There are four major classes of biomolecules - carbohydrates, proteins, nucleotides, and lipids. Carbohydrates, or saccharides, are the most abundant of the four.
Carbohydrates have several roles in living organisms, including energy transportation, as well as being structural components of plants and arthropods.
Carbohydrate derivates are actively involved in fertilization, immune systems, the development of disease, blood clotting and development.
They are called carbohydrates because the carbon, oxygen and hydrogen they contain are generally in proportion to form water with the general formula Cn (H2O)n.
Contents of this article:
Four major biomolecule classes
People think of bread, pasta and rice when they think of carbs, but all these foods are rich in carbs too.
Carbohydrates (saccharides) - Molecules consist of carbon, hydrogen and oxygen atoms. A major food source and a key form of energy for most organisms. When combined together to form polymers, carbohydrates can function as long term food storage molecules, as protective membranes for organisms and cells, and as the main structural support for plants and constituents of many cells and their contents.
Lipids (fats) - Molecules consist of carbon, hydrogen, and oxygen atoms. The main constituents of all membranes in all cells (cell walls), food storage molecules, intermediaries in signaling pathways, Vitamins A, D, E and K, cholesterol.
Proteins - Molecules contain nitrogen, carbon, hydrogen and oxygen. They act as biological catalysts (enzymes), form structural parts of organisms, participate in cell signal and recognition factors, and act as molecules of immunity. Proteins can also be a source of fuel.
Nucleic acids (nucleotides) - DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). These molecules are involved in genetic information, as well as forming structure within cells. They are involved in the storage of all heritable information of all organisms, as well as the conversion of this data into proteins.Most organic matter on earth is made up of carbohydrates1 because they are involved in so many aspects of life, including:
- Energy stores, fuels, and metabolic intermediaries.
- Ribose and deoxyribose sugars are part of the structural framework of RNA and DNA.
- The cell walls of bacteria are mainly made up of polysaccharides (types of carbohydrate).
- Cellulose (a type of carbohydrate) makes up most of plant cell walls.
- Carbohydrates are linked to many proteins and lipids (fats), where they are vitally involved in cell interactions.
What are saccharides?
Saccharides, or carbohydrates, are sugars or starches. Saccharides consist of two basic compounds: aldehydes (composed of double-bonded carbon and oxygen atoms, plus a hydrogen atom), and keytones (composed of double-bonded carbon and oxygen atoms, plus two additional carbon atoms).
There are various types of saccharides, including monosaccharides, disaccharides, and polysaccharides.
This is the smallest possible sugar unit. Examples include glucose, galactose or fructose. When we talk about blood sugar we are referring to glucose in the blood; glucose is a major source of energy for a cell.2 In human nutrition, galactose can be found most readily in milk and dairy products, while fructose is found mostly in vegetables and fruit.
When monosaccharides merge together in linked groups they are known as polysaccharides.
Two monosaccharide molecules bonded together. Disaccharides are polysaccharides - "poly..." specifies any number higher than one, while "di..." specifies exactly two. Examples of disaccharides include lactose, maltose, and sucrose. If you bond one glucose molecule with a fructose molecule you get a sucrose molecule.
Sucrose is found in table sugar, and is often formed as a result of photosynthesis (sunlight absorbed by chlorophyll reacting with other compounds in plants). If you bond one glucose molecule with a galactose molecule you get lactose, which is commonly found in milk.
A chain of two or more monosaccharides. The chain may be branched (molecule is like a tree with branches and twigs) or unbranched (molecule is a straight line with no twigs). Polysaccharide molecule chains may be made up of hundreds or thousands of monosaccharides.
Polysaccharides are polymers. A simple compound is a monomer, while a complex compound is a polymer which is made of two or more monomers. In biology, when we talk about building blocks, we are usually talking about monomers.
Types of polysaccharides
Polysaccharides may act as food stores in plants in the form of starch, or food stores in humans and other animals in the form of glycogen. Polysaccharides also have structural roles in the plant cell wall in the form of cellulose or pectin, and the tough outer skeleton of insects in the form of chitin.
A polysaccharide that humans and animals store in the liver and muscles.
These are glucose polymers made up of Amylose and Amylopectin. Amylose molecule chains are linear (long but no branches) while Amylopectin molecules are long and branch out - some Amylopectin molecules are made of several thousand glucose units.
Starches are not water soluble. Humans and animals digest them by hydrolysis - our bodies have amylases which break them down. Rich sources of starches for humans include potatoes, rice and wheat.
The structural constituents of plants are made mainly from cellulose - a type of polysaccharide. Wood is mostly made of cellulose, while paper and cotton are almost pure cellulose. Lignin, derived from wood, is a key component in the secondary walls of plant cells. Some animals, such as termites, can digest cellulose because their gut has a type of bacteria that has an enzyme which breaks down cellulose - humans cannot digest cellulose.
Chitin, a polysaccharide, is one of the most abundant natural materials in the world. Microorganisms, such as bacteria and fungi secrete chitinases, which over time can break down chitin. These microorganisms also have receptors to the simple sugars that result from this breakdown (decomposition). The bacteria and fungi convert the decomposed chitin into simple sugars and ammonia.
Chitin is the main component of fungi cell walls, the exoskeletons (hard outer shell/skin) of arthropods, such as crabs, lobsters, ants, beetles, and butterflies. Chitin is also the main component of the beaks of squid and octopuses. Chitin is useful for several industrial and medical purposes.
These are polysaccharides that are found in bacteria, especially in bacterial capsules. Pathogenic (illness causing) bacteria often produce a thick layer of mucous-like polysaccharide which cloaks the bacteria from the host's immune system. In other words, if the bacteria were in a human, that human's immune system would less likely attack the bacteria because the polysaccharide layer masks its pathogenic properties. E. coli, which can sometimes cause disease, produces hundreds of different polysaccharides.
Examples of carbohydrates
In this video, Susan Bowerman takes you through sources of good and bad carbohydrates.
On the next page we look at carbohydrates, nutrition and high-carb vs. low-carb. On the final page we discuss blood sugar levels, the glycemic index and switching to a low GI lifestyle.