What process do seaweeds use to maintain a higher concentration of iodine in their cells than in the surrounding ocean water?

Correct Answer: D

Rationale: Seaweeds concentrate iodine far above ocean levels, requiring energy to move it against its concentration gradient from low outside to high inside. Active transport, using ATP-powered pumps, achieves this, unlike diffusion or passive transport, which move substances down gradients without energy. Osmosis involves water, not solutes like iodine. Active transport's energy dependency enables seaweeds to accumulate iodine for metabolic needs, such as thyroid hormone precursors, showcasing a key adaptation in marine organisms for nutrient uptake.

Correct Answer: B

Rationale: Muscle structure descends from large to small: myofibrils (organelles within fibers) contain sarcomeres (contractile units), which comprise thin (actin) and thick (myosin) filaments. Muscle belly, the whole muscle, is larger, so starting there misorders scale. Sarcomeres, between Z-lines, house filaments, not vice versa filaments build sarcomeres, sarcomeres build myofibrils. Starting with filaments then sarcomere reverses this hierarchy, and myofibril as largest ignores its cellular role. The correct sequence reflects anatomical organization, distinguishing myofibril as a bundle of sarcomeres, each with filaments, key to understanding contraction's structural basis.

Correct Answer: D

Rationale: Myosin heads stay bound to actin post-power stroke until another ATP binds, detaching myosin by altering its conformation, allowing the cycle to repeat or relax. More Ca²⁺ affects troponin, not detachment Ca²⁺ sustains contraction. Troponin doesn't bind actin directly for release. More acetylcholine restarts excitation, not detachment. ATP's binding to myosin triggers release, distinguishing it from Ca²⁺ or neurotransmitter roles, key to cross-bridge cycling.

Correct Answer: B

Rationale: Muscle cells hierarchically organize contractile units. Myofibrils, spanning the cell's length, are bundles of sarcomeres repeating units with thick (myosin) and thin (actin) myofilaments. Sarcomeres, about 2 micrometres long, link end-to-end within myofibrils, which extend tens to hundreds of micrometres. Myofilaments are shorter segments within sarcomeres, and troponin is a tiny protein on actin. Myofibrils' full-cell length distinguishes them as the longest, critical for coordinated contraction across the fibre, unlike shorter sarcomeres or molecular-scale components.

Correct Answer: C

Rationale: Ligaments, fibrous connective tissues, link bones across joints, stabilizing skeletal structure e.g., knee ligaments. Tendons and aponeuroses attach muscles to bones, facilitating movement, while sarcomeres are muscle contractile units. Ligaments' bone-to-bone connection distinguishes them from muscle-related structures, crucial for joint integrity and distinguishing skeletal support from contractile mechanisms in musculoskeletal anatomy.