The Medical College - Page 71
on the other side of a semipermeable
membrane
(the inside of the RBC); the medium would
pos-
sess a smaller concentration of particles and
con-
sequently would lose water during osmosis.
In
our case, the red blood cells were hypertonic
in
relation to the medium and exerted a
larger
osmotic pull than the medium on the other side
of
the semipermeable membrane because it
pos-
sessed a greater concentration of particles
and
consequently accumulated water during
osmosis.
158.
D.
In this experiment, urea and glycerol
did
enter the red blood cell and water
consequently
followed because it was in greater quantity
out-
side and tried to establish an
equilibrium.
159.
D.
Cell membranes are described as
semiperme-
able because they allow some materials to
pass
while they block others. All materials that
pass
membranes must be either a liquid or dissolved
in
a liquid. Molecules also must be fairly simple,
but
molecular size is not the limiting factor
because
amino acids pass less readily than do
many
smaller molecules. Osmosis is the passage of
liq-
uids through a membrane; usually water
through
the semipermeable membrane. Diffusion is
the
passage of molecules from a more
concentrated
environment to a less concentrated region;
equal-
ization is usually the end result.
160.
D.
Active transport involves the diffusion
of
molecules against a gradient; this is an
energy
consuming phenomenon. The process of
mov-
ing substances from an area of lower
concentra-
tion to where they are in a higher
concentration
is selective, and requires respiration.
Osmosis
and diffusion are described in the
explanation
for question 159. Turgor pressure involves
the
passage of water into a cell at a faster rate
than
it can leave; the cell becomes plump and
filled
(turgid). At times the force results in
bursting
the structures. Plasmolysis is defined as
a
shrinking of protoplasm due to the loss of
water
from a cell.
161.
A.
Layers of material (probably
mucopolysac-
charide) secreted by the cell are found on the
sur-
face of the cell. The most prominent layer is
the
basement membrane, or basal lamina. These
struc-
tures are boundaries and must be traversed by
mate-
rial entering and leaving the cell. Cells must
be held
together; adjacent cell membranes interdigitate
and
intercellular cement is utilized. A desmosome
is a
specialized area of connection between
adjacent
cellular membranes (macula adherens). A
terminal
bar is a dense area surrounding the apical
cellular
surface. It includes the tight junction (zona
occlu-
dens) and the loose junction (zona adherens).
In car-
diac muscle, several cardiac muscle cells join
end to
end at a specialized junctional zone known as
an
intercalated disc.
162.
B.
Some of the proteins embedded in the
lipid
layers of membranes are shaped to form
channels
with "gates," which open only to certain
materials
or under certain conditions.
163.
A.
Osmosis is a process in which solvent
passes
from an area of lower solute concentration to
an
area of higher solute concentration. In
dialysis sol-
vent and solute both pass through the
membrane.
164.
A.
The defect cannot be dominant because
indi-
vidual 5 was produced by two normal parents
who
would have no defective alleles if the defect
is
dominant (either autosomal or X-linked).
The
defect is unlikely to be X-linked
recessive
because unless a new mutation occurred,
individ-
ual 5 should have received a normal allele for
the
gene on her X from her father (individual 2)
and
therefore could not express a recessive
X-linked
defect. Therefore the most likely
possibility
(using the assumptions of a single
controlling
gene and very rare mutations) would be an
auto-
somal gene for which the defective allele is
reces-
sive to the normal allele.
165.
C.
Unless a new mutation occurred,
individual
1 would have to have one normal allele
(which
produced the normal phenotype) and one
defec-
tive allele (which was contributed to
individual 5
along with a defective allele from individual
2) to
produce individual 5's defective
phenotype.
166.
B.
Individual 4 would have to contain two
defec-
tive alleles to produce the defective
phenotype.
167.
C.
Individual 7 would have to contain at
least
one normal allele to produce her normal
pheno-
type. Her other allele cannot be determined
with
certainty from the data given. Because her
parents
are probably both heterozygotes (see above),
she
has a
1/3 probability of having two normal
alleles
and a
2/3 probability of having one normal
and
one abnormal allele.
i.e., Nn
Nn
1/4 NN : 2/4
Nn : 1/4
nn
defective phenotype
2/3 of normal phenotype
1/3 of normal phenotype
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