How to Use This Guide
1. Learn the keywords
Many exam questions are marked by key scientific words. Use terms such as pathogen, transmission, immunity, resistance, phagocytosis and fermentation.
2. Compare similar ideas
Do not confuse antiseptic with disinfectant, or bacteria with viruses. These are common exam traps.
3. Practise short answers
For a 2-mark question, give two clear scientific points. Do not write one vague sentence.
1. Types of Micro-organisms
Micro-organisms are organisms that are too small to be seen clearly without a microscope. Some cause disease, but many are useful.
| Group | Main features | Exam example |
|---|---|---|
| Bacteria | Living, single-celled, have cell wall, cytoplasm, ribosomes and DNA. | Reproduce by binary fission; antibiotics may work against bacteria. |
| Viruses | Acellular; genetic material inside a protein coat called a capsid. | Need a host cell to replicate; antibiotics do not work. |
| Fungi | Can be unicellular or multicellular; moulds have hyphae. | Penicillium mould produces penicillin. |
| Protozoa | Single-celled eukaryotes. | Some cause disease, such as malaria parasites. |
2. Microbial Growth
Microbes grow best when environmental conditions are suitable. Bacteria reproduce by binary fission, where one cell divides into two identical cells.
Minimum growth needs
- Moisture
- Warmth
- Nutrients
- Suitable pH
Growth slows when
- Temperature is too low or too high
- Food or water is limited
- pH is unsuitable
- Preservatives are present
3. Controlling Microbial Growth
Microbial control reduces or kills microbes to prevent infection, contamination and food spoilage.
Physical methods
Heat, refrigeration, filtration, UV radiation and autoclaving.
Chemical methods
Disinfectants, antiseptics and preservatives.
Mechanical methods
Handwashing physically removes microbes from skin.
| Term | Meaning | Example |
|---|---|---|
| Sterilisation | Kills all micro-organisms, including spores. | Autoclaving lab equipment. |
| Disinfectant | Chemical used on non-living surfaces. | Bleach on a bench. |
| Antiseptic | Chemical used on living tissue. | Iodine or alcohol on skin. |
| Refrigeration | Slows microbial growth. | Keeping food cold. |
4. Disease, Pathogens and Transmission
A pathogen is a micro-organism or infectious agent that causes disease.
| Type of spread | Meaning | Example |
|---|---|---|
| Droplet | Pathogens carried in tiny droplets from coughs or sneezes. | Respiratory infections. |
| Fomite | Object that transfers pathogens. | Door handle, desk, shared equipment. |
| Vector | Living organism that carries a pathogen. | Mosquito spreading malaria. |
| Water/food | Pathogens enter through contaminated food or water. | Cholera or food poisoning. |
Localized vs systemic infection
Localized infection stays in one area. Systemic infection spreads throughout the body, often through the blood.
5. The Immune System
The immune system protects the body from pathogens. It includes barriers, white blood cells and antibodies.
Phagocytosis
White blood cells engulf and digest pathogens.
Active immunity
The body makes its own antibodies after infection or vaccination.
Passive immunity
Antibodies are received from another source, such as mother to baby.
6. Normal Flora
Normal flora are useful microbes that naturally live on or inside the body, especially on the skin and in the gut.
Strong answer: Normal flora protect the body by competing with pathogens for nutrients and space, making it harder for harmful microbes to grow.
7. Epidemiology and Global Health
Epidemiology is the study of disease patterns in populations. It helps scientists understand how diseases spread and how they can be controlled.
R₀ value
The average number of people one infected person passes the disease to.
Contact tracing
Identifying people who may have been exposed to an infected person.
Quarantine
Separating exposed individuals to reduce spread.
Why international cooperation is needed
- Diseases can cross borders through travel.
- Countries need to share data about outbreaks.
- Vaccines, medicines and equipment may need to be distributed fairly.
- Organisations such as WHO help coordinate responses.
8. Beneficial Uses of Micro-organisms
| Use | Microbe/process | What happens |
|---|---|---|
| Bread | Yeast fermentation | Yeast produces carbon dioxide, which makes dough rise. |
| Yogurt/cheese | Lactic acid bacteria | Bacteria convert sugars into lactic acid. |
| Insulin | Genetically engineered bacteria | Bacteria are modified to produce human insulin. |
| Bioremediation | Decomposer microbes | Microbes break down pollutants. |
| Agriculture | Nitrogen-fixing bacteria | Bacteria convert nitrogen into forms plants can use. |
9. Common Exam Traps
| Do not confuse... | Correct difference |
|---|---|
| Antibiotics and vaccines | Antibiotics treat bacterial infection. Vaccines prevent disease by stimulating immunity. |
| Antiseptic and disinfectant | Antiseptics are used on living tissue. Disinfectants are used on surfaces. |
| Bacteria and viruses | Bacteria are cells. Viruses are acellular and need host cells. |
| Active and passive immunity | Active = body makes antibodies. Passive = antibodies are received. |
| Localized and systemic infection | Localized stays in one area. Systemic spreads around the body. |
10. Practice Questions
Show answer
Virus.Show answer
Viruses are not cells and do not have the same bacterial structures or processes that antibiotics target.Show answer
The process by which white blood cells engulf and digest pathogens.Show answer
They compete with pathogens for space and nutrients, and they help stimulate/support the immune system.Show answer
Disease can cross borders through travel, so countries must share data, resources, vaccines and public health strategies.Short answer practice space
Explain the difference between a disinfectant and an antiseptic, giving one example of each.
Final Revision Checklist
pathogentransmissionphagocytosisimmunityantibiotic resistancefermentationepidemiology