chapter 03

Bio1151 Chapter 3 9

is the biological medium here on Earth.
- Water is the biological medium on Earth, covering 70-75% of the its surface.
  Most cells are surrounded by water, and cells themselves are about 70-95% water.
Water is a molecule, and its numerous bonds result in _ani_3,1 unique properties.
- Water properties.
  A water molecule is held together by strong, polar covalent bonds between oxygen and hydrogen atoms.
  The partially charged regions of a polar water molecule are attracted to oppositely charged parts of neighboring molecules.
  Each molecule can form weak hydrogen bonds to multiple partners, conferring water unique properties.
- Cohesion and surface tension.
- High specific heat.
- Expansion upon freezing.
- Versatile solvent for hydrophilic substances.
  Review:
- Collectively, hydrogen bonds hold water molecules together, resulting in cohesion.
  Water molecules are also attracted to other substances, such as cell walls, by adhesion.
  These properties help transport water up against gravity in plants.
  Tutorial:
- Cohesion among water molecules at the liquid surface leads to high surface tension, allowing this water strider to walk on water.
- Specific heat is the energy needed to change the temperature of 1 g of a substance by 1�C. The high specific heat of water (1 cal/g/�C):
- minimizes temperature fluctuations and moderates air temperatures in surrounding land
- allows evaporative cooling of wet surfaces.
- Ice formation. The hydrogen bonds in solid water (ice) are highly ordered and forms a 3-dimensional crystal, so ice is less dense than liquid water. Floating ice becomes a barrier that protects the liquid water from colder air, allowing organisms such as this Krill to survive under frozen surfaces.
- Water is a versatile solvent for hydrophilic (polar and ionic) substances such as NaCl.
  Water molecules form hydration shells around ions, separating them from the crystal and dissolving the salt and forming a solution.
  A solution with water as the solvent is called an aqueous solution.
  Substances dissolved in solution are called solutes.
- A water-soluble protein. Human lysozyme, a hydrophilic protein found in tears and saliva that has antibacterial action, has ionic and polar regions that allows it to dissolve in water.
Water can dissociate into H⁺ (hydrogen) and OH^- (hydroxide) .
- Water is in a state of dynamic equilibrium in which H[2]O molecules dissociate into H[3]O^+ (hydronium) and OH^- (hydroxide) ions at the same rate at which they are being reformed.
  For simplicity, H[3]O^+ can be viewed as H[2]O plus a H^+ (hydrogen ion), or proton.
  Review:
- The concentration of these ions is measured on the scale.
  - The concentration of hydrogen ions (H^+, or protons) in aqueous solution is measured in pH units.
    The H^+ concentration at 25�C ranges from 1 Molar (10^0, or pH=0) to 10^-14 M (pH=14).
    These concentrations are plotted on a negative logarithmic scale: each unit change in pH is a 10-fold change in H^+ ion concentration.
    An acid has a high concentration of H^+ ions (low pH).
    A base has a low H^+ concentration (high pH).
    Review:
- such as carbonic acid minimize changes in pH.
  - A buffer minimize changes in pH by accepting hydrogen ions from the solution when they are in excess and donating hydrogen ions to the solution when they are low.
    In human blood, dissolved CO[2] forms Carbonic acid (H[2]CO[3]).
    Carbonic acid acts as a buffer by dissociating to yield a bicarbonate ion (HCO[3]^-) and a hydrogen ion (H^+) in a dynamic equilibrium.
- precipitation is caused primarily by the combustion of fuels.
_inv_3&A How Does Acid Precipitation Affect Trees?
- Acid precipitation is caused primarily by the mixing of water vapor and sulfur and nitrogen oxides produced by the combustion of fossil fuels.
- Fossil fuels (coal, petroleum, natural gas) are formed from buried remains of dead organisms when compressed in sediment over time.
  Humans have dug up these carbon fuels to burn as energy sources, releasing excess carbon, sulfur, and nitrogen into the atmosphere.