Key Learning Objectives
By the end of this lesson and revision module, Year 7 students will be able to:
- Distinguish between the main types of microorganisms (bacteria, viruses, fungi, and protists).
- Explain the difference between beneficial microbes (e.g. gut flora, yeast in baking) and harmful pathogens.
- Understand how the human immune system and aseptic techniques prevent disease transmission.
Topic Summary & Essential Notes
Microorganisms (or microbes) are microscopic organisms invisible to the naked eye. They include bacteria (single-celled prokaryotes), viruses (non-living genetic strands that hijack host cells), fungi (yeasts and moulds), and protists. While some microbes are pathogens that cause infectious diseases (such as cholera, flu, or athlete's foot), the vast majority of microbes are beneficial or essential to life — aiding human digestion, fermenting foods (bread, yogurt, cheese), and decomposing organic waste to recycle nutrients in ecosystems.
Key Rules, Formulas & Definitions
Bacterial Binary Fission
Bacteria reproduce by splitting in two: Total Bacteria = 2ⁿ (where n = number of division cycles)
Types of Pathogens
Viruses (smallest, inside cells), Bacteria (release toxins), Fungi (hyphae/spores), Protists (vector-spread)
Primary Body Defences
Skin (barrier), Stomach Acid (kills ingested microbes), Cilia/Mucus (traps airborne pathogens), White Blood Cells (phagocytosis & antibodies)
2 Step-by-Step Worked Examples
Study these model solutions to understand how examiners award method and accuracy marks:
Worked Example 1
Worked Example 1: Bacterial Growth by Binary Fission
Question: A single bacterium divides every 20 minutes under ideal laboratory conditions. Calculate how many bacteria will be produced after 2 hours.
Step-by-Step Solution:
- Step 1: Calculate number of divisions. 2 hours = 120 minutes. Number of divisions n = 120 ÷ 20 = 6 divisions.
- Step 2: Apply exponential formula. Total = 2⁶ = 2 × 2 × 2 × 2 × 2 × 2.
- Step 3: Calculate. 2⁶ = 64 bacteria.
Final Answer: 64 bacteria
Senior Examiner Insight: Always convert total time into minutes before dividing by the mean division time.
Worked Example 2
Worked Example 2: Comparing Antibiotics and Antivirals
Question: Explain why a doctor will prescribe antibiotics to treat bacterial tonsillitis but NOT for influenza (the flu).
Step-by-Step Solution:
- Step 1: Identify pathogen types. Tonsillitis is caused by bacteria; influenza is caused by a virus.
- Step 2: Mode of action. Antibiotics kill bacteria by destroying cell walls or inhibiting bacterial metabolism.
- Step 3: Viral resistance. Viruses live and reproduce inside host cells and lack bacterial structures, so antibiotics have zero effect on them.
Final Answer: Antibiotics only kill bacteria, not viruses. Flu is a viral disease.
Senior Examiner Insight: State clearly that viruses reproduce inside human cells, which makes targeting them with antibiotics impossible.
Common Pitfalls & Examiner Warnings
Avoid these common mistakes frequently identified by examiners in Key Stage 3 assessments:
Common Mistake: Calling viruses 'living cells'
Viruses are NOT made of cells and are non-living particles consisting only of genetic material wrapped in a protein coat.
Common Mistake: Believing all bacteria are dangerous
Trillions of gut bacteria protect us against illness and digest dietary fibre.
Common Mistake: Expecting antibiotics to cure common colds
Colds and flu are viral; prescribing antibiotics for them contributes to antibiotic resistance.
Interactive Self-Check Quiz (3 Questions)
Test your understanding before checking the full worked answer and examiner mark scheme:
Question 1 of 3
Which type of microorganism cannot reproduce independently and must hijack a host cell?
Click to Reveal Answer & Mark Scheme
Correct Answer: B) Virus
Detailed Explanation: Viruses invade host cells and use the host's cellular machinery to replicate copies of themselves.
Mark Scheme & Scoring Points: 1 mark for Virus.
Question 2 of 3
What is the function of hydrochloric acid in the human stomach in terms of immune defence?
Click to Reveal Answer & Mark Scheme
Correct Answer: B) To create a low pH environment that destroys pathogens swallowed in food and saliva.
Detailed Explanation: Stomach acid (pH 1.5–2.0) denatures pathogen enzymes and kills ingested microorganisms.
Mark Scheme & Scoring Points: 1 mark for destroying/killing ingested pathogens via low pH.
Question 3 of 3
If 10 bacteria divide every 30 minutes, how many bacteria will be present after 1 hour?
Click to Reveal Answer & Mark Scheme
Correct Answer: B) 40
Detailed Explanation: After 30 mins: 10 × 2 = 20. After 60 mins: 20 × 2 = 40 bacteria.
Mark Scheme & Scoring Points: 1 mark for correct calculation: 40 bacteria.
Created by Fiaraz Iqbal
Former Headteacher & Senior Science / Maths Examiner
With over 15 years of leadership and classroom experience, Fiaraz creates high-yield revision resources aligned with National Curriculum standards to help Key Stage 3 and GCSE students secure top grades.
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