Cell Membranes and the Phospholipid Bilayer
The Structure and Function of the Plasma Membrane
Sep 26, 2009
All living cells and many organelles, including the nucleus and mitochondria, are surrounded by a cell membrane, often called the plasma membrane, which protects and supports the cell. Without the plasma membrane, cells would be unable to hold together and carry out the myriad functions an organism needs to survive.
The Structure of the Phospholipid Cell Membrane
The plasma membrane is a double membrane made up of phospholipids, molecules that look like a ball on one end with two tails hanging down. The spherical part is the phosphate, which is hydrophilic (attracted to water.) This causes the spherical end to orient outward to the liquid inside and outside of the cell. The tails are made of lipids, hydrophobic fat molecules which avoid water, making them orient towards each other so that they don't touch any liquid. Because of this natural tendency to correctly orient themselves, phospholipid molecules are able to rapidly repair any damage done to the cell membrane, making the membrane both flexible and strong.
A cell membrane is dotted with proteins which serve as channels and gates to let molecules into and out of the cell or organelle, provide structure and operate as markers to communicate with other cells. Some of these proteins only appear on one side of the membrane or are embedded entirely within it. Others, particularly the gates and channels, cross the membrane to allow molecules to travel through them in or out of the cell. Some membrane proteins even cross the membrane multiple times.
Cell Membrane Transport Across the Lipid Bilayer
The protein molecules that control transport into and out of the cell through the cell membrane operate primarily in two different ways, through passive and active transport.
Passive transport is used for small molecules, which diffuse through small protein channels from an area of high concentration to an area of lower concentration. Sometimes two molecules move through a channel simultaneously, either together or in opposite directions, balancing out the concentrations on either side of the membrane.
Active transport proteins use ATP energy to move molecules across the membrane that are too large for passive transport mechanisms or that need to cross a gradient in the opposite direction of concentration. These proteins recognize and pull the desired molecules across the membrane.
Surface Protein Receptors of the Cell Membrane
Another type of protein found on the cell membrane surface is the receptor. These molecules sit on the plasma membrane and convey important information about the cell to other cells, including neighboring cells of similar type and immune system cells. This is important because it lets the cell coordinate its growth and activity with other cells as well as allowing the cell to let the immune system know when it has been attacked or infected.
Sources:
Alberts, Bruce. Molecular Biology of the Cell. Garland Pub., 1989
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