The Cytomembrane System

A. Within the cytoplasm, newly formed polypeptide chains assembled on the ribosomes may enter the cytomembrane system:

1. The endoplasmic reticulum is a collection of interconnected tubes and flattened sacs, continuous with the nuclear membrane.

a. Rough ER consists of stacked, flattened sacs with many ribosomes attached; oligosaccharide groups are attached to polypeptides as they pass through on their way to other organelles or to secretory vesicles.

b. Smooth ER has no ribosomes; it is the area from which vesicles carrying proteins and lipids are budded; it also inactivates harmful chemicals.

2. In the Golgi bodies, proteins and lipids undergo final processing, sorting, and packaging in stacks of flattened sacs whose edges break away as vesicles.

B. A Variety of Vesicles

1. Lysosomes are vesicles that bud from Golgi bodies; they carry powerful enzymes that can digest the contents of other vesicles, worn-out cell parts, or bacteria and foreign particles.

2. Peroxisomes are membrane-bound sacs of enzymes that break down fatty acids and amino acids.

3. Caveolae are recently-discovered organelles that apparently take in proteins from the outside of the cell and slowly release them in the cytoplasm.

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