2.3 Water

Question 1

What is the most abundant molecule in cells?

Answer 1

water, so water is the medium in which the molecules of life interact

Question 2

What type of molecule is water?

Answer 2

polar

Question 3

What is a polar molecule?

Answer 3

a molecule that has regions of positive and negative charge

Question 4

What are polar covalent bonds characterized by?

Answer 4

an uneven distribution of electrons

Question 5

What do polar molecules tend to interact with?

Answer 5

other polar molecules, whereas nonpolar molecules tend not to interact with polar molecules

Question 6

What are the two general classes of molecules?

Answer 6

organized depending on how they interact with water

hydrophilic (water loving)
hydrophobic (water fearing)

Question 7

Describe hydrophilic compounds.

Answer 7

• polar
• dissolve readily in water
• water is a good solvent

Question 8

Describe hydrophobic compounds.

Answer 8

• nonpolar, these compounds don’t have regions of positive and negative charge so they arrange themselves to minimize their contact with water

Question 9

Give an example of a hydrophobic molecule.

Answer 9

oil molecules

when oil and water are mixed, the oil molecules organize themselves into droplets that limit the oil-water interface

Question 10

What is the hydrophobic effect?

Answer 10

the exclusion of nonpolar molecules by polar molecules, which drives biological processes such as the formation of cell membranes and the folding of proteins

Question 11

What is a hydrogen bond

Answer 11

a bond between a hydrogen atom in one molecule and an electronegative atom in another molecule

Question 12

How do water molecules orient themselves?

Answer 12

in a way that minimizes the repulsion of like charges so that positive charges are near negative charges because the oxygen and hydrogen atoms have slight charges

ie. because of its slight positive charge, a hydrogen atom in a water molecule tends to orient itself toward the slightly negatively charged oxygen atom in another molecule

Question 13

What can any hydrogen atom do?

Answer 13

covalently bound to an electronegative atom (ie. oxygen or nitrogen) will have a slight positive charge and can form a hydrogen bond

Question 14

Describe hydrogen bonds in the case of water.

Answer 14

the result of many such interactions is a molecular network stabilized by hydrogen bonds

hydrogen bonds are typically showed by a dotted line

*figure 2.10 textbook

Question 15

What are hydrogen bonds weaker than?

Answer 15

they are weaker than covalent bonds but hydrogen bonding is what gives water many interesting properties

Question 16

What can the presence of many weak hydrogen bonds do?

Answer 16

can help stabilize biological molecules, as in the case of nucleic acids and proteins

Question 17

What do hydrogen bonds influence?

Answer 17

the structure of both liquid water and ice

*figure 2.11 textbook

Question 18

What happens to hydrogen bonds when water freezes?

What happens when the temperature increases and the ice melts?

Answer 18

most water molecules become hydrogen bonded to four other water molecules, forming an open crystalline structure we call ice

as the temperature increases and the ice melts, some of the hydrogen bonds are destabilized and break, allowing the water molecules to pack more closely

this explains why liquid water is denser than ice

ie. ice floats on water, ponds and lakes freeze from top to bottom (and doesn’t freeze completely) which allows fish and aquatic plants to survive winter in the cold water under the layer of ice

Question 19

What is cohesion?

Answer 19

property of water given by hydrogen bonds; an attraction between molecules

Question 20

What is a consequence of cohesion?

Answer 20

high surface tension, a measure of the difficulty of breaking the surface of a liquid

Question 21

What does cohesion between molecules contribute?

Answer 21

water movement in plant

• as water evaporates from leaves, water is pulled upward through the plant’s vessels
• sometimes this water may rise as high as 100 meters above the ground in giant sequoia and coast redwood trees, which are among the tallest trees on Earth

Question 22

How do hydrogen bonds influence how water responds to heating?

Answer 22

molecules are in constant motion, and this motion increases as the temperature increases

when water is heated, some of the energy added by heating is used to break hydrogen bonds instead of causing more motion among the molecules

as a consequence, temperature increases less in these situations than if there were no hydrogen bonding

Question 23

What do the abundant hydrogen bonds do?

Answer 23

make water more resistant to temperature changes than other substances, a property that is important for living organisms on a variety of scales

Question 24

What does water resist in cells?

Answer 24

temperature variations that would otherwise result from numerous biochemical reactions

• oceans minimize temperature fluctuations, stabilizing the temperature on Earth in a range compatible with life

Question 25

What is pH?

Answer 25

a measure of the concentration of protons in solution, which is important because the pH influences many chemical reactions and biological processes

Question 26

What is the formula for pH?

Answer 26

pH = -log [H+]

Question 27

Describe the pH scale.

Answer 27

• ranges from 0-14

- logarithmic, a difference of one pH unit corresponds to a tenfold difference in hydrogen ion concentration

Question 28

What does acidic mean?

Answer 28

a solution in which the concentration of protons is higher than that of hydroxide ions (pH<7)

Question 29

What does basic mean?

Answer 29

a solution in which the concentration of protons is lower than that of hydroxide ions (pH>7)

Question 30

What does neutral mean?

Answer 30

a solution in which the concentrations of protons and hydroxide ions are equal (pH=7)

Question 31

What is an acid?

Answer 31

a molecule that releases a proton (H+)

Question 32

What is a base?

Answer 32

a molecule that accepts a proton in aqueous solution

Question 33

What is the pH of pure water>

Answer 33

7, it is neutral

Question 34

What is the pH of most cells?

Answer 34

~7 and is tightly regulated, as most chemical reactions can be carried out only in a narrow pH range

Question 35

What is the pH of blood?

Answer 35

~7.4, slightly basic

sometimes referred to as “physiological pH”

Question 36

What is the pH of freshwater lakes, ponds, and rivers?

Answer 36

tend to be slightly acidic because carbon dioxide from the air dissolves in the water and forms carbonic acid