Iron is an example of a nutrient that enters cells using simple diffusion. a membrane transport protein. a pump. endocytosis.

Iron is an essential nutrient for the human body, and its transport into cells does not occur through simple diffusion due to its charged nature and relatively large size. Instead, the most common method for iron uptake in cells is via a membrane transport protein. Specifically, iron is often transported into cells by transferrin receptors or other specialized transporters that facilitate its entry.

Here's a brief explanation of the options given:

1. Simple Diffusion: This process involves the movement of small, uncharged molecules across the cell membrane without any assistance. Iron cannot diffuse freely due to its ionic character.

2. A Membrane Transport Protein: This is the correct answer. Iron binds to transferrin in the bloodstream and is taken up by cells via specific receptors that recognize transferrin.

3. A Pump: Pumps are mechanisms that move ions or molecules against their concentration gradient using energy. This is not how iron enters cells.

4. Endocytosis: While cells can take substances in through endocytosis, iron specifically relies on binding to transport proteins for cellular uptake.

In conclusion, the most accurate choice regarding how iron enters cells is through a membrane transport protein.