The Endoplasmic Reticulum

Ribosomes, "free" or attached to membranes, participate in assembly of polypeptide chains. Ribosomes are made of two subunits, composed of RNA and protein, that work as one during protein synthesis.

The endoplasmic reticulum (ER) modifies newly formed polypeptide chains and is also involved with lipid synthesis. The endoplasmic reticulum is a collection of interconnected tubes and flattened sacs that begin at the nucleus and ramble through the cytoplasm. There are two types distinguished by the presence or absence of ribosomes:

  1. Rough ER consists of stacked, flattened sacs with many ribosomes attached.
  2. Smooth ER has no ribosomes. It is an area from which vesicles carrying proteins and lipids are budded. Steroid hormones are synthesized in the smooth endoplasmic reticulum. It also inactivates harmful chemicals.

Some of the polypeptide chains assembled on the ribosomes are stockpiled in the cytoplasm. Others pass through the cytomembrane system, where they take on their final form and become packaged in vesicles.

REVIEW: Organelles composed of a system of canals, tubes, and sacs that transport molecules inside the cytoplasm are

REVIEW: These are the primary cellular sites for the production of proteins.

REVIEW: Which organelle is the site of most of the cell's protein synthesis?

REVIEW: In which organelle are steroid hormones synthesized?

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