Plasma Membrane (Chap. 5)
Structure
http://home.earthlink.net/~shalpine/anim/Life/memb.htm
• Thickness of 7-8nm
• Primarily made up of phospholipids (Fig. 5.2 in
text)
• Have different types of proteins and cholesterol
dispersed between the phospholipids.
• The fluid mosaic model – membrane can move
and yet is very stable because of hydrophobic
and hydrophilic properties of phospholipids
• Proteins are visible by a freeze-fracture
technique where tissue is frozen (use liquid N2)
and then split along the phospholipid bilayer
through its hydrophobic tails. Viewed by SEM
or TEM.
The Plasma Membrane; Three Main
Components (other than phospholipids)
1. Cholesterol
• amphipathic (have both hydrophilic and
hydrophobic regions just like
phospholipids)
• prevents the fatty acid tails from
packaging tightly therefore, membrane is
more fluid-like.
• however at the same time, it decreases
permeability to small water-soluble
molecules. (Understand Fig. 5.8 in text)
2. Proteins
• Functions
– hormone binding sites
– some are enzymes that carry out
reactions right in the membrane
– electron carriers (more on this later)
– channels for passive and active
transport of materials in and out of the
cell
• Categories of Proteins
I. Integral proteins
– amphipathic
– some go through only one of the
phospholipid layers of the membrane (lipidanchored proteins) and others extend
completely through the membrane and are
well secured (transmembranal proteins)
II. Peripheral proteins
– lie on the surfaces of the membrane
– usually attached to the protruding portions
of integral membrane proteins
3. Glycocalyx
• visible (by TEM) as a fuzzy, outermost region
of the cell.
• a sugar coating over the membrane surface
made up of short chains of glucose attached to
the proteins (glycoprotein) and/or
phospholipids (glycolipid)
• helps:
– protect the cell
– the cell recognize, identify, and interact with
other chemicals, viruses, and bacteria