WaterWater: An absolute necessity for life

¾ of the planet is covered by itLife existed in water 2 Billion years prior to adapting to

land60% of human weight from water83% of human blood composition

Where water exists, life may be foundWater is critical to cell biochemistryWater is critical for photosynthesis

Water is the ‘universal solvent’

A generalized water cycle for Earth’s systems

Water’s structure

Simple molecular structure with Oxygen bound to 2 Hydrogen atoms by single covalent bonds

Electronegativity of O is much greater than H – so the bonds between these atoms are “polar”

Hydrogen bonding:outstanding chemical property of

water

“The polarity of water impacts water’s chemistry and the chemistry of life…”

Properties of water

• Cohesion =

• Adhesion =

• High specific heat =

• High heat of vaporization =

• Solid water less dense than liquid =

• Has solvent properties =

Fig. 2.14

Water can form ions• The covalent bonds of water can break

spontaneously to produce H+ and OH-

H2O OH- (hydroxide ion) + H+ (hydrogen ion)

• At 25° C, one liter of water contains one ten-millionth (10-7) molemole of H+ ions

• A MoleMole is equal to the weight of a substance (in grams) that corresponds to the atomic mass of the atoms forming the substance

• H+ has an atomic mass of 1, so a mole of H+ would weigh 1 gram

Water and pH

pH = -log [H+]

• Therefore, pure water, having 10-7 moles/liter concentration of H+, has a pH of 7.0

• A pH of 7.0 is determined to be “neutral” due to equal concentrations of H+ and OH- ions

pH and logarithms

• Note that the pH scale is logarithmic, meaning that a difference of 1 of pH means a 10-fold change in H+ ion concentration

• Therefore, a solution having a pH of 3 has 10X more H+ ions than a solution with a pH of 4; 100X more H+ than a solution with a pH of 5

Fig. 2.16

Acids and bases

AcidsAcids• Any solution with a pH

less than 7.0• The stronger an acid is,

the more H+ ions it releases to solution

• High amounts of H+ ions are causticcaustic = capable of burning, corroding, or destroying living tissue

BasesBases• Any solution with a pH

greater than 7.0• The stronger a base is,

the more OH- or similar ions it releases to solution

• High amounts of OH- ions are causticcaustic

Buffers help stabilize pH

• A buffer is a substance that resists changes in pH• Within organisms, buffers consist of acid-base pairings

• In human blood, carbonic acid (H2CO3) serves as a buffer which can split to form bicarbonate ion (HCO3

-) and a Hydrogen ion (H+) to aid in stabilizing the pH

Articles on Reserve for Article Review #2

• Taylor, D. 2007. Employment preferences and salary expectations of students in science and engineering. Bioscience. 57: 175-185.

• Ostfeld, R.S. et al. 2006. Controlling ticks and tick-borne zoonoses with biological and chemical agents. Bioscience. 56: 383-394.

• DeLong, E.F. 2003. A plentitude of ocean life. Natural History. 112: 40-46.

• Schmid-Hempel, P. 2003. Fight of the bumblebee. Natural History. 112: 52-59.

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