In this lesson, you will learn about the two major groups of compounds important for human life and the multiple roles water plays in the human body. Specifically, this lesson will cover:
The concepts you have learned so far govern all forms of matter and would work as a foundation for geology as well as biology. This lesson narrows the focus to the chemistry of human life; that is, the compounds important for the body’s structure and function. In general, these compounds are either inorganic or organic.
An inorganic compound is a substance that does not contain both carbon and hydrogen. A great many inorganic compounds contain hydrogen atoms, such as water (H₂O) and the hydrochloric acid (HCl) produced by your stomach. In contrast, only a handful of inorganic compounds contain carbon atoms. Carbon dioxide (CO₂) is one of the few examples.
An organic compound, then, is a substance that contains both carbon and hydrogen. Organic compounds are synthesized via covalent bonds within living organisms, including the human body. Recall that carbon and hydrogen are the second and third most abundant elements in your body. You will soon discover how these two elements combine in the foods you eat, in the compounds that make up your body structure, and in the chemicals that fuel your functioning.
The following topic examines water, one of the three groups of inorganic compounds essential to life. You will learn the other two compounds, salts and acids & bases, in the next lesson.
As much as 70 percent of an adult’s body weight is water.
Water is contained both within the cells and between the cells that make up tissues and organs. Due to its chemical structure—recall that water (H₂O) is formed by polar covalent bonds, creating partial positive and negative charges that allow for the formation of hydrogen bonds—water has several unique properties that are indispensable to human functioning.
1a. Water as a Lubricant and Cushion
Water is a major component of many of the body’s lubricating fluids. These fluids function like oil in the hinge of a door—decreasing friction in order to increase function and the life of the door (organ or tissue).
Water within the fluid around joints helps to lubricate their actions and movement.
Water in the fluid around the lungs helps them expand and recoil with breathing.
Watery fluids help keep food flowing through the digestive tract, and ensure that the movement of adjacent abdominal organs is friction-free.
Water also protects cells and organs from a physical trauma from movement.
Water in the fluid around the brain helps to cushion it within the skull.
Water in the eye protects the delicate nerve tissue located inside.
Water cushions a developing fetus in the womb.
1b. Water as a Heat Sink
A heat sink is a substance or object that absorbs and dissipates heat but does not experience a corresponding increase in temperature. Water is an excellent heat sink due to its ability to absorb large amounts of energy.
In the body, water absorbs the heat generated by chemical reactions without greatly increasing in temperature. Moreover, when the environmental temperature soars, the water stored in the body helps keep the body cool. This cooling effect happens as warm blood from the body’s core flows to the blood vessels just under the skin and is transferred to the environment. At the same time, sweat glands release warm water in sweat. As the water evaporates into the air, it carries away heat, and then, the cooler blood from the periphery (near the skin) circulates back to the body's core.
1c. Water as a Solvent
For cells in the body to survive, they must be kept moist in a water-based liquid called a solution. In chemistry, a solution consists of two substances, a solvent and a solute. A solvent is the substance, present in a greater amount, that does the dissolving while a solute, present in a smaller amount, is the substance being dissolved. When a solvent and solute are mixed so that all molecules are evenly distributed, they form a solution.
Solution Formation - Solute plus solvent equals solution.
Substances of all states of matter (solid, liquid, gas) can form solutions—nickel mixed into gold to form white gold in jewelry, coffee bean grounds mixed into water to form coffee, oxygen mixed into nitrogen to form the compressed air scuba divers breathe. However, the solutions in the human body are primarily formed with liquid water as the solvent. Water is commonly considered the “universal solvent” and it is believed that life cannot exist without water because of this. Water is certainly the most abundant solvent in the body, dissolving a wide range of polar and ionic molecules and compounds.
Because water molecules are polar, with regions of positive and negative electrical charge, water readily dissolves ionic compounds and polar covalent compounds. Such compounds are referred to as hydrophilic, or “water-loving.” Sugar dissolves well in water due to its polar bonds, making it hydrophilic. When a cube of sugar is dropped into a cup of coffee, the sugar dissolves. Nonpolar molecules, which do not readily dissolve in water, are called hydrophobic, or “water-fearing.” Oil is hydrophobic which is why oil and water do not mix. Even when vigorously shaken together, oil and water will naturally separate from one another due to the hydrophobic nature of oil.
Hydrophilic vs. Hydrophobic - Sugar cubes dissolve in coffee, but oil and water do not mix.
Water as a solvent provides several important features to the body.
Solutions store solutes in the body.
Solutions can transport certain solutes throughout the body.
Dissolved compounds with ionic bonds separate into individual ions, providing a supply of charged particles.
1d. The Role of Water in Chemical Reactions
Two types of chemical reactions involve the creation or consumption of water: dehydration synthesis and hydrolysis.
In a dehydration synthesis reaction (dehydrate, to remove water), one reactant gives up an atom of hydrogen and another reactant gives up a hydroxyl group (OH) in the synthesis of a new product. In the formation of their covalent bond, a molecule of water is released as a byproduct shown below. This is also sometimes referred to as a condensation reaction.
In a hydrolysis reaction, a molecule of water disrupts a compound, breaking its bonds. Hydro- refers to water and -lysis means to break or cut. The water is itself split into H and OH. One portion of the severed compound then bonds with the hydrogen atom and the other portion bonds with the hydroxyl group. This is a form of decomposition reaction.
These reactions are reversible and play an important role in the chemistry of organic compounds (which will be discussed in a few lessons).
Dehydration Synthesis and Hydrolysis - Monomers, the basic units for building larger molecules, form polymers (two or more chemically-bonded monomers). (a) In dehydration synthesis, two monomers are covalently bonded in a reaction in which one gives up a hydroxyl group and the other a hydrogen atom. A molecule of water is released as a byproduct during dehydration reactions. (b) In hydrolysis, the covalent bond between two monomers is split by the addition of a hydrogen atom to one and a hydroxyl group to the other, which requires the contribution of one molecule of water.
terms to know
Inorganic Compound
A substance that does not contain both carbon and hydrogen.
Organic Compound
A substance that contains both carbon and hydrogen.
Solvent
The substance in a solution that is present in a greater amount and is capable of dissolving.
Solute
The substance in a solution that is present in a smaller amount and is dissolved by the solvent.
Solution
An even mixture of solvent and solute molecules.
Hydrophilic
A substance with polar covalent or ionic bonds that readily dissolves in water.
Hydrophobic
A substance with non-polar covalent bonds that does not readily dissolve in water.
Dehydration Synthesis Reaction
A synthesis reaction in which one reactant gives up an atom of hydrogen and another reactant gives up a hydroxyl group (OH) in the synthesis of a new product.
Hydrolysis Reaction
A decomposition reaction in which a molecule of water disrupts a compound, breaking its bonds.
summary
In this lesson, you learned about compounds important for human life. First, you learned to differentiate organic and inorganic compounds based on the presence of carbon and hydrogen atoms. For the rest of the lesson, you learned about water as the first of the three groups of inorganic compounds essential to life. You learned about the multiple roles water plays in the human body including as both a lubricant and cushion and as a heat sink. You learned to identify solutes and solvents in solutions and recalled that solutions in the human body are primarily formed with liquid water as the solvent. Finally, you learned two additional roles of water in chemical reactions; dehydration synthesis reactions which remove water from reactants to create a bond and form a product and hydrolysis reactions which add water to a reactant to break a bond and form its multiple products.
A synthesis reaction in which one reactant gives up an atom of hydrogen and another reactant gives up a hydroxyl group (OH) in the synthesis of a new product.
Hydrolysis Reaction
A decomposition reaction in which a molecule of water disrupts a compound, breaking its bonds.
Hydrophilic
A substance with polar covalent or ionic bonds that readily dissolves in water.
Hydrophobic
A substance with non-polar covalent bonds that does not readily dissolve in water.
Inorganic Compound
A substance that does not contain both carbon and hydrogen.
Organic Compound
A substance that contains both carbon and hydrogen.
Solute
The substance in a solution that is present in a smaller amount and is dissolved by the solvent.
Solution
An even mixture of solvent and solute molecules.
Solvent
The substance in a solution that is present in a greater amount and is capable of dissolving.
