All life is dependent on the chemical reactions between molecules. Life begins with the simplest molecules and builds more and more complicated chemical interactions.

Follow the directions highlited in green at this website. There are many pages, reached by clicking on "Later" at the top of each page. Although it takes a while to go through, by the time you finish, you will know the complete story of life.

The chemical reactions of all living things take place in an aqueous solution. This makes water indispensable to life. The shape of the water molecule causes one side of the molecule to be slightly positively charged and the other side to be negatively charged. A molecule with an uneven charge pattern is known as a polar molecule. Even though the total charge of the subatomic particles within a polar molecule is balanced, the shape of the molecule causes one area to be slightly positive while another area is slightly positive. Water is a polar molecule because the two hydrogen atoms are slightly to one side of the molecule. In the diagram above, the oxygen atom in each water molecule is red and the two hydrogen atoms are blue. The red side of the molecule is slightly negative and the blue side is slightly positive. You can see in the diagram how the positive side of one molecule aligns with the negative side of another. The polar nature of water makes it very effective in dissolving other substances. It is these aqueous (water) solutions that life depends on.

Water must gain or lose a relatively large amount of energy for its temperature to change. As heat is added to water, most of the energy is used to break the polar bonds holding water molecules to each other. Only after these bonds have been broken can the individual molecules begin to move faster, increasing the temperature. This characteristic of water helps living things maintain homeostasis - a stable level of internal conditions.

All living things need energy to survive. Energy is defined as the ability to do work or cause change. Within any living system, it is the free energy that is important. Free energy is available to fuel the cell processes of living things. All living things need a constant flow of energy into and through their systems. Many of the chemical reactions that transfer energy in living things involve the transfer of electrons. Reactions that transfer electrons between atoms are known as oxidation-reduction reactions - redox reactions. These reactions occur most easily in the polar enviornment of a water solution.

Organic compounds contain carbon atoms that are covalently bonded to other carbon atoms and other elements like hydrogen, oxygen, and nitrogen. Carbon atoms are special for two reasons. First, carbon atoms easily bond with other carbon atoms. This is not the case with most atoms. Because of this, carbon is able to form "chains" of its atoms. Second, small clusters of atoms combined with carbon, known as monomers, have the ability to link in repeating units called polymers. Large polymers are known as macromolecules. It is these macromolecules that make the complexity of life possible.

The breakdown of complex molecules, like polymers, is called hydrolysis. The most energy efficient molecule in living things is adenosine triphosphate, ATP. When this molecule is broken down, large amounts of energy are freed for cell processes.

Groups of organic compounds:

Carbohydrates: compounds composed of carbon, hydrogen, and oxygen.

Types of carbohydrates:

• Monosaccharide - a simple sugar with the general formula CH₂O. One monosaccharide that you will learn more about when we study plants is glucose. The chemical formula for glucose is C₆H₁₂O₆. Notice that the subscripts in the formula for glucose can be reduced to the general formula for monosaccharides, CH₂O.
• Disaccharide - a double sugar composed of two simple sugars. Sucrose, common table sugar, is a combination of glucose and fructose.
• Polysaccharide - a complex molecule composed of three or more monosaccharides. Two polysaccharides that you will learn about this year are starch and cellulose.

Proteins: compounds composed of carbon, hydrogen, oxygen, and nitrogen.

The skin, muscles, and enzymes of living things are composed of proteins.

• Amino acids - these 20 chemicals are the monomer building blocks of proteins.
• Dipeptide - two amino acids bonded together.
• Polypeptide - a very long chain of bonded amino acids.
• Enzymes - complex proteins that act as catalysts - making chemical reactions more energy efficient in living things.

Lipids: compounds with a higher ratio of carbon and hydrogen to oxygen than carbohydrates.

These molecules do not dissolve in water and they store more energy than carbohydrates. You know these compounds by their common name, "fats".

• Fatty acid - an unbranched carbon chain with the carboxyl group, COOH, at one end. Fatty acids combine to make lipids.
• Triglyceride - three molecules of fatty acid combined with one molecule of the alcohol glycerol.
• Wax - a long fatty acid chain combined with a long alcohol chain.
• Steroid - an atypical lipid formed of four fused carbon rings and various functional groups. One common steroid is cholesterol.

Nucleic acids are very large and complex organic molecules that store information in cells.

• Deoxyribonucleic acid, DNA - contains information that is essential for almost all cell activities.
• Ribonucleic acid, RNA - stores and transfers information essential for the manufacturing of proteins.
• Nucleotides - the monomer molecules that link together to form DNA and RNA.

Go to chemistry class for more biochemistry details.

Concept Understanding: What structural characteristic of a water molecule makes it "polar"? What is a "redox reaction"? What "monosaccharide is produced by plants during photosynthesis? What characteristics of carbon makes it so important in biochemistry? What is the name of the sugar in DNA?

Understanding Life

Carbon is found in all organic molecules.