Final answer:
The word filling in the blank is 'pore', meaning the full sentence reads: 'All of the taste cells of a taste bud lie deep to an opening within the epithelium called a taste pore.' Taste buds, containing gustatory receptor cells, are sensitive to chemicals in foods and are located within different forms of papillae on the tongue.
Step-by-step explanation:
The taste cells of a taste bud are associated with gustatory receptor cells that detect different flavours. These cells are located within the bumps on the tongue known as papillae. Specifically, the gustatory cells are found in the part of the papillae called taste buds, which are sensitive to five types of tastes: sweet, salty, sour, bitter, and umami. These tastes are detected when chemicals in foods, called tastants, dissolve in saliva and stimulate the gustatory receptor cells.
Taste buds are present mostly on the sides and back of the tongue, within structures called fungiform, foliate, and circumvallate papillae, but not within the filiform papillae. The receptor cells pick up taste through microvilli, hair-like extensions that reach into an opening at the surface of the tongue's epithelium called the taste pore. The microvilli extend into the taste pore, allowing them to make contact with tastants dissolved in the saliva. This process leads to the generation of nerve impulses that travel to the central nervous system, allowing us to perceive taste.
The answer to the student's question is 'pore', making the complete sentence: All of the taste cells of a taste bud lie deep to an opening within the epithelium called a taste pore.