Water - Campbell Biochemistry 7th Edition - PDF

Water Campbell Biochemistry 7th Edition Chapter 2 Structure and Catalysis Chapter 2 Water Water is essential for life; Water is the principal component of most cells Making up 70% or more of the weight of most organisms Major component of many body fluids including blood, sali...

Water Campbell Biochemistry 7th Edition Chapter 2 Structure and Catalysis Chapter 2 Water Water is essential for life; Water is the principal component of most cells Making up 70% or more of the weight of most organisms Major component of many body fluids including blood, saliva, and urine It acts as a solvent for the substances we need, such as K+, glucose, adenosine triphosphate (ATP), and proteins  Many of the compounds produced in the body act as acids or bases, releasing or accepting hydrogen ions. H+ content and the amount of body water are controlled to maintain a constant environment for the cells called homeostasis. Significant deviations from a constant environment, such as acidosis or dehydration, may be life threatening. Maintenance of body pH Structure of the water molecule Two hydrogen atoms bond to one oxygen atom Each hydrogen atom of a water molecule shares an electron pair with the central oxygen atom Oxygen has two electron pairs that repel each other and two bonded pairs → bent shape Water is Polar Oxygen and hydrogen have different electronegativity values The strongly electronegative oxygen atom in a water molecule attracts electrons away from the hydrogen nuclei, leaving them with a partial positive charge, while its two unshared electron pairs constitute a region of local negative charge. As a result, the oxygen side of the molecule is much more electronegative than the hydrogen side, and the molecule is dipolar. Hydrogen Bonds Exist between Water Molecules Within a water molecule, each hydrogen bears a partial positive charge (d+) and the oxygen atom bears a partial negative charge equal to the sum of the two partial positives (2d-) As a result, there is an electrostatic attraction between the oxygen atom of one water molecule and the hydrogen of another, called a hydrogen bond Weak noncovalent bond  The negative oxygen end of one water molecule is attracted to the positive hydrogen end of another water molecule to form a hydrogen bond Each water molecule is hydrogen-bonded to approximately four close neighboring water molecules Hydrogen bonding profoundly influences the physical properties of water and accounts for its relatively high viscosity, surface tension, and boiling point. Hydrogen Bonding Gives Water Its Unusual Properties Water has a higher melting point, boiling point, and heat of vaporization than most other common solvents. These unusual properties are a consequence of attractions between adjacent water molecules that give liquid water great internal cohesion. A look at the electron structure of the H2O molecule reveals the cause of these intermolecular attractions. Melting Point, Boiling Point, and Heat of Vaporization of Some Common Solvents Why does water have such interesting and unique properties? Water has unique properties for a molecule its size, such as a very high boiling point and melting point. This is due to the extensive hydrogen bonding possible between water molecules. Each water molecule has two sources of partial positive charge and two of partial negative charge. This allows water to form an array in a solid form and to bond with many other water molecules in liquid form. The extensive hydrogen bonding requires large amounts of energy to disrupt, and therefore it melts and boils at higher temperatures than other molecules of its relative size. States of water 3 states: - Liquid - Solid (ice) - Gas  When water reaches 0ºC, water becomes locked into a crystalline lattice with each molecule bonded to its maximum of four partners. As ice starts to melt, some of the hydrogen bonds break and some water molecules can slip closer together than they can while in the ice state. Which state is less dense?? Properties of water ❑ Physical properties ❑ Chemical properties ❑ Physical properties ✓ Polar molecule ✓ Hydrophilic substances dissolve ✓ Hydrophobic substances aggregate ✓ Cohesion ✓ Adhesion ✓ Polar molecule Molecules that have ends with partial negative and positive charges are known as polar molecules It’s this polar property that allows water to separate polar solute molecules and explain why water can dissolve so many substances Why do some chemicals dissolve in water while others do not? The polar nature of water largely determines its solvent properties. Ionic compounds with full charges and polar compounds with partial charges tend to dissolve in water. The underlying physical principle is electrostatic attraction between unlike charges. The negative end of a water dipole attracts a positive ion or the positive end of another dipole. The positive end of a water molecule attracts a negative ion or the negative end of another dipole. ✓ Hydrophilic and hydrophobic Water is a polar solvent; it dissolves most biomolecules which are generally charged or polar compounds. Compounds that dissolve easily in water are hydrophilic (Greek, “water-loving”). In contrast, nonpolar solvents such as chloroform and benzene are poor solvents for polar biomolecules but easily dissolve those that are hydrophobic-nonpolar molecules such as lipids and waxes. Water as solvent Water dissolves many crystalline salts by hydrating their component ions. The NaCl crystal lattice is disrupted as water molecules cluster about the Cl- and Na+ ions ❖ Amphipathic compounds contain regions that are: o 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, called micelles, may contain hundreds or thousands of molecules. Many biomolecules are amphipathic; proteins, pigments, certain vitamins, and the sterols and phospholipids of membranes all have polar and nonpolar surface regions. Micelle formation by amphipathic molecules in aqueous solution  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. Hydrophobic interactions between nonpolar amino acids also stabilize the three-dimensional structures of proteins. Why do oil and water mixed together separate into layers? Oil molecules are amphipathic—having both polar (hydrophilic) heads and nonpolar (hydrophobic) tail portions. When oil and water separate into layers, the polar head groups of the oil molecules are in contact with the aqueous environment and the nonpolar tails are sequestered from the water. ✓ Cohesion Have you ever filled a glass of water to the very top and then slowly added a few more drops? Before it overflows, the water forms a dome-like shape above the rim of the glass. This dome-like shape forms due to the water molecules’ cohesive properties, or their tendency to stick to one another. Cohesion refers to the attraction of molecules for other molecules of the same kind, and water molecules have strong cohesive forces thanks to their ability to form hydrogen bonds with one another.  Cohesive forces are responsible for surface tension, a phenomenon that results in the tendency of a liquid’s surface to resist rupture when placed under tension or stress. Water molecules at the surface (at the water-air interface) will form hydrogen bonds with their neighbors, just like water molecules deeper within the liquid. However, because they are exposed to air on one side, they will have fewer neighboring water molecules to bond with, and will form stronger bonds with the neighbors they do have. Surface tension causes water to form spherical droplets and allows it to support small objects, like a scrap of paper or a needle, if they are placed carefully on its surface. ✓ Adhesion Adhesion is the attraction of molecules of one kind for molecules of a different kind, and it can be quite strong for water, especially with other molecules bearing positive or negative charges. For instance, adhesion enables water to “climb” upwards through thin glass tubes (called capillary tubes) placed in a beaker of water. This upward motion, known as capillary action, depends on the attraction between water molecules and the glass walls of the tube (adhesion), as well as on interactions between water molecules (cohesion). ❑ Chemical properties ✓ Dissociation ✓ Acids and bases ✓ Dissociation of water molecules Occasionally, a hydrogen atom shared by two water molecules shifts from one molecule to the other. – The hydrogen atom leaves its electron behind and is transferred as a single proton - a hydrogen ion (H+) – The water molecule that lost a proton is now a hydroxide ion (OH-) – The water molecule with the extra proton is a hydronium ion (H3O+) One water molecule dissociates into a hydrogen ion (H+) and a hydroxide ion (OH-) ✓ Acids and bases - Acids are compounds that release hydrogen ions (protons) when dissolved in aqueous solution. In other words, they are proton donors. - Bases are compounds that are proton acceptors. Medical break Dianne A. is diabetic. Because her blood levels of glucose are so high, passing from the blood into the glomerular filtrate in the kidneys and then into the urine. As a consequence of the high osmolality of the glomerular filtrate, much more water than usual is being excreted in the urine. Thus, Di has polyuria (increased urine volume). As a result of water lost from the blood into the urine, water passes from inside cells into the interstitial space and into the blood, resulting in intracellular dehydration. The dehydrated cells in the brain are unable to carry out their normal functions. As a result, Di is in a coma. Fluid Compartments in the Body Fat has relatively little water associated with it, thus obese people tend to have a lower percentage of body water than thin people, women tend to have a lower percentage than men, and older people have a lower percentage than younger people. Approximately 60% of the total body water is intracellular and 40% extracellular The extracellular water includes the fluid in plasma (blood after the cells have been removed) and interstitial water (the fluid in the tissue spaces, lying between cells). Transcellular water is a small, specialized portion of extracellular water that includes gastrointestinal secretions, urine, sweat, and fluid that has leaked through capillary walls because of such processes as increased hydrostatic pressure or inflammation. Fluid compartments in the body based on an average 70 kg man  Distribution of water in different body water compartments depends on the solute content of each compartment. Both extracellular fluid (ECF) and intracellular fluid (ICF) contain electrolytes, a general term applied to bicarbonate and inorganic anions and cations. The electrolytes are unevenly distributed between compartments; Na+ and Cl− are the major electrolytes in the ECF (plasma and interstitial fluid), and K+ and phosphates such as HPO42- are the major electrolytes in cells This distribution is maintained principally by energy-requiring transporters that pump Na+ out of cells in exchange for K+ Osmolality and Water Movement Water distributes between the different fluid compartments according to the concentration of solutes, or osmolality, of each compartment. The osmolality of a fluid is proportional to the total concentration of all dissolved molecules, including ions, organic metabolites, and proteins, and is usually expressed as milliosmoles (mOsm)/kg water. Water can move freely through ion channels of the semipermeable cellular membrane that separates the extracellular and intracellular compartments. Likewise, water can move freely through the capillaries separating the interstitial fluid and the plasma. As a result, water will move from a compartment with a low concentration of solutes (lower osmolality) to one with a higher concentration to achieve an equal osmolality on both sides of the membrane. The force it would take to keep the same amount of water on both sides of the membrane is called the osmotic pressure. Medical break In the emergency department, Dianne A. was rehydrated with intravenous saline, which is a solution of 0.9% NaCl. Why was saline used instead of water?  A solution of 0.9% NaCl is 0.9 g NaCl/100 mL, equivalent to 9 g/L. (NaCl has a molecular weight of 58 g/mol, so the concentration of NaCl in isotonic saline is 0.155 M, or 155 mM) If all of the NaCl were dissociated into Na+ and Cl− ions, the osmolality would be 310 mOsm/kg water. Because NaCl is not completely dissociated and some of the hydration shells surround undissociated NaCl molecules, the osmolality of isotonic saline is approximately 290 mOsm/kg H2O. The osmolality of plasma, interstitial fluids, and ICF is also approximately 290 mOsm/kg water, so no large shifts of water or swelling occurs when isotonic saline is given intravenously. In some cases, glucose is added to this at a 5% concentration (5 g/100 mL). The glucose provides fuel for the individual. If this is done, the saline solution has the designation of D5, for 5% dextrose.