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Multiple Choice
Which organelle completely surrounds each myofibril inside a muscle fiber?
A
Golgi apparatus
B
Mitochondrion
C
Sarcoplasmic reticulum
D
Nucleus
Verified step by step guidance
1
Step 1: Begin by understanding the structure of a muscle fiber. Muscle fibers are composed of myofibrils, which are the contractile units responsible for muscle contraction.
Step 2: Learn about the sarcoplasmic reticulum (SR). The SR is a specialized type of smooth endoplasmic reticulum found in muscle cells. Its primary function is to store and regulate calcium ions (Ca²⁺), which are essential for muscle contraction.
Step 3: Recognize the spatial relationship between the sarcoplasmic reticulum and myofibrils. The SR completely surrounds each myofibril, forming a network of tubules and cisternae that ensures efficient calcium ion delivery during contraction.
Step 4: Compare the sarcoplasmic reticulum to other organelles listed in the problem. The Golgi apparatus is involved in protein modification and transport, mitochondria generate ATP for energy, and the nucleus contains genetic material. None of these organelles are specialized to surround myofibrils.
Step 5: Conclude that the sarcoplasmic reticulum is the correct answer because it is uniquely adapted to encase myofibrils and regulate calcium ion dynamics critical for muscle function.