Eukaryotic & prokaryotic cells

Escherichia coli is a bacterium that is used widely in scientific experiments and in biotechnology.

This is a transmission electron micrograph of E. coli.

Name the structure labelled A.

Based on your knowledge, estimate the diameter of the E. coli cell.

Give your answer in m.

A student carries out a serial dilution of an E. coli culture.

This is the method the student uses:

• Transfer 10 cm³ of E. coli culture to a sterilised test tube from an original culture that has a volume of 50 cm³.

• Carry out four 10-fold serial dilutions. Each dilution involves transferring 1 cm³ of culture from one test tube to another test tube containing 9 cm³ of distilled water.

• Transfer 1 cm³ of the final 10 cm³ diluted culture to an agar plate. Evenly spread 1 cm³ of liquid across the plate using a sterilised spreader.

• Use a micropipette for each transfer.

• Incubate the agar plates for 24 h.

• Count the number of colonies that develop on the plate. Each colony is assumed to develop from a single bacterium.

• Estimate the E. coli population in the original 50 cm³ culture.

Describe two improvements to the student’s method, other than using different equipment, that could improve the accuracy of their population estimate.

The student counts 22 colonies on the agar plate.

Calculate the total number of E. coli cells in the original 50 cm³ culture.

Give your answer in standard form.

Total number of E. coli cells in 50 cm³ =........

E. coli can be genetically modified to produce useful proteins.

State two other reasons why E. coli is a suitable microorganism to use in biotechnology.

Many organisms undergo asexual reproduction by mitosis.

State two other roles of mitosis in organisms.

Plants reproduce asexually by mitosis.

Explain why mitosis, and not meiosis, is used for asexual reproduction in plants.

Bacteria reproduce asexually by a process called binary fission.

Suggest why bacteria are unable to reproduce asexually by mitosis.

During meiosis, the independent assortment of chromosomes leads to the formation of genetically different gametes.

Gametes of the western clawed frog contain 10 chromosomes.

Calculate the number of genetically different gametes resulting from the independent assortment of chromosomes that would be produced by an individual frog.

Different types of microorganism can act as pathogens.

Which of the diseases is caused by a pathogen without mitochondria in its cells?

A Black sigatoka in bananas

B Late blight in potatoes

C Ring rot in potatoes

D Ringworm in cattle

Your answer

The image below is a photomicrograph of a mitochondrion.

State the type of microscope used to produce this image.

Identify the structures labelled M and N in the photomicrograph.

Describe two ways in which the structure of mitochondrial membranes is related to the function of a mitochondrion.

The endosymbiosis theory suggests that mitochondria may have evolved from bacteria that were taken inside other cells.

These cells then evolved into eukaryotes.

Give two structural features of mitochondria that support this theory.

Explain why early eukaryotes were able to grow more quickly than cells that did not possess mitochondria.

The onion plant, Allium cepa, is grown as a food crop around the world.

The table below contains statements about the root cells of an onion.

Place ticks (✓) in the boxes in the table to indicate whether the statements are true or false.

Fig. 1 shows a cross section of the root of an onion plant.

Identify the tissues shown at M and N.

The colour of onion bulbs is determined by two genes, A/a and B/b.

• A is a dominant allele and codes for the production of a red pigment.

• Onion bulbs that are homozygous for the recessive allele, a, produce no pigment and are white.

• B is a dominant allele that inhibits the expression of allele A.

• The recessive allele, b, allows the production of the red pigment.

A white onion plant was cross-pollinated with a red onion plant. All 15 offspring had the genotype AaBb.

Identify the following:

The genotype of the white onion plant........

The genotype of the red onion plant........

The phenotype of the offspring........

State the type of gene interaction shown by the genes A/a and B/b.

Suggest how allele B inhibits the expression of allele A.