CELL COMPOUNDS
Water
Acids, Bases & Buffers
Thought Question:
• Can life exist without water?
• Why is water so important to life as we
know it?
Learning Outcomes:
• B2 - Describe the characteristics of water and its
role in biological systems
– Describe the role of water as a solvent, temperature
regulator and lubricant
– Describe how the polarity of the water molecule
results in hydrogen bonding
• B3 - Describe the role of acids, bases and
buffers in biological systems in the human body
– Differentiate among acids, bases and buffers
– Describe the importance of pH to biological systems
in the human body
Water
• Water is one of the most important
compounds for living organisms
• Many unique and important properties of
water are a result of the structure of water
molecules (polarity and hydrogen bonding)
Functions of Water in the Body
• Lubricant – in digestive system, brain and
spinal cord, joints, eyes, etc.
• Facilitates chemical reactions – ex.
Digestion
• Transport of materials – many substances
are able to dissolve in water ex. Nutrients
in blood
• Water helps to regulate temperatures in
the body
Structure of Water molecules
• The oxygen and hydrogen atoms are held
together by covalent bonds
Polarity of Water Molecules
• The oxygen end of
•
the water molecule
attracts electrons
more strongly than
the hydrogen end
Results in a slight
negative charge at
the O end, and a
slight positive charge
at the H end
Hydrogen Bonds
• The positive end of one water molecule is
attracted to the negative end of another
molecule
• Weak bonds called hydrogen bonds are
formed between molecules
Hydrogen Bonds
Special Properties of Water
•
•
•
•
•
•
•
Liquid at room temperature
“universal solvent”
Cohesive (sticks to itself)
Adhesive (sticks to other things)
Temperature changes slowly
High heat of vaporization
Less dense in solid form than liquid
• All of these properties are related to the
polarity of the water molecule and the
hydrogen bonding between the molecules
Assignment :
• Refer to pages 26-27 in your text
• For the first 4 items listed under “Water
has Unique Properties” do the following:
– Property (copy from the numbered points in
the book)
– In your own words explain:
• how this property is related to hydrogen bonding
•
and/or polarity (diagrams might help)
why this property is important to living organisms
(and specifically, in the human body)
Solubility
• The polar nature of water makes it a good
solvent (many things dissolve in it)
• Substances that dissolve in a solvent are
called solutes
• Ionic compounds are often soluble in
water
• Positive and negative ions are attracted to
different parts of the water molecule, and
ionic compounds are pulled apart
• Some molecular compounds, like sugar,
dissolve in water because parts of the
molecule are attracted to the water
• non-polar molecules do not readily
dissolve in water
Hydrophilic
• “water loving”
• Charged or polar molecules are
hydrophilic and dissolve in water
Hydrophobic
• “water fearing”
• Non-polar molecules are hydrophobic and
do not dissolve in water
Acids and Bases
• Acids release H+ ions into water, so
they contain more H+ ions than OHions
• Bases release OH- ions into water, so
they contain more OH- ions than H+
ions
• Pure water is neutral - contains
approximately equal numbers of H+
and OH- ions
• Dissociation of water
pH Scale
• pH scale represents the concentrations of
H+ ions in a solution
• Scale of 0 to 14
0 to <7 = acid (lower #, stronger acid)
>7 to 14 = base or alkali (higher #, stronger
base)
7 = neutral
• Water has 10-7 moles/L of H+ (ph = 7)
• An acid with a pH of 4 has 10-4 moles/L of
H+
• A base with a pH of 10 has 10-10 moles/L
of H+
• The pH scale is logarithmic, so the [H+]
increases by 10x for each unit of pH
• ex. a pH of 5 has 10x as much H+ as a pH
of 6
• pH simulation Link
pH in the body
• Cells require a certain pH range to live
• Too high or low pH denatures proteins and
other molecules
• Enzymes that control biochemical
reactions require the correct pH to function
Buffers
• Chemicals that prevent pH changes in
•
•
•
•
solutions
Take up extra H+ or OH- ions
Important in living organisms
Maintain correct pH in cells and tissue
fluids
Ex. Buffers in blood maintain its optimal
pH of ~7.4