BIO 101 Chapter 3

Water and the Fitness of the Environment

describe the molecule that supports all of life: water

• water is the biological medium on Earth

• all living organisms require water more than any other substance

• most cells are surrounded by water, and cells themselves are about 70-95% water

• the abundance of water is the main reason the earth is habitable

what kind of molecule is water

polar molecule

• the opposite ends have opposite charges

• polarity allows water molecules to form hydrogen bonds with each other

what are the four properties of water that facilitate an environment for life?

• cohesive behavior: the hydrogen bonds stick together

• ability to moderate temperature: water stores a lot of energy allowing it to moderate temp

• expansion upon freezing: less dense, floats

• versatility as a solvent:water is a chemical that can dissolve a vast number of other chemicals

describe cohesion

• when hydrogen bonds as a collective hold water molecules together

• cohesion helps the transport of water against gravity in plants

  • water gets into the root of the plant, goes up the root/stem, which is all due to the hydorgen atoms sticking water together

describe adhesion

• the attraction between different substances

  • ex. water and plant cell walls (attraction)

describe surface tension

• a measure of how hard it is to break the surface of a liquid

• at the air-water interface, the water forms a skin

• surface tension is related to cohesion

how does water moderate temperature

• water absorbs heat from warmer air and releases stored heat to cooler air

• water can absorb or release a large amount of heat with only a slight change in its own temperature

describe specific heat

the amount of heat that must be absorbed or lost for 1g of that substance to change its temperature by 1 celsius

what is the specific heat of water

1cal/g/ºC

• it takes 1 calorie of heat to raise the temperature of 1 gram of water by 1 degree celsius

  • it takes the same amount to lower it

• this means that water has a relatively high specific heat compared to many other substances

what is the reason for water’s high specific heat

it can be traced to hydrogen bonding

• heat is absorbed when hydrogen bonds break

• heat is released when hydrogen bonds form

• the high specific heat of water minimizes the temperature fluctuations to within the limits that permit life

define evaporation

the transformation of a substance from liquid to gas

describe heat of vaporization

the heat a liquid must absorb for 1 g to be converted to gas

what is evaporative cooling

as a liquid evaporates its remaining surface cools

• evaporate cooling of water helps stabilize temperatures in organisms and bodies of water

describe the insulation of bodies of water by floating ice

• ice floats in liquid water because hydrogen bonds in ice are more “ordered” making ice less dense

• water reaches its greatest density at 4 degrees celsius

• if ice sank, all bodies of water would eventually freeze solid making life impossible on Earth

  • if water froze from the round up there would be no life

define solution

a liquid that is homogenous mixture of substances

define solvent

the dissolving agent of a solution

define solute

the substance that is disolved

define aqueous solution

a solution in which water is the solvent

why is water a versatile solvent

due to its polarity which allows it to form hydrogen bonds easily

when an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a

hydration shell

water can also dissolve compounds made of ________ polar molecules

nonionic

even large polar molecules such as proteins can dissolve in water if they have:

ionic and polar regions

define hydrophilic

a substance that has affinity for water (likes it)

define hydrophobic

a substance that does not have affinity for water (dislikes it)

• oil is hydrophobic because they have relatively nonpolar bonds

define colloid

a stable suspension of fine particles in a liquid

most biochemical reactions occur in ______

water

what do chemical reactions depend on

the collisions of molecules and therefore on the concentration of solutes in an aqueous solution

define molecular mass

the sum of all masses of all atoms in a molecule

numbers of molecules are usually measured in ____

moles

• 1 mole = 6.02 x 10²³ molecules

• avogadro’s number and the unit of dalton were defined such that 6.02 x 10²³ = 1 g

define molarity (m)

the number of moles of solute per liter of solution

describe what happens when a hydrogen atom in a hydrogen bond between two water molecule shift from one to the other

• the hydrogen atom leaves its electron behind and is transferred as a proton, or hydrogen ion (H⁺)

• the molecule with the extra proton is now a hydronium ion (H₃O⁺) though it is often represented as H⁺

• the molecule that has lost proton is now a hydroxide ion (OH^-)

how is water in a state of dynamic equilibrium

water molecules dissociate at the same rate at which they are being reformed

though statistically rare, the ____________of water molecules has a great effect on organism

dissociation

• the changes in concentration of hydronium ion H+ and hydroxide ion OH- can drastically affect the chemistry of a cell

effects of changes in pH

• concentrations of H+ and OH- are qual in pure water

• adding certain solutes, called acids, and bases modifies the concentration of H+ and OH-

• biologists used pH scale to describe whether a solution is acidic or basic

describe acids and bases

• and acid is any substance that increases the H+ concentration of a solution

• a base is any substance that reduces the H+ concentration of a solution

in any aqueous solution at 25 Celsius the product of H+ and OH- is constant and can be written as

[H⁺][OH^–] = 10^–14

the pH of a solution is defined by

the negative logarith of H+ concentration

• written as pH = –log [H⁺]

what is te neutral aqueous solution

[H⁺] is 10^–7 = –(–7) = 7

describe the pH scale

• acidic solutions: pH less than 7

• basic solutions: pH greater than 7

• most biological fluids have pH values in the range of 6 to 8

the internal pH of most living cells must remain close to ______

pH 7

what is a buffer

substances that minimize changes in concentration of H+ and OH- in a solution

• bufferes dissolve in water and keep the concentration constant

• most buffers consist of an acid-base pair that reversibly combines with H+

• all biological systems and the human body include these different buffers

what is acid precipitation

it refers to rain, snow, or fog with a pH lower than 5.6

• it is mainly caused by the mixing of different pollutants (sulfates/nitrates) with water in the air and can fall at some distance from the source of pollutants

• it damages life in lakes and streams

• decline in forests

how does burning fossil fuels threaten water quality

CO2 is released by fossil fuel combustion and contributes to:

• warming of the earth called the “greenhouse” effect

• acidification of the oceans; this leads to a decrease in the ability of corals to form calcified reefs