Structure of Water and Hydrogen Bonding Basics
This standard is about the structure of water and hydrogen bonding and how these principles are fundamental to all life on Earth!
Structure of Water and Hydrogen Bonding Overview
The core principle of this first AP Biology lesson is that life exists as a hierarchy, with various levels interacting with one another to form the complex world of biology we see all around us.
Atoms and Molecules
Starting at the tiniest level—atoms—we can observe the principles and interactions that lay the scene for life to occur. In reality, the majority of visible biology – from animal behaviour to plant orientation to the light – is the result of molecular interactions inside individual cells. In general, each atom has a set of characteristics that are determined by the quantity of protons, electrons, and neutrons present. When these qualities are integrated into molecules, they may interact to form the overall molecule’s properties.
While scientists continue to investigate the complicated connections between biological molecules that aid in the formation of complete creatures, there is one molecule that is critical to life. This material is known as water.
Water takes up 60-90 percent of the total weight of practically all living things. Water not only makes up a large portion of most cells, but it also serves as the ideal solvent for dissolving and distributing a variety of chemicals throughout the cell.
Because water is a polar molecule, it possesses this capacity. Polar compounds do not evenly distribute their electrons. The chemical formula H2O represents water, which is made up of three atoms: one oxygen and two hydrogens. The oxygen atom has a far higher electronegative charge than the hydrogen atom. Water atom electrons spend substantially more time orbiting the oxygen atom than hydrogen atom electrons.
The structure of pure ice demonstrates this hydrogen bonding. Pure ice molecules create hydrogen bonds with one another, forming a flawless lattice structure, as seen in the figure below. In fact, the hydrogen connections between molecules keep each molecule apart more than would otherwise be the case. Because ice is less dense than liquid water, your ice cubes will float in a glass of water.
As a consequence, the molecule has a higher negative and a higher positive side. Positive charges attract the more negatively charged oxygen molecules, whereas negative charges attract the hydrogen atoms. This is why water is such an excellent polar solvent.
When other polar substances are dissolved in water, the water molecules aggressively pull them apart, ensuring that the new molecules are uniformly distributed throughout the solution. This is known as diffusion, and it allows cells and animals to effortlessly transfer certain polar chemicals throughout their bodies.
Water, on the other hand, does not mix well with non-polar compounds since it is a polar solvent. Cells employ this information to their advantage. All cells are surrounded by a lipid bilayer made up of molecules with polar heads and non-polar tails. The polar heads are drawn to the water, while the non-polar tails congregate to keep as much water out as possible. The polar areas are “hydrophilic,” meaning they attract water, while the non-polar portions are “hydrophobic,” meaning they resist it.
Properties of Hydrogen Bonding
Water molecules also interact through hydrogen bonding to provide three distinct properties: cohesion, adhesion, and surface tension.
Cohesion
Water’s capacity to “stick” to itself is known as cohesion. Due to hydrogen bonding, water molecules are more likely to cling together than break away. This feature may be seen in action in a droplet of water. Water droplets prefer to remain together rather than break apart and disperse over a surface.
Adhesion
Water’s capacity to attach to diverse hydrophilic surfaces is known as adhesion. Water adheres to the surface of some porous materials, allowing it to pass through. A droplet of water, for example, will swiftly spread out and travel through a paper towel by attaching to individual threads and “pushing” itself through the material.
Surface Tension
Although surface tension is not unique to water, it is rather high in comparison to other liquids. The ease with which an item may enter a liquid is measured by surface tension. The hydrogen bonding between water molecules increases the tension in water. This reduces the volatility of water (making it less likely to evaporate), allowing enormous quantities of water to gather and remain a suitable habitat for life.
Many biological processes rely on these three qualities. Massive trees, for example, employ the qualities of adhesion, cohesion, and surface tension to create a system of tunnels that enables water to go upward from the roots to the leaves. Water is kept going upward by adhesion, while cohesion and surface tension assist in drawing even more water to the leaves.
Overall, water is one of the most critical chemicals for life as we know it. Cells would not be able to transfer nutrients or other things if water did not have these polar qualities; cell membranes would not work; and the whole biosphere and water cycle would not exist to sustain all forms of life on Earth.
