What is Cohesion? Definition, Types, & Examples

  • Post last modified:October 2, 2021
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Cohesion Definition

Cohesion is defined as the act, state, or process of similar molecules or things adhering together. Water molecules are one example. Cohesion is the tendency of water molecules to stay together, and it is maintained by a cohesive force such as an intermolecular hydrogen bond.

The word cohesion comes from the Latin cohaesiō, which comes from the cohaereō, which means “to cling” or “to stick together.” Cohesive attraction is a synonym for cohesive force.

What is Cohesion?

Cohesion is defined as the state of similar things cohering or staying together in science. Natural cohesion may be seen in certain molecules, such as water. The intermolecular force that keeps them together is what allows them to cohere or cling together. Those attracted to other molecules or a different material, on the other hand, do not show cohesion. Adhesion is the term for this phenomena.

The concept of cohesiveness may be applied to biology as well. It is the process, act, or state in which molecules or bodily components that are similar bond or stay close together. Syncarpy, for example, is a term used in botany to describe the fusing of plant components (the fusion of carpels of a pistil).

Cohesion Examples

i. Cohesion of Water Molecules

Water is a good example of a cohesive material. Dihydrogen monoxide (HOH) molecules, which have two hydrogens and one oxygen, make up water. The molecule has polarity, which is defined as the presence of two opposing charges. The hydrogens have a slightly positive charge, which results in a partially positive pole, whereas the oxygen has a slightly negative charge, which results in a partially negative pole.

The polarity of water molecules causes them to stay together or attract one another. A cohesive force holds water molecules together. This force is an intermolecular hydrogen bond, which is a weak or transitory form of chemical connection. It is formed when one HOH’s hydrogen reacts with the hydrogen of another HOH.

As a result, as they cohere, they create a water drop. Water has adhesion in addition to cohesion. Adhesion is the attraction of different molecules to one other, whereas cohesion is the attraction of identical molecules.

Water forms droplets due to cohesion, but adhesion maintains the drop on a surface, such as the surface of leaves or flowers. If you slowly pour water through a dropper, you’ll notice that it comes out in a succession of droplets rather than a continuous stream. In addition, the drop takes on a spherical form (gravity causes the drop to lose its supposedly perfect sphere shape). Surface tension is to blame.

Have you ever noticed how a glass filled to the rim with water produces a dome-like structure on top? Surface tension is what it’s all about.

Surface tension is defined as the attractive attraction produced by molecules below the surface molecules, forcing the liquid to take on the form with the least amount of surface area. When the liquid surface comes into contact with a gas, such as air, this phrase is employed. It prevents the water’s surface from rupturing even while it is under tension or stress.

Cohesion is primarily responsible for the surface tension in water. In the shot, notice the water droplet on the leaf surface. The surface tension of water is responsible for its spherical form. Cohesion is the cause of surface tension (water molecules attracting each other).

The cohesive force (force between water molecules) appears to be stronger than the adhesive force in this case (force between a water molecule and other molecules, such as the molecules of the air and leaf surface). The attraction between water molecules is stronger than that between air molecules.

As a result, they have a tendency to display surface tension. This is crucial to plants because cohesiveness and high surface tension limit water escape through leaf stomata. Surface tension is also responsible for certain insects remaining stationary above the water or walking through it. The basic elements that enable capillarity are surface tension, cohesion, and adhesion.

This is particularly essential in vascular plants. Capillary action allows water to flow up a small tube against gravity. The molecules of the liquid are drawn inward from the surface by surface tension, resulting in the smallest feasible surface area.

The water molecules then adhere to each other due to cohesion. Adhesion aids in the binding of water molecules to the walls of a plant’s xylem tissues. As a result, water can flow upward from the root through the xylem.

ii. Cohesion of Biomolecules

Cohesion is a physicochemical phenomenon as well. It also happens physiologically, as shown by biomolecules like DNA. Sister chromatids cohere during cell division, such as mitosis and meiosis, until they split during the anaphase. Various protein complexes known collectively as cohesins mediate this cohesive process.

The four protein components of cohesin are depicted in the diagram below: SMC3, SMC1, SCC1, and SCC3. After the DNA is replicated in preparation for cell division, the cohesins hold the sister chromatids together. It guarantees that the sister chromatids will remain linked until they reach the equatorial region of the dividing cell.

Complete separation during anaphase is enabled by the lack of cohesiveness between sibling chromatids. Proper segregation may not be possible without cohesins. Cohesion is formed prior to mitosis and meiosis to keep the sister chromatids together in both mitosis and meiosis.

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