What substance would have the slowest speed of passage across a selectively permeable membrane?

Large polar molecules

Cell membrane or plasma membrane is a semi permeable membrane. it is made of lipid bilayer. Only lipid soluble molecules and small molecules can pass through it. Ions and large polar molecules cannot pass through it. Non-polar molecules are highly permeable whereas, polar molecules are low permeable.

answer:

convection currents in the magma drive plate tectonics. heat generated from the radioactive decay of elements deep in the interior of the earth creates magma (molten rock) in the aesthenosphere. the aesthenosphere (70 ~ 250 km) is part of the mantle, the middle sphere of the earth that extends to 2900 km.