Final answer:
In skeletal muscle contraction, ATP does not cause actin filaments to shorten; rather, it is used in the detachment and re-cocking of myosin heads for the sliding of actin filaments past myosin during contractions.
Step-by-step explanation:
The correct answer to the question is 'e. ATP-dependent shortening of the actin filament,' as this is not a step involved in the contraction of skeletal muscle. Contractions occur due to the sliding filament theory, which includes the following steps:
- Cross-bridge formation between actin and the myosin heads when the myosin-binding sites on actin are exposed.
- Power stroke of myosin heads that slide actin filaments past myosin, pulling the actin towards the center of the sarcomere.
- Release of Ca2+ from the sarcoplasmic reticulum which binds to troponin, causing conformational changes that expose the actin-binding sites.
- Engagement of ATP, which binds to the myosin heads, causing them to detach from actin and allowing the muscle to relax or prepare for another contraction.
Actin filaments do not shorten; they slide past the myosin filaments, which is powered by the hydrolysis of ATP, not by shortening of the actin itself.