Notes

A Cell's Boundary
Just as you have your skin, all cells have a plasma membrane as their boundary with the outside world. The plasma membrane helps keep all the stuff of the cell from floating away. But that’s not all it does. The life of a cell depends on effectively exchanging materials with the environment and on bringing in energy. Because it is at the boundary, the plasma membrane Opens in modal popup window regulates what goes into and out of the cell. It also participates in other boundary functions, like recognizing substances and other cells it comes into contact with.
Fluid Mosaic Model
The plasma membrane is about 8 nm thick. If you could stack membranes one directly on top of another, about 125,000 would fit in a millimeter.

The currently accepted model of the membrane is the fluid mosaic model, in which proteins of various types are embedded and floating around in a fluid double layer.

A mosaic is something that is made up of pieces, like a mosaic tile picture. The protein molecules are like movable mosaic pieces in the membrane. The fluid membrane double layer, often called a bilayer, is itself made of phospholipid molecules that can move around.
There are three protein positions:

integral proteins
transmembrane protein
peripheral proteins - These may attach either to the exposed parts of integral proteins or directly to the surface of the membrane.
The part of a protein that penetrates into the bilayer where the lipid part is, is hydrophobic, like the lipids themselves. This part of the molecule has a structure that does not mix easily with water. The parts that stick out of the membrane into the watery solution are hydrophilic. The hydrophobic and hydrophilic parts of the protein help stabilize its position in the membrane.
A cell requires materials to build and maintain its structure. Cells also require a way to remove waste. There are two types of transport that allow molecules to move in and out of a cell: passive transport and active transport .

Passive transport does not require energy. Molecules move across a concentration gradient . These molecules move around based on their concentration in a solution. They tend to move from areas of high concentration to low concentration, so the number of molecules over time evens out on both sides of the concentration gradient.