3.6.2Additional lessons
Barriers to Pathogens: Skin and Mucous Membranes — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 71 of 142
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5The Skin as a Barrier
The skin covers the body and prevents most pathogens from reaching deeper tissue.
Physical barrier
- Physical barrier The outer layer of the skin is made of tough, dead cells packed with keratin that pathogens find hard to penetrate.
Sebum
- Sebum Oil released into hair follicles keeps the skin supple and its acidity slows the growth of many microorganisms.
Normal flora
- Normal flora Harmless bacteria living on the skin compete with pathogens for space and nutrients.
Sealing wounds
- Sealing wounds When the skin is cut, blood clotting quickly closes the gap so microorganisms cannot enter.
Section 2 of 5Mucous Membranes
Location
- Location Mucous membranes line the airways, digestive tract and reproductive tract, which are surfaces exposed to the outside.
Mucus
- Mucus Goblet cells secrete mucus, a sticky fluid that traps microorganisms and dust particles.
Cilia
- Cilia Ciliated cells in the airways sweep trapped mucus away from the lungs towards the throat, where it is swallowed.
Stomach acid
- Stomach acid Swallowed microorganisms are killed by hydrochloric acid in the stomach, which has a very low pH.
Chemical defences
- Chemical defences Lysozyme in tears and saliva is an enzyme that breaks down bacterial cell walls.
Section 3 of 5How the Airway Is Cleaned
1
Air breathed in contains dust, spores and microorganisms that could reach the lungs.
2
Mucus produced by goblet cells traps these particles on the lining of the trachea and bronchi.
3
Cilia beat in a coordinated rhythm to move the mucus layer upwards.
4
The mucus is swallowed or coughed out, and swallowed pathogens are killed by stomach acid.
Section 4 of 5Limits of the First Line of Defence
Limits of the First Line of Defence
Non-specific
- Non-specific Barriers work against all kinds of microorganisms equally and do not target one type of pathogen.
Damage
- Damage Cuts, burns and damaged cilia, for example from smoking, allow pathogens to enter more easily.
Next defence
- Next defence If a pathogen passes the barrier, the immune system provides a second line of defence.
Section 5 of 5Exam Checklist & Tips
3.6.2 – Key Points for the Exam
- Keratin A tough protein found in the outer layer of the skin.
- Mucus A sticky secretion that traps microorganisms and particles.
- Cilia Tiny hair-like structures on cells that move mucus.
- Goblet cell A cell that secretes mucus.
- Lysozyme An enzyme in tears and saliva that damages bacterial cell walls.
Common Mistakes
- Saying the skin is a living barrier of thick cells; its outer layer is dead and keratinised.
- Writing that mucus kills pathogens; it mainly traps them, and other processes destroy them.
- Thinking barriers are specific to certain pathogens.
- Placing cilia on the goblet cells; cilia are on separate ciliated cells.
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