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Despite having a bad reputation lately, carbohydrates are the body's main source of energy. Human beings cannot function without carbohydrates. Glucose, a simple sugar, is essential for cells to function. A diet without carbohydrates can force the body to break down proteins and fats for energy, which may cause harm. The word carbohydrate tells us exactly what they are made from. 'Carbo' refers to carbon, and 'hydrate' refers to water.
The simplest carbohydrates are called monosaccharides. 'Mono' means one, and 'saccharide' means sugar. These are, therefore, single molecules of sugar. Some common examples include glucose, fructose, and galactose. Monosaccharides can have either a linear structure, meaning they are arranged in a straight line, or have a ring structure, meaning they close up like a circle.
An interesting fact about monosaccharides is that different monosaccharides can be made from the same elements. For example, glucose, fructose, and galactose are all made from 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. However, they are different sugars, because the atoms are arranged in slightly different ways. Another way to describe this is to say that they are isomers.
Polysaccharides are more complex carbohydrates. The smallest of the polysaccharides are the disaccharides, which are made from two monosaccharides joined together. An example is sugar lactose, which is a disaccharide of glucose and galactose. Generally, though, the term polysaccharide refers to a carbohydrate that has many, many monosaccharides joined together ('poly' means many).
Starch is the simplest of the complex carbohydrates. Starch is used to store energy in plant cells. It is made of a simple chain of monosaccharides joined together. Because it is so simple, it is very easy to break apart the monosaccharide molecules to get energy.
Glycogen is a bit more complex than starch. Glycogen is used to store energy in animal cells. When an animal, such as a human, needs energy, glycogen can be broken down in the liver into glucose. Glycogen is also made up of repeating monosaccharides, but they are arranged in branching chains.
Cellulose, also called fiber, is the most complex of the polysaccharides. It has a very dense structure, with winding fibrils, or fine fibers. Cellulose is made primarily by plant cells. Rather than being a way to store energy for future use, cellulose is used to maintain the structure of the plant cell. It cannot be broken down by humans, so it is passed through the body. Some animals can make a similar polysaccharide called chitin. It is found in the shells of crustaceans like crabs, for example.
Most carbohydrates are termed hydrophilic. 'Hydro' means water, and 'philic' means love. Carbohydrates love water and can easily dissolve inside it. You've seen this yourself if you've ever stirred sugar into hot water when making tea. Simple sugars such as monosaccharides and disaccharides dissolve very quickly. Larger carbohydrates, such as starch, may take more time to break down. Cellulose, because of its dense, winding structure, does not dissolve easily in water. Being hydrophilic is important because it allows carbohydrates to be easily moved through the body in the blood stream and throughout the cell.
Carbohydrates are the body's main source of energy. They can vary from simple sugars to complex starches and cellulose. Most carbohydrates can dissolve easily in water, allowing them to be moved efficiently through the body and through the cell.
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AP Biology: Help and Review28 chapters | 382 lessons
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