Transport across membranes: diffusion, osmosis and active transport

The fluid mosaic model

The membrane is a phospholipid bilayer with proteins scattered through it like tiles in a mosaic. It is 'fluid' because phospholipids move laterally. Cholesterol binds to phospholipid tails, reducing fluidity and making the membrane more rigid; glycoproteins and glycolipids act as receptors and antigens for cell recognition.

Passive processes

• Simple diffusion: small, non-polar molecules (O₂, CO₂) move down a concentration gradient directly through the bilayer.
• Facilitated diffusion: ions and polar molecules use channel proteins (water-filled pores) or carrier proteins that change shape. No ATP is used.
• Osmosis: net movement of water from higher to lower water potential across a partially permeable membrane. Pure water has a water potential of 0 kPa; adding solutes makes it more negative.

Active transport and co-transport

Active transport moves substances against a concentration gradient using carrier proteins and ATP hydrolysis. In the ileum, sodium ions are actively pumped out of epithelial cells into the blood, lowering sodium concentration inside the cell. Sodium then diffuses back in from the lumen through a co-transporter protein, carrying glucose with it against glucose's gradient. Glucose then leaves the cell into the blood by facilitated diffusion.

Factors affecting rate

• Steepness of the concentration gradient.
• Surface area — microvilli dramatically increase it.
• Number of channel or carrier proteins (rate plateaus when they are all in use).
• Thickness of the exchange surface (diffusion distance).