In a typical eukaryotic cell, which cellular structures are the primary sites where protein synthesis takes place?

Introduction / Context:
Protein synthesis is one of the most important functions of cells, because proteins act as enzymes, structural components, and signalling molecules. Understanding where in the cell proteins are actually assembled is a basic concept in cell biology. This question asks you to identify the primary structures responsible for linking amino acids together to form polypeptide chains.

Given Data / Assumptions:

• The question is about the primary sites of protein synthesis in eukaryotic cells.
• The options include ribosomes, mitochondria, Golgi apparatus, and nucleus.
• We assume standard textbook descriptions of these organelles.
• We focus on where amino acids are joined into chains, not on where DNA is stored or where proteins are processed later.

Concept / Approach:
Ribosomes are the cellular machines that read messenger RNA (mRNA) and assemble amino acids into polypeptide chains according to the genetic code. In eukaryotic cells, ribosomes may be free in the cytoplasm or attached to the rough endoplasmic reticulum. Mitochondria have their own ribosomes and can synthesise some of their own proteins, but the primary general sites of protein synthesis for the cell are ribosomes. The Golgi apparatus modifies, sorts, and packages proteins after they are synthesised. The nucleus stores DNA and is where transcription occurs, but the ribosomes perform translation, which is the actual assembly of proteins.

Step-by-Step Solution:
Step 1: Recall that translation is the process by which mRNA is decoded to build proteins from amino acids. Step 2: Remember that translation is carried out by ribosomes, which bind mRNA and transfer RNA to link amino acids together. Step 3: Note that ribosomes can be found floating freely in the cytoplasm or attached to the rough endoplasmic reticulum, forming the rough appearance. Step 4: Mitochondria and chloroplasts do have their own ribosomes and make some proteins, but when we speak of primary sites of protein synthesis in general, we are referring to ribosomes themselves. Step 5: The Golgi apparatus and nucleus have important roles in processing and genetic control, but they are not the main structures where amino acid chains are assembled.

Verification / Alternative check:
Cell biology diagrams show ribosomes as small dots on the rough endoplasmic reticulum or in the cytoplasm, directly associated with polypeptide formation. The rough endoplasmic reticulum is defined by the presence of ribosomes on its surface. Textbooks describe the Golgi apparatus as a packaging and processing centre, not as the place where polypeptides are first synthesised. The nucleus is labelled as the site of DNA storage and transcription, while protein synthesis is specifically labelled at ribosomes. This agrees with selecting ribosomes as the primary site of protein synthesis.

Why Other Options Are Wrong:
Mitochondria mainly carry out aerobic respiration to produce ATP and, although they do contain some ribosomes, they are not the general primary site of protein synthesis for the cell as a whole. Golgi apparatus modifies, sorts, and packages proteins after synthesis, but does not itself assemble amino acids into chains, so option C is wrong. The nucleus stores genetic material and is the site of transcription, not of translation, so it does not directly assemble polypeptides, making option D incorrect.

Common Pitfalls:
Some students confuse the roles of different organelles and may assume that the nucleus, as the control centre of the cell, must also synthesise proteins. Others may think of the Golgi apparatus because it is involved in handling proteins after synthesis. To avoid these mistakes, remember the division of labour: DNA stays in the nucleus, mRNA leaves the nucleus, and ribosomes in the cytoplasm or on the rough endoplasmic reticulum actually build the proteins. Linking protein synthesis directly to ribosomes will help you pick the correct answer quickly.

Final Answer:
The correct choice is Ribosomes, either free in the cytoplasm or attached to endoplasmic reticulum, because ribosomes are the primary cellular structures where protein synthesis takes place.