the case of erythrocytes, blood group antigens. Therefore, the glycocalyx tains the enzyme glucose-6-phosphatase, which converts glucose-6-
plays a significant role in organ transplant compatibility. The glycocalyx phosphate to glucose, a step in gluconeogenesis.
found on apical microvilli of enterocytes, the cells forming the lining
epithelium of the intestine, consists of enzymes involved in the diges- Rough endoplasmic reticulum
tive process. Intestinal microvilli are cylindrical projections (1–2 µm The rough endoplasmic reticulum is a site of protein synthesis; its
long and about 0.1 µm in diameter) forming a closely packed layer cytosolic surface is studded with ribosomes (Fig. 1.5E). Ribosomes only
called the brush border that increases the absorptive function of bind to the endoplasmic reticulum when proteins targeted for secretion
enterocytes. begin to be synthesized. Most proteins pass through its membranes and
accumulate within its cisternae, although some integral membrane pro-
Cytoplasm teins, e.g. plasma membrane receptors, are inserted into the rough