Drugs
A drug is defined by its effect on the body's internal chemistry. It is not just medicine; it is any substance that enters the body and changes how chemical reactions occur.
- Key Mechanism: Drugs work by interacting with specific molecules (often proteins or enzymes) to speed up, slow down, or block chemical reactions.
- Scope: This definition includes therapeutic drugs (used to treat illness), recreational drugs (used for pleasure), and poisons (which disrupt normal reactions).
Building on the concept of homeostasis, drugs are external factors that deliberately disturb or restore the balance of these internal chemical reactions.
Learning Objective 2: Describe the use of antibiotics for the treatment of bacterial infections.
Antibiotics are a specific class of drugs used to treat diseases caused by bacteria.
- Function: They kill bacteria or stop them from reproducing.
- Target: They are effective ONLY against bacterial infections (e.g., strep throat, tuberculosis).
- Limitation: They do NOT work against viruses (e.g., flu, common cold) because viruses have different structures and reproduction methods than bacteria.
Learning Objective 3: State that some bacteria are resistant to antibiotics which reduces the effectiveness of antibiotics.
Antibiotic resistance occurs when bacteria evolve mechanisms to survive exposure to antibiotics that would normally kill them.
- Result: The antibiotic becomes less effective or completely ineffective at treating the infection.
- Example: MRSA (Methicillin-resistant Staphylococcus aureus) is a strain of bacteria resistant to many common antibiotics.
Learning Objective 4: State that antibiotics kill bacteria but do not affect viruses.
It is critical to distinguish between the two:
- Antibiotics → Kill Bacteria: They target bacterial cell walls or protein synthesis.
- Antibiotics → No Effect on Viruses: Viruses are not cells and do not have the same metabolic processes; therefore, antibiotics cannot harm them.
Learning Objective 5: Explain how using antibiotics only when essential can limit the development of resistant bacteria such as MRSA.
This process is driven by natural selection. Here is the step-by-step explanation:
- Genetic Variation: Within a population of bacteria, some individuals may have a random mutation that makes them resistant to a specific antibiotic.
- Selective Pressure (The Role of Antibiotics): When antibiotics are used extensively, they kill the non-resistant bacteria. The resistant bacteria survive because the drug does not affect them.
- Reproduction: The surviving resistant bacteria reproduce and pass the resistance alleles (genes) to their offspring.
- Limiting Resistance by Essential Use Only:
- If antibiotics are used only when essential (i.e., for confirmed bacterial infections), the number of bacteria exposed to the drug is minimized.
- This reduces the selective pressure. Without the antibiotic killing off the competition, non-resistant bacteria can still thrive alongside resistant ones.
- Crucially, if no antibiotic is present, there is often a fitness cost for resistance. Resistant bacteria may reproduce more slowly than non-resistant ones in the absence of the drug. By limiting use, we allow non-resistant strains to outcompete resistant ones, thereby keeping the overall level of resistance low.
Correction: Antibiotics target specific bacterial structures (like cell walls). Viruses do not have these structures. Using antibiotics for viruses is ineffective and contributes to resistance without helping the patient.
Correction: Antibiotics kill non-resistant (sensitive) bacteria. Resistant bacteria survive the treatment. This is why resistance spreads: the drug removes the competition, allowing the resistant strain to multiply.
Why examiners accept this: Examiners look for the logical chain of natural selection. You must mention mutation, survival of the fittest, and reproduction.
Correct Phrasing Example:
"Bacteria undergo random mutations that may confer resistance. When antibiotics are used, they act as a selective agent, killing non-resistant bacteria. The resistant bacteria survive and reproduce, passing the resistance alleles to the next generation."
Key Markscheme Keywords: mutation, survive/reproduce, pass on alleles/genes.
Why examiners accept this: Examiners want practical applications of the concept. They are looking for actions that reduce the need for antibiotics or ensure they are used correctly.
Correct Phrasing Example:
"Antibiotics should only be prescribed when essential (for bacterial, not viral infections). Doctors should also ensure patients complete the full course to ensure all bacteria are killed."
Key Markscheme Keywords: use only when essential/necessary, treat bacterial infections only, avoid overuse.
- Bacteria undergo mutation (genetic variation).
2. Antibiotics kill non-resistant bacteria.
3. Resistant bacteria survive and reproduce.
4. Resistance alleles are passed to offspring / population becomes resistant.
- Use antibiotics only when essential (or necessary).
2. Ensure antibiotics are used for bacterial infections only (not viral).