Key Concepts Sugars and other carbohydrates are highly variable in structure. Monosaccharides are monomers that polymerize to form polymers called polysaccharides, and are joined by different types of glycosidic linkages. Carbohydrates perform a wide variety of functions in cells: serving as raw material for synthesizing other molecules, providing structural support, indicating cell identity, and storing chemical energy.

Monosaccharides Vary in Structure Monosaccharide monomers are simple sugars that structurally vary in four primary ways: Location of the carbonyl group Aldose: found at the end of the monosaccharide Ketose: found in the middle of the monosaccharide Number of carbon atoms present Triose: three Pentose: five Hexose: six Spatial arrangement of their atoms Different arrangement of the hydroxyl groups Linear and alternative ring forms Sugars tend to form ring structures in aqueous solutions.

Summary of Monosaccharide Structure Many distinct monosaccharides exist because so many aspects of their structure are variable: aldose or ketose placement of the carbonyl group, variation in carbon number, different arrangements of hydroxyl groups in space, and alternative ring forms. Each monosaccharide has a unique structure and function.

The Structure of Polysaccharides Polysaccharides, or complex carbohydrates, are polymers of monosaccharide monomers. The simplest polysaccharides are disaccharides, comprised of two monosaccharide monomers. The monomers can be identical or different. Simple sugars polymerize when a condensation reaction occurs between two hydroxyl groups, resulting in a covalent bond called a glycosidic linkage.

Glycosidic Linkages The glycosidic linkages can form between any two hydroxyl groups; thus, the location and geometry of these bonds vary widely.

Types of Polysaccharides 1. Plants store sugar as starch. Mixture of branched (amylopectin) and unbranched (amylose) -glucose polymer 2. Animals store sugar as glycogen. Highly branched -glucose polymer 3. Cellulose is a structural polymer found in plant cell walls. Polymer of -glucose monomers 4. Chitin is a structural polymer found in fungi cell walls, some algae, and many animal exoskeletons. Comprised of N-acetylglucosamine (NAc) monomers 5. Bacterial cell walls get structural support from peptidoglycan. Backbones of alternating monosaccharides

How Do Carbohydrates Provide Structure? Cellulose, chitin, and peptidoglycan form long strands with bonds between adjacent strands. These strands may then be organized into fibers or layered in sheets to give cells and organisms great strength and elasticity. Unlike the α-glycosidic linkages in the storage polysaccharides, the β-1,4-glycosidic linkages of structural carbohydrates are very difficult to hydrolyze – very few enzymes have active sites that accommodate their geometry or have the reactive groups necessary.

Carbohydrates and Chemical Evolution Most monosaccharides are readily synthesized under conditions that mimic early conditions; thus, it is likely that the prebiotic soup contained a wide diversity of monosaccharides. Polysaccharides, however, despite their current relative abundance on Earth, probably played little to no role in the origin of life. Monosaccharide polymerization requires specialized enzymes. Polysaccharides do not catalyze any known reactions. Polysaccharide monomers cannot provide the information required for themselves to be copied.

What Do Carbohydrates Do? Carbohydrates have diverse functions in cells: In addition to serving as precursors to larger molecules, they provide fibrous structural materials, indicate cell identity, and store chemical energy.

Glycoproteins: Cell Identity Although polysaccharides are unable to store information, they do display information on the outer surface of cells in the form of glycoproteins – proteins joined to carbohydrates by covalent bonds. Glycoproteins are key molecules in cell-cell recognition and cell-cell signaling. Each cell in your body has glycoproteins on its surface that identify it as part of your body.

Carbohydrates and Energy Storage Carbohydrates store and provide chemical energy in cells. In chemical evolution, the kinetic energy of sunlight and heat were converted into chemical energy stored in the bonds of H2CO and HCN. Today, most sugars are produced via photosynthesis, a key process that transforms the energy of sunlight into the chemical energy of C–H bonds in carbohydrates. Carbohydrates have more free energy than CO2 because the electrons in C–H bonds and C–C bonds are shared more equally and held less tightly than they are in C–O bonds.

Starch and Glycogen Are Hydrolyzed to Release Glucose The hydrolysis of -glycosidic linkages in glycogen is catalyzed by the enzyme phosphorylase. Most animal cells contain phosphorylase so they can readily break down glycogen to provide glucose. The -glycosidic linkages in starch are hydrolyzed by amylase enzymes. Amylases play a key role in carbohydrate digestion.

Energy Stored in Glucose Is Transferred to ATP When a cell needs energy, carbohydrates participate in exergonic reactions that synthesize adenosine triphosphate (ATP): CH2O + O2 + ADP + Pi  CO2 + H2O + ATP The free energy in ATP is used to drive endergonic reactions and perform cell work. Carbohydrates contain a large number of C–H bonds, which have high free energy. Fatty acids have even more C–H bonds and consequently more free energy than carbohydrates.

How Do Carbohydrates Store Energy? Starch and glycogen are efficient energy-storage molecules because the α-linkages are readily hydrolyzed, whereas the β-linkages of structural carbohydrates resist enzymatic degradation. The enzymes amylase and phosphorylase catalyze the hydrolysis of α-glycosidic linkages in glycogen and starch, respectively. The released glucose subunits can then be used in the production of ATP.