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.

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