IB Biology Topic B2.1.8: Selectivity in Membrane Permeability Notes

Selective Permeability: Cells Choose What Crosses the Membrane

1. The cell membrane maintains homeostasis by controlling what moves into and out of the cell.
2. Its selective permeability lets certain particles pass while blocking others, keeping the inside balanced.

Selectively Permeable Membranes Are More Precise Than Semi-Permeable Ones

1. Semi-permeable membranes
   1. They let small solutes through and are freely permeable to water.
   2. This describes artificial membranes such as those used in dialysis.
   3. They cannot control which specific substances cross.
2. Selectively permeable membranes
   1. The cell membrane lets certain particles pass while blocking others.
   2. This precision comes from membrane proteins that handle each substance.

Selectivity Comes From Combining Three Transport Routes

1. A membrane's overall selectivity emerges from combining three transport routes, each covered in its own subtopic.
   1. Simple diffusion lets small, hydrophobic molecules cross the bilayer on their own.
   2. Facilitated diffusion uses selective channel proteins to pass specific ions and polar molecules.
   3. Active transport uses pump proteins and ATP to move substances against their gradient.
2. Together these routes decide exactly what enters and leaves, which is what makes the membrane selective.

Selective Permeability Keeps the Cell in Balance

1. Selective permeability supports several functions that keep the cell in homeostasis.
   1. It takes up essential nutrients like glucose and amino acids.
   2. It expels waste products such as carbon dioxide.
   3. It maintains ion gradients needed for nerve signaling and muscle contraction.
   4. It controls water movement to keep osmotic balance, preventing swelling or shrinkage.