Ahern's BB 350 at OSU - 2. Water, Weak Acids, and pH

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• published: 01 Apr 2015
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1. Contact me at kgahern@davincipress.com / Friend me on Facebook (kevin.g.ahern) 2. Download my free biochemistry book at http://biochem.science.oregonstate.edu/biochemistry-free-and-easy 3. Take my free iTunes U course at https://itunes.apple.com/us/course/biochemistry/id556410409 4. Check out my free book for pre-meds at http://biochem.science.oregonstate.edu/biochemistry-free-and-easy 5. Lecturio videos for medical students - https://www.lecturio.com/medical-courses/biochemistry.course 6. Course video channel at http://www.youtube.com/user/oharow/videos?view=1 7. Check out all of my free workshops at http://oregonstate.edu/dept/biochem/ahern/123.html 8. Check out my Metabolic Melodies at http://www.davincipress.com/ 9. My courses can be taken for credit (wherever you live) via OSU's ecampus. For details, see http://ecampus.oregonstate.edu/soc/ecatalog/ecourselist.htm?termcode=all&subject;=BB 10. Course materials at http://oregonstate.edu/instruct/bb350 Highlights 1. ATP's phosphates are negatively charged and repel each other. Breaking the bond between them (with water) releases energy. 2. Water's 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, giving rise to hydrogen bonds. 3. Ionic compounds are pulled apart in water. 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. 4. Covalent bonds are extraordinarily strong compared to hydrogen bonds (by a factor of about 20). 5. The term hydrophilic refers to compounds that are soluble in water and hydrophobic refers to compounds that are not soluble in water. 'Amphiphilic' refers to compounds that have parts of them that 'like' water and parts of them that repel water. Fatty acids are perfect examples. 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 hydrogens 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. 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 side, bases (like hydroxides) can accept protons found in water. The proton concentration is therefore very critical. We measure the proton concentration using a term called pH. 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. 12. 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. We refer therefore to HA as the ACID and A- as the SALT. I will avoid using the term BASE wherever I can in this class. To reiterate, the difference between an acid and a salt is a proton. 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. It also allows one to determine the amount of salt and acid 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. Subtraction of hydrogen ions is made possible by addition of a strong BASE (the only time we will use this word in class), such as NaOH. Addition of NaOH to a solution causes the OH- to interact with H+ to form water. 17. Three important facts 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.