Microscopy

Name three structures that are found in some prokaryotic cells but are not found in all prokaryotic cells.

Table 3 shows the resolution of three types of microscope.

Table 4 shows information about yeast cells and mitochondria.

Identify the correct microscope to use in each of the following investigations.

Use your knowledge of microscopes and information in Table 3 and Table 4.

Give two reasons for each choice of microscope.

Investigation 1 – to study the living cells in a yeast culture that contains both living cells and dead cells. The living cells remain colourless and the dead cells are stained blue when a dye is added.

Type of microscope

Investigation 2 – to observe the three-dimensional structure of mitochondria.

Type of microscope

Describe how we breathe in.

A scientist prepared alveolar tissue to view using an optical microscope. The scientist cut very thin slices of the alveolar tissue.

Explain why the scientist used very thin slices of alveolar tissue with the optical microscope.

Figure 2 is an image of the lung tissue observed using an optical microscope.

Identify the tube labelled A.

The scientist used a ruler to measure the diameter of some of the alveoli.

Table 2 shows the scientist’s results.

The magnification of the image in Figure 2 is × 40

Use this information and Table 2 to calculate the mean diameter, in μm, of the alveoli.

Show your working.

Give the uncertainty associated with taking a measurement using a ruler with 1 mm graduations.

Calculate the percentage error for a measurement using the ruler of 4 mm

Describe and explain how you would use cell fractionation and ultracentrifugation to obtain a sample of nuclei from muscle tissue.

Describe the role of organelles in the production and release of enzymes by animal cells.

Do not include details of transcription in your answer.

Describe the structure of ATP.

Outline how named enzymes break down and resynthesise ATP.

Define genome and proteome.

The classification system used in the early 20th century grouped different species of bacteria according to the position and shape of flagella on bacterial cells and by the number of flagella per cell. These were observed using an optical microscope. Each species of bacterium has a characteristic cell shape and arrangement of flagella. These characteristics may be shared with other species within a genus. Flagella are fragile, difficult to stain and may extend from the cell at any angle.

Consider the accuracy and limitations of the early classification of bacteria using the arrangement of flagella.

Suggest why several bacterial species have been renamed in recent years.

Figure 6 shows an image from an optical microscope of a single bacterial cell.

This bacterial cell is 2.3 μm long (excluding the flagellum).

Calculate the magnification of this image.

Show your working.

Give two features of all prokaryotic cells that are not features of eukaryotic cells.

Many multicellular organisms produce antimicrobial polypeptides (APs) that protect them against prokaryotes. Figure 3 shows how one type of AP acts on the cell-surface membrane of prokaryotes.

This AP has a secondary structure in a helical shape.

Tick () the box to show which type of bond maintains the helical structure of the polypeptide.

The amino acids on one side of each AP helix have hydrophobic properties. The amino acids on the opposite side of each helix have hydrophilic properties. Figure 4 shows this.

Suggest how these properties of the APs allow them to become positioned across the membrane (as shown in Figure 3) and make a channel through which ions can pass.

Figure 5 shows further information about a channel formed in the cell-surface membrane by the APs.

Use Figure 5 to calculate the cross-sectional area of the channel through which ions can pass. Assume the cross-sectional area is circular. Use π = 3.14 in your calculation. Give your answer in nm² and to 1 decimal place.

The APs damage prokaryotic cells but do not damage the eukaryotic cells in the organisms that produce them. Prokaryotic cell membranes do not contain cholesterol.

Assess why the APs do not damage the eukaryotic cells of the organisms that produce them.

Scientists observed these APs on prokaryotes using a transmission electron microscope. They stained the APs using a monoclonal antibody with gold attached to it.

Suggest how these techniques allowed observation of APs on prokaryotes.

Complete Table 3 to give three differences between DNA molecules and tRNA molecules.

Scientists investigated ribosomal RNA in liver cells.

Figure 10 shows the method they used to isolate the ribosomes from the liver cells. The detergent dissolves lipids.

The scientists broke open the cells to produce a suspension of cell contents.

Describe how the scientists would remove large organelles from this suspension of cell contents.

Explain the position of the bands of ribosomes in tubes A and B in Figure 10.

Describe how a sample of chloroplasts could be isolated from leaves.

Scientists grew two groups of plants:

control plants with all the inorganic ions needed

iron-deficient plants with all the inorganic ions needed but without iron ions.

After 1 week, the scientists measured the mass of protein and the mass of chlorophyll in the chloroplasts isolated from samples of leaves of these two groups of plants.

Table 4 shows the scientists’ results.

Some proteins found inside the chloroplast are synthesised inside the chloroplast.

Give one feature of the chloroplast that allows protein to be synthesised inside the chloroplast and describe one difference between this feature in the chloroplast and similar features in the rest of the cell.

Structural difference

The ratio of protein to chlorophyll in control plants is 9:1

Use the information in Table 4 to calculate the ratio of protein to chlorophyll in iron-deficient plants.

The scientists also observed the chloroplasts from the samples of leaves using an electron microscope.

Figure 6 shows a chloroplast from a control plant (image A) and a chloroplast from an iron-deficient plant (image B).

Figure 6

This source has been removed due to third-party copyright restrictions.

Use Figure 6 to suggest why iron-deficient plants have a reduced growth rate.

Describe how a non-competitive inhibitor can reduce the rate of an enzyme-controlled reaction.

Pectin is a substance found in some fruit and vegetables.

A scientist investigated the effect of pectin on the hydrolysis of lipids by a lipase enzyme.

His results are shown in Figure 1.

The scientist concluded that pectin is a non-competitive inhibitor of the lipase enzyme.

Use Figure 1 to explain why the scientist concluded that pectin is a non-competitive inhibitor.

The scientist also found that pectin stops the action of bile salts. He prepared two suspensions:

• suspension A – lipid and bile salts

• suspension B – lipid, bile salts and pectin.

He did not add lipase to either suspension.

He observed samples from the suspensions using an optical microscope. Figure 2 shows what he saw in a typical sample from each suspension.

Calculate the maximum length of the large lipid droplet marked X in Figure 2.

Using a ruler with millimetre intervals always includes an uncertainty in the measurement. Use the uncertainty in your measurement to determine the uncertainty of your calculated maximum length.

You can assume there is no uncertainty in the magnification.

No large lipid droplets are visible with the optical microscope in the samples from suspension A.

Explain why.

Scientists investigated the role of a protein called CENP-W in mitosis. Their method involved cell fractionation and ultracentrifugation.

The scientists began by lysing (breaking open) cells and organelles using a detergent that dissolves lipids in water.

Suggest how the detergent releases CENP-W from cells.

Explain how ultracentrifugation separates CENP-W from other molecules.

CENP-W is involved in the formation of spindle fibres in mitosis. Spindle fibres are made of molecules of a protein called tubulin.

The scientists treated cells in a culture with small interfering RNA (siRNA). This siRNA causes RNA interference of expression of the CENP-W gene. The scientists took samples of cells at 0, 48 and 72 hours after adding the siRNA. They then used gel electrophoresis to separate CENP-W and tubulin from these samples.

Figure 5 shows the results of the electrophoresis. The size of each band is proportional to the amount of CENP-W or tubulin present.

Suggest how the siRNA produced these results.

Contrast how an optical microscope and a transmission electron microscope work and contrast the limitations of their use when studying cells.

Figure 6 shows an image from an optical microscope of meiosis occurring in a flower bud of a flowering plant. W and Z are undergoing meiosis.

Explain the appearance of W and Z.

An environmental scientist investigated a possible relationship between air pollution and the size of seeds produced by one species of tree.

He was provided with a very large number of seeds collected from a population of trees in the centre of a city and also a very large number of seeds collected from a population of trees in the countryside.

Describe how he should collect and process data from these seeds to investigate whether there is a difference in seed size between these two populations of trees.

Bacteriophages are viruses that kill bacteria.

Figure 2 shows drawings of a bacteriophage and a bacterium.

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.

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.

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.