Final answer:
Small hydrophobic molecules with a molecular weight under 500 Da can diffuse through the cell membrane, aided by the semipermeable nature of the membrane and factors like temperature.
Step-by-step explanation:
The subject of this question falls under the domain of Biology, specifically cell biology. Small molecules can generally only diffuse across a cell membrane if they are hydrophobic and have a molecular weight less than 500 Dalton (Da). This is because molecules with a higher molecular weight move more slowly and may not be able to penetrate the nonpolar interior of the membrane or pass through its small pores. The semipermeable nature of the membrane allows these smaller molecules to pass through, which is vital for processes like gas exchange and cell signaling. Temperature also affects diffusion, as an increase in temperature leads to an increase in average molecular speed, enhancing the diffusion process.