Which of the following is incorrect?

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.