CELL COMPARTMENTS

1. Every cell is surrounded by a plasma membrane.
2. Within each cell are numerous membrane-bound compartments, nonmembranous particles, and filaments, known collectively as cell organelles.
3. A cell is divided into two regions, the nucleus and the cytoplasm, the latter composed of the cytosol and cell organelles other than the nucleus.
4. The membranes that surround the cell and cell organelles regulate the movements of molecules and ions into and out of the cell and its compartments.
   1. Membranes consist of a bimolecular lipid layer, composed of phospholipids and cholesterol, in which proteins are embedded.
   2. Integral membrane proteins are amphipathic proteins that often span the membrane, whereas peripheral membrane proteins are confined to the surfaces of the membrane.

5. Three types of membrane junctions link adjacent cells.
   1. Desmosomes link cells that are subject to considerable stretching.
   2. Tight junctions, found primarily in epithelial cells, limit the passage of molecules through the extracellular space between the cells.
   3. Gap junctions form channels between the cytosols of adjacent cells.

1. The nucleus transmits and expresses genetic information.
   1. Threads of chromatin, composed of DNA and protein, condense to form chromosomes when a cell divides.
   2. The nucleolus is the site at which ribosomal subunits are formed.

2. Ribosomes, composed of RNA and protein, are the sites of protein synthesis.
3. The endoplasmic reticulum is a network of flattened sacs and tubules in the cytoplasm.
   1. Granular endoplasmic reticulum has attached ribosomes and is primarily involved in the packaging of proteins that are to be secreted by the cell or distributed to other organelles.
   2. Agranular endoplasmic reticulum is tubular, lacks ribosomes, and is the site of lipid synthesis and calcium accumulation and release.

4. The Golgi apparatus modifies and sorts the proteins that are synthesized on the rough endoplasmic reticulum and packages them into secretory vesicles.
5. Mitochondria are the major cell sites that consume oxygen and produce carbon dioxide in chemical processes that transfer energy to ATP that can then be used to perform energy-requiring cell functions.
6. Lysosomes digest particulate matter that enters the cell.
7. Peroxisomes break down certain toxic products formed from oxygen.
8. The cytoplasm contains a network of four types of filaments that form the cytoskeleton: (1) microfilaments, (2) intermediate filaments, (3) muscle thick filaments, and (4) microtubules.