Drs. Danielli and Davson's "sandwich"
model of proteins and lipid seemed reasonable at the time and was accepted for several
decades. But more research revealed that some portions of proteins
were anchored IN the double layer of lipids.
Membrane proteins are classified into two groups,
depending on how they are anchored to the membrane.
Peripheral proteins are anchored on the side
of the membrane and do not go completely through the membrane.
Integral proteins pass
completely through the
membrane, with parts
of the protein
hanging out of either side of the membrane.
Because integral proteins pass completely through
the membrane, they can provide a channel to let substances move
into and out of cells (see the channel in the picture to the right.) You
might think of this like a miniature sponge. The size and shape
of the channels regulate the size of the molecules that can pass
through these protein gateways.