Which of the following microscopic structures is only found in the cardiac muscle tissue?
a) myosin
b) tropomyosin
c) sarcomeres
d) intercalated discs
e) striations

The correct answer is: d) intercalated discs

Cardiac muscle tissue is muscle tissue found in the heart.  It is similar to skeletal muscle in that both are striated and organized into sarcomeres but cardiac muscle fibers are shorter and usually contain only one nucleus in the central region of the myocyte (muscle cell). Cardiac muscle fibers are rich in mitochondria and myoglobin, (ATP is produced primarily through aerobic metabolism), are extensively branched and are connected to one another by intercalated discs. An intercalated disc is a structure that enables the propagation of contraction among muscle cells: it allows the muscle cells to contract in a wave-like pattern.

in the year 2850, humans successfully colonized mars. the martian modules that were constructed could hold only a small population of people. it is now a century later, and the population on mars has grown to 600 people. of the current mars population, 45 people express the recessive phenotype for phenylketon- uria (pku) syndrome (pp genotype).

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