IB DP Biology Topic 2: Molecular biology 2.2 Water Question Bank HL Paper 2


The Chinese pangolin (Manis pentadactyla) is a critically endangered species that has declined in numbers by 80% since 2000. It inhabits both forest and grassland, where it uses long, powerful claws to open ant and termite nests and ingests the insects using a long, sticky tongue.

(a) (i) State with a reason whether pangolins are autotrophic or heterotrophic.
(ii) Explain what information is needed to find the trophic level of pangolins.

(b) Pangolins are unique among mammals in having evolved scales, which are a recognition feature of reptiles. Explain which features you expect pangolins to have, which would show that they are mammals, not reptiles.

(c) The Chinese pangolin, Manis pentadactyla, has a diploid chromosome number of 40.
(i) State how many chromosomes there would be in gametes of this species.
(ii) Sex is determined in the same way in pangolins as in humans. State how many autosomes there are in somatic cells of M. pentadactyla.



(i) heterotrophic because it feeds on/eats food/other organisms /eats ants/termites/ doesn’t photosynthesise/does not produce its own food;
a. what (prey) it eats/feeds on/ stomach content;
b. the trophic level of what (prey) it eats/feeds on/the trophic level of
c. trophic level is the position an organism occupies in the food chain/web;

a. hair;
b. mammary glands/milk secretion;
c. alveoli in lungs;
d. lower mandible/jaw consisting of just one bone;
e. giving birth to live young/are placental (apart from duck-billed
f. external ears/ pinna;
g. warm-blooded/endothermic/constant body temperature;

(i) 20;
(ii) 38
19 pairs;


The diagram shows water molecules as they might be arranged in liquid water and the interactions between them.

(a) (i) State how many water molecules are shown in the diagram.
(ii) Identify the interactions that are shown between the water molecules.

(b) (i) With reference to the diagram, explain how water in sweat evaporates.
(ii) Outline the reasons for secretion of sweat in humans.



(i) 10;
(ii) hydrogen bonds/H bonds;

(i) a. heat increases molecular motion/vibration;
b. (hydrogen) bonds break;
c. evaporation is separation of water molecules / water changes from liquid to gas/vapour;
d. heat removed from skin surface/body;
(ii) a. cooling/removing heat/lowering body temperature;
b. to prevent overheating
to help maintain body temperature/for temperature homeostasis/for
to keep temperature at 37 °C;


This image shows a normal red blood cell.

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These images show two red blood cells that have been placed in solutions with different concentrations of solutes.

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Outline the properties of water molecules that permit them to move upwards in plants.[2]


Define osmolarity. [1]


Deduce, with a reason, which red blood cell has been placed in a hypertonic solution.[1]


State what change there has been in the cell surface area to volume ratio in red blood cell 1.[1]



a. water molecules are polar
can form hydrogen bonds 

b. cohesion between water molecules allows continuous water columns
cohesion between water molecules allows transpiration stream «to form in xylem» 

c. adhesion of water to the walls of xylem vessel «helps water rise» 

d. water evaporates at environmental temperatures allowing transpiration pull OWTTE


«measurement of» solute concentration of a solution OWTTE


cell 2 because it has plasmolized/lost water/volume has decreased






The Chinese soft-shelled turtle, Pelodiscus sinensis, lives in salt water marshes. The turtle can live under water and out of water.

These turtles have fully developed lungs and kidneys, however, many microvilli have been discovered in the mouth of P. sinensis. A study was undertaken to test the hypothesis that oxygen uptake and urea excretion can simultaneously occur in the mouth.

Initial experiments involved collecting nitrogen excretion data from P. sinensis. The turtle urinates both in water and out of water. When in water it allows waste products to be washed out of its mouth. When out of water it regularly dips its head into shallow water to wash its mouth. The table shows the mean rates of ammonia and urea excretion from the mouth and kidney over six days.

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It was noted that during long periods out of water, turtles rhythmically moved their mouths to take in water from a shallow source and then discharge it. Changes in the dissolved oxygen and the quantity of accumulated urea in the rinse water discharged by the turtles were monitored over time as shown in this graph.

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In order to test whether a urea transporter was present in the mouth tissues of the turtles, phloretin (a known inhibitor of membrane proteins that transport urea) was added to the water in which a further set of turtles submerged their heads. The results of that treatment are shown.

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Further research was conducted to determine where mRNA expression of a urea transporter gene might be occurring in P. sinensis. Gel electrophoresis was used to analyse different tissue samples for mRNA activity.

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Expression of the urea transporter gene by cells in the turtle’s mouth was assessed by measuring mRNA activity. Turtles were kept out of water for 24 hours and then injected with either a salt solution that matched the salt concentration of the turtle, dissolved ammonia or urea, followed by another 24 hours out of water.

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Deduce whether the excretion of ammonia or urea changes more when a turtle emerges from water.[2]


Compare and contrast the changes in urea excretion in the mouth with the changes in urea excretion in the kidney when a turtle emerges from the water.[3]


Describe the trends shown by the graph for dissolved oxygen in water discharged from the mouth.[1]


Suggest reasons for these trends in dissolved oxygen.[2]


Deduce with a reason whether a urea transporter is present in the mouth of P. sinensis.[2]


Outline the additional evidence provided by the gel electrophoresis results shown above.[2]


Identify which of these turtle groups represent the control, giving a reason for your answer.[1]


Suggest a reason for the greater expression of the gene for the urea transporter after an injection with dissolved ammonia than an injection of urea.[2]


The salt marshes where these turtles live periodically dry up to small pools. Discuss the problems that this will cause for nitrogen excretion in the turtles and how their behaviour might overcome the problems.[3]



a. urea 

b. for both mouth and kidney 

c. percentage change/change in μmol day−1 g−1 greater with urea/other acceptable numerical comparison


a. both higher/increased on emergence from/with turtle out of water 

b. both increased by 0.66 «μmol−1 g−1 when turtle emerges from water» 

c. % increase is higher in kidney / kidney 940% versus mouth 73/75% / increase is higher proportionately higher in kidney / kidney x10 versus mouth nearly double/x1.73 

d. urea excretion by mouth greater than kidney out of water «despite larger % increase in kidney excretion»


decrease «when head is submerged» and increase when head is out of water


a. oxygen absorbed from water/exchanged for urea when head dipped in water«so oxygen concentration decreases» 

b. lungs cannot be used with head in water / can «only» be used with head out of water 

c. oxygen from water «in mouth» used in «aerobic cell» respiration 

d. oxygen from air dissolves in water when head out of water «so oxygen concentration increases»


a. urea transporter is present 

b. less urea «excreted»/ lower rate «of urea excretion» / excretion almost zero when phloretin/inhibitor was present


a. mRNA only in mouth and tongue/in mouth and tongue but not esophagus intestine kidney or bladder 

b. bands / lines indicate mRNA for/expression of urea transporter gene 

c. urea transporter gene expressed / urea transporters in mouth/tongue / not expressed/made in esophagus/intestine/kidneys/bladder 

d. mRNA/transcription/gene expression/urea transporters higher in tongue/more in tongue «than mouth»


salt solution is control because it does not contain a nitrogenous/excretory waste product / it matches the salt concentration of the turtle / the turtle’s body already contains salt / because the turtle lives in salt water/salt marshes / because nothing has been altered


a. ammonia is «highly» toxic/harmful 

b. ammonia is more toxic than urea/converse 

c. ammonia converted to urea 

d. urea concentration raised «by injecting ammonia» 

e. difference between ammonia and urea «possibly» not «statistically» significant



a. urea becomes more concentrated «in small pools» / lower concentration gradient «between tongue/mouth and water» 

b. less water available for urine production/excretion by kidney
less water in ponds for mouth rinsing/more competition for pools (to use for mouth rinsing)

Behaviour to overcome problems:

c. «still able to» dip mouth into/mouth rinse in water/pools 

d. «still able to» excrete urea «though the mouth» in the small pools 

e. more conversion of ammonia to urea/urea excretion rather than ammonia

f. more urea transporters/expression of urea transporter gene 

g. urea excreted «in mouth/via microvilli» by active transport/using ATP 

h. excretion with little/no loss of water


a. Explain how abiotic factors affect the rate of transpiration in terrestrial plants.[8]

b.Describe the importance of water to living organisms.[5]


a. less transpiration/water loss as (atmospheric) humidity rises;

b. air spaces inside leaf are saturated/nearly saturated (with water vapour);

c. smaller concentration gradient with higher atmospheric humidity;

d. more transpiration/water loss as temperature rises/with more heat;

e. faster diffusion / more kinetic energy (of water molecules);

f. faster evaporation (due to more latent heat available);

g. more transpiration/water loss as wind (speed) increases;

h. humid air/water vapour blown away from the leaf;

i. increasing the concentration gradient (of water vapour);

j. more transpiration/water loss in the light;

k. light causes stomata to open / stomata closed in darkness;

l. low COconcentration inside leaf in bright light so stomata open wider;

Accept any of the points if clearly made on an annotated graph.


a. coolant in sweat/in transpiration;

b. water has a high heat of vaporisation / heat taken when hydrogen bonds break;

c. water is cohesive so can pulled up/so can be moved under tension in xylem;

d. water is an excellent/universal solvent/dissolves many different substances;

e. medium for transport in blood/xylem/phloem;

f. medium for metabolic reactions / (metabolic) reactions happen dissolved in water;

g. surface tension due to cohesion allows organisms to live on water surface;

h. water has high heat capacity so much energy required to change its temperature;

i. ice floats so lakes/oceans do not freeze allowing life under the ice;

j. high heat capacity so stable habitat/so temperature of water changes slowly;

k. used in chemical reactions/photosynthesis/hydrolysis in organisms;

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