Protein synthesis occurs on ribosomes in eukaryotic cells

Steps:

• Identify common structures visible in eukaryotic cell electron micrographs: nucleus, mitochondria, endoplasmic reticulum, ribosomes.
• Recall that protein synthesis involves translation of mRNA into polypeptides.
• Recognize ribosomes as the organelles where this translation happens, either free in cytoplasm or attached to rough ER.
• Match the structure labeled B to ribosomes based on its appearance (small granules or clusters).

Why B is correct:

• Ribosomes are the cellular machinery for protein synthesis, assembling amino acids into chains via translation of mRNA.

Why the others are wrong:

• A (nucleus): Stores genetic material but does not synthesize proteins; transcription occurs here.
• C (mitochondria): Site of cellular respiration and ATP production, not protein synthesis.
• D (smooth ER): Involved in lipid synthesis and detoxification, lacks ribosomes for protein production.

Final answer: B