Carbohydrates (glucose, sucrose and cellulose)
Carbohydrates (also called saccharides) are molecular compounds made from just three elements: carbon, hydrogen and oxygen. Monosaccharides (e.g. glucose) and disaccharides (e.g. sucrose) are relatively small molecules. They are often called sugars. Other carbohydrate molecules are very large (polysaccharides such as starch and cellulose).
- a source of energy for the body e.g. glucose and a store of energy, e.g. starch in plants
- building blocks for polysaccharides (giant carbohydrates), e.g. cellulose in plants and glycogen in the human body
- components of other molecules eg DNA, RNA, glycolipids, glycoproteins, ATP
Types of carbohydrates
There are various types of carbohydrate. They include monosaccharides, disaccharides, and polysaccharides.
This is the smallest possible sugar unit. Examples include glucose, galactose, or fructose. Glucose is a major source of energy for a cell. “Blood sugar” means “glucose in the blood.”
In human nutrition, these include:
- galactose, most readily available in milk and dairy products
- fructose, mostly in vegetables and fruit
Disaccharides are two monosaccharide molecules bonded together, for example, lactose, maltose, and sucrose.Bonding one glucose molecule with a galactose molecule produces lactose. Lactose is commonly found in milk.Bonding one glucose molecule with a fructose molecule, produces a sucrose molecule.Sucrose is found in table sugar. It is often results from photosynthesis, when sunlight absorbed by chlorophyll reacts with other compounds in plants.
Different polysaccharides act as food stores in plants and animals. They also play a structural role in the plant cell wall and the tough outer skeleton of insects. Polysaccharides are a chain of two or more monosaccharides.
The chain may be:
- branched, so that the molecule looks like a tree with branches and twigs
- unbranched, where the molecule is a straight line
Polysaccharide molecule chains may consist of hundreds or thousands of monosaccharides. Glycogen is a polysaccharide that humans and animals store in the liver and muscles. Starches are glucose polymers that are made up of amylose and amylopectin. Rich sources include potatoes, rice, and wheat. Starches are not water soluble. Humans and animals digest them using amylase enzymes. Cellulose is one of the main structural constituents of plants. Wood, paper, and cotton are mostly made of cellulose.
Glucose, also called dextrose, one of a group of carbohydrates known as simple sugars (monosaccharides). Glucose has the molecular formula C6H12O6. It is found in fruits and honey and is the major free sugar circulating in the blood of higher animals. It is the source of energy in cell function, and the regulation of its metabolism is of great importance (see fermentation; gluconeogenesis). Molecules of starch, the major energy-reserve carbohydrate of plants, consist of thousands of linear glucose units. Another major compound composed of glucose is cellulose, which is also linear. Dextrose is the molecule D-glucose.
A related molecule in animals is glycogen, the reserve carbohydrate in most vertebrate and invertebrate animal cells, as well as those of numerous fungi and protozoans. See also polysaccharide.
Sucrose, commonly known as “table sugar” or “cane sugar”, is a carbohydrate formed from the combination of glucose and fructose. Glucose is the simple carbohydrate formed as a result of photosynthesis. Fructose is nearly identical, except for the location of a double-bonded oxygen. They are both six-carbon molecules, but fructose has a slightly different configuration. When the two combine, they become sucrose.
Plants use sucrose as a storage molecule. For quick energy, cells may store the sugar for later use. If far too much is accumulated, plants may begin to combine the complex sugars like sucrose into even large and denser molecules, like starches. These molecules, and oily lipids, are the main storage chemicals used by plants. In turn, animals eat these sugars and starches, break them back down into glucose, and use the energy within the bonds of glucose to power our cells.
Sucrose has been an important sugar for humans because it is easy extracted from plants such as sugar cane and sugar beets. These plants tend to store an excess of sugar, and from this we produce the majority of the sugar that we use. Even most “natural” sweeteners, which claim to be healthier than sucrose, are simply a different version of glucose combined in a different manner by plants.
Sucrose is the most common form of carbohydrate used to transport carbon within a plant. Sucrose is able to be dissolved into water, while maintaining a stable structure. Sucrose can then be exported by plant cells into the phloem, the special vascular tissue designed to transport sugars. From the cells in which it was produces, the sucrose travels through the intercellular spaces within the leaf. It arrives at the vascular bundle, where specialized cells pump it into the phloem. The xylem, or vascular tube which carries water, adds small amounts of water to the phloem to keep the sugar mixture from solidifying. The sucrose mixture then makes its way down the phloem, arriving at cells in the stem and roots which have no chloroplasts and rely on the leaves for energy.
The sucrose is absorbed into these cells, and enzymes begin breaking the sucrose back into its constituent parts. The six-carbon glucose and fructose can be broken down into 3-carbon molecules, which are imported into the mitochondria, where they go through the citric acid cycle (AKA the Krebs Cycle). This process reduces coenzymes, which are then used in oxidative phosphorylation to create ATP. The energy within the bonds of ATP can power many of the reactions these cells need to complete in order to maintain the stem and roots.
Likewise, all other life on Earth is dependent upon sucrose and other carbs produced by plants. Sucrose was one of the first substances to be extracted from plants on a mass-scale, creating the white table sugar we know today. These sugars are extracted and purified from large crops, including sugar cane and sugar beets. To extract the sugar, the plants are usually boiled or heated, releasing the sugar. “Sugar in the Raw” is sugar which has not been treated further, while white table sugar undergoes more purification.
Cellulose is the substance that makes up most of a plant’s cell walls. Since it is made by all plants, it is probably the most abundant organic compound on Earth. Aside from being the primary building material for plants, cellulose has many others uses. According to how it is treated, cellulose can be used to make paper, film, explosives, and plastics, in addition to having many other industrial uses. The paper in this book contains cellulose, as do some of the clothes you are wearing. For humans, cellulose is also a major source of needed fiber in our diet.
The structure of cellulose
Cellulose is usually described by chemists and biologists as a complex carbohydrate (pronounced car-bow-HI-drayt). Carbohydrates are organic compounds made up of carbon, hydrogen, and oxygen that function as sources of energy for living things. Plants are able to make their own carbohydrates that they use for energy and to build their cell walls. According to how many atoms they have, there are several different types of carbohydrates, but the simplest and most common in a plant is glucose. Plants make glucose (formed by photosynthesis) to use for energy or to store as starch for later use. A plant uses glucose to make cellulose when it links many simple units of glucose together to form long chains. These long chains are called polysaccharides.
Cellulose and plant cells Since cellulose is the main building material out of which plants are made, and plants are the primary or first link in what is known as the food chain (which describes the feeding relationships of all living things), cellulose is a very important substance. It was first isolated in 1834 by the French chemist Anselme Payen (1795–1871), who earlier had isolated the first enzyme. While studying different types of wood, Payen obtained a substance that he knew was not starch (glucose or sugar in its stored form), but which still could be broken down into its basic units of glucose just as starch can. He named this new substance “cellulose” because he had obtained it from the cell walls of plants.
Human uses of cellulose
Cellulose is one of the most widely used natural substances and has become one of the most important commercial raw materials. The major sources of cellulose are plant fibers (cotton, hemp, flax, and jute are almost all cellulose) and, of course, wood (about 42 percent cellulose). Since cellulose is insoluble in water, it is easily separated from the other constituents of a plant. Cellulose has been used to make paper since the Chinese first invented the process around a.d. 100. Cellulose is separated from wood by a pulping process that grinds woodchips under flowing water. The pulp that remains is then washed, bleached, and poured over a vibrating mesh. When the water finally drains from the pulp, what remains is an interlocking web of fibers that, when dried, pressed, and smoothed, becomes a sheet of paper.
Despite the fact that humans (and many other animals) cannot digest cellulose (meaning that their digestive systems cannot break it down into its basic constituents), cellulose is nonetheless a very important part of the healthy human diet. This is because it forms a major part of the dietary fiber that we know is important for proper digestion. Since we cannot break cellulose down and it passes through our systems basically unchanged, it acts as what we call bulk or roughage that helps the movements of our intestines. Among mammals, only those that are ruminants (cudchewing animals like cows and horses) can process cellulose. This is because they have special bacteria and microorganisms in their digestive tracts that do it for them. They are then able to absorb the broken-down cellulose and use its sugar as a food source. Fungi are also able to break down cellulose into sugar that they can absorb, and they play a major role in the decomposition (rotting) of wood and other plant material.