Which of the following regarding proteins is incorrect?
a. they form peptide bonds
b. all are enzymes
c. play a role in transport of material across a membrane
d. may serve as structural proteins

b. All are enzymes  

Explanation:

"Every enzyme is a protein, but not every protein is an enzyme." You may have heard that phrase already.

With the exception of one group of RNA molecules with enzymatic activity, every enzyme is a protein.

Proteins, in addition to enzymes, can perform many other functions in the cell, such as structural and transport functions. Protein is a type of substance formed from a set of linked amino acids (called peptide bonds). In other words, proteins are composed of carbon, hydrogen, oxygen and nitrogen molecules.

the endomembrane system is a series of compartments that work together to package, label, and ship proteins and molecules. in your cells, the endomembrane system is made up of both the endoplasmic reticulum and the golgi apparatus. these compartments are folds of membranes that form tubes and sacs in your cells.