Final answer:
Tight junctions in the gut create a watertight seal with proteins called claudins and occludins, but become leakier as you move down the gut to allow some substances to pass through.
Step-by-step explanation:
Tight junctions are an essential feature of epithelial cells that line the lumens of organs, such as the intestines and lungs. These junctions create a watertight seal that prevents extracellular fluids from passing between the cells. The proteins predominantly involved in this are called claudins and occludins, which function to hold the adjacent cells tightly against each other, therefore preventing leakage into the extracellular space.
As you move down the gut, tight junctions become more leaky. These leaky tight junctions have fewer sealing strands of proteins and are incomplete, allowing limited intercellular movement of solvent and solutes. This regulated permeability allows some substances to pass through, adapting to the different functional needs along the gut.