Biochemistry 2 - Water, Acid-Bases, pH PDF

Biochemistry Water, acids-bases, blood buffers (PFYB033C and PFYB102C)  Water is essential for life. It covers 70% of the earth's surface and every living thing is dependent upon it  The human body (and the single cell mass) is comprised of over 70% wat...

Biochemistry Water, acids-bases, blood buffers (PFYB033C and PFYB102C)  Water is essential for life. It covers 70% of the earth's surface and every living thing is dependent upon it  The human body (and the single cell mass) is comprised of over 70% water, and it is a major component of many bodily fluids including blood, urine, and saliva. Water constitutes 45%-75% of total human body weight.  It is distributed in both intracellular and extracellular compartments % Body weight %Total body water Total body water 60 Extracellular fluid 20 33 Vascular fluid 5 8 Interstitial fluid 15 25 Intracellular fluid 40 67  Water is the biological solvent in which all biochemistry goes on! Water plays a major role in all aspects of metabolism: absorption transport digestion excretion maintenance of body temperature Water is also a direct participant in reactions such as hydrolysis and condensation THE PROPERTIES OF WATER  The structure of water Water is a hydride of oxygen in which the highly electronegative oxygen atom attracts the bonding (covalent) of two hydrogen atoms  Water Basics Polar: part of a molecule is + + slightly positive, while 1 another part is slightly 1 1 1 negative Oxygen “hogs” electrons 1 2 from hydrogen 6 – 8 protons: 10 electrons 1 – Negative charge 5 2 3 Hydrogen – 1 proton: 0 electrons 4 – Positive charge -  This leads to polar H-O bonds in which the hydrogen atoms have a slight positive charge and the oxygen atom has a slight negative charge (dipol structure) Neighboring liquid water molecules interact with one another This type of attraction involving a hydrogen atom is known as hydrogen bond  1) Due to its polarity, water has a high specific heat (resist temp changes) Takes lots of energy to break bonds (helps to maintain body temp) The weight of our bodies is mostly water. So, even when we exercise, the temperature of our body does not change much.  2) Due to its polarity, water is cohesive (water molecules stick to each other) Why the paper clip does not sink? Hydrogen bonds are weaker than covalent bonds. However, the cumulative effect of many hydrogen bonds is equivalent to the stabilizing effect of covalent bonds In proteins, nucleic acids and water, hydrogen bonds are essential to stabilize overall structure  3) Due to its polarity, water is adhesive (water molecules stick to other molecules) Capillary action! 3 states of water: liquid, solid and gas  Hydrophilic & Hydrophobic properity of water Water is a polar solvent – It readily dissolves most biomolecules, which are generally charged or polar compounds (salts, ions, sugars, urea and alcohols) by hydrating and stabilizing or weakening the electrostatic interactions between them and polar by forming hydrogen bonds The same factors apply to charged biomolecules The forces that hold the nonpolar regions of the molecules together are called hydrophobic interactions – Hydrophobic interactions among lipids, and between lipids and proteins, are the most important determinants of structure in biological membranes  The Properties of water 1. Physical Properties Polar molecule Hydrophilic substances dissolve Hydrophobic substances aggregate Osmosis - the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration Diffusion  The Properties of water 2. Chemical Properties : Ionizes to H+ and OH- Acids and bases Buffers system  Solution: mixture where 1 Solutions substance dissolves in another 2 parts to a solution Solution 1) Solute: substance that dissolves Atoms, ions, molecules 2) Solvent: substance in SUGAR which the solute is dissolved Usually water  Solutions Example: Human blood – Solvent: Plasma (watery part of blood, 92%) – Solutes: Carbs, proteins, sugars, etc… Plasma WBCs & Platelets RBCs Composition of blood  Major cations of the intracellular medium: + K and Mg ++ Major anions of the intracellular medium: Organic acids,proteins, HCO3- and Cl- Major cations of the plasma: Na+, Ca++, Mg++,K+ Major anions of the plasma: Cl- and HCO3-  Amphipathic compounds = contain regions that are both: Polar (or charged) and regions that are nonpolar When an amphipathic compound is mixed with water, the polar hydrophilic region interacts favorably with the solvent and tends to dissolve but the nonpolar, hydrophobic region tends to avoid contact with the water These stable structures of amphipathic compounds in water are called micelles  Many biomolecules are amphipathic; e.g. Proteins, pigments, certain vitamins, and the sterols and phospholipids of membranes all have polar and nonpolar surface regions Liposomes can carry water-soluble drugs and fat-soluble drugs, which is partly why they are so fantastic for nutrient delivery  (Auto)Ionization of water = spontaneous generation of hydrogen ions Water molecules exist in equilibrium with hydrogen ions and hydroxide ions H2O H+ + OH- (H3O+ hydronium) The product [H+] [OH–]—the ion product of water—is constant even when additional acid–base pairs are dissolved in the water M - Unit used for quantities of atoms and At 25 °C, pure water contains H+ and OH– at molecules concentrations of 1 × 10–7 mol L–1 each; it is neutral and has a pH value of exactly 7 But when chemicals are mixed with water, the mixture (solution) can become either acidic or basic As it is a log scale, one unit reflects a 10-fold change in proton concentration 26  The water equilibrium constant is written as: Kw = [H+] [OH-] Experimentally, it has been found that the concentration of: H+ = OH- = 10-7 Therefore: Kw = [10-7][ 10-7] = [10-14]  pH and Acids and Bases pH = - log [H+] Range is from 0 - 14 If [H+] is high, the solution is acidic; pH < 7 If [H+] is low, the solution is basic or alkaline ; pH > 7 Acids are H+ donors Bases are H+ acceptors, or give up OH- in solution Acids and bases can be: – Strong – dissociate completely in solution HCl, NaOH – Weak – dissociate only partially in solution Lactic acid, carbonic acid 28  Acid pH Scale Base neutral Blood pH scale measures the amount of H+ ions Some molecules release H+ ions when dissolved H+ ions accumulate (acids) Some molecules release OH- ions when dissolved OH- ions accumulate (bases)  Practice Quiz 1) Vocabulary: Polar, Adhesion, Cohesion, High Specific Heat, Solution, Solute, Solvent, pH 2) Name 4 basic features of water. 3) Which water property explains that one atom is positively charged, while another is negatively charged? 4) How does cohension and adhesion differ? 5) Which is a stronger acid? Substance A with a pH of 5 or substance B with a pH of 3? 6) Which has more H+ ions? Substance A with a pH of 5 or substance B with a pH of 3? 7) Which has more OH- ions? Substance A with a pH of 5 or substance B with a pH of 3? 8) How does a solute differ from a solvent? 9) What is the solute and solvent of blood? 31  The Body and pH Homeostasis of pH is tightly controlled Extracellular fluid = 7.4 Blood = 7.35 – 7.45 < 6.8 or > 8.0 death occurs Acidosis - below 7.35 (H+ too high) Alkalosis - above 7.45 (OH-) 32  Small changes in pH can produce major disturbances Most enzymes function only with narrow pH ranges Acid-base balance can also affect electrolytes (Na+, K+, Cl-) Can also affect hormones An increase of H+ concentration can easily alter the charges and functions of proteins, enzymes, nucleic acids, some hormones and membranes Normal blood pH is 7,35-7,45. Values below 6,8 or above 7,70 are seldom compatible with life In living organisms, pH of the body fluids are tightly regulated by biological buffers and some organs (lungs and kidneys) 33  Blood pH OH- OH- H+ H+ H+ H+ OH- OH- H+ H+ H+ H+ H+ H+ H+ OH- H+ OH- H+ H+ OH- H+ H+ OH- OH- OH- H+ H+ OH- H+ H+ Normally: Balance of H+ and OH- ions in blood (7.4 pH) Alcohol abuse: Excess alcohol lowers blood pH (H+ ions accumulate) Kidney removes excess H+ Can lead to kidney damage 35  The body produces more acids than bases Acids are taken in with foods Acids produced by metabolism of lipids and proteins Cellular metabolism produces CO2 CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3- 36  Control of Acids 1. Buffer systems Take up H+ or release H+ as conditions change Buffer pairs – weak acid and a base And exchange a strong acid or base for a weak one Results in a much smaller pH change 37  Buffering Systems Buffers are aqueous systems that tend to resist changes in pH when small amounts of strong acid [H+] or strong base [OH-] are added. A buffer system consists of a weak acid (the proton donor) and its conjugate base (the proton acceptor)  Biological Buffering Systems 1. Bicarbonate/carbonic acid buffer system 2. Protein buffer system 3. Hemoglobin buffer system 4. Phosphate buffer system https://www.youtube.com/watch?v=VzEEs 00v-JU  Bicarbonate/carbonic acid buffer system The most important buffer of the plasma is the bicarbonate/carbonic acid buffer system The ratio of base to acid (HCO3-/H2CO3) is nearly 20/1 in plasma under physiological conditions This buffer system is more complex than others, because carbonic acid (H2CO3) is formed from dissolved CO2 which is produced in tissues and diffused to plasma  CO2 + H2O H2CO3 HCO3- + H+ This reaction is slow in plasma but in erythrocytes, carbonic anhydrase increases the rate of this reaction HCO3-/H2CO3 = 20/1 when plasma pH=7,4 When hydrogen ion concentration increases in plasma, HCO3- ions bind H+ forming H2CO3 H2CO3 is converted to CO2 + H2O CO2 is released to atmosphere by lungs  Hemoglobin buffer system Hemoglobin (Hb) is a protein which carries O2 to tissues and CO2 from tissues to lungs and is an effective buffer The most important buffer groups of Hb are histidines. Each globin chain contains 9 histidine. %95 of CO2which is released from tissues to plasma is diffused into erythrocytes.  In erythrocytes, carbonic anhydrase constitutes H2CO3 from CO2 and H2O and then HCO3- and H+ are released by the ionization of H2CO3. Carbonic anhydrase CO2 + H2O H2CO3 HCO3- + H+ Released protons take part in the formation of salt bridges between globin chains of Hb, and lead the change in the conformation of Hb molecule in tissue capillaries.  Bicarbonate buffer Sodium Bicarbonate (NaHCO3) and carbonic acid (H2CO3) Maintain a 20:1 ratio : HCO3- : H2CO3 HCl + NaHCO3 ↔ H2CO3 + NaCl NaOH + H2CO3 ↔ NaHCO3 + H2O 44  Phosphate buffer Major intracellular buffer H+ + HPO42- ↔ H2PO4- OH- + H2PO4- ↔ H2O + H2PO42- 45  Protein Buffers Includes hemoglobin, work in blood and ICF Carboxyl group gives up H+ Amino Group accepts H+ Side chains that can buffer H+ are present on 27 amino acids. 46 47 48 49 50