Molecules

Adhesion and Cohesion

By definition, multicellular life does not exist without biological binding of the various components (see Biological Bonds). The modern world would fall apart literally if the mechanical interlocking (see Mechanical Bonds) is suddenly switched off. The crystal is held together in one piece by chemical bonds (see Chemical bonds). They become rocks and minerals via actions in nature.

Adhesion and cohesion are very general concepts to express whether the binding parts are different or similar (Figure 12-25d). The interface between water and solid is often used as example to illustrate binding in daily life. Due to the surface tension, a water droplet by itself is always in spherical shape. As shown by the various illustrations in Figure 12-25d, it more or less retains its round shape if the cohesion is strong; otherwise the liquid would spread out called wetting.

Figure 12-25d Adhesion and Cohesion [view large image]

The degree of wetting can be expressed roughly with the contact angle

formed by the tangential at the edge. It is inversely proportional to wetting (see formulas in Figure 12-25d).

Mechanical Bond - The earliest type of bonding was developed in late stone age about 10000 years ago in the form of weaving. The method involves interlacing two sets of threads at right angles to each other. The parts are held together by mechanical force if the edges or ends are sealed tight (Figure 12-25e-a,b). Nails, as a means of fastening wooden objects, date back to approximately 3400 BC in ancient Egypt. The use of Screws, on the other hand, can be traced to about 3rd Century BC in Greece. They are the most versatile tools

to build large size structures (Figure 12-25e-c,d). The glues on the other hand offer many advantages over other binding techniques such as sewing, mechanical fastening, etc. These include cost effectiveness, easy application, ability to bind different materials of relatively small size. The adhesion property of glue comes from mechanical interlocking via the porous as well as the molecular attraction of the two substances (Figure 12-25e-e,f).

Figure 12-25e Mechanical Interlocking [view large image]

Biological Bond - This kind of bonding depends on the adhesion molecules located on the cell surface. There are two main types responsible for cell-cell (C-C) and cell-extracellular matrix (C-ECM) adhesions respectively (Figure 12-25f). As implied by the definition of adhesion, it is absolutely necessary for multicellular organisms to maintain the body form and structure. Adhesion molecules are also present on the surface of prokaryotes and unicellular eukcaryotes as a first step toward colonization and its use for pathogenesis. Adhesion also involves in cell movement and cell migration during embryonic development. Flexible cells can maintain more contacting area and hence can maintain stronger adhesion. Ultimately, it is the contraction of the filaments within the cell to

Figure 12-25f Biological Adhesions

prevent disintegration by external force. Cellular adhesion is more or less permanent except for migrating cells, or in developmental changes. It turns loose when becomes mature or contracts disease.

Table 10-02a below provides a legend to explain all the seemingly inscrutable terminologies about biological adhesions in Figure 12-25f.

Name	Component(s)	Function(s)
CAM	Transmembrane receptor	Umbrella designation for "Cell Adhesion Molecule"
Integrin	Surface receptor (a protein)	Responsible for C-C and C-ECM monitoring and communication
Selectin	Transmembrane polypeptide	Mediating migration of circulatory cells
Gap Junction	Connexin + Channel	Providing direct contact between the cytoplasm of two cells
Tight Junction	Claudin + Occludin + Actin filaments + ... etc	Creating very close contact between the membrane of two cells
Adhesion Belt	Cadherin + Catenins + Actin filaments + ... etc	Regulating cell shape, maintain tissue integrity, etc
Desmosome	Cadherin + Desmoplakin + Cytoskeletal filaments	Providing strong adhesion between cells
Hemi-desmosome	~ Desmosome with Cadherin replaced by Integrin	Promoting adhesion of epithelial cell to the basement membrane.
Focal Adhesion	Integrin + Talin + Actin filaments	Providing mechanical links between actin filaments and ECM
Proteoglycan	Polysaccharide chains	Regulating the movement of molecules in ECM, and signaling

Table 12-02a A Legend for Biological Adhesions

Chemical Bond - Atoms can bind together to form molecules by the attraction of opposite electric charges. Depending on the electronic configurations of the atoms, the bonds are classified (in order of the attractive strength) as ionic, covalent, metallic, hydrogen, and the weakest dipole-dipole bond via van der Waals force (Figure 12-25g). Usually, individual molecules are invisible with unaided eyes (too small in size). It is through the cycle of condensation, melting, dissolving (in liquid), and precipitation that the individual atoms and molecules come together to form small crystals of a few centimeter in size (enough to be seen). The elements are naturally scattered all over, they come together to form rocks and minerals by condensation or precipitation at different temperature, but they do not cleanly segregate one type from the others. That's why it is so valuable to have mineral of relatively pure compound weighed a few carat (called gemstone, 1 carat = 0.2 gm) shown below :

Figure 12-25g Chemical Bonds
[view large image]

See more from "Rocks, Minerals, and Gemstones" about this kind of adhesion on inanimate objects in nature.