生物化學(xué)基礎(chǔ)知識(shí)2課件

1、Chapter 2 生物化學(xué)基礎(chǔ)知識(shí)Some Important Chemical Concepts and Principles for Studying Biochemistry 2.4 水（water，H2O）生物化學(xué)基礎(chǔ)2.4.1 The structure and characteristics of waterThe water molecule is dipolar and highly cohesive.The electron outer shell of the oxygen atom is roughly tetrahedral, with two hydrogen at

2、oms at two corners, and two pairs of electrons (lone pairs) at the other two corners. 生物化學(xué)基礎(chǔ)The oxygen atom is partially negative and the two hydrogen atoms are partially positive.Each water molecule can serve as both H donor and acceptor, forming hydrogen bonds with as many as four neighboring wate

3、r molecules, providing strong cohesive forces between them.生物化學(xué)基礎(chǔ)The structure of water molecule生物化學(xué)基礎(chǔ)Hydrogen bond in ice (solid)生物化學(xué)基礎(chǔ)水：生命中不可承受之無生物化學(xué)基礎(chǔ)Water interacts electrostatically with charged solutesWater is a polar solvent. It readily dissolves most biomolecules, which are generally charged

4、 or polar compoundsCompounds 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 hydrophobicnonpolar molecules such as lipids and waxes.生物化學(xué)基礎(chǔ)

5、Water is an excellent solvent for polar molecules.It competes for hydrogen bonds or forms oriented/structured shells around ions.Biomolecules are mostly water soluble (hydrophilic, “water-loving”) molecules.Water-free microenvironments are also formed in biological systems to maximize polar interact

6、ions.生物化學(xué)基礎(chǔ)Water as solventWater dissolves many crystalines salts by hydrating their component ions.生物化學(xué)基礎(chǔ)Hydrophobic (“water hating”, nonpolar) groups tend to be squeezed/driven together by water, forming specific biological structures (interior of globular proteins, biomembranes).生物化學(xué)基礎(chǔ)Amphipathic

7、 compounds in aqueous solution生物化學(xué)基礎(chǔ)Water molecules have a slight tendency to undergo reversible ionization to yield H+ and OH-.The product of H+OH- in aqueous solutions at 25C is always 110-14 M2 (the measured Keq for pure water is 1.810-16 M at 25C and the concentration of water is 55.5 M).pH is d

8、efined as the negative logarithm of the molar concentration H+ (pH scale is logarithm, not arithmetic).2.4.2 Ionization of water, weak acids, and weak bases生物化學(xué)基礎(chǔ)What is pH？ pH is abbreviation of latin words “Pondus hydrogenii” （Pondus=壓強(qiáng)、壓力；hydrogenium=氫）。 pH is in close relationship with the conce

9、ntration of H+ in aqueous solution. pH = -log H+生物化學(xué)基礎(chǔ)pH scale designates the actual concentration of H+ (thus of OH-, remember their product is a constant), in any solution in the range between 1.0 M H+ and 1.0 M OH- (pH value from 0 to 14.0).pH can be accurately measured using glass electrodes (wh

10、ich is selectively sensitive to H+). (The pH meter)The structure and function of biomolecules are widely affected by the pH of the solutions.生物化學(xué)基礎(chǔ)The pH meter生物化學(xué)基礎(chǔ)生物化學(xué)基礎(chǔ)A-H+ Weak acids and bases have characteristicdissociation constants生物化學(xué)基礎(chǔ) HA A- + H+ Acid Base Ka =A- H+HAKaKa: dissociation cons

11、tants生物化學(xué)基礎(chǔ)生物化學(xué)基礎(chǔ)生物化學(xué)基礎(chǔ) Titration Curves Reveal the pKa of Weak AcidsTitration is used to determine the amount of an acid in a given solution. A measured volume of the acid is titrated with a solution of a strong base, usually NaOH, of known concentration. The NaOH is added in small increments until

12、 the acid is consumed, as determined with an indicator dye or a pH meter. The concentration of the acid in the original solution can be calculated from the volume and concentration of NaOH added.A plot of pH against the amount of NaOH added (a titration curve) reveals the pKa of the weak acid.生物化學(xué)基礎(chǔ)

13、生物化學(xué)基礎(chǔ)The titration curve of acetic acidWhen Ac-=HAc,pKa =pAc- H+HAc= pH生物化學(xué)基礎(chǔ)Many biochemical structures and processes are affected by pH due to the involvement of groups behaving as weak acids and weak bases.pH values in biological systems are usually strictly kept constant (near pH 7.0) by buffer

14、ing pairs of conjugate acid-bases (e.g., the phosphate buffer H2PO4-/HPO42- in cytoplasm and the bicarbonate buffer H2CO3/HCO3- in blood, better with CO2 gas to dissolved H2CO3 conversion).生物化學(xué)基礎(chǔ)生物化學(xué)基礎(chǔ)生物化學(xué)基礎(chǔ)生物化學(xué)基礎(chǔ)2.4.3 Water as a ReactantWater is not just the solvent in which the chemical reactions of living cells occur, it is very often a direct participant in those reactions. Hydrolysis reaction