About half the total area of membrane in a eukaryotic cell encloses the labyrinthine spaces of the endoplasmic reticulum (ER). The rough ER has many ribosomes bound to its cytosolic surface. Ribosomes are organelles that are not membrane-enclosed; they synthesize both soluble and integral membrane proteins, most of which are destined either for secretion to the cell exterior or for other organelles. We shall see that, whereas proteins are transported into other membrane-enclosed organelles only after their synthesis is complete, they are transported into the ER as they are synthesized. This explains why the ER membrane is unique in having ribosomes tethered to it. The ER also produces most of the lipid for the rest of the cell and functions as a store for Ca2+ ions. Regions of the ER that lack bound ribosomes are called smooth ER. The ER sends many of its proteins and lipids to the Golgi apparatus, which often consists of organized stacks of disc-like compartments called Golgi cisternae. The Golgi apparatus receives lipids and proteins from the ER and dispatches them to various destinations, usually covalently modifying them en route.
Mitochondria and chloroplasts generate most of the ATP that cells use to drive reactions requiring an input of free energy; chloroplasts are a specialized version of plastids (present in plants, algae, and some protozoa), which can also have other functions, such as the storage of food or pigment molecules. Lysosomes contain digestive enzymes that degrade defunct intracellular organelles, as well as macromolecules and particles taken in from outside the cell by endocytosis. On the way to lysosomes, endocytosed material must first pass through a series of organelles called endosomes. Finally, peroxisomes are small vesicular compartments that contain enzymes used in various oxidative reactions.
In general, each membrane-enclosed organelle performs the same set of basic functions in all cell types. But to serve the specialized functions of cells, these organelles vary in abundance and can have additional properties that differ from cell type to cell type.
On average, the membrane-enclosed compartments together occupy nearly half the volume of a cell (Table 12–1), and a large amount of intracellular membrane is required to make them. In liver and pancreatic cells, for example, the
Figure 12–1 The major intracellular compartments of an animal cell. The cytosol (gray), endoplasmic reticulum, Golgi apparatus, nucleus, mitochondrion, endosome, lysosome, and peroxisome are distinct compartments isolated from the rest of the cell by at least one selectively permeable membrane (see movie 9.2).
endoplasmic reticulum has a total membrane surface area that is, respectively, 25 times and 12 times that of the plasma membrane (Table 12–2). The membrane-enclosed organelles are packed tightly in the cytoplasm, and, in terms of area and mass, the plasma membrane is only a minor membrane in most eukaryotic cells (Figure 12–2).