The unit-membrane model still is accepted as a starting point,
but now is considered oversimplified. Enough of the proteins must
extend all the way through the lipid bilayer to hold the membranes
together, since all of the lipids can be extracted from a membrane
with ether and still leave the bilayer structure intact, as can
be seen in electron micrographs.
Conversely, the membrane surface cannot be covered entirely with
proteins, since the membrane can be attacked by the enzyme phospholipase,
which acts only on lipids. Other evidence suggests that biological
membranes have pores, which allow the passage of small neutral molecules,
and other molecular machinery, for moving ions and molecules from
one side to the other.
A better picture of a probable membrane structure than the simple
protein-coated lipid bilayer is the "potato" model at
the right. Still, the simple unit-membrane model is the best beginning
approximation for thinking about actual membrane structures.
The unit-membrane model is like a child's drawing of an automobile.
It is not an accurate representation of all autos, or even of any
one car, but is recognizably symbolic of the type. Perhaps as we
grow up chemically, we shall do better with our picture of membranes,