Cohesion means unity. In writing as well as in speech we use certain words and expressions to establish connections among ideas in a sentence and paragraph. These cohesive devices add unity to your answer. If you do not use them often enough, your essay will become a collection of disjointed sentences.

Your sentences may be grammatically correct, but if you fail to establish the connection between them, a great deal of meaning will be lost. The most common cohesive devices are personal pronouns (he, she, they), demonstrative pronouns (this, that, these, those), articles and transitional adverbs (therefore, furthermore).

Synonyms and different forms of the same word can also be used to unify a paragraph. An advantage of using synonyms is that they make it possible to avoid the repetition of words. While repetition is not grammatically incorrect, it will easily put off the reader.

Read the following paragraph. It contains a lot of repetition.

Atmospheric pollution deteriorates the quality of air. Pollution comes from a variety of sources like vehicles, factories, air conditioning and air travel. Vehicles, factories and air conditioners put harmful chemicals into the air.  Chemicals like bromine and chlorine are particularly dangerous. Bromine and chlorine rise into the atmosphere, and the chemicals react with the ozone layer.

As you can see the sentences given above are all grammatically correct. But they contain a lot of repetition. The problem with repetition is that it lengthens your writing and bores the reader. By using cohesive devices properly, you can make your writing more interesting.

Atmospheric pollution deteriorates the quality of air. Pollution comes from a variety of sources such as vehicles, factories and air conditioners. They release harmful chemicals into the air.  Chemicals like bromine and chlorine are particularly dangerous. They rise into the atmosphere, and then react with the ozone layer

This paragraph is shorter than the original paragraph. It is also a whole lot better. In this paragraph the writer employs several cohesive devices to make his sentences more interesting and well-connected.