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(a) You are required to make simple dilutions of the 10M solution which reduce the concentration between each successive dilution. You will need to prepare 10 cm$^3$ of each concentration.
(i) Table 1.1 shows how to make up one of the concentrations of molecule M you will use. Decide which concentrations of molecule M to prepare using simple dilutions of the 10M solution. Complete Table 1.1 to show how you will prepare the other concentrations.
$$\text{Table 1.1}$$
| volume of 10M /cm$^3$ | volume of distilled water, W /cm$^3$ | percentage concentration of molecule M |
|----------------------|------------------------------------|--------------------------------------------|
| 10 | 0 | 10 |
Proceed as follows:
1. Prepare the concentrations of molecule M as shown in Table 1.1.
2. Put 1 cm$^3$ of A into a test-tube.
3. Put 1 cm$^3$ of K into the same test-tube and mix well.
4. Put 1 cm$^3$ of 10M into the same test-tube and mix well. Start timing.
5. Record the time taken to reach the end-point in (a)(ii).
If the end-point is not reached in 4 minutes (240 seconds) record 'more than 240' and record the colour of the solution.
6. Repeat step 2 to step 5 for each of the concentrations of molecule M prepared in step 1.
(ii) Prepare the space below and record your results for the known concentrations of molecule M.
You are now required to estimate the concentration of molecule M in a sample of plant extract, U.
7. Repeat step 2 to step 4 with U. Record the time taken to reach the end-point in (a)(iii).
(iii) State the time taken to reach the end-point for sample U. .............................................. [1]
(iv) Use your results in (a)(ii) and (a)(iii) to estimate the concentration of molecule M in sample U.
concentration = .............................................................. [1]
(v) Describe how you could use this procedure to produce a more accurate estimate of the concentration of molecule M in the sample of plant extract U than the one given in (a)(iv).
(b) A student suggested that molecule M might act as an antibiotic.
In order to test this suggestion the student carried out the following investigation:
- bacteria were spread over the surface of a strip of agar gel containing nutrients
- bacteria were allowed to grow, shown by the shaded area in Fig. 1.1
- small drops (2 µm$^3$) of different concentrations of molecule M were put onto the surface of the agar gel strip
- after 24 hours, the inhibition area (where the bacteria were no longer observed) was measured for each concentration of molecule M.
$$\text{Fig. 1.1 shows a diagram of the strip of agar gel after 24 hours. This is not to scale.}$$
The results are shown in Table 1.2.
$$\text{Table 1.2}$$
| concentration of solution of molecule M /µg cm$^{-3}$ | inhibition area /mm$^2$ |
|------------------------------------------------------|-----------------------|
| 0 | 0 |
| 1 | 30 |
| 6 | 50 |
| 10 | 70 |
| 30 | 106 |
| 100 | 120 |
(i) Plot a graph of the data shown in Table 1.2. [4]
(ii) Use your graph to estimate the inhibition area for a concentration of molecule M of 46 µg cm$^{-3}$.
inhibition area = ......................................................... [1]
(iii) Explain how the data support the statement that molecule M might act as an antibiotic. [1]
(iv) Suggest how molecule M may act as an antibiotic. [2]
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(a) Select a part of the leaf on N1 which shows the four tissue layers $L$ ($L1$ and $L2$), $P$ and $Q$.
Do not include a vascular bundle.
(i) Use the eyepiece graticule in the microscope to measure:
• the depth of the whole leaf, $T$
• the depth of each of the tissues, $L$ ($L1$ and $L2$), $P$ and $Q$, as shown in Fig. 2.1.
$T = \text{................} \text{eyepiece graticule units}$
$L1 = \text{................} \text{eyepiece graticule units}$
$P = \text{................} \text{eyepiece graticule units}$
$Q = \text{................} \text{eyepiece graticule units}$
$L2 = \text{................} \text{eyepiece graticule units}$
(ii) Use the measurements from (a)(i) to determine the simplest ratio of the depth of the leaf ($T$) to the depth of the palisade layer.
You may lose marks if you do not show your working.
simplest ratio ......................................................
(iii) Use the measurements from (a)(i) to help you draw a large plan diagram of the part of the leaf on N1, as shown by the shaded area in Fig. 2.2.
This must include at least one vascular bundle.
You are expected to draw the correct shape and proportions of the different tissues.
Use one ruled label line and label to identify the palisade layer.
(iv) Observe the cells of the epidermis at the end of the leaf on N1 as shown in Fig. 2.2. These cells are not identical.
Select one group of four adjacent (touching) cells which show some of the differences between these cells. Each cell must touch at least one of the other cells.
Make a large drawing of this group of four cells.
Use one ruled label line and label to identify the cell wall of one cell.
(b) Fig. 2.3 is a photomicrograph of a stained transverse section through a different type of leaf.
You are not expected to be familiar with this specimen.
Annotate Fig. 2.3 to describe three observable differences between the leaf sections in Fig. 2.3 and on N1 by:
• drawing label lines to three features in Fig. 2.3 that show these differences
• describing next to each line how each feature is different from the specimen N1.
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