Items 222-226

Examine the high power transmission electron micrograph of cell surface projections in Tissue-culture microscope 3.3 below. These projections are cut perpendicular to their long axis. For each numbered item of a functional role choose the most appropriate component labeled with a letter in the electron micrograph.

222. These structures are actin-rich microfilemts. They serve to stiffen the surface projections.

223. This structure is a phospholipoprotein bilayer serving as a selective permeability barrier surrounding the projections as well as the entire cell.

224. The projections shown here were are called

(A) cilia
(B) microvilli
(C) dendrited
(D) spindle fiber
(E) flagella

226. These surface projections have which primary physiological function?

(A) movement of luminal contents
(B) transduction of mechanical into electrical energy
(C) transduction of chemical into electrical energy
(D) absorption of luminal contests
(E) gas exchange

ANSWERS AND TUTORIAL ON ITEMS 222.-226

The answers are: 222-C;223-A;224-F;225-B;226-D. Tissue-culture microscope 3.3 is a high power transmission electron micrograph of sections cut perpendicular to the long axis of the microvilli (B) in the jejunum of the small intestine. Microvilli are apical surface projections designed to increase the cell surface are. They are particularly well developed in areas where luminal contents are being absorbed, e.g., in the small and large intestine and proximal and distal convoluted tubules in the kidneys. Like all other cell surface projections, microvilli are covered by a phospholipoprotein bilayer of plasma membrane (A). The cores of microvilli are filled with many microfilaments (C) with a diameter of 6nm.

These microfilaments are rich in actin and are involved in stabilizing the microvilli. The plasma membrane of intestinal microvilli has a thick glycocalys (F) which contains the glycoprotein rich extracellular domains of integral membrane proteins. This fuzzy coat on the outer leaflet of the plasma membrane consists of minute filaments, 2.5-5 nm in diameter. They often project 0.1 to 0.5 ?m beyond the apical tips of microvilli. The glycocalyx prevents digestive enzymes in the lumen of the small intestine from gaining access to epithelial cells. In addition, the glycocalyx contains digestive enzymes, such as disaccharidases, that complete the final steps in nutrient digestion. Professional High Power Microscopes