The rough endoplasmic reticulum (RER) is primarily involved in the synthesis of proteins. This organelle is called ‘rough’ because it is studded with ribosomes, which are the cellular structures responsible for protein synthesis. When ribosomes are attached to the RER, they translate mRNA into polypeptide chains, which then fold into functional proteins. These proteins are often destined for secretion, incorporation into the cell membrane, or for use within lysosomes. Therefore, the macromolecule made in the rough endoplasmic reticulum is protein.
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