lateral movement of proteins in lipid bilayer

[113zami]

I have tried to understand this sentence but I just don't get it :

"Lipids and proteins are capable of lateral movement , but movement of either from one face of the bilayer to the other is prohibited. In the fluid mosaic model the proteins can float within the lipid bilayer while traversing the plane of the membrane."

so first it says proteins can't move from one face to another then it says they traverse the plane of the membrane which means go through the membrane, so I don't get it how can they go through and be prohibited from going through at the same time

any1 please explain to me, if you have a picture or something it would make things clearer, thanks alot

[MisterATP]

The situation of my English isn't very good, but I guess there is talking about integral proteins whose transfix lipid bilayer (both upper and lower layers).

[canalon]

Yep: A protein can float on either face, or depending on the size, can cross the whole membrane (think channels for example) but they cannot invert the side that is facing out with the one facing in and reciprocally. Basically because the extremity popinting out are hydrophilic and cannot cross the hydrophobic membrane.

[jonmoulton]

The author's phrase "traverse the plane of the membrane" refers to the ability of transmembrane proteins to move about side-to-side in the membrane without leaving the membrane. One reason that this is a confusing statement is that the membrane is not a plane but is a bag, with a more-or-less spherical shape (though for a cell with extensive long parts extending from the central cell, such as a neuron, the cell can be far from spherical). However, when "viewed" on a small enough scale the membrane approximates a plane and then the statement that a transmembrane protein can "traverse the plane of the membrane" makes some sense. If you picture the cell membrane as a horizontal plane, the transmembrane protein can move forward or back, left or right, but cannot move up and down or rotate on most axes (though the protein can rotate on an axis at right angles to the plane of the membrane).

One way of picturing this might be to think of corks floating on calm water. They can't push underwater and they can's fly off the water into the air, but they can move in any direction as long as they are floating on the water's surface.

[Wingnut]

I think the use of the term "traversing" in the passage is somewhat confusing. It does sound, as written, as though the protein can pass through the membrane, whereas I think the author really means to say that the protein can move laterally within the plane of the membrane. (Assuming, as soneone has already pointed out, that you flatten the curved membrane into a 2-D sheet.)