Which of the following is incorrect?

Glycogen is not found in yeast and fungi

The correct answer is option 3. Glycogen is not found in yeast and fungi.

Let us delve into each statement:

1. In amylose, \(\alpha\)-D-(+)-glucose units are held by \(C_1 - C_4\) glycosidic linkage.

This statement is correct. Amylose is a form of starch, and its structure consists of linear chains of \(\alpha\)-D-glucose units connected by \(C_1 - C_4\) glycosidic bonds.

2. In amylopectin, \(\alpha\)-D-glucose units are held by \(C_1 - C_4\) and \(C_1 - C_6\) glycosidic linkage.

This statement is also correct. Amylopectin is another form of starch characterized by its branched structure. It contains both \(C_1 - C_4\) and \(C_1 - C_6\) glycosidic bonds, linking the \(\alpha\)-D-glucose units in both linear and branched regions.

3. Glycogen is not found in yeast and fungi.

This statement is incorrect. Glycogen is a polysaccharide similar to starch but with more extensive branching. It serves as a storage form of glucose in animals, fungi, and bacteria, including yeast.

4. Cellulose is composed of \(\beta\)-D-glucose units only.

This statement is true. Cellulose is a structural polysaccharide found in the cell walls of plants. It is composed of linear chains of \(\beta\)-D-glucose units linked by \(C_1 - C_4\) glycosidic bonds. The \(\beta\)-configuration of glucose units in cellulose gives it its distinct structural properties.

Therefore, statement 3 is the incorrect one. Glycogen is indeed found in yeast and fungi, serving as an energy storage molecule in these organisms.