This standard focuses on the Structure of Water and Hydrogen Bonding, specifically how these concepts are important to all life on Earth!

Video Tutorial

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Resources for this Standard

• Overview & Video Tutorial (This article)
• Quick Test Prep
• Crossword Puzzle

• PowerPoint

ENDURING UNDERSTANDING

SYI-1

Living systems are organized in a hierarchy of structural levels that interact.

LEARNING OBJECTIVE

SYI-1.A

Explain how the properties of water that result from its polarity and hydrogen bonding affect its biological function.

ESSENTIAL KNOWLEDGE

SYI-1.A.1

The subcomponents of biological molecules and their sequence determine the properties of that molecule.

SYI-1.A.2

Living systems depend on properties of water that result from its polarity and hydrogen bonding.

 SYI-1.A.3

The hydrogen bonds between water molecules result in cohesion, adhesion, and surface tension.

Structure of Water and Hydrogen Bonding Overview

The big idea in this first AP Biology lesson is that life exists as a hierarchy from atoms to the entire planet, with different levels interacting with one another to create the complex world of biology we see all around us.

Starting at the smallest level – atoms – we can begin to see the rules and interactions that set up the stage for life to take place. In fact, most of the observable biology that we can see – from animal behavior to the way plants turn towards the sun – is a product of molecular interactions within individual cells. In general, each atom inherently contains a number of properties that arise from the number of protons, electrons, and neutrons in an atom. When combined into molecules, these properties can interact to create the properties of the overall molecule.

While we are still researching the complex interactions between biological molecules that help create entire organisms, there is one molecule that is of the utmost importance for life. We call this substance water.

Water makes up approximately 60-90% of the total weight of almost every organism. Not only does water make up a massive percentage of most cells, but water provides the perfect solvent to dissolve and distribute a number of molecules within the cell.

Water has this ability because it is a polar molecule. Polar molecules do not share their electrons equally. Water is made of three atoms – one oxygen and two hydrogens – represented by the chemical formula H₂O. The oxygen atom is far more electronegative than the hydrogen atoms. The electrons within a water atom spend much more time circling the oxygen atom than the hydrogen atoms.

This hydrogen bonding can be clearly seen in the structure of pure ice. The water molecules within pure ice form hydrogen bonds with each other, creating a perfect lattice structure, as seen in the image below. In fact, the hydrogen bonds between molecules hold each molecule further apart than would normally happen. This makes ice less dense than liquid water, which is why your ice cubes float in a glass of water.

This results in a molecule that has a more negative side and a more positive side. The more negatively charged oxygen molecule tends to attract positive charges, while the hydrogen atoms tend to attract negative charges. This is what makes water such a great polar solvent.

When other polar substances are dissolved in water, the water molecules actively pull them apart – evenly distributing the introduced molecules throughout the solution. This is called diffusion, and it gives cells and organisms the ability to easily distribute certain polar substances throughout their cells and body.

On the other hand, because water is a polar solvent, it does not mix well with non-polar substances. Cells use this fact to their advantage. The lipid bilayer that surrounds all cells is composed of molecules with a polar head and a non-polar tail. The polar heads are attracted to the water, while the non-polar tails group together to exclude as much water as possible. The polar regions are “hydrophilic” because they are attracted to water, while the non-polar regions are “hydrophobic” because they tend to repel water.

Furthermore, water molecules interact through hydrogen bonding to create 3 very unique properties: cohesion, adhesion, and surface tension.

Cohesion is the ability of water to “stick” to itself. The hydrogen bonding between water molecules means they are more likely to stick together than break apart. You can see this property in action in a droplet of water. Instead of breaking apart and spreading across a surface, water droplets tend to stay intact.

Adhesion, on the other hand, is water’s ability to stick to various hydrophilic surfaces. This gives water the ability to move through certain porous materials by adhering to their surface. For example, if you place a droplet of water on a paper towel it will quickly spread out and move through the towel by adhering to individual fibers and “pulling” itself through the material.

Surface tension is not unique to water, but water does have a high surface tension compared to other liquids. Surface tension is a measure of how easily an object can penetrate a liquid. In water, this tension is increased by all the hydrogen bonding between water molecules. In effect, this makes water less volatile (less likely to evaporate), which allows large bodies of water to collect and remain a viable environment for life.

These three properties are important for many biological processes. For example, massive trees use the properties of adhesion, cohesion, and surface tension to help move water molecules from the roots to the leaves by creating a series of passageways that allow water to travel upward. Adhesion ensures that water keeps moving upward, while cohesion and surface tension help pull even more water to the leaves.

In all, this makes water one of the most important molecules for life as we know if. If water did not have these polar properties, cells would not be able to distribute nutrients or other substances, cell membranes would not function, and the entire biosphere and water cycle would not exist to support all kinds of life on Earth.