Solubility and Nutrients
1 The human body is an
open system with a
constant exchange of
nutrients and energy
between the surroundings
and the body. Food is
eaten to provide the energy
that is stored in the molecular structure of the food molecules. Food
is first broken down in the stomach and then passes through the
intestinal tract where it
is either absorbed or emitted by the body. If the minerals and
nutrients are absorbed, they pass into the blood stream, where they
are transported to various parts of the body. Once in the proper
location, the nutrients are either utilized or reorganized into new
molecules. Whether or not the minerals and nutrients are absorbed,
and where they are absorbed, depends on the solubility of those
nutrients.
2 Several factors affect the solubility of the nutrients that we eat.
The term solubility means the amount of a substance that dissolves
in a given quantity of solvent at specified conditions of temperature
and pressure to produce a saturated solution. Remember that a
solvent is the dissolving medium in a solution. As the blood stream is
a fluid environment that transports nutrients, the solubility of these
nutrients is vital to the well-being of the body.
3 Some nutrients are water soluble, meaning that they dissolve in
aqueous solutions. Other nutrients are fat soluble, meaning that they
dissolve in fatty tissues and oils. Some nutrients are also solubilized
by the proteins that we eat and transported through the bloodstream
by the proteins. It is very important that we understand the solubility
of various nutrients so that we can understand both how our body
uses those nutrients and how often we should eat those nutrients.
4 Several factors affect nutrient solubility. One of those factors is
the temperature of the solution. Temperature is the average kinetic
energy of the particles of a substances. The higher the temperature,
the faster the particles move. This means that at a higher
temperature, there is an increased chance that the solute particles
will make contact with the solvent particles, increasing the rate that
the solute dissolves. Most of the vitamins and minerals that we eat
are soluble at body temperature, or 37ºC. A second factor has to do
with the concentration of the solute in the solvent. There needs to be
enough solvent to surround the solute particle so that it can be
dissolved. This goes for both aqueous and fatty solutions in our
bodies.
5 A third factor, and perhaps the most important factor for the
solubility of nutrients, has to do with the molecular structure of the
nutrients. There is an old saying with solubility rules: “Like dissolves
like.” This means that the structures of “like” substances will dissolve
in each other. What does this mean? Molecules can be polar (have a
charge) or nonpolar. Polar molecules tend to dissolve in polar
solutions, such as water. Nonpolar fatty molecules tend to dissolve in
lipids, such as fats.
6 The vitamins that we eat can be either water soluble or fat
soluble. Water soluble vitamins are not stored in the body, and
therefore must be replenished each day, as they are washed out with
the urine. These vitamins include the B-complex vitamins and
vitamin C. On the other hand, fat soluble vitamins become dissolved
in fat before entering the bloodstream, and the excess is stored in the
liver. These types of vitamins, which include vitamins A, D, E, and K,
do not need to be replenished each day as they can be stored in fat
tissues within the body.
7 Why are these important? Vitamin C is a critical antioxidant that
aids in the health of cell membranes. The B-complex vitamins aid in
metabolism and cell growth and division. Vitamin D aids in the
absorption of calcium, another essential mineral. Calcium controls
such functions as muscle contraction, nerve impulses, and blood
clotting. If the concentration of calcium in our bloodstream (body
fluids) gets too low, the calcium must be replenished by food intake
or the body will take the necessary calcium from the bones.
Water and Life on Earth
1 Water is everywhere on our planet.
You can find it in the air, on the surface,
even underground. In fact, water
covers over 70% of the surface of
Earth. As you can imagine, with this
much water present on Earth, it must be
a pretty important compound, and
in fact, it is. Life on Earth could not
exist without water, and all life is highly
dependent on its unique properties. Most of the metabolic processes
in biological organisms take place in an aqueous solution, enabling
biological organisms to obtain energy to live and grow. The ability of
water to act as an acid or a base in a solution also plays an important
role in these processes. But, what does this have to do with aqueous
solutions and the unique role of water in both chemical and biological
systems?
2 In order to better understand the role that water plays in these
systems, it is important to first understand some of the terms. A
solution is a liquid mixture that contains both a solvent (the liquid
medium) and a solute (the particles that are dissolved in that solvent).
The solute and the solvent together create a solution. Therefore, an
aqueous solution is a solution that contains water as the solvent.
Most solutions on our planet are some form of an aqueous solution.
In fact, two of the most important metabolic processes that occur in
biological organisms occur in aqueous solutions. One of those
processes is the process of photosynthesis that occurs in plants. The
other process is the process of cellular respiration that occurs in all
organisms.
3 Why is water so important in these processes? Water has the
unique ability to act as both an acid and a base, making the water
molecule “amphoteric.” The Brønsted-Lowry definition of acids and
bases states that an acid is a proton donor, and a base is a proton
acceptor. This is why water is considered amphoteric: it can do both,
depending on the solution that it is in. This property has to do with
the polar nature of water. Water contains the second most
electronegative element, oxygen. This element, combined with
hydrogen, causes water’s high polarity, and this polarity is what
allows it to act as both an acid and a base in solution.
4 How water behaves depends on the type of solution or system it
is in. Electrons are accepted or donated in the form of hydrogen
atoms. Hydrogen can either have a positive or negative charge,
depending on its form. When water acts as an acid, it donates its
hydrogen ions (protons) to solution. If water is placed in a solution
that is less acidic than itself, then it will act as an acid and will work
to “dissolve” the compound. This is why certain compounds
dissolve when placed in water. On the other hand, water acts as a
base when it is placed in solutions that are more acid than it is.
However, pure water without any additives is completely neutral, with
a pH of 7.
5 Water is critical in both biological and chemical systems in other
ways, as well. Water is a crucial reactant in photosynthesis and acts
as an electron donor in this process. In fact, photosynthesis could
not occur without water molecules. This means that there would be
no plant life on Earth without water. This is a critical issue: animal life
could not exist on the planet without plant life, because animals
depend on the sugars and other compounds produced by plants for
their metabolic processes. In animals, oxygen acts as the final
electron acceptor, which creates water as a byproduct of this
process. Animals breath in the oxygen gas produced by the plants.
The high electronegativity of oxygen allows it to easily accept the
hydrogen atoms, which drives the process of cellular respiration. As
you can see, water is essential not only to the biological processes of
life, but more specifically to the chemical processes that support
biological life.
6 Other properties of water are critical to biological life, such as the
surface tension that produces capillary forces. These capillary forces
are used to “draw” water from the roots to the stems of plants.
Density and viscosity, which are properties of fluids that have to do
with the resistance of that fluid to flow, are critical to capillary flow. If
water were any more dense, or if the viscosity were higher or lower, it
would not be able to move through the vascular system of plants
effectively.