The Biological Importance
of Water
Why is water important?
•Major component of all living systems and
our planet.
•Occupies most of a cell’s volume.
•Has major properties that living systems
require.
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Water is dipolar- made up of two elements, hydrogen
and oxygen
– Neutral in charge and polar covalent.
– Polarity: unequal sharing of electrons, leading to a partially
positive and partially negative charge.
– The oxygen atom is slightly negative and the hydrogen atoms are
slightly positive.
– Covalent bonds are between the hydrogen and oxygen atoms,
resulting in an uneven sharing of electrons = polarity.
– Water molecules form hydrogen bonds with other neighboring
water molecules.
– Each H2O molecule can form 4 hydrogen bonds with four other
water molecules.
– Groups of hydrogen bonds are very strong, but bonds by
themselves are weak.
– Also find hydrogen bonds holding our DNA and proteins
together.
Label the
hydrogen
and
covalent
bond
Covalent Bond
Polar
• Water soluble
•Asymmetrical: uneven electron
distribution, do not share electrons
• Examples: water, sugar, salt, ethanol,
hydrogen sulfide
• The electrons of the polar covalent
bonds spend more time near the oxygen
than the hydrogen.
• Polarity allows water to form hydrogen
bonds with each other.
•Ionic compounds dissolve in water to
form ions: for most biological reactions to
occur the reactants MUST dissolve in
water.
Nonpolar
• Fat soluble
• Symmetrical: electrons are evenly
distributed.
• Strongly covalently bonded
•Examples: fats, oils, cell membranes and
cell walls, carbon dioxide, gasoline.
• Dissolves almost all substances (mostly polar), and
provides a medium in which other molecules can
interact.
• Soluble: able to be dissolved, especially in water.
• Solution: uniform mixture of two or more substances.
– Solvent: liquid in which a solute dissolves
• Important solutions in our body (water = solvent): blood plasma,
sweat, urine, and tears
– Solute: substance that dissolves in a solvent
• Important solutes in our body: oxygen, salt, sugars, and calcium
– Examples: water= solvent, Kool-Aid powder / sugar= solute,
Kool-Aid= solution
Hydrophilic Hydrophobic Amphipathic
“water loving”
“water fearing”
Molecules
Nonpolar
dissolve in water. molecules
separate in
water.
Molecules have
both polar and
nonpolar
regions.
Ex- Cell
Membrane
Recall Questions
Take 5- 10 minutes and answer the
questions in your notes.
Cohesion & Surface Tension
• Water molecules stick together
– Why? Hydrogen bonding
• Causes: surface tension, which means how
difficult it is to break the surface.
• Surface tension: creates a skin on the surface of
the water.
• Examples:
– Skipping rocks, rain drops beading
up on a car, water droplets on a penny,
spider walking on water
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• Water molecules stick to other objects
• Examples:
– Meniscus (graduated cylinder), wet
microscope slides sticking together
• Movement of liquid through a
narrow passage / H2O
molecules stick to tubes of
small diameter.
• Result of the cohesion of water
molecules sticking to each
other, and adhesion of water
molecules sticking to another
surface.
• Examples:
– Drinking straw
– Food coloring added to water to
dye flowers
– Plants transporting water from
roots to leaves
• Water resists changes in temperature; therefore
water must absorb more heat energy to increase
temperature.
• Very important because our cells release a lot of
heat, and water absorbs that heat, which allows
us to regulate cell temperatures= HOMEOSTASIS
• Specific heat is the energy required to raise 1g of
water by 1ºC.
• Examples: oceans cool slower than land due to
the high heat capacity of water.
•
•
•
•
•
Vaporization = evaporation & boiling
Water is changing from a liquid to a gas
Liquid molecules enter the air
Evaporation produces a cooling effect
Hydrogen bonds must be broken before water
can evaporate and this requires a lot of energy.
• Example: sweating (humans) or panting in dogs=
dogs can’t sweat through their skin, they pant to
circulate air through their bodies to cool down=
evaporative cooling.
• Water freezes = crystalline structure maintained by hydrogen
bonding
• O C / 32F = Freezing
• Ice is less dense than liquid water because the hydrogen bonds
are positioned in a way that pushes the molecules apart= lowers
density.
• When ice melts to liquid water, the structure collapses and the
density of the liquid increases.
• Ecosystems:
– Advantages: protects aquatic ecosystems.
– If ice filled an entire lake or ocean, from bottom to top, all of the living
organisms would die.
– Also allows turnover of nutrients.
• In the spring, the ice melts; water sinks and pushes up the nutrient rich water.
Guess the property
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Videos
• http://viewpure.com/HP5dDJGz8b4?start=0&
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• https://vimeo.com/83880203
Review Table
• Complete the review table without looking
back in your notes!
• You can do it, have faith in yourself!