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Diseases and Immunity - Pathogens and transmission

Grade 11IGCSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A pathogen is a disease-causing organism. Pathogens include bacteria, viruses, fungi, and protoctists.

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Transmissible diseases are infections where the pathogen can be passed from one host to another.

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Transmission can occur via direct contact (e.g., through blood or body fluids) or indirect contact (e.g., contaminated surfaces, food, water, or air).

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Cholera is caused by the bacterium VibrioVibrio choleraecholerae, which is transmitted through contaminated water. The bacteria produce a toxin that causes Cl−Cl^- (chloride ions) to be secreted into the small intestine.

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The secretion of Cl−Cl^- ions lowers the water potential (Ψ\Psi) in the gut lumen, causing water to move out of the blood and cells into the intestine by osmosis, leading to diarrhea and dehydration.

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Mechanical barriers to pathogens include the skin and hairs in the nose. Chemical barriers include HClHCl (hydrochloric acid) in the stomach and mucus.

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Cellular defense involves white blood cells: phagocytes perform phagocytosis (engulfing pathogens), and lymphocytes produce antibodies (proteinsproteins) that bind to specific antigens on pathogens.

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Vectors are organisms that transmit pathogens from one host to another without being affected by the disease themselves (e.g., the female Anopheles mosquito transmitting PlasmodiumPlasmodium which causes malaria).

📐Formulae

Magnification=Image sizeActual sizeMagnification = \frac{Image \ size}{Actual \ size}

Actual size=Image sizeMagnificationActual \ size = \frac{Image \ size}{Magnification}

Ψintracellular>Ψlumen  ⟹  Osmotic water loss\Psi_{intracellular} > \Psi_{lumen} \implies \text{Osmotic water loss}

💡Examples

Problem 1:

Explain the mechanism by which the cholera bacterium causes severe dehydration using the concept of water potential (Ψ\Psi).

Solution:

  1. The bacterium VibrioVibrio choleraecholerae releases a toxin. 2. This toxin stimulates the secretion of Cl−Cl^- ions from the intestinal wall into the lumen. 3. This increases the solute concentration in the lumen, significantly lowering the water potential (Ψlumen\Psi_{lumen}). 4. Water moves from the blood and cells (higher Ψ\Psi) into the intestine (lower Ψ\Psi) by osmosis down a water potential gradient.

Explanation:

In cholera, the loss of Cl−Cl^- ions creates an osmotic gradient that pulls water out of the body, resulting in watery feces and dehydration.

Problem 2:

A virus particle is measured under an electron microscope. The image size is 4040 mmmm and the magnification is ×200,000\times 200,000. Calculate the actual size of the virus in micrometers (μm\mu m).

Solution:

Actual size=40 mm200,000Actual \ size = \frac{40 \ mm}{200,000} Actual size=0.0002 mmActual \ size = 0.0002 \ mm Since 1 mm=1,000 μm1 \ mm = 1,000 \ \mu m: 0.0002×1,000=0.2 μm0.0002 \times 1,000 = 0.2 \ \mu m

Explanation:

To find the actual size, divide the image length by the magnification. Always ensure units are converted correctly; 1 mm=103 μm1 \ mm = 10^3 \ \mu m and 1 μm=103 nm1 \ \mu m = 10^3 \ nm.

Pathogens and transmission - Revision Notes & Key Diagrams | IGCSE Grade 11 Biology