You are given a sentence and your task is to generate another sentence that supports the given input.

One oxygen atom combines with two hydrogen atoms to form a water molecule.
2 hydrogen, 1 oxygen atoms combine to make up a molecule of water.

Cell membrane functions as a barrier to the movement of substances between the intracellular and extracellular spaces 3.
A cell's membrane provides a barrier to keep extracellular materials from mixing with its internal components.

The duodenum is the first part of the small intestine and most of the chemical digestion occurs here.

In the body, chemical digestion mainly takes place in the small intestine.