Final answer:
The striations in cardiac muscle cells are not modified gap junctions; rather, they are caused by the arrangement of actin and myosin filaments. Cardiac cells are connected by intercalated discs with desmosomes for mechanical stability and gap junctions for electrical coupling, enabling synchronized contraction. Thus, the statement is false.
Step-by-step explanation:
The statement that the striations in cardiac muscle cells are modified gap junctions that unite the cells and permit the heart to contract as a unit is false. Striations in cardiac muscle cells are due to the arrangement of actin and myosin filaments within the cells, which are similar to those found in skeletal muscle. However, cardiac muscle cells are unique in that they are connected by intercalated discs, which contain both desmosomes and gap junctions. Desmosomes hold the cells together and resist mechanical stress, while gap junctions create channels between cells that allow ions and electrical impulses to pass rapidly—a critical feature for the synchronized contraction of the heart muscle, or syncytium.