What happens during the process of simple diffusion?

During the process of simple diffusion, small or hydrophobic molecules move across a cell membrane passively and down their concentration gradient. This means that these molecules tend to move from areas of higher concentration to areas of lower concentration without the need for any energy expenditure by the cell. This is a fundamental concept in cellular biology, as it reflects how substances can cross the semi-permeable lipid bilayer of the cell membrane based on their size and solubility.

Small nonpolar molecules, such as oxygen and carbon dioxide, are able to easily pass through the lipid layers because they can interact favorably with the hydrophobic regions of the membrane. This passive movement is essential for processes such as gas exchange in respiration.

In contrast, larger molecules cannot easily pass through the membrane by simple diffusion, and their movement requires different mechanisms, such as facilitated diffusion or active transport. Additionally, active transport involves an energy expenditure, which is contrary to the passive nature of simple diffusion. Water absorption by cells often occurs through specific channels, known as aquaporins, rather than via simple diffusion. Thus, the correct understanding of simple diffusion emphasizes the passive movement of small or hydrophobic molecules down their concentration gradient.