Using Figure 2 and your own knowledge, put a tick (✓) in the box next to the only correct statement about the structures of the bacteriophage and the bacterium.

☐ Both have ribosomes.

☐ Both have a cell-surface membrane.

☐ The bacteriophage has a capsid and the bacterium has a cell-surface membrane.

☐ The bacteriophage has a cell wall and the bacterium has a capsid.

Using the scales in Figure 2, calculate how many times longer the bacterium is than the bacteriophage.

Use the distance between the points labelled A and B on each drawing in your calculations. Show your working.

The bacterium is ______________ times longer

Scientists investigated the use of bacteriophages to treat lung infections caused by bacteria. They infected the lungs of mice with a pathogenic species of bacterium.

The mice were then divided into two groups, A and B. • The mice in group A were not treated with bacteriophage. • The mice in group B were treated by breathing in a spray containing bacteriophage particles.

After 3 days, the scientists killed the mice and removed their lungs. They washed out each set of lungs with a set volume of liquid. The scientists determined the number of live bacteria in the liquid.

Figure 3 shows the scientists' results. Figure 3 shows the mean and the range of the data about the mean for each group. Standard deviations of the means are not shown.

What would the scientists' null hypothesis be for this investigation?

With some samples, the scientists decided they needed to carry out a series of dilutions of the sample before counting the bacteria.

Use evidence from Figure 3 to explain why dilutions were necessary for some samples but not for others.

Using only Figure 3, what can you conclude from these data about the effectiveness of the bacteriophage in treating this lung infection in mice?

Do not consider statistical analyses in your answer.