Figure

Fluid-mosaic model of cell membrane structure.

Redrawn from S. J. Singer and G. L. Nicholson, Science, 175:723. Copyright 1972 by the American Association for the Advancement of Science.

of blood cells. Most cells, however, are packaged into tissues and are not free to move around the body. But even in tissues there is usually a space between the plasma membranes of adjacent cells. This space is filled with extracellular fluid and provides the pathway for substances to pass between cells on their way to and from the blood.

The forces that organize cells into tissues and organs are poorly understood, but they depend, at least in part, on the ability of certain transmembrane proteins in the plasma membrane, known as integrins, to bind to specific proteins in the extracellular matrix and to membrane proteins on adjacent cells. Integrins also transmit signals from the extracellular matrix to the cell interior that can influence cell shape and growth.

Many cells are physically joined at discrete locations along their membranes by specialized types of junctions known as desmosomes, tight junctions, and gap junctions. Desmosomes (Figure 3-10a) consist of a region between two adjacent cells where the apposed plasma membranes are separated by about 20 nm and have a dense accumulation of protein at the cytoplas-mic surface of each membrane and in the space between the two membranes. Protein fibers extend from the cytoplasmic surface of desmosomes into the cell and are linked to other desmosomes on the opposite side of the cell. Desmosomes function to hold adjacent cells firmly together in areas that are subject to considerable stretching, such as in the skin. The specialized area of the membrane in the region of a desmo-some is usually disk-shaped, and these membrane junctions could be likened to rivets or spot-welds.

Asecond type of membrane junction, the tight junction, (Figure 3-10b) is formed when the extracellular

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions

I. Basic Cell Functions

3. Cell Structure

© The McGraw-Hill Companies, 2001

(a) Desmosome

Plasma membrane

Plasma membrane

Extracellular space

(c) Gap junction

Plasma membrane

Gap-junction membrane protein

Lumen side

Blood side

Extracellular space

(b) Tight junction

Plasma membrane

Tight — junction

Extracellular pathway blocked by tight junction -,

Extracellular space

Lumen side

Blood side

Transcellular pathway across epithelium

1.5 nm diameter channels linking cytosol of adjacent cells

Lumen side liTXTil

Blood side

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