THE CYTOPLASM AND ITS ORGANELLES

The cytoplasm surrounds the nucleus, and it is in the cyto­plasm that the work of the cell takes place. Cytoplasm is essentially a colloidal solution that contains water, elec­trolytes, suspended proteins, neutral fats, and glycogen molecules. Although they do not contribute to the cell's function, pigments may also accumulate in the cytoplasm. Some pigments, such as melanin, which gives skin its color, are normal constituents of the cell. Bilirubin is a normal major pigment of bile; however, excess accumulation of bilirubin within cells is abnormal. This is evidenced clini­cally by a yellowish discoloration of the skin and sclera, a condition called jaundice.

Embedded in the cytoplasm are various organelles, which function as the organs of the cell. These organelles include the ribosomes, endoplasmic reticulum (ER), Golgi complex, mitochondria, lysosomes, microtubules, filaments, peroxisomes, and centrioles.

Ribosomes

The ribosomes serve as sites of protein synthesis in the cell. They are small particles of nucleoproteins (rRNA and proteins) that can be found attached to the wall of the ER or as free ribosomes (Fig. 4-3). Scattered in the cytoplasm individually or joined by strands of mRNA are the func­tional units called polyribosomes. Free ribosomes are in­volved in the synthesis of proteins, mainly enzymes that aid in the control of cell function.

Endoplasmic Reticulum

The ER is an extensive system of paired membranes and flat vesicles that connects various parts of the inner cell (see Fig. 4-3). Between the paired ER membranes is a fluid-filled space called the matrix. The matrix connects the space between the two membranes of the double-layered nuclear membrane, the cell membrane, and various cyto-plasmic organelles. It functions as a tubular communica­tion system transporting various substances from one part of the cell to another. A large surface area and multiple en­zyme systems attached to the ER membranes also provide the machinery for a major share of the metabolic func­tions of the cell.

Two forms of ER exist in cells: rough and smooth. Rough ER is studded with ribosomes attached to specific binding sites on the membrane. Ribosomes, with their ac­companying strand of mRNA, synthesize proteins. Pro­teins produced by the rough ER are usually destined for incorporation into cell membranes and lysosomal enzymes or for exportation from the cell. The rough ER segregates these proteins from other components of the cytoplasm and modifies their structure for a specific function. For ex­ample, the synthesis of both digestive enzymes by pancre­atic acinar cells and plasma proteins by liver cells takes place in the rough ER. All cells require a rough ER for the synthesis of lysosomal enzymes.

The smooth ER is free of ribosomes and is continuous with the rough ER. It does not participate in protein syn­thesis; instead, its enzymes are involved in the synthesis of lipid molecules, regulation of intracellular calcium, and metabolism and detoxification of certain hormones and drugs. It is the site of lipid, lipoprotein, and steroid hor­mone synthesis. The sarcoplasmic reticulum of skeletal and cardiac muscle cells is a form of smooth ER. Calcium ions needed for muscle contraction are stored and released from cisternae of the sarcoplasmic reticulum. Smooth ER of the liver is involved in glycogen storage and metabolism of lipid-soluble drugs.