Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Bones and exoskeletons provide anchorage for muscles and act as levers to facilitate movement. In humans, the elbow joint acts as a lever where the fulcrum is the joint, the effort is provided by the muscles, and the load is the forearm.
Movement at a joint requires antagonistic muscle pairs. For example, in the human arm, the biceps acts as a flexor and the triceps acts as an extensor. In an insect leg (e.g., grasshopper), the extensor muscle relaxes and the flexor muscle contracts to pull the tibia toward the femur.
Synovial joints allow specific movements. Key structures include: Cartilage (reduces friction and absorbs shock), Synovial fluid (lubricates the joint), Ligaments (connect bone to bone), and Tendons (connect muscle to bone).
Skeletal muscle fibers are multinucleate and contain specialized endoplasmic reticulum known as the sarcoplasmic reticulum, which stores ions.
Myofibrils are made up of repeating units called sarcomeres. A sarcomere is the functional unit of contraction, delimited by -lines, containing thin actin filaments and thick myosin filaments.
The sliding filament theory explains contraction: Myosin heads bind to actin to form cross-bridges. hydrolysis provides the energy for the power stroke, pulling the actin filaments toward the center of the sarcomere (the -line).
Calcium ions () play a regulatory role: they bind to troponin, causing a conformational change in tropomyosin, which uncovers the binding sites on actin for myosin heads.
The H-zone and I-band shorten during contraction, but the A-band (length of myosin) remains constant.
📐Formulae
💡Examples
Problem 1:
During a muscle contraction, a sarcomere is observed to shorten from to . If the width of the -band is , what is the total width of the -bands in the contracted state?
Solution:
In the contracted state, the -band width is .
Explanation:
The total length of the sarcomere is the sum of the -band and the -bands. Since the -band length (myosin filament length) remains constant at during contraction, the -band width is calculated as: .
Problem 2:
Explain the role of in the detachment of myosin heads.
Solution:
The binding of a new molecule of to the myosin head causes it to detach from the actin binding site.
Explanation:
In the sliding filament model, once the power stroke is complete, the myosin head remains bound to actin (the rigor state) until a new molecule of binds to the myosin head. This binding reduces the affinity of myosin for actin, allowing the cycle to repeat.
Problem 3:
Label the state of the muscles in a grasshopper leg during jumping.
Solution:
Extensor muscle: Contracts; Flexor muscle: Relaxes.
Explanation:
To jump, the grasshopper needs to extend its hind leg rapidly. This is achieved by the antagonistic pair where the extensor muscle (the larger muscle) contracts powerfully, while the flexor muscle relaxes to allow the joint to straighten.