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AP®BiologyCollege BoardExam QuestionsUnit 1: Chemistry of LifeWater & Chemical Elements

Water & Chemical Elements (College Board AP® Biology): Exam Questions

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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 hydronium ion bonded to two water molecules through hydrogen bonds, showing oxygen and hydrogen atoms with bonding lines and angles.
  • Diagram of a hydrogen peroxide molecule showing two oxygen atoms each bonded to two hydrogen atoms, forming a bent structure.
  • Diagram showing three water molecules with hydrogen bonds: two outer molecules aligned with an oxygen atom bonded to two hydrogen atoms each.
  • Chemical structure diagram of hydrogen peroxide, showing two oxygen atoms single-bonded, each with two hydrogen atoms attached.

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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?

  • Molecular structure of a phospholipid, featuring a glycerol backbone, two fatty acid tails, and a phosphate group with an 'R' variable group.
  • Chemical structure of a phospholipid with a phosphate group and three fatty acid chains, including unsaturated bonds, depicted in zigzag lines.
  • Chemical structure of a triglyceride molecule with three ester linkages and three fatty acid chains, one with a double bond, depicting its composition.
  • Chemical structure of a phospholipid with a glycerol backbone, two fatty acid chains, and a phosphate group linked to a variable R group.

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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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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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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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