5.1: What are carbohydrates
- Page ID
- 423676
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- Define carbohydrates, monosaccharides, polysaccharides, and write their general formulas.
- Draw and interpret fisher projections of monosaccharides.
What are carbohydrates
Carbohydrates are primary energy storage compounds, e.g., glucose (\(\ce{C6H12O6}\)) synthesized by using carbon dioxide (\(\ce{CO2}\)) from the air, water (\(\ce{H2O}\)) from the soil, and energy from sunlight, along with the release of oxygen (\(\ce{O2}\)), as illustrated in Figure \(\PageIndex{1}\).
Oxidation of carbohydrates, e.g., the reverse of the photosynthesis reaction, release energy that plant and animals use for activities. Carbohydrates are the support structure material in plants, e.g., cell-wall and wood, shell material of crustaceans, and connective tissues in animals.
Glucose is a polyhydroxy aldehyde, and fructose, i.e., another carbohydrate found in honey, is a polyhydroxy ketone, as illustrated in Figure \(\PageIndex{2}\).
Carbohydrates are polyhydroxy aldehydes, polyhydroxy ketones, or other compounds that hydrolyze to polyhydroxy aldehydes or polyhydroxy ketones.
The general formula of simple carbohydrates is \(\ce{C_{n}H_{2n}O_{n}}\), which can also be written as \(\ce{C_{n}.(H2O)_{n}}\) which is the origin of the name carbohydrates, i.e., hydrates of carbon.
Fisher projections are two-dimensional representations of molecules for showing the configuration of chiral centers:
- the chiral center is in the plane of the page,
- horizontal lines represent bonds projecting toward the viewers,
- and vertical lines represent bonds projecting away from the viewer.
- The parent \(\ce{C}\) chain is placed on the vertical line, with the most oxidized \(\ce{C}\), i.e., the \(\ce{C=O}\) in carbohydrates, at the top or near the top end, and the numbering starts from the top most \(\ce{C}\), as illustrated in Figure \(\PageIndex{2}\).
(a)
(b)Monosaccharides and polysaccharides
Simple carbohydrates that can not be hydrolyzed to more simple ones are called monosaccharides. For example, D-glucose, D-fructose, and D-threose shown in previous figures are monosaccharides.
Linear or branched chain polymers comprised of monosaccharide repeat units (monomers) are called polysaccharides. For example, starch is a polysaccharide with a D-glucose monomer, illustrated in Figure \(\PageIndex{3}\).
Polysaccharides hydrolyze to monosaccharides, as illustrated in the following general hydrolysis reaction.
