How do you explain the change in membrane permeability as temperature increases?

Generally, increasing the temperature increases membrane permeability. At temperatures below 0 ^oC the phospholipids in the membrane don't have much energy and so they can't move much, which means that they're closely packed together and the membrane is rigid. Between 0 ^oC and 45 ^oC the permeability increases as phospholipids start to move around more, as they have more energy, and therefore they're no longer as tightly packed, meaning that the membrane is partially permeable. At temperatures about 45 ^oC the phospholipid bilayer starts to melt and the membrane becomes more permaeable. Water inside the cell expands, putting pressure on the membrane, and transport proteins deform (due to denaturing of proteins) so they can't control what enters or leaves the cell.