Final answer:
The organelle that proteins can enter as they are being synthesized is the rough endoplasmic reticulum (RER). Ribosomes attached to the RER initiate protein synthesis, with the proteins entering the RER for possible modifications, packaging, and sorting to other organelles or secretion from the cell.
Step-by-step explanation:
The organelle that proteins can enter as they are being synthesized is known as the rough endoplasmic reticulum (RER). Proteins synthesized on the ribosomes attached to the RER enter the lumen where they may undergo modifications such as folding or addition of side chains. These proteins can either become part of the RER membrane, be incorporated into other cellular membranes, or be secreted from the cell. Additionally, proteins are synthesized in the cytoplasm by ribosomes that may be free or attached to the RER. Ribosomes give the RER its rough appearance and serve as the site where protein assembly begins.
Once in the RER, proteins can be packaged into transport vesicles and sent to the Golgi apparatus for further processing. This pathway is typical for secretory and membrane-bound proteins. Proteins synthesized by free ribosomes in the cytoplasm typically do not enter the RER and may function within the cytoplasm or be targeted to other organelles, such as mitochondria or peroxisomes, based on specific signal sequences they contain.