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Drugs - Medicinal drugs (Antibiotics)

Grade 12IGCSEBiology

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

🔑Concepts

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Antibiotics are chemical substances, often derived from microorganisms like PenicilliumPenicillium fungi, that kill or inhibit the growth of bacteria without harming the host cells (selective toxicity).

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Antibiotics work by disrupting bacterial processes, such as the synthesis of the peptidoglycan cell wall, the function of 70S70S ribosomes during protein synthesis, or the replication of bacterial DNADNA.

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Antibiotics are ineffective against viruses because viruses lack a cell wall, do not have their own metabolic pathways, and replicate inside host cells where drugs cannot easily reach them without damaging the host.

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Antibiotic resistance is a significant global health issue. It arises through random genetic mutations in bacteria. When antibiotics are overused or courses are not finished, resistant bacteria survive and reproduce via NaturalSelectionNatural Selection.

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Resistance genes are often carried on small, circular pieces of DNADNA called plasmids, which can be transferred between bacteria through a process called conjugation.

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The zone of inhibition in an agar plate experiment is used to measure the effectiveness of an antibiotic; a larger diameter indicates a more effective drug against that specific bacterial strain.

📐Formulae

Nt=N0×2nN_t = N_0 \times 2^n

n=log⁡10Nt−log⁡10N0log⁡102n = \frac{\log_{10} N_t - \log_{10} N_0}{\log_{10} 2}

Area of Inhibition=πr2Area\ of\ Inhibition = \pi r^2

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

💡Examples

Problem 1:

A population of bacteria starts with 500500 individuals. If they divide every 3030 minutes, calculate the total number of bacteria after 44 hours, assuming no antibiotics are present.

Solution:

Nt=500×28=128,000N_t = 500 \times 2^8 = 128,000 bacteria

Explanation:

In 44 hours (240240 minutes), there are 240/30=8240 / 30 = 8 generations. Using the formula Nt=N0×2nN_t = N_0 \times 2^n, where N0=500N_0 = 500 and n=8n = 8.

Problem 2:

In an experiment testing the efficacy of Penicillin, the diameter of the zone of inhibition is measured as 1414 mmmm. Calculate the area of the bacterial growth that was inhibited.

Solution:

Area=π×(7 mm)2≈153.94 mm2Area = \pi \times (7\ mm)^2 \approx 153.94\ mm^2

Explanation:

The radius rr is half of the diameter (14/2=7 mm14 / 2 = 7\ mm). The area is calculated using the formula for the area of a circle: A=πr2A = \pi r^2.

Problem 3:

Explain why penicillin is effective against Gram-positive bacteria but not against human cells.

Solution:

Penicillin inhibits the enzyme responsible for cross-linking peptidoglycan chains in the bacterial cell wall. Human cells do not possess a cell wall or peptidoglycan, meaning the drug has no target to act upon.

Explanation:

This is the principle of selective toxicity, where the drug targets structures specific to the pathogen (BacteriaBacteria) that are absent in the host (HumansHumans).

Medicinal drugs (Antibiotics) - Revision Notes & Key Diagrams | IGCSE Grade 12 Biology