3.7.2Additional lessons
Barriers to Pathogens: Skin and Mucous Membranes — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 1 of 101
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.
Part 1 of 4Skin as a Physical Barrier
The skin is the body's largest organ and covers almost all of its outer surface.
Keratinised surface
- Keratinised surface The outer layer of the epidermis is made of dead cells filled with tough keratin,
- forming a dry, waterproof layer that most pathogens cannot penetrate.
Constant renewal
- Constant renewal Cells at the surface are shed continuously and replaced from below, carrying away microorganisms that have landed on the skin.
Dry, acidic surface
- Dry, acidic surface Skin has little moisture and a slightly acidic pH, conditions that make it hard for many bacteria and fungi to grow and divide.
Normal flora
- Normal flora Harmless microorganisms living on the skin compete with pathogens for space and nutrients, so invaders struggle to become established.
Sebum and sweat
- Sebum and sweat Oily and watery secretions contain substances that slow microbial growth, adding a chemical layer to the physical barrier.
Part 2 of 4Mucous Membranes and Secretions
Mucous membranes line the openings of the body that are in contact with the outside world.
Mucous Membranes and Secretions
Mucus
- Mucus Goblet cells secrete sticky mucus that traps bacteria, viruses and dust particles in the airways, gut and reproductive tract before they reach deeper tissues.
Cilia
- Cilia Ciliated epithelial cells in the trachea and bronchi beat in coordinated waves, sweeping the mucus and its trapped microbes upwards to the throat.
Swallowing and coughing
- Swallowing and coughing Mucus moved up to the throat is swallowed,
- and the strongly acidic stomach juice then destroys many of the microorganisms it contains.
Lysozyme
- Lysozyme This enzyme in tears, saliva and nasal mucus breaks down peptidoglycan in bacterial cell walls, bursting the bacteria.
Stomach acid
- Stomach acid Hydrochloric acid in gastric juice gives a very low pH that kills most pathogens swallowed in food, drink or mucus.
Part 3 of 4What Happens When the Barrier Fails
1
A cut, burn or bite breaks the keratinised epidermis, leaving an opening through which bacteria and viruses can enter the underlying tissue.
2
Platelets and clotting factors respond within seconds to seal the wound, which limits blood loss and blocks the entry route.
3
Pathogens that do enter are met by the innate immune system, in which phagocytes engulf and digest non-self material.
4
Surviving pathogens are presented as antigens to lymphocytes, which start the slower but specific adaptive immune response.
| Barrier | Type of defence | How it protects |
|---|---|---|
| Keratinised skin | Physical | Dry, tough layer that blocks entry of microbes |
| Mucus and cilia | Physical and mechanical | Traps microbes and moves them out of the airways |
| Lysozyme | Chemical | Digests bacterial cell walls |
| Stomach acid | Chemical | Low pH kills ingested microorganisms |
Part 4 of 4Exam Checklist & Tips
3.7.2 – Key Points for the Exam
- Pathogen A microorganism or virus that causes disease in its host.
- Mucus A sticky glycoprotein secretion that traps microorganisms and particles.
- Cilia Hair-like projections on epithelial cells that beat to move mucus.
- Lysozyme An enzyme that digests peptidoglycan in bacterial cell walls.
- Primary defence Non-specific barriers that stop pathogens entering the body at all.
Common Mistakes
- Saying the skin kills pathogensit mainly blocks them physically, while chemical features only slow their growth.
- Forgetting that barriers are non-specific, since they act in the same way against every type of pathogen.
- Describing cilia as moving pathogens down towards the lungs; they sweep mucus upwards, away from the lungs.
- Stating that mucous membranes are found only in the airways, when they also line the gut and urinogenital tract.
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