Simple Diffusion Factors

Three primary factors influence a molecule's ability to diffuse via simple diffusion:

Size:
- Macromolecules generally cannot pass through the membrane.
- Very small entities, like ions, may also face difficulty.
- There's an optimal size range for diffusion.
Unexpected Diffusion Rates:
- Small molecules diffuse more slowly than expected.
- Water diffuses faster than anticipated.
Theories on Water Diffusion:
- Membrane Pores: Some believe the membrane contains pores that facilitate water diffusion.
- Lipid Movement and Gaps: As lipids move, they create transient gaps that water molecules can pass through. These gaps then reseal.
Polarity:
- Nonpolar substances are more permeable.
- Lipids, despite their size, can move through the membrane.
- Polar substances like proteins cannot pass through easily.
- Steroid hormones (estrogen, progesterone, testosterone) can enter cells directly without binding to cell membrane receptors.
Ions and Charge:
- Ions are small, but charged.
- Ions are surrounded by water molecules, forming a "shell of hydration."
- The shell of hydration increases the effective size of the ion, preventing it from passing through the membrane.
- Removing water molecules from the ion requires cellular effort to facilitate its movement across the membrane.