empty. Peroxisomes are the sole site of plasmalogen synthesis. Plasmalogen is
a group of glycerol-based phospholipids in which the aliphatic side chains are
not attached by ester linkages. They have a widespread distribution with high-
est concentrations in the brain, spinal cord, liver, and kidney. Energy produc-
tion(answer a),exocytosis(answer c),detoxification(answer d),and the
synthesis of lysosomal enzymes (answer e)would not be affected in that dis-
ease. In Zellweger syndrome, peroxisomes are empty because of the failure of
the signal system that sorts protein to the peroxisome. Because the peroxi-
some lacks a genome (DNA) or synthetic machinery (ribosomes), it must
import all proteins. The defect appears to be in the peroxisomal membrane
(peroxins), but errors or absence of the peroxisomal signal sequence would
result in the same symptoms. Peroxisomes have only a single membrane
around them and contain catalase. Peroxisomes carry out oxidation reactions
to protect the cell. Those oxidation reactions remove hydrogen atoms from
molecules like alcohol and phenols to form hydrogen peroxide. In the perox-
idative reaction, catalase breaks down hydrogen peroxide to water and oxy-
gen. Most of the alcohol humans consume is broken down by the ADH
(alcohol dehydrogenase) and MEOS (microsomal ethanol-oxidizing system,
that is, P450 cytochrome) pathways in the hepatocyte (liver cell) cytoplasm;
the remaining small percentage is broken down by oxidation in the peroxi-
somes of hepatocytes using catalase. See “High-Yield Facts” page 30 for more
information on alcohol metabolism.
46.The answer is d. (Alberts, pp 928, 966–968, 976, 1041–1042.)
Cytochalasins are potent inhibitors of cell motility and other cellular events
that depend on actin assembly: cytokinesis, which is conducted by the
actin-containing contractile ring; phagocytosis; and formation of lamel-
lipodia. Cytochalasins bind to the plus end of actin filaments and prevent
further polymerization. The movement of chromosomes in anaphase of the
cell cycle depends on disassembly of microtubules at the kinetochore in
anaphase A and addition at the plus end of the polar microtubules in
anaphase B (answer a).Ciliary movement, vesicular transport, and the
structure of centrioles depend on microtubules (answers b, c, and e).
47.The answer is a.(Alberts, pp 758–761.)Chloroquine neutralizes acidic
compartments such as the secretory vesicles. Chloroquine treatment
inhibits the conversion of proinsulin to insulin, resulting in decreased for-
mation of insulin within secretory vesicles. Acidification causes concentration
120 Anatomy, Histology, and Cell Biology