Carbohydrates, also known as saccharides, are the basis for all life on Earth. They are compounds of carbon, hydrogen, and oxygen. Along with helium, these elements are the four most abundant in the universe. Both hydrogen and oxygen are crucial for life, especially when they come together to form water. However, even together as water, hydrogen and oxygen are inorganic, and therefore incapable of forming and sustaining life. It is only through bonding with carbon that organic molecules are formed and life is possible.
Carbon is sometimes called the king of the elements, and for good reason. Carbon is unique in that multiple carbon atoms can from strong covalent bonds to each other and form carbon chains, or even rings in which the first carbon in a chain is bonded to the first. The longer the carbon chain, or carbon skeleton, the more available bonds there are for the molecule. Since carbon is the only element that has this ability, carbon has more bonding options than any other element. Carbon is known to form almost ten million different compounds, which account for approximately 95% of all known compounds.
Hydrates of Carbon
Carbohydrates are hydrates of carbon, meaning that they are water molecules bonded to carbon. The general formula is Cm(H2O)n, where m and n don't necessarily have to be the same number, but often are.
Monosaccharides, or simple sugars, are sugars made of single saccharides that can not be hydrolyzed to give a simpler sugar. These are the fundamental units of carbohydrates, and are usually colorless, water soluble, and crystalline solids. Monosaccharides are further classified by the number of carbon atoms in the base chain. Tiroses have three carbons, tetroses have four, pentoses have five, hexoses have six, heptoses have seven, etc. Additionally, due to carbon's versatile bonding potential, different sugars can form that have the same chemical formula but different structures. These sugars are called isomers, and while they do share some traits, they can be quite different as well. Monosaccharides are small enough to move in and out of cells, and there are several that are very important for life.
C6H12O6: There are three primary isomers of this hexose that are important to life. Glucose is the most common, and is produced during photosynthesis. Galactose is found in dairy products, sugar beets, and gums, and can be used in place of glucose, though it is not as easy to break down as glucose. Fructose is found in honey, tree and vine fruits, flowers, berries, and most root vegetables, and is considered the sweetest of all naturally occurring carbohydrates.
In the presence of oxygen, eukaryotic cells break simple sugars, most commonly glucose, into water and carbon dioxide through the process of metabolic respiration. When oxygen is not present, fermentation occurs instead, resulting in carbon dioxide and alcohol. While both processes release energy, metabolic respiration releases much more energy than fermentation. The various processes of animal life require large amounts of energy, and would be impossible without metabolic respiration.
C5H10O5: Ribose is an important pentose that forms part of the backbone of DNA. The phosphorylated derivatives of ribose, ATP and NADH, play crucial roles in metabolism as energy storage and electron transfer molecules.
C5H10O4: Deoxyribose is formed when a ribose loses an oxygen atom. DNA is the primary repository of genetic information, and it consists of a long chain of nucleotides. Each nucleotide consists of a deoxyribose molecule with an organic base (adenine, thymine, guanine, or cytosine) attached to the primary carbon of a ribose.
Simple sugars can bind together and form chains through dehydration. For example, the first carbon of one glucose molecule can bond with the oxygen of the fourth carbon of a fructose, resulting in the loss of a water molecule and the disaccharide sucrose, which we know as common table sugar. This type of bond is called a glycosidic linkage. Oligosaccharides are chains of anywhere between two and ten monosaccharides bonded together through these glycosidic linkages.
Oligosaccharides are too large to pass through cell membranes, so sugars locked into these chains are not available for cellular respiration or fermentation. Smaller oligosaccharides, like disaccharides and trisaccharides, can generally be broken down into monosaccharides by hydrolysis, or the addition of water. For example, when lactose or sucrose are consumed, the body adds water to break the glycosidic linkages and release glucose and fructose from the sucrose, and glucose and galactose from the lactose. Once these simple sugars are freed, they can be used either in metabolic respiration or fermentation, or they can be grouped together and reformed into either structural or food storage polysaccharides.
When eleven or more simple sugars are linked together, very large carbohydrates called polysaccharides are formed. Polysaccharides are still be made up of the same simple sugars as monosaccharides and smaller oligosaccharides, but they are not as easily soluble in water. Starch and glycogen are both polysaccharides of glucose. Each starch or glycogen contains thousands of glucose monosaccharides linked together. Plants form the polysaccharide starch, and animals form the polysaccharide glycogen. Both are used for food storage. When a plant or animal is eaten, these polysaccharides are broken down into simpler sugars during digestion.
Other polysaccharides, like chitin and cellulose, are built for structural reasons rather than food storage. Plant cell walls are made of cellulose. Chitin is found in fungal cell walls, as well as the exoskeletons of some insects and mollusks and the beaks of some birds. For most animals, these polysaccharides are indigestible. However, some animals have specialized digestive systems that can break these down into simpler sugars.
Carbohydrates, or saccharides, are hydrates of carbon, meaning that they consist of one or more water molecules bonding covalently to one or more carbon atoms. Carbon is unique for many reasons, but perhaps the most important is that it is the only element that can form strong covalent bonds to itself. The resulting carbon chains and rings, leaving at lest two remaining bonds for each carbon atom, grant carbon more bonding options than any other element.
Monosaccharides are the simplest forms of carbohydrates, and are called simple sugars. Monosaccharides can form glycosidic linkages with each other through dehydration and produce longer chains of carbohydrates. Oligosaccharides consist of anywhere from two to ten monosaccharides linked together, and polysaccharides are eleven or more. Some polysaccharides, like starch and glycogen, consist of thousands of simple sugars linked together.
Some polysaccharides provide structural support for plants and animals, while others store food as long chains of glucose. When these food storage polysaccharides are eaten, the glycosidic linkages are broken and thousands of glucose monosaccharides are released. Glucose is the most common carbohydrate in plants and animals. In the presence of oxygen, simple sugars like glucose are broken down into carbon dioxide and water through the process of metabolic respiration, which releases large amounts of energy. When oxygen is not present, the simple sugar can be broken down through fermentation, but far less energy is released this way.
Every organism requires some form of monosaccharide as a source of chemical energy in order carry out even the most basic cellular functions of life. Additionally, other monosaccharides, like ribose and deoxyribose, are essential building blocks for DNA and other cellular structures. Cells that have cell walls build those walls out of structural polysaccharides. Life on Earth is often referred to as carbon based life simply because without carbon compounds, specifically carbohydrates, life could not exist.