Questions to Practice Critical Thinking

Questions to practice critical thinking

(These questions are all from AQA Spec B, 678B papers – except 7 which is from a specimen Human Biol4 paper. Mark schemes are published for these questions)

1. A large number of roots from many genetically identical bean plants were cut into thin sections. The sections were sorted into groups, depending upon their distance from the root tip. Some sections were examined with a light microscope to find the proportion of dividing cells. Other sections were used to find the mean dry mass of their cells.

The graph shows the results. The diagrams below the graph show the appearance of cells in light microscope sections at different distances from the root tip.

(a) Suggest two other variables which need to be controlled to ensure similar

root growth in different plants. In each case give the reason for your answer. 2

(b) Describe how you would find the dry mass of a sample of root sections. 3

d) Explain the change in the proportion of dividing cells with increasing distance

from the root tip. 2

(e) Using the diagrams, suggest how a root tip gets longer. 3

2. Clover plants have leaves all through the year. Some clover plants have leaves that produce poisonous hydrogen cyanide gas when damaged. These cyanogenic plants are less likely to be eaten by snails. However, the leaves of these plants can be damaged by frost, resulting in the production of enough hydrogen cyanide to kill the plant. Acyanogenic plants do not produce hydrogen cyanide.

The characteristic is genetically controlled. The map shows the proportion of the allele leading to hydrogen cyanide production in populations of clover from different areas in Europe. It also shows isotherms, lines joining places with the same mean January temperature.

3. Scientists measured the rate of aerobic respiration in bees at different environmental temperatures. They measured the rate of oxygen consumption of individual bees using the apparatus shown in the diagram.

(a)(i) The scientists left the tube in the water bath until it reached the required temperature.

Then they closed the tap. The drop of coloured liquid started to move steadily.

Which way did the liquid move? Explain your answer. 2

(ii) What measurements would the scientists need to make to compare the rate of

oxygen consumption of different bees. 3

(iii) What is the syringe used for? 1

(iv) Why is the substance to absorb carbon dioxide in the tube? 2

At low temperatures, some bees respire at a much higher rate than others. The body temperature of these bees is much higher than their surroundings. It is also higher than the body temperature of other bees. These bees are described as endothermic bees. The diagram shows the body structure of a bee.

(b) At low temperatures, endothermic bees vibrate their wings rapidly. Vibrating their

wings produces heat. Suggest how. 2

queen bee. The worker bees are all offspring of the queen bee. Worker bees are

unable to reproduce.

In winter, the colony comes together in a cluster. The queen is at the centre of the

cluster. Endothermic bees are also found near to the centre of the cluster.

Using the information given, suggest how endothermy evolved in honeybees. 3

4. The growth of young rice plants from seeds is affected by a plant hormone called

gibberellic acid. Rice plants of the ‘slender’ variety have very long shoots.

The diagram shows the structure of a seed from a rice plant.

Scientists investigated amylase activity in seeds from normal and ‘slender’ rice plants.

They removed endosperm tissue from the seeds of both varieties of plants and placed the

samples on agar plates. The agar in all of the plates contained starch and in half of the

plates it also contained gibberellic acid. After a few days, the plates were flooded with

iodine in potassium iodide solution. Four typical plates are shown below.

(a) Give one variable that must be controlled when preparing the starch agar plates.

Explain why it must be controlled. 2

(b) There are clear areas around the endosperm on some of the plates. Explain why. 2

to seeds of the normal variety? 2

(d) Slender plants have the genotype ss. Crosses were carried out between plants

that were heterozygous Ss. The offspring were 311 normal and 89 slender plants,

giving an observed ratio of 3.49:1. A chi-squared test was carried out, to see if

this was significantly different from the expected 3:1 ratio. A value for c2 of 1.61

was obtained. The table shows c2 values and corresponding probability values

for one degree of freedom.

Probability values / 0.50 / 0.20 / 0.10 / 0.05 / 0.01 / 0.001
c2 values / 0.46 / 1.64 / 2.71 / 3.84 / 6.64 / 10.3

(i) What was the null hypothesis for this test? 1

(ii) Explain what a c2 value of 1.61 indicates about the difference between the

observed and expected ratios. 2

5. Coleoptiles are the first green shoots that grow from grass seeds. Scientists

investigated the effects of different concentrations of the substance, IAA, on the growth

of sections taken from coleoptiles.

Figure 1 shows the apparatus the scientists used.

The growth of the six sections of coleoptiles in buffer solution was measured for 50 minutes. IAA was then added to the buffer solution and growth was recorded for another 50 minutes.

Figure 2 shows the photographic film recording of one experiment.

(a) Sections from six coleoptiles were used in each experiment, rather than just

one section from one coleoptile.

This made it easier to make measurements of growth. Suggest why. 2

(b) Suggest two reasons for using the metal weight. 2

(d) IAA leads to a softening of plant cell walls. Describe and explain the results

shown in Figure 2.

(e) The scientists repeated the experiment several times with different

concentrations of IAA.

Suggest two factors they would have to keep constant in all the experiments.

Give a reason for keeping each factor constant. 2

(f) The vertical axis on Figure 2 was calibrated by raising the metal weight by

known amounts. Explain why this calibration was necessary. 1

6. Crabs are invertebrates that live on the seashore. Scientists investigated the digestive enzymes produced by three species of crab. They measured the activity of three enzymes;

· endopeptidase,

· cellulase (digests cellulose),

· laminarinase (digests a substance only found in seaweeds).

The diagram shows part of the food web to which the three species of crab belong.

(a) Use information in the question to name the species of crabs Y and Z. Give

reasons for your answers. 4

(b) The activity of the endopeptidase from one species of crab was measured. A

sample of the enzyme was added to a solution of a colourless substance called

BAPNA. A yellow compound is produced when BAPNA is broken down by the enzyme.

endopeptidase + BAPNA (colourless) yellow compound

Describe how you would carry out an investigation to find the optimum temperature

for the activity of the endopeptidase. 3

7. Human oestrogen may be released in very low concentrations into rivers in water

from sewage works. A genetically engineered yeast has been produced to detect this oestrogen.

Two plasmids were inserted into the yeast. One plasmid carried the gene for a human oestrogen receptor protein. The other plasmid carried a promoter gene and the gene for an enzyme, beta-galactosidase.

When oestrogen enters the yeast cell it binds to the receptor protein, forming a complex. This complex binds to the promoter gene which then causes transcription of the beta-galactosidase gene. The higher the concentration of oestrogen, the greater the amount of enzyme produced. The enzyme reacts with a colourless indicator called ONPG. A yellow compound is produced when ONPG is broken down by the enzyme.

beta-galactosidase + BAPNA (colourless) yellow compound

The concentration of the enzyme was measured by the time it took for a particular depth of yellow colour to form.

(a) Scientists have recorded many cases of abnormal sexual development in male animals living in rivers. Some scientists have linked this abnormal development to the presence of oestrogen in rivers. Suggest and explain why. 2

(b) There has been an increase in the amount of oestrogen in rivers over the

past forty years. Suggest two reasons why. 2

oestrogen receptor protein might be isolated. 2

(d) This test is very specific for oestrogen. Explain why. 2

(e) When carrying out this test, suggest two variables which have to be controlled.

Explain why each variable has to be controlled. 4

(f) The yeast test was calibrated using known concentrations of oestrogen. The

graph shows the enzyme activity produced by different concentrations of oestrogen.

The concentrations are measured in ng per litre. A ng is 10-9 grammes.

The test is only accurate for a certain range of concentrations of oestrogen.

Suggest what the range is and explain your answer.

Range ……………… to …………………..ng per litre 1

Explanation 2

(g) Low concentrations of oestrogen cause the yeast cells to produce large

numbers of enzyme molecules. Suggest how. 3

(h) Describe how you would carry out an investigation to find the optimum

temperature at which to carry out the test. 3

(i) The hepatitis B virus causes life-threatening disease in humans.

One vaccine for hepatitis B contains inactivated viruses. Scientists have

produced a different vaccine using a strain of yeast that has been

genetically engineered to produce a protein of the hepatitis B virus. The

protein is purified and used as a vaccine.

The scientists believe that the vaccine produced by the yeast cells is safer

and at least as effective as the vaccine containing inactivated virus.

Suggest why they believe the vaccine from yeast is better. 4

8. The pH of soil affects the growth of plants. Scientists tried to grow two species of plant on

large areas of bare soil of different pH. They grew each species on its own.

In a different investigation, the scientists recorded the growth of the same species in natural communities. The pH of the soil where each species grew was also measured.

The graphs show the results of both investigations.

(a) The results are very similar when common sorrel and sheep’s fescue are grown

on their own. Suggest one explanation. 2

(b) The results obtained when these plants are growing in communities are different

from when they are grown on their own. Explain the difference. 3

9. The housefly is an insect. The egg of the housefly hatches into a larva that feeds and grows, before changing into an adult fly. The larva and the adult feed on similar types of food.

Scientists investigated the digestive enzymes produced by larvae and adults. The diagram shows how they extracted the enzymes.

(a) Explain why the tissues were homogenised. 1

(b) Explain why a buffer solution was used. 2

(c) Describe how differential centrifugation was used to produce the supernatant solution and the pellet containing cell surface membranes. 2

(d) Treatment of the membrane pellet with the detergent solution produced a

solution containing exopeptidase and an enzyme that digests a disaccharide. Explain how. 2

(e) The scientists analysed the amylase and lysozyme enzymes from the larva

and adult fly. They found that

· both amylases digested starch but that they had a different protein structure

· both lysozymes had the same protein structure but the amounts produced by

the larva and adult were significantly different.

(i) Use your knowledge of genes to explain how the amylase enzymes in the

larva and adult can have a different structure. 2

(ii) Use your knowledge of transcription to explain the different amounts of

lysozyme produced by the larva and adult. 2