Water & Hydrogen Bonding

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Function of Water

Water is a major component of cells and makes up 60-70% of the human body. Life evolved in an environment where water was abundant. It has several properties that are important in biology.

Importance of water

Water is one of the most useful molecules for life. Its uses include:
- As a reactant in cells (e.g. photosynthesis, hydrolysis).
- Provides structural support in cells.
- Keeps organisms cool to maintain an optimum body temperature.

Properties of water

Special properties of water are:
- Metabolic importance.
- High heat capacity.
- Heat of vaporization.
- Cohesive properties.
- Useful as a solvent.

The Structure of Water

The structure of a water molecule helps us to understand hydrogen bonding and the function of water.

Contents of a water molecule

Water molecules (H₂O) are made from:
- One oxygen atom.
- Two hydrogen atoms.

Polarity

Water is a polar molecule.
- The oxygen atoms in water are slightly negatively charged.
- The hydrogen atoms in water are slightly positively charged.

Hydrogen bonding

When polar covalent bonds containing hydrogen form, the hydrogen in that bond has a slightly positive charge because hydrogen’s electron is pulled more strongly toward the other element and away from the hydrogen.
Because the hydrogen is slightly positive, it will be attracted to neighboring negative charges.
- When this happens, a weak interaction occurs between the δ+of the hydrogen from one molecule and the δ– charge on the more electronegative atoms of another molecule.

Hydrogen bonding - importance

The polarity of water molecules means that a hydrogen atom on one water molecule is attracted to the oxygen atom on another water molecule.
This attraction is called hydrogen bonding.
Hydrogen bonds provide many of the critical, life-sustaining properties of water,
- Hydrogen bonding also stabilizes the structures of proteins and DNA, the building block of cells.

Hydrogen Bonding & Water's Properties

Hydrogen bonding gives water its special characteristics, including cohesion, adhesion, and surface tension.

Cohesion

Have you ever filled a glass of water to the very top and then slowly added a few more drops?
Before it overflows, the water forms a dome-like shape above the rim of the glass.
- This water can stay above the glass because of the property of cohesion.
- In cohesion, water molecules are attracted to each other (because of hydrogen bonding), keeping the molecules together at the liquid-gas (water-air) interface, although there is no more room in the glass.

Surface tension

Cohesion allows for the development of surface tension, the capacity of a substance to withstand being ruptured when placed under tension or stress.
- This is also why water forms droplets when placed on a dry surface rather than being flattened out by gravity.
Cohesion and surface tension keep the hydrogen bonds of water molecules intact and support the item floating on the top.

Adhesion

Cohesive forces are related to water’s property of adhesion, or the attraction between water molecules and other molecules.
- This attraction is sometimes stronger than water’s cohesive forces, especially when the water is exposed to charged surfaces such as those found on the inside of thin glass tubes known as capillary tubes.

Adhesion & capillary action

Adhesion is observed when water “climbs” up the tube placed in a glass of water.
- Notice that the water appears to be higher on the sides of the tube than in the middle.
This is because the water molecules are attracted to the charged glass walls of the capillary more than they are to each other and therefore adhere to it.
- This type of adhesion is called capillary action.

Capillary action

Cohesion & adhesion - importance

Cohesive and adhesive forces are important for the transport of water from the roots to the leaves in plants.
- These forces create a “pull” on the water column.
This pull results from the tendency of water molecules being evaporated on the surface of the plant to stay connected to water molecules below them, and so they are pulled along.
Plants use this natural phenomenon to help transport water from their roots to their leaves.