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Interaction and interdependence - Defence against disease

Grade 12IBBiology

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

🔑Concepts

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Pathogens are organisms or viruses that cause disease. These include bacteria, viruses, fungi, and protozoa.

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The first line of defense includes the skin and mucous membranes. The skin provides a physical barrier and secretes sebaceous glands that lower the pH to between 4.54.5 and 6.06.0 to inhibit bacterial growth.

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Blood clotting is a cascade of reactions. When platelets or damaged tissue release clotting factors, the enzyme thrombin is produced, which converts the soluble protein fibrinogen into the insoluble fibrous protein fibrin.

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The second line of defense involves non-specific ingestion of pathogens by phagocytic white blood cells (macrophages) through the process of endocytosis.

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The third line of defense is specific immunity. B-lymphocytes produce antibodies (IgIg) that bind to specific antigens on the surface of pathogens.

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Antibiotics block specific metabolic pathways in prokaryotic cells, such as 70S70S ribosome function or cell wall synthesis, but are ineffective against viruses because viruses use the host cell's metabolic pathways.

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A primary immune response occurs on the first exposure to an antigen, characterized by a lag phase. A secondary immune response, facilitated by memory cells, is much faster and produces a higher concentration of antibodies ([Ab][Ab]).

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The structure of an antibody is a Y-shaped protein consisting of two heavy chains and two light chains, with a variable region specific to an antigen.

📐Formulae

Actual Size=Image SizeMagnification\text{Actual Size} = \frac{\text{Image Size}}{\text{Magnification}}

Prothrombin→Clotting FactorsThrombin\text{Prothrombin} \xrightarrow{\text{Clotting Factors}} \text{Thrombin}

Fibrinogen→ThrombinFibrin\text{Fibrinogen} \xrightarrow{\text{Thrombin}} \text{Fibrin}

Magnification=Measured Length of Scale BarLabel on Scale Bar\text{Magnification} = \frac{\text{Measured Length of Scale Bar}}{\text{Label on Scale Bar}}

💡Examples

Problem 1:

A micrograph of a StaphylococcusStaphylococcus bacterium shows the cell with a diameter of 25 mm25 \text{ mm}. If the magnification of the image is ×50,000\times 50,000, calculate the actual diameter of the bacterium in micrometers (μm\mu m).

Solution:

Step 1: Use the formula Actual Size=Image SizeMagnification\text{Actual Size} = \frac{\text{Image Size}}{\text{Magnification}}. Step 2: Convert 25 mm25 \text{ mm} to μm\mu m: 25×1000=25,000μm25 \times 1000 = 25,000 \mu m. Step 3: Calculate: 25,000μm50,000=0.5μm\frac{25,000 \mu m}{50,000} = 0.5 \mu m.

Explanation:

The actual size is determined by dividing the measured image size by the magnification factor, ensuring units are consistent (usually μm\mu m or nmnm for microbiology).

Problem 2:

Explain why penicillin is effective against StreptococcusStreptococcus bacteria but not against the Influenza virus.

Solution:

Penicillin inhibits the synthesis of peptidoglycan in bacterial cell walls. Bacteria are prokaryotes with their own metabolic pathways. Viruses like Influenza lack a cell wall and do not have their own metabolism; they rely on the host's 80S80S ribosomes and enzymes.

Explanation:

Antibiotics target specific prokaryotic structures (like the cell wall or 70S70S ribosomes) which are absent in viruses and distinct from eukaryotic host cells.