**Core Concept**
Penicillins and cephalosporins are a class of beta-lactam antibiotics that exert their antibacterial effects by inhibiting cell wall synthesis in susceptible bacteria. This is achieved through the inhibition of enzymes involved in the cross-linking of peptidoglycan layers.

**Why the Correct Answer is Right**
The mechanism of action of penicillins and cephalosporins involves the inhibition of transpeptidase enzymes, specifically penicillin-binding proteins (PBPs), which are responsible for the cross-linking of peptidoglycan layers in the bacterial cell wall. This inhibition disrupts the structural integrity of the cell wall, leading to osmotic instability and ultimately, bacterial cell lysis. The binding of these antibiotics to PBPs is highly specific and reversible, which is why they are effective against rapidly dividing bacteria.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because aminoglycosides, not penicillins and cephalosporins, exert their effects by interfering with protein synthesis.

**Option B:** This option is incorrect because sulfonamides, not penicillins and cephalosporins, inhibit folic acid synthesis in bacteria.

**Option C:** This option is incorrect because tetracyclines, not penicillins and cephalosporins, bind to the 30S ribosomal subunit and inhibit protein synthesis.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that beta-lactam antibiotics, including penicillins and cephalosporins, are ineffective against bacteria that lack a functional cell wall or have a modified cell wall, such as Mycoplasma species and some strains of Neisseria gonorrhoeae.

**Correct Answer: D. Inhibit transpeptidase enzymes.**