Cambridge (CIE) AS Your notes
Biology
Antibiotics
Contents
Penicillin
Antibiotic Resistance
Consequences of Antibiotic Resistance
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 1
, Penicillin
Your notes
Penicillin affects bacteria & not viruses
Antibiotics are drugs that kill or stop the growth of bacteria but that do not harm the
cells of the infected organism
Some antibiotics are derived from living organisms, e.g. penicillin is produced by some
fungi in the genus Penicillium, whilst other are made synthetically in a laboratory
Antibiotics work by interfering with the growth or metabolism of the target bacterium,
e.g.
synthesis of bacterial cell walls
activity of proteins in bacterial cell surface membranes
bacterial enzyme action
bacterial DNA synthesis
bacterial protein synthesis
The effect of penicillin on bacteria
When a new bacterial cell is growing it secretes enzymes that create small holes in the
bacterial cell wall
These holes allow the bacterial cell wall to stretch as the cell grows, and new
peptidoglycan molecules join up via formation of cross-link attachments
Penicillin stops the formation of cross-links in the cell wall
However, the enzymes keep creating holes in the bacterial cell wall, making the walls
weaker and weaker
As bacteria live in watery environments and take up water by osmosis, their weakened
cell walls eventually burst as they can no longer withstand the pressure exerted on them
from within the cell
This means penicillin is only effective against bacteria that are still growing, as
autolysins no longer create holes and no more cross-links form once the growth of a
bacterium is complete
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 2
Biology
Antibiotics
Contents
Penicillin
Antibiotic Resistance
Consequences of Antibiotic Resistance
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 1
, Penicillin
Your notes
Penicillin affects bacteria & not viruses
Antibiotics are drugs that kill or stop the growth of bacteria but that do not harm the
cells of the infected organism
Some antibiotics are derived from living organisms, e.g. penicillin is produced by some
fungi in the genus Penicillium, whilst other are made synthetically in a laboratory
Antibiotics work by interfering with the growth or metabolism of the target bacterium,
e.g.
synthesis of bacterial cell walls
activity of proteins in bacterial cell surface membranes
bacterial enzyme action
bacterial DNA synthesis
bacterial protein synthesis
The effect of penicillin on bacteria
When a new bacterial cell is growing it secretes enzymes that create small holes in the
bacterial cell wall
These holes allow the bacterial cell wall to stretch as the cell grows, and new
peptidoglycan molecules join up via formation of cross-link attachments
Penicillin stops the formation of cross-links in the cell wall
However, the enzymes keep creating holes in the bacterial cell wall, making the walls
weaker and weaker
As bacteria live in watery environments and take up water by osmosis, their weakened
cell walls eventually burst as they can no longer withstand the pressure exerted on them
from within the cell
This means penicillin is only effective against bacteria that are still growing, as
autolysins no longer create holes and no more cross-links form once the growth of a
bacterium is complete
© 2025 Save My Exams, Ltd. Get more and ace your exams at savemyexams.com 2
