Items 259-264

259. The projections indicated as letter B are

(A) cilia
(B) mcrovilli
(C) flagella
(D) basal bodies
(E) focal adhesion points

260. The structures indicated at the letter A is

(A) the basement membrane
(B) an endocytic vesicle
(C) a microtubules cut in longitudinal section
(D) the plasma membrane
(E) part of the Golgi apparatus

261. Which protein is most likely to be found in the linear structures labelled C?

(A) histone H-1
(B) collagen
(C) actin
(D) desmin
(E) clathrin

262. Materials at letter E is part of the

(A) Extracellular matrix
(B) terminal web
(C) smooth endoplasmic reticulum
(D) phagolysosome
(E) basal lamina

263. At the letters D, which process is most likely to occur?

(A) fatty acid synthesis
(B) steroid secretion
(C) photosynthesis
(D) receptor-mediated endocytosis
(E) nucleation of microtubules assembly

264. The spacing between adjacent structures labelled B is closest to which of the following values?

(A) 1.0 ?m
(B) 0.1 ?m
(C) 10 nm
(D) 10 ?m
(E) 5 ?m

ANSWERS AND TUTORIAL ON ITEMS 259-264

The answers are: 259-B; 260-D; 261-C; 262-B; 263-D; 264-B. Tissue-culture microscope 3.8 is a transmission electron micrograph of the brush border of intestinal epithelial absorptive cells. The brush border consists of numerous closely packed cellular projections called microvilli (B). Microvilli are about 0.1 ?m in diameter and 0.5 to 1.0 ?m long; in the brush border they are tightly packed together so that spacing between adjacent microvilli is about the same as the diameter of each individual mcrovillus is surrounded by a plasma membrane (A) that is part of the plasma membrane of the epithelial cell. Each microvillus contains a core bundle of microfilaments (C) made of the protein actin. These microfilaments project into the terminal web (E), a structure that lends support to the microvilli. The microvilli serve to provide an extended surface of plasma membrane, providing the space for the transport channels and enzymes needed so these absorptive cells can carry out their principal function. These epithelia are also involved in the active uptake of small macromolecules (peptides, fragments of nucleic acids, etc.) via receptor mediated endocytosis. This involves the formation of numerous endosomes, a process that occurs as the bases of the clefts adjacent microvilli (D). Boom Mounted Microscopes