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Carbohydrates are naturally occuring polyhydroxyaldehydes or polyhydroxyketones, or they are compounds that react with water to give these compounds.
They include sugars, starches, cellulose, and similar substances.
Carbohydrates that are unable to react with water and usually are the units making up larger carbohydrates are called monosaccharides.
The most common is glucose, a pentahydroxyaldehyde, and it is the most widely occuring structural unit in the entire living world.
It is the builing unit for many important polysaccarides such as cellulose and starch.
Fructose, a pentahydroxylketone, is another monosaccharide that makes sugar (sucrose) when combined with glucose.
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OH OH OH OH OH
glucose (open-chain form)
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OH OH OH OH OH
fructose (open-chain form)
The open-chain forms account for less than 0.1% of all the molecules.
Monosaccharides usually come in cyclic forms.
Cyclic Forms of Monsaccharides
When dissolved in water, molecules of carbohydrates exist in equilibrium involving more than one structure.
Glucose, exists as two cyclic forms and one open chain form (rare) in water.
The equilibrium shifts to supply more of any of its members when a specific reaction occurs to only one.
Galactose, another monosaccharide, differs by the projection of the OH group at carbon 4 (far left carbon in ring).
Disaccharides are carbohydrates composed of two monosaccharide molecules.
They are created through dehydration synthesis (removal of water) and broken apart through hydrolysis (addition of water).
For example, these are molecules of sucrose and lactose:
glucose + fructose ==> sucrose + H2O
galactose + glucose ==> lactose + H2O
The above structures both come in cyclic forms and are linked through oxygen bridges.
Polymers of of carbohydrates are called polysaccharides, and make up some of the most inportant naturally occurring compounds.
They have thousands of monosaccharide units linked to each other by oxygen bridges.
They incluse starch, glycogen, and cellulose, all three of which yield only glucose when completely hydrolyzed.
Starch occurs naturally in plants, which use it to store glucose units for energy.
It is often found in seeds and tubers (e.g., potatoes).
It consists of two kinds of polymers of glucose.
The simpler kind is called amylose, and it makes up about 20% of starch.
It is basically a chain of glucose units linked by oxygen bridges.
The average amylose moleculse has over 1000 units.
During digestion, the oxygen bridges are hydrolyzed and the glucose units are broken up.
amylose + nH2O ==> n glucose
In the above diagram, n is very large, usually over 1000.
The other part of starch, the part that makes up most of it, is made up of amylopectin.
It is even larger than amylose.
Basically, it consists of several amylose molecules linked and cross-linked with oxygen bonds.
Glycogen is used to store glucose units for energy in animals.
Its structures is much like amylopectin.
Any unused glucose molecules in our bloodstream is put into the liver and muscles to be stored as glycogen until needed.
Cellulose is also a poolymer of glucose.
It is much like amylose, but it has the oxygen bridges diferently oriented because it uses -glucose molecules.
It makes up cell walls in plants.
Humans lack the enzyme to hydrolyze it, and plant materials made with it, like leafy vegetables, are not digested and used as "roughage" or fiber.
Some animals have bacteria living in their digestive tracts that can hydrolyze cellulose for them.