1. Endoplasmic Reticulum

Within the cytoplasm of cells is a 3-dimensional maze of connecting and branching channels made by a continuous membrane. This is called the endoplasmic reticulum (ER). ER can be classified as rough ER when ribosomes are attached to the cytosolic side of the membrane or smooth ER when no ribosomes are present. Rough ER is prominent in cells that are making proteins for export such as digestive enzymes, hormones, structural proteins or antibodies. The main function of rough ER is the segregation of proteins destined for export from the cell or for intracellular use. Proteins are modified within the ER by the addition of carbohydrate, removal of a signal sequence and other post-translational modifications. Phospholipid synthesis and assembly of multichain proteins also occur there.

Smooth ER lacks attached ribosomes and often appears more tubular than rough ER. The two types of ER are continuous in some sections. Smooth ER confers on the cell the ability to perform a variety of specialized functions. It is necessary for steroid sysnthesis, metabolism and detoxification of substances in the liver, phospholipid synthesis, and excitation-contraction coupling in skeletal muscle.

2. Golgi complex

The Golgi, a curved membrane stack resembling a stack of pancakes, finishes the post-transitional modifications, concentrates and packages proteins for export or storage. It also adds directions for the destination of the protein package. Proteins made within the rough ER bud off in vesicles and are trasported to the Golgi where the vesicles fuse with the membrane and the components are further modified, concentrated and packaged by the time they bud off as vesicles on the opposite side of the the Golgi. Therefore, the Golgi shows a polarity in that one side accepts incoming vesicles (convex or cis face) and the final product vesicles bud off the opposite side (concave or trans face). In fact, biochemical studies have shown that the enzymes present within the Golgi are different at different levels of the membrane stack.

3. Lysosomes

Lysosomes are membrane bound vesicles (0.05 to 0.5 micron) containing more than 40 hydrolytic enzymes that can digest most biological macromolecules. These organelles are the sites of intracellular digestion that are more numerous in cells performing phagocytosis. The limiting membrane keeps the digestive enzymes separate from the cytoplasm. The most common lysosomal enzymes are acid phosphatase, ribonuclease, deoxyribonuclease, proteases, sulfatases, and lipases. The enzymes function optimally at pH 5 and are mostly inactive at the pH of the cytosol (7.2). This taken with the limiting membrane protects the cell from digesting itself. Lysosomal enzymes are synthesized on the rough ER, trasferred to the Golgi for modification and packaging. The cellular machinery attaches a directional signal to the enzymes (mannose-6-phosphate) that allows the ER and Golgi to sort these proteins and, via a receptor mediated process, segregate them to forming lysosomes.

Primary lysosomes are small concentrated sacs of enzymes that are not digesting anything. Primary lysosomes fuse with a phagocytic vacuole to become secondary lysosomes or phagolysosomes where digestion begins. As the substances are digested the nutrients diffuse through the lysosomal membrane to the cytosol. Residual bodies are formed when indigestable things remain in the vacuoles. In cells with a long life span such as cardiac muscle cells, residual bodies are more numerous and are referred to lipofuscin or age pigment.

Lysosomes also participate in the turnover of cellular organelles. Cytoplasmic components become enclosed in a membrane that fuses with a primary lysosome to become an autophagosome. In bone, the lysosomal enzymes are released from osteoclasts to digest surrounding bone during the process of remodeling. Lysosomal enzymes are also involved in the process of inflammation.

4. Peroxisomes

These small (0.5 to 1.2 microns) containing oxidative enzymes. Peroxisomes contain amino oxidases, hydroxyacid oxidase and catalase. Catalase protects the cell from hydrogen peroxide damage. Enzymes involved in lipid metabolism are also found in peroxisomes. Peroxisomal enzymes are synthesized on the free cytosolic ribosomes with a signal sequence that directs them to peroxisomes. As enzymes are added the peroxisome grows and then splits into two smaller peroxisomes.