Highlights Lecture #2 Spring 2017

Highlights Water/Acids

1. ATP is the “gasoline” of the cell. It is able to store energy in the three phosphates it contains. The phosphates are negatively charged at physiological pH and repel each other. Breaking the bond between them (with water) causes the phosphate to fly away and release energy.

2. Water is a bent molecule. Electrons are not shared equally by oxygen and the hydrogens, resulting in the hydrogens having a partial positive charge and the oxygen having a partial negative charge. These partial charges give rise to hydrogen bonds. Note that the attraction of an atom’s nucleus for electrons is measured by electronegativity. The higher the electronegativity, the stronger the attraction for electrons.

3. Ionic (polar) compounds will be pulled apart in water. We say that they dissolve. In this case, the ions (charged molecules, such as K+) are surrounded by the correspondingly opposite partial charge of the water molecule. Thus, K+ is most closely associated with the oxygen component of the water molecule since the oxygen is the most negatively charged. This is an example of an ion-dipole interaction and is very common in solutions involving water.

4. Bond energies are measures of the amount of energy it takes to break a bond. Covalent bonds (such as those between oxygen and hydrogen in water) are extraordinarily strong compared to hydrogen bonds (by a factor of about 20). Nonetheless, hydrogen bonds contribute significantly to the properties of water and biological molecules. This is due to their very large numbers.

5. We use the term hydrophilic to refer to compounds that are soluble in water and hydrophobic to refer to compounds that are not soluble in water. The term ‘amphiphilic’ or amphipathic is used to refer to compounds that have parts of them that ‘like’ water and parts of them that repel water. Fatty acids (in soaps) are a good example of an amphiphilic compound. We shall see later that the lipids in membranes are amphiphilic.

6. Many biological compounds are hydrophilic. Examples include sugars, amino acids, nucleic acids, and most proteins. Fat is the predominant hydrophobic compound found in cells. Fatty acids (which are components of fats) are amphiphilic.

7. Hydrogen bonds can occur between many different molecules. All it takes is hydrogen atoms in a molecule with partial positive charges and a nearby molecule with a partial negative charge. The hydrogen is referred to as the hydrogen bond donor and the partial negative molecule is a hydrogen bond acceptor. The partial negative atoms are most commonly nitrogen or oxygen.

8. Hydrogen bonds are responsible for giving water its extremely high melting and boiling points for a molecule of its small molecular weight.

9. In water, molecules like acids can donate protons (H+) to the solution. This has a drastic effect on the properties of water. On the other hand, bases (like hydroxides) can accept protons and create water when they combine. The proton concentration of a solution is therefore very critical. We measure the proton concentration using a term called pH. pH is equal to the negative log of the hydrogen ion concentration of a solution (pH = -log[H+])

10. Strong acids and weak acids differ in their properties. HCl (hydrochloric acid) is a strong acid. If you put 10 million molecules of HCl in water, all 10 million molecules will dissociate into H+ and Cl ions.

11. Many acids we find in cells are weak acids. Examples include acetic acid, which is a stronger acid than water, but a weaker acid than HCl. When we describe weak acids, we designate them by the letters HA. When the acid loses a proton, we refer to what is left as A. The difference between HA and A is clearly the proton that is lost. In this class, we refer therefore to HA as the ACID and A as the SALT. To reiterate, the difference between an acid and a salt is a proton.

12. I will avoid using the term BASE wherever I can in this class and will generally reserve the term for strong bases, such as NaOH. These strong bases completely dissociate in watr.

13. If one has an acid that loses one proton per 1000 molecules, it is a stronger acid than one that loses one proton per 100,000 molecules.

14. A weak acid has the ability, under proper circumstances to absorb or donate protons to resist changes that happen to the solution in which the buffer is located. This ability to resist changes in proton concentration means that solutions of weak acids resist (within certain ranges), changes in pH.

15. The Henderson-Hasselbalch equation (pH = pKa + log [A]/[HA] (where A is what I called the ‘salt’ and HA is the acid) allows one to measure the pH of a solution of a weak acid if one knows the pKa and the amount of salt and acid in a fixed volume. It also allows one to determine the amount of salt and acid in a fixed volume if one knows the pH and pKa.

16. Addition of hydrogen ions (protons = H+) to a solution is made possible by adding a strong acid, such as HCl. Removal of hydrogen ions is made possible by addition of a strong BASE, such as NaOH. Addition of NaOH to a solution causes the OH to interact with H+ to form water.

17. Three important facts to know about logarithms. a) the log of 1 is zero; b) the log of a number less than one is negative; c) the log of a number greater than one is positive.

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