1. Diversity of Structure and Function in Carbohydrate Molecules
Carbohydrates
Diversity of Structure and Function in Carbohydrate Molecules
Carbohydrates, also called saccharides, are compounds formed from carbon, hydrogen, and oxygen.
Simple sugars termed monosaccharides are the building blocks of carbohydrates.
More complex carbohydrates include oligosaccharides (typically two to 10 monomers) and polysaccharides, which consist of many repeating monosaccharides.
Sugars have the chemical formula (CH2O)n, where n represents the number of carbon-hydrate groups.
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2. Monosaccharides are classified according to molecular structure.
Carbohydrates
Monosaccharides are classified according to molecular structure.
A monosaccharide molecule comprises a carbonyl group (C=O), carbons single-bonded to other carbons in a backbone, and multiple functional hydroxyl groups (OH).
First classification criterion for a monosaccharide: the placement of the carbonyl group.
Second classification criterion for a monosaccharide: the number of carbon atoms in the structural backbone, which can range from three to seven.
Third classification criterion for a monosaccharide: how their hydroxyl groups are situated around their carbon atoms.
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3. Figure 1 Same formula, different functions.
Carbohydrates
Figure 1 Same formula, different functions.
The carbohydrates within each box have the same chemical formula, but each pair differs in the arrangement of its carbons, hydrogens, and in some cases, hydroxyl groups (–OH). These structural differences cause the paired molecules to possess vastly different biological functions.
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4. Interpreting the Ring Structure of a Carbohydrate Molecule
Carbohydrates
BIOSKILL
Interpreting the Ring Structure of a Carbohydrate Molecule
Structural formulas commonly represent sugars as linear chain models, but this simplified form rarely exists in nature. Especially in solutions, sugars form a ring structure, e.g., glucose forms a ring with bonds that result in its carbonyl group becoming a hydroxyl group.
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5. Figure 2 Sugars can transform between linear and ring structures.
Carbohydrates
Figure 2 Sugars can transform between linear and ring structures.
The linear form of glucose rarely exists in nature (a). More often, particularly in aqueous solutions, glucose forms a ring structure (b). The ring can assume two different conformations referred to as the α (alpha) (–OH group angled toward the carbon-carbon bond) and β (beta) forms (–OH group angled toward the carbon-oxygen bond). The β form is more prevalent because it is more stable than the α form.
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6. Carbohydrates Figure 3 Fructose.
The linear and ring form of the carbohydrate fructose, with its intermediate form in between.
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7. Carbohydrates Figure 4 Benzene.
An example of a six carbon ring structure.
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8. Glycosidic linkages join monosaccharides into large polymers.
Carbohydrates
Glycosidic linkages join monosaccharides into large polymers.
Monosaccharides are relatively bulky and are therefore inefficient to store, so organisms convert those not immediately needed into polysaccharides.
A covalent bond called a glycosidic linkage connects structures made of multiple monomers.
The simplest polymer is two linked monosaccharides-a disaccharide.
Glycosidic linkages are formed when a hydroxyl (OH) group is lost from one monomer and a hydrogen (H) atom is lost from the other, resulting in the release of a water (H2O) molecule (a dehydration or condensation reaction), creating polysaccharides.
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9. Carbohydrates Figure 5 Disaccharides.
Maltose and sucrose are disaccharide sugars made up of two monomers.
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10. Organisms use polysaccharides for energy storage.
Carbohydrates
Organisms use polysaccharides for energy storage.
Polysaccharides serve two primary functions in living organisms: energy storage and structural support.
The primary energy storage polysaccharide in plants is starch, whereas animals (and some fungi) store glucose as glycogen. Both are glucose polymers.
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11. Carbohydrates
Figure 7 Microscopy images of storage polysaccharides, glycogen and starch.
a) Mouse liver cells reveal accumulated glycogen granules, magenta staining (400x magnification). b) Transmission electron microscopy of a chloroplast with starch granules ("st") (Scale bar: 500 nm).
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12. Organisms use polysaccharides for cellular structures.
Carbohydrates
Organisms use polysaccharides for cellular structures.
Cellulose, a linear chain of several hundreds or thousands of glucose monomers, is one of the most important structural polysaccharides.
Cellulose is the primary component of the tough walls that enclose plant cells.
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13. Carbohydrates Figure 8 Cellulose.
A dissected hemp stalk shows cellulose fibers.
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14. Organisms use polysaccharides for cellular structures.
Carbohydrates
Organisms use polysaccharides for cellular structures.
Cellulose has a branch-free linear structure, which allows hydrogen bonding to occur between adjacent cellulose molecules, resulting in strong, parallel groupings of molecules called microfibrils.
Chitin occurs in the cell walls of fungi, in certain algae, in the exoskeletons of arthropods, and in some cephalopods.
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15. Carbohydrates Figure 9 Chitin.
The exoskeletons of arthropods such as millipedes (example shown here) and centipedes are made up of the carbohydrate chitin.
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16. Figure 10 Distinguishing features of important polysaccharides.
Carbohydrates
Figure 10 Distinguishing features of important polysaccharides.
Cellulose, a structural support protein for plants, consists of β-glucose monomer strands bonded together by hydrogen bonds. Starch, the primary carbohydrate storage form in plants, consists of strands of α-glucose monomers that are either branched or unbranched. These different monomers result in the opposite orientation of the hydroxyl groups. Glycogen, the primary carbohydrate storage form in animals, consists of strands of α-glucose monomers that are highly branched.
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17. Organisms use polysaccharides for cell-cell recognition.
Carbohydrates
Organisms use polysaccharides for cell-cell recognition.
All cells feature glycoproteins on their surface.
Glycoproteins are composed of proteins covalently bonded to a relatively short, unique polysaccharide.
The suite of a cell's glycoproteins conveys information about the cell type and species.
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18. Carbohydrates
Figure 11 Researchers have focused on developing advanced diagnostics for diabetes.
Studies of diabetes patients show that the blood self-test like the image above deters patients who need regular insulin. As a solution, scientists have developed non-invasive methods such as a test to measure acetone in the breath, another indicator, to diagnose and treat Type 2 diabetes.
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