Membrane Structure

external image plasmamembranefigure1.jpg

A thin, pliable, layer of tissue that separate the cells from the extracelluar world.

Every cell is enclosed in a membrane, a double layer of phospholipids (lipid bilayer). The exposed heads of the bilayer are "hydrophilic" (water loving), meaning that they are compatible with water both within the cytosol and outside of the cell. However, the hidden tails of the phospholipids are "hydrophobic" (water fearing), so the cell membrane acts as a protective barrier to the uncontrolled flow of water. The membrane is made more complex by the presence of numerous proteins that are crucial to cell activity. These proteins include receptors for odors, tastes and hormones, as well as pores responsible for the controlled entry and exit of ions like sodium (Na+) potassium (K+), calcium (Ca++) and chloride (Cl-).

The job of the cell membrane is to separate the intracellular components from the extracellular environment. The cell membrane also anchors the cytoskeleton to provide a shape for the cell, and by attaching the extracellular matrix and other cells to help group the cells together to form a tissue. It also able to regulate what enters and exits the cell like a filter, therefore facilitating the transport of materials needed for survival. It also maintains the cell potential

All membranes contain proteins and lipid. However, the proportion of each varies depending on the membrane. For example:
  • Myelin, which insulates nerve fibers, contains only 18% protein and 76% lipid. A electron micrograph of myelin is to the right.
  • Mitochondrial inner membrane contain 76% protein and only 24% lipid.
  • Plasma membranes of human red blood cells and mouse liver contain nearly equal amounts of proteins (44, 49%) and lipids (43, 52%).

The membrane structure is that of a lipid bilayer. The lipids are amphipathic which means that they have hydrophilic (water attracting) polar heads pointing out and the hydrophobic (water repelling) portion forming the core. The fatty acid hydrophobic tails always line up in the center of a double row of phospholipids while the hydrophilic head points outward attracting the water

Fluid Mosaic Model
The membrane is described as a fluid mosaic model because like a mosaic, it is made up of different parts. It is a lipid bilayer made of phospholipid component. It is composed of different kinds of macromolecules:
  • Two layers of phospholipids
  • Each phospholipids is free to move, but cannot leave the bilayer because of their hydrophobic tails repelling water
  • Many proteins are embedded in the bilayer


Work Sited:
"Molecular Expressions Cell Biology: Plasma Membrane." Molecular Expressions: Images from the Microscope. Web. 11 Sept. 2011. <>.
"Fluid Mosiac Model - YouTube." YouTube - Broadcast Yourself. Web. 11 Sept. 2011. <>.
"[[File:///\\Anatomy%20and%20Physiology%20of%20Animals%20Structure%20Plasma%20Membrane.jpg.|File:Anatomy and Physiology of Animals Structure Plasma Membrane.jpg.]]" Wikimedia Commons. Web. 11 Sept. 2011. <>.