Page 49 - Anatomy and Physiology of Farm Animals, 8th Edition

34 / Anatomy and Physiology of Farm Animals

Integral
proteins
VetBooks.ir Cholesterol

Extracellular

Intracellular
Channel Phospholipids

Peripheral protein
Figure 2-8. Fluid mosaic model of the cell membrane and proteins associated with the membrane.

phospholipids, proteins, and cholesterol. to move substances across the cell mem­
According to the fluid mosaic model, the brane. In Figure 2‐8 a transmembrane
phospholipids are arranged with their protein forms a channel to permit passage
polar (hydrophilic) ends facing the pro­ across the cell membrane. Proteins may
tein layers, while their nonpolar (hydro­ also be found bound to the surface of the
phobic) ends face each other in the center membrane, and such proteins are classified
of the membrane (Fig. 2‐8). Because of as peripheral proteins (Fig. 2‐8). Most of
the hydrophobic properties of the phos­ these are on the cytosolic surface of the
pholipids, the membrane forms a water‐ membrane, and they are often found bound
impermeable barrier that separates the to an integral protein.
interior of the cell from the ECF. This The functions of cell membrane pro­
barrier protects the cell by preventing the teins include: (1) transport of substances
simple diffusion of water and water‐soluble across the membrane; (2) provision of a
particles. Plasma membranes contain site of binding for substances found in
varying amounts of cholesterol, which is extracellular fluid, such as hormones; (3)
found between the phospholipid molecules contribution to the formation of cell‐to‐
and adjusts the fluidity and flexibility of cell junctions; (4) provision of enzymes
the membrane. with active sites facing either the cytosol
The protein composition of the outer or the extracellular fluid; and (5) identi­
cell membrane is extremely variable among fication or recognition of cell type or cell
different types of cells, and this variation origin by other cells. These functions
has a great deal of influence on functional are discussed in more detail later in this
differences between cells. Some mem­ chapter.
brane proteins are firmly inserted into the Structural modifications of the cell
membrane among the phospholipids membrane occur largely on the free sur­
(Fig. 2‐8). These are integral proteins and face of cells (a surface not adjacent to any
they may be arranged so that they extend other cells). These modifications usually
completely across the membrane. Proteins increase the cell surface and presumably
that are exposed to both the cytosol inside function in absorption or secretion. The
the cell and the extracellular fluid sur­ striated or brush border seen in light
rounding the cell are transmembrane microscopy appears in electron micrographs
proteins (Fig. 2‐8). Many transmembrane to consist of uniform fingerlike projections
proteins are involved in transport processes called microvilli. Less regular projections,