## **Core Concept**
Carriers play a significant role in the transport of substances across cell membranes, particularly for molecules that are lipid-insoluble or require facilitated diffusion. They are crucial in various physiological processes, including the absorption of nutrients and the elimination of waste products.

## **Why the Correct Answer is Right**
The correct answer involves understanding the role of carriers in different biological processes. Carriers are essential for the transport of glucose, amino acids, and ions across cell membranes. However, in the case of lipid-soluble substances, such as oxygen and carbon dioxide, they can diffuse directly through the cell membrane without the need for carriers.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Carriers are crucial for glucose transport into cells, especially in the case of facilitated diffusion by glucose transporters (GLUT).
- **Option B:** Amino acids often require carrier-mediated transport to enter cells, as they are polar and cannot diffuse through the lipid bilayer.
- **Option D:** Ion transport across cell membranes frequently involves carrier proteins or channels to regulate the balance of ions.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that substances like glucose and amino acids rely on carrier proteins for their transport across cell membranes, which is vital for cellular nutrition and function. In contrast, lipid-soluble substances can diffuse directly through the membrane.

## **Correct Answer:** .