Antibiotics, culturing microorganisms and new medicines — Edexcel GCSE Biology
How antibiotics work, aseptic technique, the antimicrobial core practical, developing new medicines and monoclonal antibodies.
Revision notes
7 short notes, in the order of the specification. Each one in short:
Antibiotics are medicines that treat bacterial infections by inhibiting cell processes in the bacterium, without harming the host's own cells. They cannot treat viral diseases such as HIV, colds or flu, because viruses are not cells. Viruses live and reproduce inside host cells, so they do not have the cell processes that antibiotics inhibit.
Aseptic techniques
Triple onlyAseptic technique stops unwanted microorganisms getting into a culture and stops the culture escaping. The agar and petri dishes are sterilised in an autoclave, and the inoculating loop is heated in a flame until red hot, then cooled. Lids are lifted only briefly, then taped on, and the dishes are incubated upside down.
In this core practical, bacteria are spread over sterile agar to make a lawn, and paper discs soaked in antiseptics, antibiotics or plant extracts are placed on top, with a water control disc. After incubating at 25 °C for about 48 hours, the clear zone around each disc is measured. A larger clear zone means a more effective substance.
Calculating cross-sectional areas
Triple onlyThe area of a clear zone or bacterial colony is calculated with the equation for the area of a circle, area = πr², because they are roughly circular. The radius, r, is half the diameter, so measure the diameter, halve it, square it and multiply by π. A clear zone 14 mm across has an area of about 154 mm².
Developing a new medicine has four stages: discovery, development, preclinical testing and clinical testing. In preclinical testing the medicine is tested on cells, tissues and then live animals to check whether it is toxic and whether it works. In clinical testing it is given first to a few healthy volunteers, then to patients to find the right dose.
Producing monoclonal antibodies
Triple onlyHigher tierMonoclonal antibodies are identical antibodies, all made from one type of cell, that bind to only one specific antigen. An animal is exposed to the antigen, and its lymphocytes that make the desired antibody are collected. These are fused with tumour cells to make hybridoma cells, which divide repeatedly and produce the antibody, which is then purified.
Uses of monoclonal antibodies
Triple onlyHigher tierMonoclonal antibodies bind to one specific antigen, so they can target particular molecules or cells. They are used in pregnancy tests to detect a hormone in urine, in diagnosis to show the position of blood clots or cancer cells, and in treatment to carry a drug directly to cancer cells, so fewer healthy cells are damaged.
Exam questions
7 exam-style questions (20 marks), each with its mark scheme.
Answer the questionsFlashcards
14 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of health, disease and the development of medicines on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheet
Method, variables, risks and a results table, with questions to answer.
Print it
Free PDFs to print or save.
Quick check questions
Why can antibiotics not be used to treat viral infections?
Show answer
Antibiotics inhibit cell processes in bacteria; viruses are not cells and reproduce inside host cells.
Triple only: why is an inoculating loop heated in a flame before use?
Show answer
To sterilise it by killing any microorganisms on it.
Triple only: in the core practical, what does a larger clear zone show?
Show answer
The substance is more effective at stopping bacteria growing.
Name the four stages in developing a new medicine in order.
Show answer
Discovery, development, preclinical testing, clinical testing.
Triple only, Higher tier: what is a hybridoma cell?
Show answer
A cell made by fusing an antibody-producing lymphocyte with a tumour cell; it divides and produces antibodies.
Frequently asked questions
Why can't antibiotics kill viruses?
Antibiotics cannot kill viruses because they work by inhibiting cell processes in bacteria, and viruses are not cells. Viruses live and reproduce inside host cells, so they do not have the cell processes that antibiotics target. This is why antibiotics cannot be used to treat viral diseases such as HIV, colds or flu.
What are the stages of developing a new medicine?
New medicines go through four stages: discovery of a chemical that might treat the disease, development to improve it, preclinical testing on cells, tissues and live animals to check toxicity and effectiveness, and clinical testing on people. Clinical testing starts with a few healthy volunteers to check safety, then patients to find the dose.
Why are bacterial cultures incubated at 25 °C?
Triple only Bacterial cultures are incubated at 25 °C because a lower temperature makes it less likely that harmful pathogens, which grow best at body temperature, will grow. The lid is taped on but not fully sealed, so microorganisms cannot escape or get in, but oxygen can still enter. The dish is incubated upside down.
How do you calculate the area of a clear zone?
Triple only Calculate the area of a clear zone using area = πr², because the zone is roughly circular. Measure the diameter across the middle, halve it to get the radius, square it and multiply by π. For example, a 14 mm diameter gives a radius of 7 mm and an area of about 154 mm². Never use the diameter instead.
What is a monoclonal antibody?
Triple onlyHigher tier A monoclonal antibody is one of many identical antibodies, all made from one type of cell, that bind to only one specific antigen. They are made by hybridoma cells, formed by fusing a lymphocyte with a tumour cell. Because they are specific, they are used in pregnancy tests, diagnosis and targeting drugs to cancer cells.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026