During muscle contraction, myosin heads attach to binding sites on which protein in the sarcomere?

Correct Answer: A

Rationale: In the sarcomere, myosin heads bind to actin filaments during contraction, as per the sliding filament theory. This interaction pulls actin toward the sarcomere center, shortening it. Troponin and tropomyosin regulate access to actin’s binding sites, while collagen is not involved, making 'Actin' the correct answer.

Correct Answer: C

Rationale: The pelvic girdle, part of the appendicular skeleton, supports the lower limbs (via hip joints) and protects pelvic organs (e.g., bladder). It does not protect the spinal cord (vertebral column’s role) or aid respiration, making 'Supporting the lower limbs and organs' the correct answer.

Correct Answer: B

Rationale: Troponin, a protein on actin filaments, binds calcium ions released from the sarcoplasmic reticulum. This binding shifts tropomyosin, exposing actin’s myosin-binding sites, enabling contraction. Tropomyosin prevents binding in a relaxed state, making 'Binding to calcium ions' the correct answer.

Correct Answer: C

Rationale: Myoglobin, a protein in muscle cells, binds and stores oxygen, releasing it for aerobic respiration during contraction when oxygen demand rises. ATP and creatine phosphate provide energy, while troponin regulates contraction, making 'Myoglobin' the correct answer.

Correct Answer: C

Rationale: The ribs, part of the thoracic cage in the axial skeleton, form a protective enclosure around vital organs. The 12 pairs of ribs, along with the sternum and thoracic vertebrae, shield the heart and lungs directly. The liver, located below the diaphragm, is partially protected by the lower ribs. Vertebrae protect the spinal cord, the skull protects the brain, and the femur is a leg bone, making 'ribs' the correct answer.