Motion will occur at a lever system when

Correct Answer: D

Rationale: In a lever system, motion occurs when the effort (muscle contraction force) at the insertion exceeds the load (resistance), causing the movable bone to shift. For example, in a bicep curl, the biceps' effort at the radius (insertion) must overcome the weight to flex the elbow. If resistance at the insertion exceeds the load, it implies no motion, as the load wins. If the force is less than the load, no movement happens. Effort at the origin (stationary point) doesn't directly cause motion. Effort at the insertion exceeding the load is the correct biomechanical condition for lever movement, making it the right answer, reflecting how muscles drive skeletal levers.

Correct Answer: C

Rationale: Nodding yes involves extension (head tilts back) and flexion (head tilts forward) at the neck, driven by muscles like the sternocleidomastoid and splenius. Rotation and circumduction involve turning or circling, not nodding. Lateral/medial rotation twists the head, not tilting it. Abduction/adduction don't apply to neck nodding. Extension and flexion are the correct actions, as they describe the up-and-down motion, fundamental to neck kinematics in this gesture.

Correct Answer: C

Rationale: The human body contains approximately 640 muscles, a widely accepted estimate in anatomy reflecting skeletal muscles primarily, which vary slightly by individual due to small accessory muscles. This number excludes smooth and cardiac muscles' microscopic units but focuses on distinct, named skeletal muscles enabling movement. Fewer, like 340, undercounts the extensive network across limbs, torso, and face. Higher figures like 860 might exaggerate by including minor variants or non-skeletal types, but 640 aligns with standard texts. It accounts for paired muscles (e.g., biceps) and smaller ones (e.g., in the ear), balancing precision with practicality. This figure underscores the muscular system's complexity, supporting diverse functions from walking to facial expressions, and provides a reliable benchmark for anatomical study, distinguishing it from overly broad or narrow estimates.

Correct Answer: C

Rationale: Cellular respiration encompasses breaking down food (glucose) with oxygen to produce energy (ATP) in cells, mainly via aerobic means in mitochondria, releasing carbon dioxide and water. Anaerobic respiration skips oxygen, yielding less energy and lactic acid. Aerobic respiration is a subset, but cellular respiration covers all energy-making processes, fitting broadly. Oxygen is a reactant, not the process. This oxygen-dependent breakdown fuels most organisms, contrasting anaerobic's limited scope, and its cellular scope includes glycolysis and beyond, making it the precise term for energy formation across living systems.

Correct Answer: C

Rationale: In striated muscles (skeletal and cardiac), the sarcomere is the functional contractile unit, spanning from one Z line to the next. It contains actin and myosin filaments that slide during contraction, shortening the sarcomere to produce force, as per the sliding filament theory. The Z band anchors actin but isn't the unit itself. Cross bridges are myosin heads interacting with actin, a mechanism within the sarcomere, not the unit. A myofibril is a larger structure of many sarcomeres. The sarcomere's role as the basic repeating segment driving contraction makes it the best answer, central to understanding muscle mechanics and striation patterns.