NDA (II) 2017 General Ability Test · Question 63 of 150

Correct — B, Collagen. A tendon is dense regular connective tissue, and almost all of its dry weight is collagen laid down in parallel bundles running along the line of pull. Collagen's molecule is a triple helix of three polypeptide chains wound round one another, and those helices pack into fibrils and then into fibres, which is why the tissue resists stretching so strongly along one direction. That is precisely what a tendon has to do — transmit the force of a contracting muscle to a bone without giving way. Collagen is the most abundant protein in the mammalian body and is also the main fibre in ligaments, skin, cartilage and the organic part of bone.

• (a)Fibrin — Fibrin is the insoluble mesh that forms during blood clotting, made when thrombin acts on the circulating plasma protein fibrinogen. It is a temporary net over a wound, not a permanent structural fibre.
• (c)Elastin — Elastin is a genuine connective-tissue protein, but its job is the opposite one — it lets a tissue stretch and spring back. It dominates in the walls of large arteries, the lungs and the skin, and in elastic ligaments, whereas a tendon needs to resist stretch, not permit it.
• (d)Cellulose — Cellulose is not a protein at all. It is a polysaccharide of glucose units and it forms the cell wall of plants; humans cannot even digest it, and it is nowhere in animal connective tissue.

Connective tissue is built from cells scattered in a matrix of fibres and ground substance, and the character of a tissue depends on which fibre dominates. Collagen fibres give tensile strength, elastic fibres give recoil, and reticular fibres form the fine supporting network of soft organs. Tendons join muscle to bone; ligaments join bone to bone; both are collagen-rich, but ligaments carry rather more elastic fibre because a joint must move.

The quickest route through this item is to eliminate rather than recall. Cellulose is a carbohydrate and belongs to plants, so it fails the word 'protein' in the stem. Fibrin belongs to clotting. That leaves two real connective-tissue proteins, and the one that resists a pull is collagen while the one that yields to a pull is elastin. Since a tendon is a tow-rope, collagen wins. Vitamin C is needed for collagen to be made properly, which is why scurvy shows up as bleeding gums, poor wound healing and joint pain.

• Tendons attach muscle to bone; ligaments attach bone to bone.
• Collagen is the most abundant protein in the mammalian body and gives connective tissue its tensile strength.
• A collagen molecule is a triple helix of three polypeptide chains.
• Elastin allows stretch and recoil and is abundant in artery walls, lung tissue and skin.
• Vitamin C is required for normal collagen synthesis, and its deficiency causes scurvy.

• Swapping tendon and ligament; tendon joins muscle to bone.
• Choosing elastin because it sounds like a fibre for movement — a tendon must not stretch.
• Treating cellulose as an animal protein; it is a plant carbohydrate.

NDA asks either for the protein of a named tissue or for which tissue joins which structures, so learn the fibre and the attachment together.

• practice — not a real PYQ

  Which tissue connects one bone to another at a joint?

  • (a)Ligament
  • (b)Tendon
  • (c)Areolar tissue
  • (d)Adipose tissue

  Answer(a) Ligament — tendons join muscle to bone, ligaments join bone to bone.

• practice — not a real PYQ

  Deficiency of which vitamin impairs collagen synthesis and causes scurvy?

  • (a)Vitamin C
  • (b)Vitamin A
  • (c)Vitamin D
  • (d)Vitamin K

  Answer(a) Vitamin C — it is required for collagen to be made properly, so its lack shows as bleeding gums, poor wound healing and joint pain.