more hydrogen ions are secreted into the tubular lumen than bicarbonate ions are filtered into the glomerular filtrate.. more bicarbonate ions are filtered into the glomerular filtrate t
Trang 1BIOLOGY TOPICAL:
Digestive and Excretory Systems
Test 1
Time: 21 Minutes*
Number of Questions: 16
* The timing restrictions for the science topical tests are optional
If you are using this test for the sole purpose of content
reinforcement, you may want to disregard the time limit
Trang 2DIRECTIONS: Most of the questions in the following
test are organized into groups, with a descriptive passage preceding each group of questions Study the passage, then select the single best answer to each question in the group Some of the questions are not based on a descriptive passage; you must also select the best answer to these questions If you are unsure of the best answer, eliminate the choices that you know are incorrect, then select an answer from the choices that remain Indicate your selection
by blackening the corresponding circle on your answer sheet A periodic table is provided below for your use with the questions
PERIODIC TABLE OF THE ELEMENTS
1
H
1.0
2
He
4.0 3
Li
6.9
4
Be
9.0
5
B
10.8
6
C
12.0
7
N
14.0
8
O
16.0
9
F
19.0
10
Ne
20.2 11
Na
23.0
12
Mg
24.3
13
Al
27.0
14
Si
28.1
15
P
31.0
16
S
32.1
17
Cl
35.5
18
Ar
39.9 19
K
39.1
20
Ca
40.1
21
Sc
45.0
22
Ti
47.9
23
V
50.9
24
Cr
52.0
25
Mn
54.9
26
Fe
55.8
27
Co
58.9
28
Ni
58.7
29
Cu
63.5
30
Zn
65.4
31
Ga
69.7
32
Ge
72.6
33
As
74.9
34
Se
79.0
35
Br
79.9
36
Kr
83.8 37
Rb
85.5
38
Sr
87.6
39
Y
88.9
40
Zr
91.2
41
Nb
92.9
42
Mo
95.9
43
Tc
(98)
44
Ru
101.1
45
Rh
102.9
46
Pd
106.4
47
Ag
107.9
48
Cd
112.4
49
In
114.8
50
Sn
118.7
51
Sb
121.8
52
Te
127.6
53
I
126.9
54
Xe
131.3 55
Cs
132.9
56
Ba
137.3
57
La *
138.9
72
Hf
178.5
73
Ta
180.9
74
W
183.9
75
Re
186.2
76
Os
190.2
77
Ir
192.2
78
Pt
195.1
79
Au
197.0
80
Hg
200.6
81
Tl
204.4
82
Pb
207.2
83
Bi
209.0
84
Po
(209)
85
At
(210)
86
Rn
(222) 87
Fr
(223)
88
Ra
226.0
89
Ac †
227.0
104
Unq
(261)
105
Unp
(262)
106
Unh
(263)
107
Uns
(262)
108
Uno
(265)
109
Une
(267)
*
58
Ce
140.1
59
Pr
140.9
60
Nd
144.2
61
Pm
(145)
62
Sm
150.4
63
Eu
152.0
64
Gd
157.3
65
Tb
158.9
66
Dy
162.5
67
Ho
164.9
68
Er
167.3
69
Tm
168.9
70
Yb
173.0
71
Lu
175.0
†
90
Th
232.0
91
Pa
(231)
92
U
238.0
93
Np
(237)
94
Pu
(244)
95
Am
(243)
96
Cm
(247)
97
Bk
(247)
98
Cf
(251)
99
Es
(252)
100
Fm
(257)
101
Md
(258)
102
No
(259)
103
Lr
(260)
GO ON TO THE NEXT PAGE.
Trang 3Passage I (Questions 1–5)
The kidneys regulate hydrogen ion (H+)
concentration in extracellular fluid primarily by
controlling the concentration of bicarbonate ion
(HCO3 ) The process begins inside the epithelial cells of
the proximal tubule, where the enzyme carbonic anhydrase
catalyzes the formation of carbonic acid (H2CO3) from
CO2 and H2O The H2CO3 then dissociates into HCO3
and H+ The HCO3 enters the extracellular fluid, while
the H+ is secreted into the tubule lumen via a Na+–H+
countertransport mechanism, which takes advantage of the
Na+ gradient established by the Na+–K+ pump
Since the renal tubule is not very permeable to the
H C O3 filtered into the glomerular filtrate, the
reabsorption of HCO3 from the lumen into the tubular
cells occurs in a roundabout way Carbonic anhydrase
causes this HCO3 to combine with the secreted H+,
forming H2CO3 The H2CO3 then dissociates into CO2
and H2O The H2O remains in the lumen, while the CO2
enters the tubular cells
Looking at Figure 1, it can be seen that inside the
cells, every H+ secreted into the lumen is countered by an
HCO3 entering the extracellular fluid Thus, the basic
mechanism by which the kidneys regulate body fluid pH
is by the titration of H+ with HCO3
Na + + HCO3
extracellular
fluid
tubule lumen
tubular cell
Na +
H + + HCO3
H2CO3
CO2 + H2O
Na +
HCO3 + H +
HCO3
3Na +
2K +
K +
H2CO3
H2O
CO2 +
CO2
carbonic anhydrase
[ ]
carbonic anhydrase
[ ]
Figure 1 The drug acetozolamide is a potent noncompetitive
inhibitor of carbonic anhydrase Acetozolamide is a
diuretic—a drug that increases the rate of urine formation
and thereby increases the excretion of water and other
solutes Diuretics can be used to maintain adequate urine
output or to rid the body of excess fluid
1 Based on information in the passage, which of the
following is the mechanism by which Na+ is transported into the tubular cells?
A Facilitated diffusion
B Passive diffusion
C Active transport
D Endocytosis
2 Administration of acetozolamide to a patient would
result in an increase in the excretion of:
I Na+
II HCO3 III H+
IV H2O
A II only
B IV only
C I, II, and IV only
D I, II, III, and IV
3 The kidneys excrete acidic urine when:
A more hydrogen ions are secreted into the tubular
lumen than bicarbonate ions are filtered into the glomerular filtrate
B more bicarbonate ions are filtered into the
glomerular filtrate than hydrogen ions are secreted into the tubular lumen
C more hydrogen ions are filtered into the
glomerular filtrate than bicarbonate ions are secreted into the tubular lumen
D more bicarbonate ions are secreted into the
tubular lumen than hydrogen ions are filtered into the glomerular filtrate
GO ON TO THE NEXT PAGE.
Trang 44 Which of the following hormones would have the
opposite effect on blood volume as acetozolamide?
A ADH
B FSH
C Calcitonin
D Somatostatin
5 Spironolactone is an adrenocorticosteroid that acts as
a diuretic by inhibiting aldosterone A patient taking
this drug would most likely have a(n):
A increased blood volume and a decreased plasma
concentration of Na+
B increased blood volume and an increased plasma
concentration of Na+
C decreased blood volume and a decreased plasma
concentration of Na+
D decreased blood volume and an increased plasma
concentration of Na+
GO ON TO THE NEXT PAGE.
Trang 5Passage II (Questions 6–10)
Despite its bad reputation, cholesterol plays a vital
role in health maintenance Cholesterol serves as a
precursor for bile salts, steroid hormones, and other
specialized molecules, and is a constituent of membranes
Cholesterol is primarily supplied by the diet or
synthesized by the liver, though all cells have cholesterol
biosynthesis capability
Most of the cholesterol in plasma is carried by LDL
(low-density lipoproteins), which is the product of VLDL
(very low-density lipoproteins) metabolism Almost every
body cell has LDL membrane receptors Binding of LDL
to these receptors triggers an invagination of the cell
membrane and engulfment of the LDL-cholesterol
complex Inside the cell, enzymes break down the LDL,
releasing the cholesterol for incorporation into membranes
or other specialized molecules Intracellular free
cholesterol inhibits its own biosynthesis by inhibiting the
rate-limiting enzyme involved in the process Thus,
dietary and liver-produced cholesterol is utilized before the
cell produces its own When intracellular free cholesterol
is low, the cells produce more LDL receptors in order to
increase cholesterol uptake from the plasma
Another carrier, called HDL (high-density
lipoproteins), lowers plasma cholesterol concentration by
binding to free cholesterol from dying plasma membranes
These HDL-cholesterol complexes are metabolized by
liver cells, yielding free cholesterol that is secreted into
bile The bile eventually empties into the duodenum,
where it is reabsorbed from the intestine or excreted into
the feces Alternatively, liver cells may metabolize the
free cholesterol into bile acids The bile acids then pass
through the bile duct and into the small intestine, where
the biliary cholesterol is reabsorbed from the intestine or
excreted into the feces
6 Which cell organelle is responsible for the enzymatic
degradation of LDL-cholesterol complexes?
A Lysosomes
B Mitochondria
C Endoplasmic reticulum
D Ribosomes
7 Normal VLDL production by the liver is inhibited
by the ingestion of high concentrations of the vitamin niacin Which of the following would most likely occur in an individual consuming excess niacin?
A Decreased bile cholesterol levels
B Decreased cholesterol secretion into the small
intestine
C Increased activity of cellular
cholesterol-synthesizing machinery
D Increased plasma concentration of VLDL
8 Research has shown that ingestion of saturated fatty
acids (SFAs) tends to raise plasma cholesterol both
by inhibiting the conversion of free cholesterol to bile acids and by stimulating cholesterol biosynthesis Which of the following would best account for these findings?
A Chylomicrons containing SFAs inhibit the
rate-limiting enzyme involved in cellular cholesterol biosynthesis
B SFAs resemble cholesterol molecules to the
extent that the body is often unable to differentiate between them
C Chylomicrons containing SFAs bind to LDL
receptors, thereby blocking the receptor-mediated endocytosis of LDL-cholesterol complexes
D SFAs mimic the action of HDL molecules,
thereby increasing cholesterol transport and increasing cholesterol metabolism in liver cells
GO ON TO THE NEXT PAGE.
Trang 69 An individual with an average daily intake of
cholesterol discovers that she has abnormally high
plasma cholesterol Which of the following steps
would be effective in lowering her plasma cholesterol
concentration?
I Increase bile cholesterol
II Increase plasma HDL level
III Decrease plasma HDL level
IV Decrease dietary cholesterol intake
A IV only
B II and IV only
C III and IV only
D I, III, and IV only
1 0 Familial hypercholesterolemia is a disease in which
certain afflicted individuals maintain a plasma
cholesterol level five times higher than the normal
level Which of the following best explains this
phenomenon?
A Afflicted individuals have abnormally low
cholesterol requirements for normal body
functioning
B Afflicted individuals are unable to produce the
rate-limiting enzyme involved in cellular
cholesterol biosynthesis
C Afflicted individuals have defective livers that
metabolize LDL-cholesterol complexes as well as
HDL-cholesterol complexes
D Afflicted individuals lack the gene that codes for
LDL receptors
GO ON TO THE NEXT PAGE.
Trang 7Questions 11 through 16 are
NOT based on a descriptive
passage
1 1 If the hormone secretin were not released when
chyme entered the small intestine, which of the
following would NOT occur?
A Secretion of enterogastrone
B Secretion of cholecystokinin
C Secretion of insulin
D Secretion of bicarbonate ion
1 2 The “hunger pangs” that may be experienced in the
stomach 12 to 24 hours after the last ingestion of
food are due to:
A the emptying of the stomach contents into the
small intestine through the pyloric sphincter
B peristaltic contractions of the stomach.
C the excess secretion of mucus in the stomach.
D the highly acidic environment of the stomach.
1 3 In the process of fat emulsification, bile salts make
fats more susceptible to the action of lipases by:
A transporting fat globules to the region of lipase
activity
B increasing the surface area of the fat globules.
C functioning as a catalyst to the lipases.
D lowering the pH of the small intestine.
1 4 Children who develop acute glomerulonephritis, in
which the renal glomeruli are inflamed and therefore fail to filter adequate quantities of fluid, would be expected to develop all of the following EXCEPT:
A an excess of extracellular fluid in body tissue.
B a high concentration of urea in the blood.
C a decreased urine output.
D a high concentration of sodium in the urine.
1 5 Vitamin K cannot be synthesized by human cells.
While the amount of vitamin K in ingested foods is not normally sufficient to maintain adequate blood coagulation, the normal plasma concentration of vitamin K is sufficient Which of the following best accounts for this apparent contradiction?
A Vitamin K is synthesized in the liver using
hormones absorbed from ingested plant cells
B Vitamin K is a byproduct of protein degradation
during digestion
C Degraded vitamin K is reabsorbed into the blood
from the kidneys
D Vitamin K is synthesized by bacteria that inhabit
the colon
1 6 Which of the following would most likely cause
malabsorption in the small intestine?
A Presence of extremely dilute chyme in the small
intestine
B Destruction of the intestinal villi
C Deceased secretion of HCl in the stomach
D Presence of alkaline fluid in the small intestine
END OF TEST
Trang 8ANSWER KEY:
Trang 9DIGESTIVE AND EXCRETORY SYSTEMS TEST 1 TRANSCRIPT
Passage I (Questions 1-6)
1 The correct answer is choice A Referring to both Figure 1 and the passage, you see that H+ is secreted into the lumen via a Na+-H+ countertransport mechanism After carbonic anhydrase catalyzes the formation of carbonic acid inside the tubular cell, the carbonic acid spontaneously degrades into bicarbonate ion and H+, causing a buildup of H+ within the cell The H+ is secreted into the lumen in exchange for Na+ The Na+ binds to a carrier protein on the luminal side of the cell membrane, while at the same time, an H+ binds to the opposite side of the same carrier protein Since there is a greater concentration of Na+ outside the cell than inside, due to the Na+ -K+ pump, Na+ moves down its concentration gradient into the cell This movement supplies the energy for moving the H+ into the tubular lumen Therefore, it is said that the Na+ moves
into the cell via facilitated diffusion; no energy is required because movement is with the concentration gradient, not against
it Thus, choice A is the correct answer Choice C is wrong because although active transport requires a protein carrier, it also requires APT Choice D can also be eliminated Endocytosis is the uptake of extracellular material via invagination of the plasma membrane Clearly, that's not occurring here In this case, one ion is being exchanged for another ion Choice B, passive diffusion, is also incorrect Passive diffusion is the movement of an ion down its concentration gradient without the aid of a carrier protein and the countertransport of another ion Again, the correct answer is choice A
2 The correct answer is choice C The key to solving this question is understanding the effects of carbonic anhydrase From Figure 1, we know that carbonic anhydrase is found in both the lumen and within the tubular cells of the kidney In the lumen, hydrogen ions and bicarbonate ions are converted to carbonic acid, which then dissociates into carbon dioxide and water This reaction is catalyzed by carbonic anhydrase The water remains in the lumen, while the carbon dioxide enters the cells Inside the cells, carbonic anhydrase catalyzes the formation of carbonic acid from water and carbon dioxide The carbonic acid dissociates into a bicarbonate ion and a hydrogen ion The bicarbonate ion enters the extracellular fluid, while the hydrogen ion is secreted into the lumen in exchange for a sodium ion According to the passage, acetozolamide is an inhibitor
of carbonic anhydrase Thus, administering acetozolamide would prevent those reactions that are catalyzed by carbonic anhydrase And this would mean that the sodium ion-hydrogen ion countertransport would come to a halt, as would the re-absorption of bicarbonate ion from the lumen into the tubular cells Since the fluid remaining in the lumen is eventually excreted in the urine, acetozolamide would result in a decrease in urinary hydrogen ion, an increase in urinary bicarbonate ion, and an increase in urinary sodium ion This increase in urine osmolarity would draw water into the lumen, to counter the increase in lumen fluid osmolarity Thus, there will be an increase in the urinary amounts of Roman numerals I, II, and IV, and a decrease in Roman numeral III, and so choice C is the right answer Once you had determined that Roman numeral I was
a component of the correct answer, you should have ruled out choices A and B, narrowing it down to C and D Again, choice
C is correct
3 Choice A is correct According to the passage, the secretion of hydrogen ions into the lumen is countered by the movement of bicarbonate ions into the extracellular fluid from the filtrate, and that this titration is the means by which the kidneys regulate body fluid pH Acidity is a function of hydrogen ion concentration Therefore, when the kidneys excrete acidic urine, it means that there is a high concentration of hydrogen ions in the urine relative to the concentration of bicarbonate ions Since urine is formed from the fluid in the tubule lumen, acidic urine results when the lumen fluid has a high hydrogen ion concentration relative to the bicarbonate ion concentration This means that more hydrogen ions are secreted into the tubule than bicarbonate ions are filtered into the lumen from the glomerulus Thus, choice A is correct and choice B is wrong Now even if you weren't able to figure out the origin of acidic urine, you should have been able to narrow
it down to choice A and B right off the bat based on Figure 1 alone As you can clearly see, it is the hydrogen ions that are secreted INTO the tubular lumen, thereby eliminating choice C and D Again, choice A is the correct answer
4 The correct answer is A Answering this question requires an understanding of the effect acetozolamide and various hormones have on the body's fluid status Acetozolamide inhibits carbonic anhydrase and thus inhibits both the excretion of hydrogen ions into the lumen and the re-absorption of bicarbonate The excretion of hydrogen ions into the lumen is important because it provides the means by which sodium is taken up into the tubular cell If no hydrogen ions are produced
in the tubular cell due to the inhibition of carbonic anhydrase, the sodium cannot be taken up and so remains within the lumen, increasing its osmolarity Water will diffuse back into the lumen due to the increased osmolarity of the lumen, and thus, urine volume will increase Indeed, this is the way in which this diuretic acts to rid the body of excess fluid The hormone with the opposite effect then, is one that causes a decrease in water excretion In other words, this hormone must cause an increase in water re-absorption and ADH is the hormone we're looking for ADH, or antidiuretic hormone, increases the re-absorption of water in the kidneys Even its name, ANTI-diuretic hormone, should have made this question a dead giveaway, since acetozolamide is a diuretic Thus choice A is the correct answer Choice B is incorrect because FSH, or
Trang 10the hypothalamus and is a potent inhibitor of many other hormones, including: growth hormone, TRH (thyroid-releasing hormone), ACTH, insulin, glucagon, gastrin, and renin Once again, the correct answer is choice A
5 Choice C is the correct answer Coming up with the answer to this question required no information from the passage itself According to the question stem, spironolactone is a diuretic that inhibits aldosterone Aldosterone is the hormone that causes an increase in the re-absorption of sodium ions from the lumen of the nephron and an increase in the secretion of potassium ions into the lumen of the nephron Thus, aldosterone increases the plasma concentration of sodium ions Therefore, a drug that inhibits aldosterone will inhibit the re-absorption of sodium ions and will thereby decrease the plasma concentration of sodium ions Based on that piece of information, you know that we can eliminate choices B and D
To distinguish between choices A and C, you need to know about aldosterone's effect on blood volume Osmolarity is the number of solute particles in a certain volume of liquid And you know that if you have two compartments divided by a membrane permeable to water, water will diffuse from the area of lower osmolarity to the area of higher osmolarity Sodium
is re-absorbed from the tubular filtrate into the tubular cells, and eventually enters the bloodstream This movement of sodium from the tubular lumen to the cells, and finally to the blood results in an increase in the osmolarity of these regions, resulting
in a concomitant movement of water So where the sodium ions go, the water follows, which means than an increase in the plasma concentration of sodium will be accompanied by an increase in blood volume Therefore, when you inhibit aldosterone, not only do you inhibit the re-absorption of sodium, but you inhibit the movement of the water as well, resulting in a decrease in blood volume And so, choice C is the correct answer Even if you didn't remember the effects of aldosterone there was still a way to get the right answer You're told in the question stem that spironolactone is a diuretic And from the passage you know that diuretics increase the loss of water and other solutes Therefore, upon administration of a diuretic, you would expect a decreased plasma concentration of solutes along with a decrease in blood volume, since urine excretion has increased So by this line of reasoning, it is obvious that choice C must be the correct answer
Passage II (Questions 6-10)
6 The correct answer is choice A To answer this question, you needed to remember the basic functions of these four cell organelles Lysosomes are the garbage disposal system of cells Lysosomes make use of hydrolytic enzymes to accelerate the breakdown of unwanted proteins, polysaccharides, nucleic acids, and lipids In this passage, we have lysosomes separating the LDL from the cholesterol molecules, thereby freeing the cholesterol for use by the cell Thus, choice A is right Choice B
is wrong because the primary function of mitochondria is cellular respiration the process by which usable energy is derived from food materials and stored in the high-energy phosphate bonds of ATP Choice C is wrong because the endoplasmic reticulum (ER) serves as the main intracellular transport mechanism for molecules, especially proteins While some cells do contain large amounts of ER, some of which has associated degradative enzymes, intracellular degradation is not the primary function of most cellular ER and certainly cannot be considered the main degradative source for the breakdown of LDL-cholesterol complexes Ribosomes, choice D, either float freely within the cytoplasm or are connected to the membranes of the ER (rough endoplasmic reticulum) Ribosomes are responsible for translating messenger RNA transcripts into peptide chains Thus, choice D is wrong Again, choice A is correct
7 Choice C is the correct answer VLDL is synthesized by the liver, but it is soon cleaved into LDL by removal of a lipid moiety Thus, if the vitamin niacin interferes with VLDL production, which is exactly what happens according to the question stem, then ingestion of excessive amounts of niacin will effectively reduce the LDL concentration in that individual's plasma Thus, right off the bat, you should have eliminated choice D, because VLDL plasma concentration will obviously be decreased, not increased An individual with decreased amounts of available LDL will have fewer LDL-cholesterol complexes The net result is that cholesterol uptake will decrease, and the rate-limiting enzyme that controls cellular cholesterol
biosynthesis will not be inhibited This will result in increased activity of the cellular cholesterol-synthesizing machinery to
compensate for the apparent lack of dietary cholesterol Thus, choice C is correct Choice A is incorrect because, as stated in the passage, bile cholesterol levels are primarily a function of HDL concentration Since VLDL is not the precursor of HDL,
a decrease in VLDL production would not affect HDL levels On the contrary, if less cholesterol is made available to cells, and more cholesterol is synthesized intracellularly, then the resulting increase in plasma cholesterol levels would initiate
plasma cholesterol clearance mechanisms For this reason, liver cells would secrete more cholesterol into the bile, where it
could be excreted into the feces So, choice A is wrong Choice B is also wrong, for many of these same reasons HDL is responsible for controlling how much cholesterol is transported into liver cells, where it is broken down into bile acids and secreted into the small intestine VLDL concentration does not regulate HDL concentration Also, since a low LDL level would result in increased plasma cholesterol, this would result in an increase in cholesterol secretion into the small intestine Again, choice C is the correct answer
8 Choice C is correct To get this question right, you need to understand the underlying factors responsible for the regulation of plasma cholesterol levels that are described for you in the passage The question presents a situation in which