Molecules Important to Life

Most matter in your body that is not water is made of organic compounds. Organic compounds contain carbon atoms that are covalently bonded to other elements - typically hydrogen, oxygen, and other carbon atoms. The four main types of organic compounds found in living things include familiar names like carbohydrates, lipids (like fats and steroids), nucleic acids, and proteins. These larger molecules create the structural parts of the cell (like phospholipids in cell membranes), carry information (like DNA in our genes), and control the rate of chemical reactions in the cells (like enzymes). Without these compounds, cells could not function. Water, oxygen, carbon dioxide and the minerals needed to sustain life are inorganic.

biological molecules chart

Carbohydrates are the most abundant biological compounds. They are made up of carbon, hydrogen, and oxygen atoms with a ratio of two hydrogen atoms and one oxygen atom for every carbon atom.


Carbohydrates are normally found in the breads and cereals as well as fruits and vegetables. You may have heard of runners eating large quantities of spaghetti or other starchy foods the day before a race. This practice is called "carbohydrate loading." It works because carbohydrates are used by cells to store and release energy. Carbohydrates are a key source of energy, and they are found in most foods - especially fruits, vegetables, and grains.

The building blocks of carbohydrates are single sugars, called monosaccharides such as glucose, C6H12O6, and fructose. Simple sugars such as glucose are very important to the body because they are a major source of energy in cells because they contain large amounts of energy.

Disaccharides are double sugars formed when two monosaccharides are joined together. When glucose and fructose combine, a molecule of sucrose, known as table sugar, is formed.

The largest carbohydrate molecules are polysaccharides and consist of many monosaccharides joined together. These carbohydrates are also made from glucose molecules, but they are stuck together in different ways. Examples are:

The illustration below shows the structure of these molecules.

Lipids are nonpolar molecules that are not soluble in water.
Lipids are insoluble in water because their molecules are non-polar. Many of our common substances are lipids, which include fats, oils, and waxes along with a variety of related substances. Lipids are an important part of the structure and functioning of cell membranes. Phospholipids make up the lipid bilayer of cell membranes. Steroids include cholesterol, which is found in animal cell membranes. Other lipids include some light-absorbing compounds called pigments, such as the plant pigment chlorophyll. Lipids provide twice as much energy per gram as the same amount of carbohydrate.

The most common fats in the human body which is used for long term energy storage are triglycerides, three long chain fatty acids connected to a glycerol.

saturated and unsaturated fatty acids

Fats are lipids that store energy. A typical fat contains three fatty acids bonded to a glycerol molecule backbone. Glycerol is a three-carbon organic molecule. A fatty acid is a long chain of carbon atoms, with hydrogen atoms bonded to them. Because bonds between carbon and hydrogen are rich in energy, fats can store a lot of energy.

Saturated Fat
In a saturated fatty acid, all of the carbon atoms in the chain are bonded to two hydrogen atoms (except the carbon atom on the end which is bonded to three hydrogen atoms.) Most animal fats such as those in butter, lard, and grease from cooked meats contain primarily saturated fatty acids. Saturated fatty acids are straight molecules and are generally solid at room temperature.

saturated and unsaturated fatty acids

Unsaturated Fat
In an unsaturated fatty acid, some of the carbon atoms are linked by a double covalent bond, each with only one hydrogen atom, producing kinks in the molecule. Most plant oils, such as olive oil, and some fish oils contain mainly unsaturated fatty acids and are generally liquid at room temperature. Hydrogenated vegetable oils contain naturally unsaturated fatty acids that have been saturated artificially by the addition of hydrogen atoms. This means hydrogenated vegetable oils, like those found in margarine and vegetable shortening, are generally solid at room temperature.

Phospolipids and Cholesterol
Other important lipids are phospholipids and cholesterol. Phospholipids have two long fatty acid chains and a phosphate compound attached to glycerol. It is a major component of cell membranes because of polar/non-polar structure. Cholesterol is an important component of cell membranes and is the precursor to many steroid hormones.

Lipids are found in fatty meats such as bacon, as well as products such as whole milk, cream and butter, and from the oils of many plants such as olives, corn, and peanuts. Because fat can be concentrated in droplets, it is the preferred way to store energy by broad-tailed hummingbirds, Selasphorus, and other birds that migrate long distances. The comb of a honeybee, Apis, is composed of wax, a compound consisting of fatty acids and alcohols. The wax is secreted by the bees' special abdominal glands.

Proteins are essential to all life. They provide structure for tissues and organs and carry out cell metabolism. Some examples are: muscle, keratin in hair and nails, and collagen in connective tissues. A protein is a large, molecule composed of carbon, hydrogen, oxygen, nitrogen. Some proteins also contain sulfur and phosphorus. The number of proteins in your body is virtually countless. Proteins are found everywhere, in cartilage, bones and muscles, hormones, antibodies, and even enzymes. The basic building blocks of proteins are called amino acids. There are 20 common amino acids. These 20 building blocks, in various combinations, make thousands of proteins.
Proteins tend to fold into compact shapes determined by how the protein's amino acids interact with water and one another.

Enzymes are chemicals found in living things that act to speed up specific chemical reactions. Enzymes are catalysts for biochemical (living) reactions. If there is any chemical reaction in any living thing there is an enzyme that works to bring it about.
All living cells make enzymes, but enzymes are not alive. Enzyme molecules function by altering other molecules. Enzymes are involved in nearly all metabolic processes. Enzymes are important because they purify blood, strengthen the immune system, break down fats, lower cholesterol, enhance mental capacity, cleanse the colon, enhance sleep, and improve aging skin.

A simple definition of life would be: the transfer of energy through the breakdown of nutrients. In other words, all living things get their energy for life by breaking down the chemicals in other living things. Sounds simple enough but, the activity of getting nutrients and energy from food is maybe the most complex group of chemical reactions in the universe. This is because chemical reactions that occur inside living organisms can't happen without a catalyst that would make these reactions happen and control them.

Catalysts are chemicals that while helping a reaction come about, are not themselves changed. Catalysts not used in biochemical reactions, generally are common inorganic substances which have uses outside of being catalyst for a reaction. For example, platinum is used as a catalyst in the reaction that breaks down nitrogen oxides in car exhaust, yet platinum has many other uses. Most biological catalysts, or enzymes, on the other hand are very specific. They exist and are created with only one purpose, to act as a catalyst for one specific reaction biochemical reaction.

Enzymes are proteins which are used as catalysts for a specific reaction. Made up of a complex of amino-acids, enzymes are part of every chemical reaction in living things. Examples of enzyme aided reactions include all digestion, growth and building of cells, any breakdown of substances such as vitamins, and nutrients, and all reactions involving transformation of energy. Chemical reactions in the body are also controlled by enzymes. The rate and location or site of a reaction is also controlled by enzyme action. A good example of the involvement of enzyme action is in the building of living material within the cell.

Inside the cell, enzymes create RNA and DNA by facilitating the reaction of ribose with adenosine. They also specify the sites for linking to build RNA along a DNA template. Once the RNA is formed, it is the enzymes that catalyze the construction of proteins from amino acids. It is the catalytic action of particular enzymes that create specific structures within living cells.

Lack of specific enzymes is the cause of many disorders. Disorders such as albinism, diabetes, and cystic fibrosis are traceable to either a lack of a specific enzyme or an imbalance of one.

Some examples of where enzymes are visible in abundance would be in human saliva and in the human digestive tract. Saliva contains an enzyme that breaks down starches into their component sugars. While the stomach combines the enzyme pepsin with acid to speed the digestion of proteins. Enzymes are carried to the intestines to facilitate the digestion of fats.

Another benefit of enzymes in biochemical reactions is that they control the release of energy in living reactions. The breakdown of chemical bonds releases energy. If you were to measure the amount of energy from a candy bar, you would see that it might have 200 calories or more. While the body needs energy to function, the immediate release of chemical energy from the breakdown of food and nutrients would be disastrous. The small candy bar mentioned here would have the ability to release enough energy to raise the body temperature of a 200 pound man 3 1/2 degrees Fahrenheit! It is the work of enzymes that allow for the controlled release of the energy in living chemical reactions.

Plants turn the energy of sunlight directly into food by using sunlight energy for chemical bonds in the form of sugar. Enzymes are responsible here, too, they control the absorption of radiant energy. Think about it, have you ever sat under a tree during a hot summer afternoon and wondered what keeps the leaves so cool? The sunshine will be hot enough to melt tar on the streets, but plant leaves remain barely warm to the touch. Where does all the energy go? It is slowly being tied up in the chemical bonds of sugar. The process is photosynthesis and it is the basis of all life on earth, but without enzyme controls this process would be impossible.

Enzymes are at work wherever there is life. Yeast use enzymes to leaven bread and ferment sugar into alcohol. Bacteria use enzymes to break down cellulose fiber in the stomachs of cows and the stomachs of termites. Plants, animals, bacteria, or fungi, if they are alive, use enzymes to control all living chemical reactions. Reproduction, growth, metabolism, synthesis, are all enzyme regulated reactions in living things.


Nucleic Acids
All of your cells contain nucleic acids. Nucleic acids are another important type of organic compound that is necessary for life. A nucleic acid is a long chain of smaller molecules that store cellular information in the form of a code. These smaller molecules are called nucleotides. A nucleotide has three parts: a sugar, a nitrogen base, and a phosphate group, which contains phosphorus and oxygen atoms. There are two types of nucleic acids, and they are DNA and RNA - and each type contains four kinds of nucleotides. These two substances were first found in the nucleus of a cell.

DNA, or deoxyribonucleic acid, consists of two strands of nucleotides that spiral around each other. Chromosomes contain long strands of DNA, which contains the instructions used to form all of an organism's enzymes and structural proteins. Thus, DNA forms the genetic code that determines how an organism looks and acts. DNA's instructions are passed on every time a cell divides and from one generation of an organism to the next.

RNA, or ribonucleic acid, consists of a single strand of nucleotides. RNA is a nucleic acid that forms a copy of DNA for use in making proteins. RNA can also act as an enzyme, promoting the chemical reactions that link amino acids to form proteins.

RNA and DNA compared


Adenosine triphosphate is an extremely important molecule formed by a nucleotide. It gets its name from the three phosphate groups that are attached to the five-carbon sugar portion of the molecule. It is the energy carrier in all organisms including humans. When food molecules are broken down inside cells, some of the energy in the molecules is stored temporarily in ATP. Energy is stored in the bonds between the phosphate groups. The last two phosphate bonds are unstable and are easily broken. The energy that is released by ATP breakdown is used by all cells to synthesize molecules like carbohydrates and proteins. In muscle cells, the energy is used for muscle contraction, and in nerve cells, it is used for the conduction of nerve impulses. As you can see, cells need a steady supply of ATP to function.