# Carbohydrates Carbohydrates are a group of organic molecules encompassing sugars (monosaccharides and disaccharides) and their polymers. Most carbohydrates have a general formula of Cn(H2O)m. The nutritional sources of carbohydrates include table sugar (a common name for sucrose) and starchy products such as potatoes and cereals.

Table sugar and starch – the major dietary sources of carbohydrates
Image sources:
Emilian Robert Vicol from Com. Balanesti, Romania - White-Sugar-Crystals_91637-480x360, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=38382827
MoRntashamL GOLD - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=125835813

## Monosaccharides Monosaccharides are simple sugars typically having a general formula of (CH2O)n. The number of carbon atoms in monosaccharides ranges from 3 to 7. The common example is a six-carbon sugar – **glucose**, with a general formula of C6H12O6. Like most monosaccharides, glucose adopts a ring structure in aqueous solutions.

Glucose is the major source of energy for the cell. The release of energy from monosaccharides occurs through cellular **respiration**, a process that involves breaking them down into smaller molecules.

Equation summarising cellular respiration
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BChristinelmiller - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=98216277

Another function of monosaccharides is to serve as building material for other kinds of molecules, such as amino acids. ## Dehydration synthesis Two monosaccharide units can be joined together in a dehydration reaction.

Two glucose units are joined via a dehydration reaction to form a disaccharide called maltose
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CNX OpenStax - http://cnx.org/contents/GFy_h8cu@10.53:rZudN6XP@2/Introduction, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=49922728

The most common **disaccharides** formed by the dehydration reaction are sucrose ("table sugar"), maltose and lactose. The addition of more sugar units to the chain results in the synthesis of polysaccharides. ## Polysaccharides Polysaccharides are large polymers consisting of hundreds to thousands of monosaccharide units. Polysaccharides serve either energy storage or structural functions. Both plant and animal cells use polymers made of glucose monomers to store energy. Glucose can be quickly retrieved from these polymers by a reaction called **hydrolysis.** Unlike glucose, these polysaccharides are insoluble, resulting in compact storage. The plant storage polysaccharide is **starch**. Starch consists of two types of molecules: **amylose**, a linear molecule, and **amylopectin**, a more complex, branched molecule. Amylose allows a slower release of glucose, while amylopectin can be broken down faster due to more free ends.

Starch consists of two types of molecules
Image source: Biorender.com

The animal storage polysaccharide **glycogen** is similar to amylopectin but is more branched. Glycogen is predominantly stored in liver and muscle cells, though other cells also contain a limited amount of glycogen.

Glycogen has plenty of branching points
Image source: Biorender.com

**Cellulose**, another polymer of glucose, is the most abundant structural polysaccharide in the world. Numerous linear and unbranched cellulose molecules align and are connected by weak electrostatic attractions known as **hydrogen bonds**. Aggregated cellulose molecules, forming structures called microfibrils, constitute a major component of plant cell walls.

Cellulose molecules make up microfibrils in the plant cell wall

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