**Core Concept**
Proteoglycans are a type of glycoprotein composed of a core protein covalently attached to one or more glycosaminoglycan (GAG) chains. They play a crucial role in maintaining the structure and function of connective tissue, including the extracellular matrix and the cell surface.

**Why the Correct Answer is Right**
Proteoglycans are highly negatively charged due to the presence of sulfate and glucuronic acid residues in their GAG chains. This negative charge enables proteoglycans to bind and sequester water, contributing to the compressive properties of cartilage and other connective tissues. The core protein of proteoglycans serves as a scaffold for GAG chain attachment and can also interact with other proteins and molecules to regulate cellular behavior.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because proteoglycans do not primarily function as structural proteins like collagen or elastin. While they do contribute to the structural integrity of connective tissue, their unique composition and properties set them apart from traditional structural proteins.
**Option B:** This option is incorrect because proteoglycans are not primarily involved in cell signaling. While they can interact with growth factors and other signaling molecules, their primary role is in maintaining tissue structure and function.
**Option C:** This option is incorrect because proteoglycans are not exclusively found in cartilage. While they are abundant in cartilage, they are also present in other connective tissues, including bone, ligaments, and tendons.

**Clinical Pearl / High-Yield Fact**
Proteoglycans are essential for maintaining the health and function of articular cartilage. Degradation of proteoglycans, such as in osteoarthritis, can lead to cartilage breakdown and joint degeneration.

**Correct Answer: D.**