Day 107 MCAT Practice Question

Proteins have a variety of non-enzymatic functions in the body,

including structural, immunological, transport, and other functions. For this reason,

Answer D is correct because all three statements are true regarding the nonenzymatic functions of proteins in the human body.

As shown in the image below, there are a variety of ways that molecules, transport

vesicles, and other cargo can be transported within and across cells. Some of these

mechanisms include microtubules and the kinesin-dynein system, which allows for

either anterograde or retrograde transport, or the actin-myosin binding system that

is responsible for muscle contraction. These systems rely on various proteins, such

as the tubulins that make up microtubules, the kinesin and dynein proteins, and the

myosin and actin proteins. For this reason, Statement I is correct.

Below is an image of an antibody, with its Fc and Fab portions. Antibodies are

protein molecules that function to recognize certain antigens, or molecular

markers, on either foreign- or self-molecules. Antibodies are highly specific and

are produced when the immune system is exposed to particular antigens, such

as a food, an antigenic protein delivered via vaccine, or environmental exposures

to things like mold, dust, or animals. Because proteins play a wide variety of

immunological roles in the body, Statement II is correct.

Finally, below is a schematic of an aquaporin embedded within a lipid membrane

bilayer. Aquaporins are protein structures that function as water channels, allowing

water to pass in or out through its hydrophilic center as a means of bypassing the

hydrophobic lipid bilayer, which water cannot normally permeate. By controlling the

passage of water, aquaporins subsequently regulate osmotic pressure by controlling

the concentration of solutes across membranes. Aquaporins are classified as

“integral membrane proteins,” and for this reason, Statement III is correct.