HTHSCI 1LL3 Study Guide - Winter 2018, Comprehensive Midterm Notes - Metabolism, Bicarbonate, Sodium
12 Oct 2018
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HTHSCI 1LL3
MIDTERM EXAM
STUDY GUIDE
Fall 2018
Acid, pH, Buffers
What is an Acid?
● An acid is a substance that releases protons when dissolved
● Extent of dissociation, that is, the amount of protons released compared to the total amount of
compound, is a measure of the strength of the acid
● HCl is a strong acid, because it dissociates completely in water, generating free [H+] and Cl-
● Acidity can be measured on a scale called pH
pH
● Most living cells have a very narrow range of tolerance for pH
● The H+ concentration will be important (either explicitly or implicitly) for many other topics in
biology
● [H+] is controlled in all biological organisms
● Each pH unit represents a factor of 10 difference in H+
The conceptual problem with pH
● Every factor of 10 difference in H+ represents 1.0 pH units
○ Every factor of 2 difference in H+ represents 0.3 pH units
How can you determine the pH of the solution?
● Use a pH meter or pH paper
● Titrate the solution with precise amounts of base or acids in conjunction with a soluble dye
The hydration of carbon dioxide in water
● As CO2 goes into solution, carbonic acid is formed, which partially dissociates, liberating protons
(H+) and thus causing the solution to become more acidic, lowering the pH
Weak acid
● Some substances, like acetic acid (vinegar!) dissociate poorly in water.
● Thus, they release protons, but only a small fraction of their molecules dissociate (ionize).
● Such compounds are considered to be weak acids.
● Thus, while 1 M HCl is pH = 0, 1 M acetic acid is only pH = 2.4
● Thus are in equilibrium with their ionized species
○ Governed by the Law of Mass action and characterized by an equilibrium constant
Water, Acids, Bases & Buffers
Water
● Performs functions like:
○ Transport
○ Component of chemical rxns
○ Controls body temperature
● Body water is almost constant at about 60% of our weight
● Adipose tissues contains little water
○ Children have a lot of water while obese people have less
Fluid Compartments
● Total body water: 25L of intracellular fluid + 15L of extracellular fluid = 40L total
● Extracellular fluid: 10L interstitial + 5L blood = 15L total
Polarity of Water
● Shared electrons of the H-O bond are attracted to the oxygen molecule
● Gives hydrogen atoms a partial positive charge
● Also gives oxygen atom a partial negative charge
Properties of Water
● Polarity of water allows polarity molecules to dissolve
● Hydrogen bonds are formed with polar compounds and ‘hydration shells surround ions
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● Hydrogen bonds are weak, constantly breaking and reforming, so that solutes can move in solution
and water can move through pores in cell membranes
Thermal Regulation
● Water helps maintain body temperature in the narrow range compatible with life
● It is resistant to temperature change and can dissipate heat from areas that generate it such as the
brain
● Water also maintains or dissipates heat when blood vessels in the skin constrict/dilate respectively
Electrolytes & Osmolality
● Electrolytes are anions (Cl-, HCO3-, PO42-) and cations (K+, Na+)
● Osmolality is the concentration of all dissolved solutes in the blood (electrolytes, proteins)
● Water will move between compartments to keep the osmolality the same
pH
● pH is a measure of the acidity or alkalinity of a solution
● pH = -log[H+]
● pH in the blood must be maintained in a narrow range (7.35-7.45)
Acids and Bases
● Acids donate H+ to a solution, while bases accept an H+
● Strong acids dissociate completely, weak acids dissociate only to a limited extent in water
● Weak acids (HA) dissociate into H+ and the conjugate base (A-)
● The tendency to dissociate is defined by the equilibrium constant Ka
AND HA ⇔ A- + H+
● The larger the Ka, the greater tendency to dissociate
Henderson-Hasselbalch Equation
[A-]/[HA]
● When pH = pKa, 50% of the acid is dissociated
○ When log(1) = 0, so when conjugate base divided by acid concentration is 1
● This is when a buffer is most effective, although it is still useful to ±1 pH unit of the pKa
Buffers
● Buffers are combos of weak acids and their conjugate base
● Buffers resist changes in pH, they are most effective when pH = pKa
because equal amounts of both acid + base are present
● Because both acid and bases are present, protons can be accepted and
donated to maintain pH
● Buffers work best within a range of 1 pH unit above or below the pKa of
the buffer, where the pKa is the negative log of the Ka
Biologically Important Buffers
● There are a number of buffering systems in the body to deal with the
metabolic acids produced
● Dihydrogen phosphate-hydrogen phosphate (H2PO4- ⇔ HPO42-, pK
=7.2) system is active inside cells
● Hemoglobin has an amino acid side chain that can accept H+, there are
other proteins in cells that can do this as well
● Normal metabolism generates metabolic acids (lactate, ketone bodies),
inorganic acids (sulfuric acid, hydrochloric acid) and carbon dioxide
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Document Summary
An acid is a substance that releases protons when dissolved. Extent of dissociation, that is, the amount of protons released compared to the total amount of compound, is a measure of the strength of the acid. Hcl is a strong acid, because it dissociates completely in water, generating free [h+] and cl- Acidity can be measured on a scale called ph ph. Most living cells have a very narrow range of tolerance for ph. The h+ concentration will be important (either explicitly or implicitly) for many other topics in biology. Each ph unit represents a factor of 10 difference in h+ Every factor of 10 difference in h+ represents 1. 0 ph units. Every factor of 2 difference in h+ represents 0. 3 ph units. Use a ph meter or ph paper. Titrate the solution with precise amounts of base or acids in conjunction with a soluble dye.
