CAIE A-Level Biology A2 14.1 Homeostasis in Mammals Questions

Practise mammalian homeostasis by interpreting kidney and nephron diagrams, tracing urine and ADH responses, and explaining insulin or glucagon control of blood glucose.

Exam points

Explain homeostasis using narrow limits for body temperature, blood glucose and water potential.
Trace receptors, coordination and effectors in negative feedback that restores a set point.
Explain liver deamination of excess amino acids and urea formation for excretion.
Label the kidney cortex, medulla, pelvis, ureter and renal blood-vessel branches in sections.
Identify nephron structures and associated vessels in diagrams or micrographs.
Explain glomerular ultrafiltration and selective reabsorption in the proximal convoluted tubule.
Relate Bowman’s capsule filtration barriers and proximal-tubule adaptations to their functions.
Trace falling blood water potential through hypothalamus, pituitary, ADH and collecting-duct aquaporins.
Trace glucagon signalling from its receptor through G protein, cAMP and PKA to glycogen breakdown.
Interpret blood-glucose curves to explain insulin or glucagon feedback restoring the set point.
Compare test-strip colour from glucose oxidase/peroxidase with biosensor current for glucose concentration.

Covered syllabus points

14.1.1—Homeostasis definition and importance 14.1.2—Homeostasis control principles 14.1.3—Urea production 14.1.4—Kidney gross structure 14.1.5—Nephron structure 14.1.6—Urine formation 14.1.7—Nephron structure and function 14.1.8—Osmoregulation with ADH 14.1.9—Glucagon cell signalling 14.1.10—Negative feedback control of blood glucose 14.1.11—Glucose test strips and biosensors

[Maximum number: 6]

Explain what is meant by homeostasis in a mammal and explain why it is important to maintain body temperature, blood glucose concentration and the water potential of blood.

any six from:
1 maintain / keep / regulate, internal, environment / conditions ;
2 within, narrow / set, limits or around, optimum value / set point / norm ;
3 low (body) temperature and consequence ;
4 high (body) temperature and consequence ;
5 low blood glucose (concentration) and consequence ;
6 high blood glucose concentration and consequence ;
7 low, (blood) water potential / $\psi$ , and consequence ;
8 high, (blood) water potential / $\psi$ , and consequence ;

[Maximum number: 5]

Glucagon-like peptide-1 (GLP-1) is a hormone consisting of 31 amino acids. It is secreted by specialised intestinal epithelial cells known as L cells.

Fig. 6.1 outlines the secretion of GLP-1 and its effects on the pancreas, brain and stomach.

(a)

GLP-1 inhibits the secretion of glucagon by $\alpha$ cells in the pancreas.

Describe two processes in the liver that are stimulated by glucagon.

[ 2 ]

(b)

GLP-1 inhibits the emptying of the stomach. This is an example of negative feedback.

Explain what is meant by negative feedback.

[ 3 ]

[Maximum number: 11]

(a)

In mammals, excess amino acids cannot be stored in the body.

Outline the formation of urea from excess amino acids by liver cells.

[ 3 ]

deamination/amine group removed;
A amino/ $\mathrm{NH}_{2}$
ammonia/ $\mathrm{NH}_{3}$ , formed ;
combined with carbon dioxide ;
urea cycle ;
A ornithine cycle

(b)

The mammalian kidney is composed of many nephrons.

Describe the process of ultrafiltration in the nephron.

[ 4 ]

(c)

Fig. 7.1 is a diagram of a nephron.

Fig. 7.1

Complete Table 7.1 by naming a part of the nephron that corresponds to each of the statements.

Table 7.1

[ 4 ]

statement	part of nephron
(passes through the medulla)	collecting duct/loop of Henle ;
(glucose is reabsorbed into the blood)	proximal convoluted tubule/PCT ;
(ADH acts on its walls)	collecting duct/distal convoluted tubule/DCT ;
(most of the water is reabsorbed into the blood)	proximal convoluted tubule/PCT ;

[Maximum number: 3]

Glucagon-like peptide-1 (GLP-1) is a hormone consisting of 31 amino acids. It is secreted by specialised intestinal epithelial cells known as L cells.

Fig. 6.1 outlines the secretion of GLP-1 and its effects on the pancreas, brain and stomach.

GLP-1 inhibits the emptying of the stomach. This is an example of negative feedback.

Explain what is meant by negative feedback.

[Maximum number: 3]

In mammals, excess amino acids cannot be stored in the body.

Outline the formation of urea from excess amino acids by liver cells.

deamination/amine group removed;
A amino/ $\mathrm{NH}_{2}$
ammonia/ $\mathrm{NH}_{3}$ , formed ;
combined with carbon dioxide ;
urea cycle ;
A ornithine cycle

[Maximum number: 4]

Fig. 1.1 is a drawing of a longitudinal section (LS) of a human kidney.

Fig. 1.1

Use the letters A, B, C and D in Fig. 1.1 to complete Table 1.1.
Each letter may be used once, more than once or not at all.
For each description, list all the letters that are correct.

Table 1.1

description	region of kidney
location of loops of Henle	C
location of Bowman's capsules	A
location of glomeruli	A
contains urine at final concentration	B and D

[Maximum number: 4]

Fig. 1.1 is a diagram of a nephron.

Fig. 1.1

Label Fig. 1.1 using:
- one labelling line and the letter A to identify a region that contains urine
- one labelling line and the letter B to identify a region that contains podocytes
- one labelling line and the letter C to identify a region of the nephron that is within the medulla of the kidney
- one labelling line and the letter D to identify the afferent arteriole.

[Maximum number: 2]

The main component of the glomerular filtrate formed in the Bowman's capsule is water. Most of this water is reabsorbed as the filtrate passes along the nephron.
- $180 \mathrm{dm}^{3}$ of water forms part of the glomerular filtrate each day.
- $1.4 \mathrm{dm}^{3}$ of water leaves the body in urine each day.

Calculate the percentage of water in the glomerular filtrate that is reabsorbed as the glomerular filtrate passes along the nephron.

Show your working and give your answer to one decimal place.

Calculation:
$\frac{180 - 1.4}{180} \times 100$
or
$\frac{178.6}{180} \times 100$

Answer:
$99.2\%$

[Maximum number: 4]

The cells of the proximal convoluted tubule are adapted to carry out selective reabsorption. Describe and explain how these cells are adapted to carry out selective reabsorption.

any four from:
1 microvilli, for large surface area / increases surface area (for reabsorption) ;
2 cotransporter (proteins) for movement of, glucose / amino acids (with sodium ions) ;
3 tight junctions / described, to, stop substances passing in between cells / cause substances to pass though cells ;
4 many mitochondria to provide ATP for, active transport / pumping of $\mathrm{Na}^{+}$ ;
5 folded basal membrane for many sodium (potassium) pumps ;

[Maximum number: 8]

Osmoregulation is the control of the water potential of body fluids such as blood.

(a)

Osmoreceptors and antidiuretic hormone (ADH) have an important role in osmoregulation.

[ 3 ]

(i)

Name the location of osmoreceptors.

[ 1 ]

(ii)

State the stimulus that is detected by osmoreceptors.

[ 1 ]

(changes in) water potential (of blood) ; $\mathbf{A} \psi$

(iii)

Name the structure that secretes ADH into the blood.

[ 1 ]

(b)

ADH acts on the cells of the collecting duct, resulting in changes in the volume and concentration of urine.

Fig. 1.1 shows the relationship between the concentration of ADH in the blood and the rate of production of urine by the kidneys.

Fig. 1.1

[ 5 ]

(i)

As the concentration of ADH in the blood increases, the rate of production of urine decreases.

Using Fig. 1.1, calculate the percentage decrease in the rate of production of urine between ADH concentrations of 1 and 3 arbitrary units.

Show your working.
Give your answer to one decimal place.
answer =

[ 2 ]

(ii)

Explain the relationship shown in Fig. 1.1.

[ 3 ]

[Maximum number: 4]

When blood glucose concentration decreases, glucagon is released by the pancreas into the blood and is transported to the cells.

Fig. 6.1 outlines the effect of glucagon on liver cells.

Fig. 6.1

Identify P, Q, R and S shown in Fig. 6.1.

P
Q
R
S

[Maximum number: 4]

(a)

In mammals, blood glucose concentration is maintained around a set point so that the mammal can function efficiently. This is an example of homeostasis.

Name the mechanism that maintains blood glucose concentration around a set point.

[ 1 ]

(b)

Fig. 10.1 shows how the blood glucose concentration varies before, during and after a period of exercise.

Fig. 10.1

Explain the results shown in Fig. 10.1 between 15 minutes and 70 minutes.

[ 3 ]

(c)

Type 1 diabetes in humans is a condition where the pancreas does not produce enough insulin to control blood glucose concentration.

Test strips are used to measure the concentration of glucose in a sample of urine. Two reactions take place on the test strip and a colour change occurs if glucose is present in the urine.

The equations for these reactions are:

\text { glucose }+ \text { oxygen } \xrightarrow{\text { glucose oxidase }} \mathbf{A} \quad+\text { hydrogen peroxide }

peroxidase
hydrogen peroxide $+\mathbf{B} \longrightarrow$ oxidised B+ water

[ 4 ]

(i)

Identify compound A and the type of substrate represented by B.

A
B

[ 2 ]

(ii)

Most people with diabetes mellitus use a biosensor to obtain their blood glucose concentration.

Suggest the advantages of using a biosensor rather than test strips.

[ 2 ]

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