2.3 Water Flashcards
What is the most abundant molecule in cells?
water, so water is the medium in which the molecules of life interact
What type of molecule is water?
polar
What is a polar molecule?
a molecule that has regions of positive and negative charge
What are polar covalent bonds characterized by?
an uneven distribution of electrons
What do polar molecules tend to interact with?
other polar molecules, whereas nonpolar molecules tend not to interact with polar molecules
What are the two general classes of molecules?
organized depending on how they interact with water
hydrophilic (water loving)
hydrophobic (water fearing)
Describe hydrophilic compounds.
- polar
- dissolve readily in water
- water is a good solvent
Describe hydrophobic compounds.
- nonpolar, these compounds don’t have regions of positive and negative charge so they arrange themselves to minimize their contact with water
Give an example of a hydrophobic molecule.
oil molecules
when oil and water are mixed, the oil molecules organize themselves into droplets that limit the oil-water interface
What is the hydrophobic effect?
the exclusion of nonpolar molecules by polar molecules, which drives biological processes such as the formation of cell membranes and the folding of proteins
What is a hydrogen bond
a bond between a hydrogen atom in one molecule and an electronegative atom in another molecule
How do water molecules orient themselves?
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
What can any hydrogen atom do?
covalently bound to an electronegative atom (ie. oxygen or nitrogen) will have a slight positive charge and can form a hydrogen bond
Describe hydrogen bonds in the case of water.
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
What are hydrogen bonds weaker than?
they are weaker than covalent bonds but hydrogen bonding is what gives water many interesting properties
What can the presence of many weak hydrogen bonds do?
can help stabilize biological molecules, as in the case of nucleic acids and proteins
What do hydrogen bonds influence?
the structure of both liquid water and ice
*figure 2.11 textbook
What happens to hydrogen bonds when water freezes?
What happens when the temperature increases and the ice melts?
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
What is cohesion?
property of water given by hydrogen bonds; an attraction between molecules
What is a consequence of cohesion?
high surface tension, a measure of the difficulty of breaking the surface of a liquid
What does cohesion between molecules contribute?
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
How do hydrogen bonds influence how water responds to heating?
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
What do the abundant hydrogen bonds do?
make water more resistant to temperature changes than other substances, a property that is important for living organisms on a variety of scales
What does water resist in cells?
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
