

  • Diagram of a water molecule cluster showing two water molecules connected by hydrogen bonds, with each oxygen atom bonded to two hydrogen atoms.
  • Structural formula showing hydrogen peroxide molecules: central oxygen bonded to two hydrogens, each oxygen also linked to another hydrogen.
  • Diagram of two water molecules with hydrogen bonds; oxygen atoms are at the centre of each, and hydrogen atoms are attached by solid and dashed lines.
  • Chemical structure of hydrogen peroxide, showing two oxygen atoms bonded, with each bonded to two hydrogen atoms, illustrating the molecule's composition.

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2
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Exploring the molecular pathways and physiological importance of carbon, nitrogen, and phosphorus in different biological systems has highlighted their contributions to life’s fundamental processes. Carbon forms the framework of biological molecules, nitrogen is critical for proteins and nucleic acids, and phosphorus is essential for nucleic acids, membrane structure, and energy metabolism.

Cellular membranes contain phospholipids which provide structure and specific physiological characteristics to support the molecular pathways of the cell. Which of the following models represents the correct arrangement of phosphorus within a phospholipid?

  • Chemical structure of a phospholipid with two hydrophobic tails, glycerol backbone, phosphate group, and variable R-group at the top.
  • Chemical structure of a phospholipid with a phosphate group, variable group R, and glycerol backbone linked to three fatty acid chains.
  • Chemical structure of a triglyceride molecule with three fatty acid chains, each linked to a glycerol backbone via ester bonds.
  • Chemical structure of a phospholipid molecule, featuring a phosphate group, glycerol backbone, and two fatty acid chains with zigzag tails.

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Nitrogen is a key part of building biomass because of its presence in all amino acids and therefore proteins. Which of the following best describes nitrogen within a protein?

  • Each amino acid contains an amine group (-NH₂)

  • Each amino acid contains a nitrate group (-NO2-)

  •  Each protein molecule contains an amine group (-NH₂)

  • Each protein molecule contains a nitrate group (-NO2-)

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4
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Water is known as a cohesive molecule due to the presence of hydrogen bonding. Water is also adhesive to other molecules. Which of the following does not use these properties of water to function?

  • Mass transport through the xylem of plants and blood vessels in animals.

  • Surface tension on a body of water meets the air to create a film on the body of water.

  • Capillary action within xylem vessels.

  • Evaporation of water from the skin during sweating.

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5
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The covalent bonds within water molecules are described as polar covalent bonds. Which of the following best explains why the covalent bonds within water are polar?

  • Hydrogen is less electronegative than oxygen, generating a partial negative charge near the hydrogen atom.

  • Hydrogen is more electronegative than oxygen, generating a partial positive charge near the hydrogen atom.

  • Oxygen is more electronegative than hydrogen, generating a partial negative charge near the oxygen atoms.

  • Oxygen is less electronegative than hydrogen, generating a partial positive charge near the oxygen atoms.

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