High School Biology : Understanding Sliding Filament Theory
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Example Questions
Example Question #1 : Understanding Sliding Filament Theory
In sarcomeres, the thick filament is composed of which protein?
Titin
Actin
Myosin
Collagen
Myosin
Sarcomeres are composed of thick and thin filaments. The thin filament is composed of polymerized actin, while the thick filament is composed of myosin. Titin is a protein that spans the full range of the sarcomere, and is involved in stability and elasticity in the muscle. Collagen is not a primary component of sarcomeres.
Example Question #1 : Understanding Sliding Filament Theory
Which statement is incorrect in describing sliding filament theory?
Actin and myosin filaments stay the same size during contraction
The protein complex formed is classically named actomyosin and helps facilitate the "stroke" part of muscle contraction
The actin filaments lengthen, while the myosin filaments shorten
Actin and myosin form a "crossbridge" when myosin binds to actin
The actin and myosin filaments slide past one another
The actin filaments lengthen, while the myosin filaments shorten
The sliding filament theory describes the mechanism that allows muscles to contract. According to this theory, myosin (a motor protein) binds to actin. The myosin then alters its configuration, resulting in a "stroke" that pulls on the actin filament and causes it to slide across the myosin filament. The overall process shortens the sarcomere structure, but does not change the actual length of either filament.
Example Question #3 : Understanding Sliding Filament Theory
In order for muscle contraction to occur, what molecules/ions must be readily available?
Glycogen
ADP + Pi
NADPH and GADPH
Calcium ions and ATP
GTP and chloride ions
Calcium ions and ATP
The correct answer is ATP and calcium ions. Myosin head activation to form a cross-bridge with actin requires ATP, and the cleavage of ATP to ADP + Pi contracts the myosin head and pulls the actin. Calcium is required to expose actin binding sites for myosin in conjunction with troponin.
Example Question #4 : Understanding Sliding Filament Theory
Muscles require a supply of ATP in order to contract. What function is enabled by the release of energy from ATP?
Myosin detaching from actin
Myosin attaching to actin
Shortening of myosin
Myosin attaching to the Z-disc
Myosin bending to pull actin
Myosin detaching from actin
In the sliding filament theory, myosin heads attach to an actin filament, bend to pull the actin filaments closer together, then release, reattach, and pull again. Energy from ATP is required for the myosin head to release from the actin filament—otherwise the myosin heads would remain in the same place, and the muscle would not contract. Even though the filaments are moving, the filaments themselves never actually get shorter or longer.
When ATP stores are depleted, myosin becomes incapable of detaching from actin, and the muscle remains in a taut, flexed state. This is the cause of rigor mortis.
Example Question #5 : Understanding Sliding Filament Theory
When is ATP required for muscles according to the sliding filament theory?
To perform the power stroke, where the myosin heads rotate away from the sarcomere
For the myosin heads to bind the actin
For crossbridge formation
For the crossbridges to detach from the actin and eventually reorient the myosin heads.
To perform the power stroke, where the myosin heads rotate toward the sarcomere
For the crossbridges to detach from the actin and eventually reorient the myosin heads.
The myosin head will hydrolyze the 
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