In skeletal muscle Catt is essential for the contraction process. For skeletal
muscle cells, what is the source of Cat+.?

The source of Calcium (Ca⁺⁺) for the skeletal muscle cells is the sarcoplasmic reticulum, the main store of intracellular Ca⁺⁺.

Explanation:

Calcium (Ca⁺⁺) is an ion that plays an important role in muscle contraction, a process that requires high cytoplasmic concentrations of this ion to promote myofibrillary activity.

The sarcoplasmic reticulum (SR) -equivalent to the endoplasmic smooth reticulum- is an organelle that regulates the contractile activity of skeletal muscle. The two terminal cisterns of the SR attach to the T-tube, forming a triad, which surrounds the myofibrilla.

Once the muscle cell receives the signal to contract, the Ca⁺⁺ channels are opened and allow Ca⁺⁺ to enter the cell, promoting the exit of this cation from the SR and increased its cytoplasmic levels. This stimulates muscle contraction.

fabric is silk wool ext. paper isnt

explanation:

u cant make fabric out of paper cause paper comes from wood while fabric comes from animals

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link: