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

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To determine which of the options is NOT a function of proteins, we need to understand the functions of proteins and then compare them to the options provided.

Proteins are large molecules made up of amino acids, and they serve various functions in cells. Some common functions of proteins include:

1. Acting as structural components: Proteins provide support and structure to cells and tissues. Examples include structural proteins like collagen in our skin and bones.

2. Enzymatic activity: Proteins called enzymes catalyze chemical reactions in cells. Enzymes help speed up biochemical reactions necessary for cell function.

3. Cell signaling: Proteins can act as messengers and transmit signals within cells or between cells. They are involved in communication processes, allowing cells to respond to changes in their environment.

4. Transport: Certain proteins, such as transporters and channels, facilitate the movement of molecules in and out of cells or across membranes.

5. Defense and protection: Some proteins, like antibodies, play a crucial role in the immune system by recognizing and neutralizing foreign substances in the body.

Now, let's compare the provided options:

a: Act as a main fuel for cells - This is a valid function of proteins. While carbohydrates and fats are the primary sources of energy for cells, in certain situations, proteins can be used as a source of energy through a process called gluconeogenesis.

B: Control chemical reactions in a cell - This is also a valid function of proteins. Enzymes, which are proteins, regulate and control the chemical reactions within cells by acting as catalysts.

Based on the analysis, both options (a and B) are functions of proteins. Therefore, none of the options provided is NOT a function of proteins.