Final answer:
The jelly-like substance within cells that helps maintain shape, hold organelles, and allows biochemical reactions is known as the cytoplasm.
Step-by-step explanation:
The jelly-like substance within cells is known as cytoplasm. This gel-like material fills the interior of the cell, contained by the plasma membrane, and provides a medium for cellular processes. The cytoplasm encompasses not only the cytosol, the liquid part, but also various organelles and particles like ribosomes, which are responsible for protein synthesis. It is crucial for the maintenance of cell shape, holding organelles in place, and for many biochemical reactions necessary for the life of the cell.