ropelike arrangement is well suited to providing mechanical stability to the
cell and resisting stretch, allowing the cell to respond to tension. The differ-
ent types of intermediate filaments all have a similar structural pattern: non-
helical head and tail segments with a helical arrangement in the center of the
intermediate filament structure. Movement is generated by motor proteins
(answer a)such as myosin, dynein, and kinesin. There is a good mnemonic
device for remembering the direction of movement directed by kinesin and
dynein. Kinesin kicks the molecules out; dynein drags them in; also, the plus
end of the microtubule is oriented toward the plasma membrane, so minus
end toward nucleus. This works for fibroblasts as well as neurons, to be dis-
cussed in a later chapter. Nucleation of microtubules is conducted by cen-
trosomes (answer c);microtubule-associated proteins (MAPs) stabilize or
destabilize microtubules (answer d).Microtubules function in organellar
transport—for example, axonal transport (answer e).
42.The answer is a.(Alberts, pp 691–693. Ross and Pawlina, pp 47–48.)
Oxidative metabolism by cytochrome p450 enzymes in hepatocytes is a
primary mechanism for drug metabolism. Barbiturates are modified in the
liver by oxidative demethylation through the P450 oxidase system found in
the smooth endoplasmic reticulum (SER) (the structure shown in the elec-
tron micrograph). The SER in hepatocytes responds to Phenobarbital
ingestion by increasing its volume. The proliferation (hypertrophy) of the
SER facilitates metabolism of drugs. There is a concomitant increase in
enzymatic activity, however, the synthesis of those enzymes occurs in the
rough endoplasmic reticulum (RER) not in the SER (answer d).The pur-
pose of drug metabolism is to make drugs more water soluble (answer c)
so they can be more easily excreted from the liver through the bile. Increase
in enzymatic activity following Phenobarbital ingestion catalyzes reactions
that increase the solubility of various xenobiotics including toxins, alcohol,
steroids, eicosanoids, carcinogens, insecticides, and other environmental
pollutants. Lysosomes not the SER contain acid hydrolases (answer e).The
P450 system and the SER are involved in drug interactions. Hepatocytes
adapted to metabolize one drug may develop increased capability to
metabolize other drugs. For example, if patients taking Phenobarbital for
epilepsy increase their alcohol intake they may be ingesting subtherapeutic
levels of the antiseizure medication because of induction of smooth ER in
response to the alcohol.
118 Anatomy, Histology, and Cell Biology