Which of the following is NOT a function of proteins?

a: act as a main fuel for cells
B: control chemical reactions in a cell

To determine which of the given options is NOT a function of proteins, we need to understand the functions of proteins and then examine each option.

Proteins are large biomolecules composed of amino acids. They play a crucial role in various biological processes and perform a wide range of functions in living organisms. Some common functions of proteins include:

1. Structural support: Certain proteins, such as collagen, act as the building blocks of tissues and provide structural support to cells and organs.

2. Enzymatic activity: Proteins called enzymes facilitate and control chemical reactions within cells. Enzymes act as catalysts, speeding up the rate of biochemical reactions.

3. Transport: Many proteins aid in the transport and movement of substances across cell membranes and within the body. Examples include transporters and channels that facilitate the movement of ions and molecules across membranes.

4. Defense: Antibodies are specialized proteins that play a crucial role in the immune system by recognizing and neutralizing foreign substances, such as bacteria or viruses.

5. Hormonal regulation: Some proteins act as hormones that regulate various physiological processes in the body, including growth, metabolism, and reproduction.

Now, let's examine the options:

a: Act as a main fuel for cells - This option describes the function of carbohydrates, such as glucose, which is the primary source of energy for cells. While proteins can be broken down and used as an energy source, their main function is not to act as the main fuel for cells.

B: Control chemical reactions in a cell - This option correctly identifies one of the functions of proteins, specifically their role as enzymes to control and catalyze chemical reactions in a cell.

Based on the explanations above, option a: act as a main fuel for cells is NOT a function of proteins.