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19. The correct answer is (E). Dominated by the presence of conifers is the taiga. This biome is south
of the tundra. Moose, bears, and birds are a few of the characteristic animals in the taiga.
20. The correct answer is (A). The offspring of a homozygous dominant-homozygous recessive cross
will demonstrate the dominant phenotype. This result can quickly be ascertained from a Punnett
square:
G = dominant, green leaf color
g = recessive, yellow leaf color
GG 
× gg
Thus, it is observed that 100% of the offspring will be heterozygous for leaf color. However, since
green leaf color is dominant, the phenotype will accordingly be green.
21. The correct answer is (B). Archaeopteryx is the oldest known fossil of the class Aves (birds). This
animal possessed characteristics of both reptiles and birds. Hence, it is believed to represent the
evolutionary link between the classes Reptilia and Aves. Although Archeopteryx had a long tail and
teeth, it also had wings, leading systematists to classify this animal as a bird.
22. The correct answer is (D). Enzymes do not affect the equilibrium constant for a given reaction.
Instead, enzymatic catalysis simply allows equilibrium to be reached more quickly than would occur
in the non-catalyzed reaction. Enzymes accomplish such a task by allowing reactions to occur more
readily; reactive portions of substrates are brought into close approximation. Consequently, the acti-
vation energy is lowered, and the forward and reverse reactions will occur more readily.
23. The correct answer is (C). Carbon fixation occurs in the dark reactions. In these reactions, CO
2
is
used to synthesize carbohydrates. The energy sources for this set of reactions are ATP and NADPH.
The ATP and NADPH are products of the light-dependent reactions.
24. The correct answer is (E). The all-or-none response is a property of neurons. This concept refers to the
response of a neuron to a given stimulus. If a stimulus is of threshold intensity (or greater), then the
neuron will “fire.” However, if the stimulus is of subthreshold intensity, the neuron will not propagate a
signal.