science icon Biochemistry - Chemistry of Life

Many science theories relate to the historywww link of the universe. They all begin with the assumption that we know how matter behaves.

Chemistry is the study of the structure and properties of matter.

The chemical reactions in all living things take place in an aqueous (water) 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. Water is a polar molecule because the two hydrogen atoms are slightly to one side of the molecule. The polar nature of water makes it very effective in dissolving other substances.

Water has a high "heat capacity" - meaning it must gain or lose a relatively large amount of heat energy for its temperature to change. As heat is added to water, most of the energy is used to break the bonds holding the 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.

ATP

The most energy efficient molecule in living things is adenosine triphosphate, ATP. Cells get almost all their energy by breaking the bond indicated on the diagram, converting ATP and ADP.

The energy available in food is measured in calories, a unit of heat. In chemistry and physics, a calorie is defined as the amount of heat needed to raise the temperature of one gram of water by one Celsius degree.

A food Calorie (notice the capital C) is actually a kilocalorie - the amount of heat needed to raise the temperature of 1000 grams of water by one Celsius degree. Since the density of water is one gram per cubic centimeter, 1000 grams of water is equal to a volume of 1000 cubic centimeters, or one liter.

Organic compounds of living things:

Carbohydrates: composed of carbon, hydrogen, and oxygen.

As a food, carbohydrates provide about 4 Calories per gram.

  • 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 (6 carbons) and fructose (five carbons).
     
  • Polysaccharide - a complex molecule composed of three or more monosaccharides.
    You will learn about the polysaccharide cellulose this year.
Proteins: composed of carbon, hydrogen, oxygen, and nitrogen. Proteins make up a large portion of the bodies of all living things - including skin, muscles, and enzymes.

As a food, proteins about provide 4 Calories per gram.

  • 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: contain 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".

As a food, fats provide about 9 Calories per gram.

  • 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 the DNA and RNA polymers.