Page 51 - Medicinal Plants_PharmD general

Membranes

A model of biological membranes that seems to account for many of
their properties is the phospholipid-globular mosaic model recently
proposed by several workers such as "fluid mosaic" model of the cell
membrane by Singer and Nicolson in 1972. The fluid mosaic model
describes the structure of the cell membrane (Figure 5) as a mosaic of
components —including phospholipids, proteins, and carbohydrates—that
gives the membrane a fluid character.
1- Phospholipids are a class of lipids that are a major component of all

cell membranes. They can form lipid bilayers because of their
amphiphilic characteristic. The structure of the phospholipid molecule
(Figure 6) generally consists of two hydrophobic fatty acid "tails" and
a hydrophilic "head" consisting of a phosphate groupattached to a
glycerol molecule.
2- Proteins make up the second major component of plasma membranes.
Integral proteins that are embedded in the phospholipid bilayer as a
permanent part of a cell membrane; may or may not extend through both
layers. They may serve as passages for the movement of water soluble
solutes into and out of the cytoplasm. In addition to, peripheral proteins
on the inner or outer surface of the phospholipid bilayer, but not
embedded in its hydrophobic core.
3- Carbohydrates are the third major component of plasma membranes.
They are always found on the exterior surface of cells and are bound
either to proteins (forming glycoproteins) or to lipids (forming
glycolipids).

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