Penicillin is an antibiotic that is widely used to treat bacterial infections. It is derived from the fungus Penicillium and works by blocking biochemical pathways that are important for bacterial growth and survival.

The correct answer to the question is option 'C' - Penicillin acts against bacteria by blocking biochemical pathways important for bacteria.

Here is a detailed explanation of why this is the correct answer:

1. Introduction to Penicillin:
Penicillin is a type of antibiotic that belongs to the class of drugs known as beta-lactams. It was the first antibiotic discovered and is still widely used today to treat a variety of bacterial infections.

2. Mechanism of Action:
Penicillin works by inhibiting the synthesis of the bacterial cell wall, which is crucial for the survival and replication of bacteria. It does so by targeting an enzyme called transpeptidase, also known as penicillin-binding protein (PBP), which is responsible for cross-linking the peptidoglycan chains in the bacterial cell wall.

3. Blocking Biochemical Pathways:
The bacterial cell wall is composed of peptidoglycan, a complex structure made up of sugars and amino acids. The cross-linking of peptidoglycan chains by transpeptidase enzymes gives the cell wall its strength and rigidity. Penicillin acts by binding to the active site of transpeptidase and inhibiting its activity. This prevents the cross-linking of peptidoglycan chains and weakens the bacterial cell wall.

4. Effects on Bacterial Cells:
Without a strong and intact cell wall, bacteria become vulnerable to osmotic pressure and eventually lyse (burst) due to the influx of water. This leads to the death of the bacterial cells. Additionally, the weakened cell wall makes it easier for the immune system and other antibiotics to attack and destroy the bacteria.

5. Selective Toxicity:
One of the key advantages of penicillin is its selective toxicity, meaning it targets bacterial cells while sparing human cells. This is because human cells do not have a peptidoglycan cell wall and do not rely on transpeptidase enzymes for growth and survival.

In conclusion, penicillin acts against bacteria by blocking the biochemical pathways that are important for bacterial cell wall synthesis. This weakens the cell wall, leading to the death of bacterial cells. It is important to note that penicillin is effective only against bacteria and not against viruses or other types of microorganisms.