The tiny projections in the small intestine adapted for absorption are called:

Correct Answer: B

Rationale: The small intestine maximizes nutrient absorption via villi finger-like projections lining its walls. Covered in microvilli, they vastly increase surface area, allowing efficient uptake of sugars, amino acids, and fats into the bloodstream. Venules are blood vessels, alveoli handle lung gas exchange, and nephrons filter kidney blood. Villi's specialized structure and location enhance digestion, distinguishing them as the key absorptive feature, critical for nutrient distribution in the body.

Correct Answer: C

Rationale: Muscle contraction follows a precise sequence. Calcium ions first enter the cytoplasm from the sarcoplasmic reticulum, triggered by a nerve impulse depolarizing the sarcolemma. This calcium binds troponin, shifting tropomyosin to expose actin's binding sites. Only then can myosin heads, energized by prior ATP hydrolysis, engage actin, pulling filaments past each other. Calcium's entry is the initial cytosolic event, preceding troponin's action, site exposure, and cross-bridge formation. This order, rooted in excitation-contraction coupling, ensures contraction starts with a neural signal, distinguishing it from subsequent mechanical steps.

Correct Answer: D

Rationale: Smooth muscle, found in organ walls, lacks striations due to unaligned actin and myosin, operates involuntarily under autonomic control, and has one nucleus per cell. Skeletal muscle is striated, voluntary, and multinucleate; cardiac muscle is striated, involuntary, and uninucleate with intercalated discs. Smooth muscle's non-striated, single-nucleus, involuntary nature suits its role in visceral functions like digestion, distinguishing it from skeletal and cardiac types in structure and regulation.

Correct Answer: C

Rationale: Actin's myosin-binding site is exposed when calcium ions bind troponin, shifting tropomyosin away via a conformational change. ATP energizes myosin post-binding, nerve impulses trigger calcium release, and acetylcholine initiates the impulse but only calcium directly uncovers the site. This regulatory step, within the sarcomere, distinguishes it from nerve or energy events, enabling cross-bridge cycling, fundamental to contraction mechanics.

Correct Answer: A

Rationale: Tendons, fibrous extensions of muscle, anchor it to bone, transmitting force for movement e.g., Achilles tendon. Fasciculi are fibre bundles, sarcomeres are contractile units, and intercostals are rib muscles, not attachment structures. Tendons' role in linking muscle to skeleton distinguishes them, essential for biomechanical function and contrasting with ligaments' bone-to-bone role.