IB Biology HLDisease & ImmunityPaper 1 & 2~9 min read
Antibiotics
An antibiotic works because bacteria are built differently from you. It attacks something a prokaryotic cell has and a eukaryotic cell does not. Once you see that, both big exam questions answer themselves: why antibiotics are safe for us, and why they are useless against viruses.
📘 What you need to know
Antibiotics are drugs that inhibit the growth of microorganisms.
Most antibiotics kill bacteria or stop them growing, but do not harm the cells of the infected organism.
They work by blocking processes that occur in prokaryotic cells but do not have the same effect on eukaryotic cells.
Processes targeted include cell wall formation, ribosome function, DNA replication, transcription and translation.
Some antibiotics come from living organisms such as saprotrophic fungi. Penicillin is produced by fungi in the genus Penicillium.
In the wild, those antimicrobial secretions help the fungus compete by killing nearby saprotrophic bacteria.
Antibiotics can also be made synthetically in a laboratory.
Antibiotics are ineffective against viruses, because viruses are non–living particles with no metabolism or cell structure. Antivirals target viral enzymes instead.
Why antibiotics do not hurt you
This is the central idea of the whole page. An antibiotic is not a general poison. It jams a specific process, and that process either does not exist in your cells or works differently enough that the drug does not affect it.
Notice how much of this is revision from cell biology. Prokaryotic cells have a cell wall, a loop of DNA and their own type of ribosome, and each of those is a target.
What an antibiotic can block
Cell wall formation — your cells do not have a cell wall at all.
Ribosome function — prokaryotic ribosomes are a different type from yours, so a drug can bind to theirs and not to yours.
DNA replication
Transcription
Translation
If a question asks why antibiotics do not damage human cells, do not answer “because they only target bacteria”. That just restates the question. Name a structural difference — no cell wall, or a different type of ribosome — and you have the mark.
Where antibiotics come from
Some antibiotics are made by living organisms, particularly saprotrophic fungi. Penicillin is produced by certain fungi in the genus Penicillium.
This is not charity on the fungus’s part. A saprotrophic fungus feeds on dead material, and so do plenty of bacteria. Releasing antimicrobial secretions kills nearby saprotrophic bacteria, which means less competition for the same food. We simply borrowed a weapon from a turf war.
Antibiotics can also be made synthetically in a laboratory.
Penicillin and the cell wall
The bacterium is not poisoned. It is left unable to build a wall strong enough to hold it together, so it ruptures when it next tries to grow.
Why penicillin does not work on every bacterium
Penicillin is not effective against all bacteria — tuberculosis is the standard example. There are two reasons the syllabus wants:
Thicker cell walls, which reduce permeability so the drug cannot get through.
Enzymes that break down penicillin, destroying the drug before it can act.
This is why there are many different antibiotics rather than one. Each is effective against a different range of bacterial diseases.
Antibiotics and viruses
Antibiotics do nothing to a virus, and the reasoning is worth writing out carefully because it is a very common exam question.
The reason
What it means
Viruses are non–living
They are particles, not cells, so there is no bacterial–style structure to attack
No metabolism
There are no metabolic reactions of their own to block, and no cell wall or ribosomes
They borrow your machinery
When a virus replicates it uses the host cell’s mechanisms for transcription, translation and other metabolic pathways
So those cannot be targeted
Antibiotics do not bind to the proteins host cells use in these processes, and a drug that did would damage your own cells
What is used instead
Antivirals, which target viral enzymes without harming the host cell
Why this matters beyond the exam. Every antibiotic prescribed for a cold or flu does nothing for the patient, because those are viral. It still applies selective pressure to the bacteria living in them — which is the subject of the next page.
Worked examples
WE 1
Explain why antibiotics do not harm human cells
Explain how antibiotics can kill bacteria inside the body without damaging human cells. (3 marks)
Point 1: what they target
Antibiotics block specific processes that occur in prokaryotic cells, such as cell wall formation or ribosome function.
Point 2: the difference
Human cells are eukaryotic. They have no cell wall, and their ribosomes are a different type, so the drug does not bind to them.
Point 3: the outcome
The same processes are therefore not affected in human cells, so the bacteria are killed or stopped from growing while the patient’s cells are unharmed.
Target a structure prokaryotes have and we do notname at least one specific target — a general “they are different” is not enough
WE 2
Explain why antibiotics are useless against viruses
A patient with influenza is told that antibiotics will not help. Explain why. (3 marks)
Point 1: what a virus is
Viruses are non–living particles, not cells, so they have no cell wall, no ribosomes and no metabolism of their own to block.
Point 2: replication
When a virus replicates it uses the host cell’s mechanisms for transcription and translation.
Point 3: why we cannot target that
Antibiotics do not bind to the proteins host cells use, and a drug that did would damage the patient’s own cells. Antivirals are used instead, targeting viral enzymes.
Nothing bacterial to attack, and attacking the host would cause harmfinish with antivirals — it shows you know there is an alternative treatment
WE 3
Suggest why a fungus produces an antibiotic
Suggest why saprotrophic fungi such as Penicillium produce antimicrobial secretions in the wild. (2 marks)
Point 1: the shared resource
Saprotrophic fungi feed on dead organic material, and saprotrophic bacteria growing nearby feed on the same material.
Point 2: the advantage
Killing those bacteria reduces competition, so the fungus obtains more nutrients and grows better. It is a competitive advantage, not a defence against disease.
Kill the competition, keep the food“suggest” questions reward reasoning — explicitly use the word competition
💡 Exam tips
Define antibiotics as drugs that inhibit the growth of microorganisms.
Learn the target list: cell wall formation, ribosome function, DNA replication, transcription, translation.
Use the words prokaryotic and eukaryotic when explaining safety.
Name Penicillium as the genus and say it is a saprotrophic fungus.
Give both reasons penicillin fails on some bacteria: thicker walls and enzymes that break it down.
Always end a virus question with antivirals target viral enzymes.
⚠ Common mistakes
Saying antibiotics kill all microorganisms. They act on bacteria, not viruses.
Saying viruses are killed by antivirals. Viruses are not alive — antivirals stop them replicating by targeting viral enzymes.
Writing that antibiotics “boost the immune system”. They act directly on the bacteria.
Saying human cells have no ribosomes. They do — the ribosomes are a different type.
Confusing penicillin with Penicillium. One is the drug, the other is the fungus that makes it.
Assuming one antibiotic works on every bacterium. Cell wall thickness and penicillin–destroying enzymes get in the way.
Up next: Antibiotic Resistance. We have just described a perfect weapon. The next page explains how bacteria are steadily taking it away from us, and why the mechanism is pure natural selection.
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