Polysaccharides that are synthesized in plant cells are starch (used as a reserve substance) and cellulose (used to build the cell wall).
Both of these compounds are biopolymers, i.e. consist of simpler molecules (monomers) that are repeated many times. Moreover, both starch and cellulose have glucose as the monomer.
It is much easier to synthesize at once not a huge molecule of a polysaccharide, but a multitude of molecules of its monomer, which will then “stick” together.
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