IGCSE · Practice questions · Why bacteria are used

Why bacteria do the work.

Six original Cambridge-style questions on why bacteria are used in biotechnology and genetic modification, including population doubling calculations and (Extended) plasmids and ethics.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Fast, clever, uncontroversial.

Core reasons: rapid reproduction rate; ability to make complex molecules (e.g. proteins).
Extended reasons: few ethical concerns over their manipulation and growth; they contain plasmids.
Doubling: number = 2ⁿ, where n = time ÷ time for one division.
Plasmid: small circular DNA, separate from the chromosome, copied every time the cell divides.

Recall

[2 marks]

State two reasons why bacteria are useful in biotechnology.

They have a rapid reproduction rate, so large populations are grown quickly. ✓
They can make complex molecules, such as proteins (for example enzymes or insulin). ✓

Calculation

[3 marks]

A bacterium divides once every 20 minutes. Starting with a single bacterium, calculate the number of bacteria present after 4 hours. Show your working.

4 hours = 240 minutes; 240 ÷ 20 = 12 divisions. ✓
Number = 2¹². ✓
= 4096 bacteria. ✓

Data

[3 marks]

The table shows the number of bacteria in a culture.

Time / min	0	25	50	75	100
Number of bacteria	100	200	400	800	1600

(a) State the time taken for the population to double. (b) Predict the number of bacteria at 150 minutes.

(a) 25 minutes (the number doubles every 25 minutes). ✓
(b) 150 minutes is two more doubling times after 100 minutes. ✓
1600 × 2 × 2 = 6400 bacteria. ✓

Explain

[4 marks]

Extended. Discuss why bacteria, rather than animals, are usually used when an organism is genetically modified to make a human protein.

Bacteria reproduce very rapidly, so a large population making the protein is produced quickly. ✓
They can make complex molecules such as proteins using the instructions in a gene. ✓
There are few ethical concerns over the manipulation and growth of bacteria, because they are single-celled with no nervous system; modifying animals raises more objections. ✓
Bacteria contain plasmids, which can be used to carry the human gene into the cell and are copied every time the cell divides. ✓

Recall

[3 marks]

Extended. Describe what a plasmid is and state where it is found.

A small, circular piece of DNA. ✓
Found in the cytoplasm of bacteria (prokaryotic cells). ✓
Separate from the main circular chromosome; it carries a few genes and is copied when the cell divides. ✓

Analysis

[3 marks]

A company can make an enzyme either by extracting it from the pancreas of cattle or by growing bacteria that make it. Suggest three advantages of using bacteria.

Bacteria reproduce rapidly, so large quantities can be made quickly, all year round. ✓
No animals need to be killed, so there are fewer ethical concerns. ✓
The supply does not depend on the number of cattle slaughtered; the product is consistent and there is less risk of passing on animal diseases. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.