Plasma membranes are semi-fluid with a consistency similar to olive oil. Individual phospholipids are able to move laterally and spin, much like an individual in a crowded room can move through the groups of people. Phospholipids cannot, however, flip over from one side of a lipid bilayer to the other leaflet- to do this requires the protein called flippase. Flipping through the leaflet would require the polar heads of the phosolipid to pass through the nonpolar region of the bilayer where the tails of the phospholipids congregate- phospholipds avoid mixing their hydrophobic and hydrophilic parts. Because phosolipids cannot flip from one leaflet to another, they are only semi-fluid, not fluid.
The mosaic structure comes from the phospholipid leaflet being spotted with proteins, sugars, and cholesterol. The additional components of the plasma membrane give an appearance similar to a tiled floor.
The function of the plasma membrane is to keep certain things inside the cell, keep other things outside the cell, and to allow for the transport of certain things into and outside of the cell, as well as keeping adjacent cells together and signaling other cells. The plasma membrane is selectively permeable- that is, it is selective about what enters and leaves the cell. Transport proteins embedded in the phospholipid bilayer provides a gate for certain things to be moved in or out of the cell. Transport, diffusion, and osmosis rely on gradients, or the relative disequilibrium of certain substances on each side of the membrane. Cell adhesion is important to keep all the liver cells in the liver and all the brain cells in the brain. Cells signal each other to alert changes in the environment and to coordinate their activities.
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