Which of the following describes tight junctions?
a. cell junctions in areas that are subjected to great mechanical stress, such as the skin and heart muscles
b. junctions among epithelial cells lining the digestive tract
c. junctions within electrically excitable tissues, such as the heart and smooth muscle
d. junctions where cells are held together by cadherins

Junctions among epithelial cells lining the digestive tract. (Ans. B)

Explanation:

Tight junction are defined as a barrier is form where the membrane of two adjacent cells join together. Cell membrane of two adjacent cells connected by two main proteins claudins and occludins (trans-membrane proteins).

Tight junction helping to prevent molecules to passing between the cells and helping to maintain the polarity present in the cell with the help of bind cells together.

Tight junction have several functions:

1) Tight junction are present in the epithelial cells, these cells lines the body cavity and the surface which is present in the body. They are helping to separate epithelial cells from the nearby environment and also helping to separate from the surface which is present in the body.

2) Most important function of the tight junction is to help in forming a barrier which prevent molecules from moving around.

1. a tropical dry forest carries out more rapid nutrient cycling