The Chemistry of Nutrition and The Chemical Level of Organization
Atoms are the smallest units of matter that are stable. Atoms themselves are made up of positive protons, neutral neutrons (both in the center of the atom – the nucleus), and negative electrons in regular orbits about the nucleus.
Molecular bonding – Ionic bonds
Ultimately, atoms are defined by the number of protons they have in the nucleus (the atom’s “Atomic Number”). Atoms will react with each other by either transferring or sharing electrons, with the ultimate goal of having a complete outer valence shell of electrons – this is the creation of molecules. It can be said that atoms are greedy or lazy when it comes to completing their outer shell of electrons. Some are greedy in that they will take electrons from a neighboring atom. In turn, the neighboring atom is lazy in that it would rather give up its outer shell electrons and drop down to the next full orbital. The atom which gains an electron will become a negative ion (anion) and the atom which loses an electron will become a positive ion (cation). In this case, an ionic bond is formed by the attraction between the two oppositely charged ions.
Molecular bonding – Covalent bonds
In other cases, both atoms are lazy, and rather than transferring electrons, it’s easier to simply share electrons in their outer shell. This is called a covalent bond. However, some atoms will have a greater attraction to electrons than others (electronegativity). When an atom in a covalent bond has a greater electronegativity than its neighbor, there will be an uneven sharing of valence electrons. The electrons would gather more around the more electronegative atom and that atom would have a partial negative charge. Likewise, the atom in the bond that has a smaller electronegativity will have a partially positive charge. This is known as a polar covalent bond and a great example of such a bond is in water, where the highly electronegative Oxygen will pull the electrons more around it. Therefore, Oxygen will have a partial negative charge, and the associated Hydro- gens will have a partial positive charge.
If the atoms have the same electronegativity (e.g. if the atoms are the same, such as the two oxygen atoms in O2) or nearly the same electronegativity (e.g. carbon and hydrogen have almost the same electronegativity, C-H), then the electrons would be shared evenly between the atoms, resulting in a non-polar covalent bond.
At this point, one may ask, “why is it important to understand these fundamental chemistry concepts in order to give nutrition recommendations to athletes?” Remember that nutrients are simply the molecules and ions that make up our cells. To truly comprehend how molecules such as carbohydrates, lipids, and proteins function, you must understand their unique structures and how their structures react in one more important molecule (and our other macronutrient), water.
Water and Hydrogen Bonds
With its unique properties, water is an incredibly important molecule for life. So that begs the question, what are the properties of water?
- Liquid at room temperature
- Liquid water does not change tempera-
- High heat of vaporization
- Frozen water is less dense than liquid water
- Molecules of water cling together
- A solvent for other polar molecules
Of course, this leads us to another question — Why does water have these unique properties? It is due to its shape, its polarity, and its creation of Hydrogen bonds (“1” on image to right).
Looking at Water
Water is formed by the covalent bond between an oxygen atom and 2 hydrogen atoms, therefore there is no “net” charge. However, oxygen has a greater electronegativity for electrons than hydrogen. Therefore, there is an unequal distribution of electrons, and a polar covalent bond is formed.
So how does this lead to the unique properties of water? What bond holds water molecules together? The creation of hydrogen bonds. Hydrogen bonds (H-bonds) occur between hydrogen in a covalent bond and a negatively charged atom of another molecule. These are relatively weak intermolecular (between molecules) bonds (approx. 5kcal/mol). Although relatively weak, the bonds are still bigger than room temperature (however, if one raises the temperature to boiling, the H-bonds will rip apart).
Hydrophobic vs Hydrophilic
Other polar molecules (e.g. carbohydrates) will dissolve in water, as the partial positive ends are attracted to the partially negative oxygen in water, and the partial negative end will be attracted to the hydrogen (hence, hydrogen bonds form between water molecules as well as between other molecules). Ions (e.g. Na+, Ca+2, Cl–) will also dissolve in water, with negative ions (anions) being sur- rounded by water’s partially positive hydrogen atoms and positive ions (cations) surrounded by the partially negative oxygens.
Nonpolar molecules (e.g. fats, oils, cholesterol) will not dissolve in water. This is the case for lipids, which are made primarily of C-C and C-H bonds. There are two reasons it will not dissolve in water. First, nonpolar molecules cannot form H-bonds with H20. Second, water would have to order itself around each molecule in what is known as a loss of entropy (chaos or randomness). This organization would require energy.
From this point forward, we can classify molecules not only as polar and nonpolar but also if they will dissolve in water. Lipids are hydrophobic (hydro– mean- ing water, and –phobic meaning fear) molecules and will not dissolve in water while carbohydrates and proteins are hydrophilic (-philic meaning love) that CAN form H-bonds and dissolve in H2O.
Amphipathic molecules, Polar and Nonpolar at the same time
Some very specific (in our in case, very important) molecules are hydrophilic/polar at one end and hydrophobic/ nonpolar at the other. These are known as amphipathic molecules. If put in water, the hydrophilic end of these molecules will point out in the water and the hydrophobic end will associate with other hydrophobic ends. In this case, a micelle is created.Read more here