Which one of the following statements about proteins in mammalian cells is correct?
Proteins in mammalian cells are synthesized in cell membrane.
Proteins in mammalian cells are not synthesized in cell membrane but directly absorbed from food.
Proteins in mammalian cells are synthesized in rough endoplasmic reticulum.
Proteins in mammalian cells are synthesized in Golgi apparatus.
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UPSC CAPF – 2018
Protein synthesis, or translation, occurs on ribosomes. Ribosomes can be found freely in the cytoplasm or attached to the surface of the endoplasmic reticulum, forming the rough endoplasmic reticulum (RER). Proteins destined for secretion out of the cell, insertion into cell membranes (including the plasma membrane and membranes of organelles like RER, Golgi, lysosomes), or delivery to organelles within the secretory pathway (like lysosomes) are synthesized on ribosomes attached to the RER. Proteins destined for the cytoplasm, nucleus, mitochondria, or peroxisomes are typically synthesized on free ribosomes in the cytoplasm. Therefore, the rough endoplasmic reticulum is a major site of protein synthesis in mammalian cells, particularly for proteins entering the secretory pathway.