2019 Free Response

[Figure 1. Model of two-step enzymatic plant pathway for synthesis of IAA from tryptophan: Gene Trp-T -> (arrow down) mRNA -> (arrow down) Enzyme Trp-T; Tryptophan --(Enzyme Trp-T)--> I3PA. Gene YUC -> (arrow down) mRNA -> (arrow down) Enzyme YUC; I3PA --(Enzyme YUC)--> IAA.]

Auxins are plant hormones that coordinate several aspects of root growth and development. Indole-3-acetic acid (IAA) is an auxin that is usually synthesized from the amino acid tryptophan (Figure 1). Gene Trp-T encodes an enzyme that converts tryptophan to indole-3-pyruvic acid (I3PA), which is then converted to IAA by an enzyme encoded by the gene YUC.

A student studying two different aquatic, plant-eating, unicellular protist species (species A and B) designed an experiment to investigate the ecological relationship between the two species (Table 1).

TABLE 1. EXPERIMENTAL TREATMENT GROUPS

In treatment group I, the student placed 10 individuals of species A into a container with liquid growth medium and 10 individuals of species B into a separate container with an equal amount of the same liquid growth medium. In treatment group II, the student placed 5 individuals of each species into a single container with the liquid growth medium. The student then maintained the containers under the same environmental conditions and recorded the number of individuals in each population at various time points. The results are shown in Table 2.

TABLE 2. NUMBER OF INDIVIDUALS IN EACH PROTIST POPULATION IN BOTH TREATMENT GROUPS

[Graph template: a blank grid (graph paper) with two curves already plotted (shaded columns of Table 2): "Species A, Group II" - an S-shaped curve rising steeply then leveling off at the top right; "Species B, Group I" - an S-shaped curve rising then leveling off at a lower plateau, labeled partway up the right side. Axes are unlabeled on the template, to be completed by the student with the unshaded columns (Species A, Group I and Species B, Group II).]

The pyruvate dehydrogenase complex (PDC) catalyzes the conversion of pyruvate to acetyl-CoA, a substrate for the Krebs (citric acid) cycle. The rate of pyruvate conversion is greatly reduced in individuals with PDC deficiency, a rare disorder.

[Figure 1. Release of neurotransmitters into the synapse in response to an action potential: a diagram of a presynaptic neuron terminal releasing "Neurotransmitter" molecules (packaged in vesicles) into the "Synapse", which are received by receptor channels on the "Postsynaptic Neuron" membrane.]

[Figure 2. Model of a typical action potential in a neuron: a graph of Membrane Potential (mV) on the y-axis (labeled -70, -55, 0, 30) versus Time (milliseconds) on the x-axis (0 to 5); a dashed horizontal line at -55 mV labeled "Threshold"; the curve starts at resting potential -70 mV, rises sharply to a peak of about +30 mV around t=1.5 ms, then falls rapidly, undershoots to about -75 mV around t=3 ms, and returns gradually toward -70 mV by t=5 ms.]

Acetylcholine is a neurotransmitter that can activate an action potential in a postsynaptic neuron (Figures 1 and 2). A researcher is investigating the effect of a particular neurotoxin that causes the amount of acetylcholine released from presynaptic neurons to increase.

TABLE 1. DIVERGENCE (IN PERCENT) OF MITOCHONDRIAL DNA SEQUENCES AMONG FIVE PRIMATE SPECIES

A researcher studying the evolutionary relationship among five primate species obtained data from a sequence of mitochondrial DNA (mtDNA) from a representative individual of each species. The researcher then calculated the percent divergence in the sequences between each pair of primate species (Table 1).

TABLE 1 (data table referenced by the question, reproduced below as Table 1 with the growth results for the three haploid strains):

Table 1. The data show the growth of haploid Saccharomyces cerevisiae yeast strains on media that differ in amino acid content. A plus sign (+) indicates that the yeast strains grow, and a minus sign (−) indicates that the strains do not grow.

The yeast Saccharomyces cerevisiae is a single-celled organism. Amino acid synthesis in yeast cells occurs through metabolic pathways, and enzymes in the synthesis pathways are encoded by different genes. The synthesis of a particular amino acid can be prevented by mutation of a gene encoding an enzyme in the required pathway.

A researcher conducted an experiment to determine the ability of yeast to grow on media that differ in amino acid content. Yeast can grow as both haploid and diploid cells. The researcher tested two different haploid yeast strains (Mutant 1 and Mutant 2), each of which has a single recessive mutation, and a haploid wild-type strain. The resulting data are shown in Table 1.

mRNA EXPRESSION LEVELS

[Figure 1. mRNA expression levels of six genes — a table with rows Liver, Heart, Brain, Kidney, Pancreas, Skeletal Muscle and columns Gene E, Gene F, Gene G, Gene H, Gene I, Gene J; each cell is shaded white (No mRNA), gray (Moderate amount of mRNA), or black (High amount of mRNA), as follows:

A researcher is studying patterns of gene expression in mice. The researcher collected samples from six different tissues in a healthy mouse and measured the amount of mRNA from six genes. The data are shown in Figure 1.

TABLE 1. CHANGES IN MORNING GLORY PETAL CELLS DURING FLOWER OPENING

[Diagram, BUD: a petal cell containing a central vacuole. On the plasma membrane: a "K+ Channel" (dark circle) allowing K+ to cross; a "Proton Pump" (light gray circle) pumping H+ out of the cell (labelled H+, H+ outside). On the vacuole membrane: a "K+/H+ Transport Protein (inactive)" (black oval, inactive) with H+ and K+ labelled nearby but not being actively transported.]

[Diagram, OPEN FLOWER: a larger petal cell/vacuole. On the plasma membrane: "K+ Channel" (dark circle) with K+ moving in; "Proton Pump" (light gray circle) pumping K+ in and H+ out (labelled K+, H+). On the vacuole membrane: "K+/H+ Transport Protein (active)" (black oval, active) transporting H+ out of the vacuole and K+ into the vacuole (labelled H+, K+, with multiple K+ ions shown accumulating inside the vacuole); H2O arrow entering the cell, labelled H2O.]

The petal color of the Mexican morning glory (Ipomoea tricolor) changes from red to blue, and the petal cells swell during flower opening. The pigment heavenly blue anthocyanin is found in the vacuole of petal cells. Petal color is determined by the pH of the vacuole. A model of a morning glory petal cell before and after flower opening is shown in Table 1.