**Core Concept**
Proteoglycans are a type of glycoprotein that consists of a core protein covalently attached to one or more glycosaminoglycan (GAG) chains. They play a crucial role in various physiological processes, including cell signaling, cell adhesion, and the maintenance of the extracellular matrix structure.

**Why the Correct Answer is Right**
Proteoglycans are indeed involved in the regulation of cell growth, differentiation, and migration through interactions with growth factors, cytokines, and other signaling molecules. They also contribute to the maintenance of the extracellular matrix structure by forming a network with collagen and elastin fibers. Additionally, proteoglycans are essential components of cartilage, where they help maintain the structural integrity of the tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because proteoglycans are not primarily involved in the transport of molecules across cell membranes. While they can interact with signaling molecules, they do not function as transport proteins.
**Option B:** This option is also incorrect because proteoglycans do not play a significant role in the regulation of ion channels. Their primary functions are related to cell signaling, cell adhesion, and the maintenance of the extracellular matrix structure.
**Option C:** This option is incorrect because proteoglycans are not directly involved in the breakdown of extracellular matrix components. While they can interact with enzymes involved in matrix degradation, their primary functions are related to the maintenance of the matrix structure.

**Clinical Pearl / High-Yield Fact**
Proteoglycans are essential components of the extracellular matrix, and their dysregulation has been implicated in various diseases, including osteoarthritis, fibrosis, and cancer.

**Correct Answer:**
D