How does a membrane create a semi-impermeable barrier separating the inside and

This is achieved by the chemical properties of the phospholypids that form a major part of the membrane. Phospholypids have an hydrophilic head of glycerol or another related compound and an hydrophobic tail of hydrocarbon. The external environment and the inner containt of a cell are hydrophilics, so the phospholypids tend to form a characteristic double bilayer with the hydrophilic heads facing the hydrophilic content of the cell and the environment, and the hydrophobic tails protected inside.

Then we have an hydrophobic barrier between 2 hydrophilic compartments. This makes the semipermeable barrier, preventing the simple diffusion of hydrophilic molecules (except the small ones like water) and charged molecules.

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