How do neutral solutes move across the plasma membrane? Can the polar molecules also move across it in the same way? If not, then how are these transported across the membrane?

Neutral solutes such as O₂, CO₂ and steroid hormones move across the plasma membrane by simple diffusion

• Small, nonpolar molecules dissolve in the hydrophobic lipid bilayer and cross down their concentration gradients without assistance

• No metabolic energy is required and transport rate obeys Fick’s law, depending on concentration difference, membrane thickness, surface area and the solute’s lipid-water partition coefficient

Polar molecules and ions cannot diffuse appreciably through the hydrophobic core of the lipid bilayer

• The energetic barrier is too high for charged or highly electronegative species to partition into the membrane

Polar solutes are transported by membrane proteins in one of three ways

• Facilitated diffusion

  • Transport proteins (channel proteins and carrier proteins) provide specific pathways for down-gradient movement

  • Channel proteins form aqueous pores for rapid ion or water flux; carrier proteins bind substrates (e.g. glucose, amino acids) and undergo conformational change to shuttle them across

• Active transport

  • Primary active transport uses ATP-driven pumps (e.g. Na⁺/K⁺-ATPase) to move solutes against their electrochemical gradients

  • Secondary active transport couples the downhill movement of one solute to the uphill transport of another via symporters or antiporters

• Vesicular transport

  • Large molecules (e.g. proteins, polysaccharides) and bulk fluid are internalized or expelled by endocytosis and exocytosis, respectively

Summary of transport modes

• Simple diffusion for small, nonpolar solutes without proteins or energy

• Facilitated diffusion for polar solutes down gradients via specific proteins

• Active transport for uphill movement requiring energy

• Vesicular pathways for macromolecules and bulk transport