Assertion: The ground substance of cartilage is made up of proteoglycan molecules Reason: These molecules with water content form a firm gel that gives cartilage its firm consistency
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Assertion-Reason Analysis
Answer: Both the Assertion (A) and the Reason (R) are TRUE, and R is the correct explanation of A.
Assertion - TRUE
The ground substance of cartilage is indeed made up of proteoglycan molecules. Specifically, the ground substance of hyaline cartilage contains three types of glycosaminoglycans (GAGs) - hyaluronan, chondroitin sulfate, and keratan sulfate - where the latter two are bound to core proteins to form proteoglycan monomers (most importantly, aggrecan). Multiple aggrecan monomers link to a single hyaluronan backbone via link proteins to form large proteoglycan aggregates.
"The ground substance of hyaline cartilage contains three types of GAGs: hyaluronan, chondroitin sulfate, and keratan sulfate. The last two bind to a core protein to form a proteoglycan monomer. Aggrecan is the most abundant proteoglycan monomer in hyaline cartilage." - Histology: A Text and Atlas
Reason - TRUE and correctly explains A
The reason accurately describes the mechanism behind cartilage's firm consistency. The proteoglycan aggregates (particularly aggrecan) carry a large negative charge due to their sulfate groups, which gives them a high affinity for water molecules. This water-binding creates a firm gel with significant turgor pressure:
"Their negative charges bind and hold large amounts of water molecules... The pressure, or turgor, that occurs in these giant hydrophilic proteoglycan aggregates accounts for the ability of cartilage to resist compression without inhibiting flexibility, making them excellent shock absorbers." - Histology: A Text and Atlas
The entrapment of these proteoglycan aggregates (swollen with water) within the collagen fibril meshwork is precisely what gives cartilage its characteristic firm-yet-resilient consistency.
Summary Table
| Statement | Verdict | |
|---|---|---|
| Assertion | Ground substance of cartilage = proteoglycan molecules | TRUE |
| Reason | Proteoglycans + water → firm gel → firm consistency | TRUE |
| Link | R correctly explains A | YES |
The key molecular player here is aggrecan, a large proteoglycan (~250 kDa) with ~100 chondroitin sulfate chains and ~60 keratan sulfate chains. Its highly negative charge attracts and retains water, generating the osmotic turgor that resists compressive forces. This is the structural basis for cartilage functioning as a shock absorber in joints.
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