Unit 1: Chemistry of Life

A water molecule is described as polar. Which statement best explains why?

A model of liquid water shows each molecule as one oxygen atom bonded to two hydrogen atoms by solid lines. Dashed lines connect a hydrogen atom of one molecule to the oxygen atom of a neighboring molecule. What do the dashed lines represent?

A student recorded the air temperature at the shore of a lake and the temperature of the lake water 1 m below the surface on the same summer day. Which claim is best supported by the data and by the properties of water?

During exercise, a person sweats and the sweat evaporates from the skin. Which property of water best explains how this cools the body?

In a tall tree, water moves up from the roots to the leaves through narrow tubes called xylem. Which explanation of the unbroken column of water in the xylem is most accurate?

A student observes that water rises 3 cm up a narrow glass tube dipped in a beaker. The student claims the rise is caused by adhesion of water to the glass. Which additional test would best evaluate this claim?

A water strider can stand on the surface of a pond without sinking. Which property of water makes this possible?

Bacteria are grown in a medium in which the only source of sulfur is a radioactive isotope of sulfur. After several generations, the cells are broken open and their macromolecules are separated. Which macromolecules will contain the most radioactivity?

Algae were grown for 10 days in media with different phosphate concentrations, with all other nutrients in excess. The cell density reached is shown. Which statement best describes the data and explains it?

A farmer adds a nitrogen fertilizer to a field of corn. In the corn plants, the nitrogen absorbed by the roots is used directly to build which molecules?

During the hydrolysis of a disaccharide into two monosaccharides, what happens to the water molecule that takes part in the reaction?

A cell builds a single polypeptide of 150 amino acids by dehydration synthesis. How many molecules of water are released in building the chain?

A model shows two monomers. One has −OH at its end and the other has −H at its end. An arrow leads to a single molecule in which the monomers are joined, plus one molecule of H₂O. Which process does the model represent?

Starch, glycogen and cellulose differ in function, but they are all built from the same kind of monomer. What is that monomer?

Glycogen, which animals use to store glucose, is a highly branched polymer. When blood glucose falls, liver enzymes remove glucose monomers one at a time from the ends of the chains. Which claim best explains how branching suits this function?

The table shows the melting points of four fatty acids that each have a chain of 18 carbon atoms. Which statement best describes the pattern and explains it?

Phospholipids are shaken in water and then examined. Which arrangement of the molecules is most likely to be seen?

Cholesterol is a steroid found in the plasma membranes of animal cells. What is its main role there?

Testosterone and estrogen are steroid hormones. Which description of these molecules is accurate?

Which list gives the three parts of a single nucleotide?

The base composition of three nucleic acid samples was measured. Which sample is most likely single-stranded DNA?

One strand of a DNA molecule reads 5′-ATGCCA-3′. Which sequence and orientation is the complementary strand?

When a new strand of DNA or RNA is made, to which part of the growing strand is each new nucleotide attached?

A peptide bond joins two amino acids in a polypeptide. Between which groups does the bond form?

A chemical treatment breaks the hydrogen bonds between atoms of the polypeptide backbone but leaves the covalent bonds of the chain intact. Which feature of a protein is most directly lost?

An enzyme from the small intestine contains several disulfide bridges. A reducing agent breaks these covalent bonds. The enzyme's activity was measured under three conditions. Which conclusion is best supported?

A purified protein is unfolded completely in a solution that breaks its hydrogen bonds and other weak interactions. When the solution is slowly removed, the protein folds back into its original shape and regains full activity, with no other molecules present. Which claim does this result best support?

A protein dissolves in the watery cytosol. A mutation replaces an amino acid with a charged R group on the protein's outer surface by an amino acid with a nonpolar R group. Which effect is most likely?

In a narrow glass cylinder, the surface of water curves upward where it meets the glass. In a cylinder made of wax, the surface of water curves downward at the edge. Which statement best explains the difference?

A student mixes a protein-digesting enzyme with a protein in buffer at 37 °C. After an hour, the protein has been hydrolyzed to amino acids. Which control would best show that the breakdown depends on the enzyme?

Variety A of a crop plant keeps its cell membranes working after a night at 2 °C; variety B does not. A researcher hypothesizes that variety A has more unsaturated fatty acids in its membrane phospholipids. Which investigation best tests this hypothesis?