Fix the mistake: Natural Organic Compounds and Polymers

Sugars (Carbohydrates)

Sugars have the general formula (CH₂O)_n and are energy sources and structural substances produced by plants through photosynthesis. They are one of the major components of living organisms.

Basic Knowledge

Monosaccharides: The smallest units that cannot be hydrolyzed any further.
・Glucose C₆H₁₂O₆: an aldohexose; mainly α/β-D-glucopyranose (six-membered ring). Because it can take an open-chain -CHO structure, it is reducing (positive silver-mirror and Fehling's tests).
・Fructose: a ketohexose, mainly in the furanose form (five-membered ring); it is reducing because of its α-hydroxy ketone structure.
・Galactose: a component of lactose and a C4 stereoisomer of glucose.

Disaccharides: Formed by dehydration condensation of two monosaccharide molecules.
・Maltose (malt sugar): glucose ×2, α-1,4 bond, reducing, hydrolyzed by the enzyme maltase.
・Sucrose (table sugar): glucose + fructose, α-1,β-2 bond, nonreducing (both hemiacetal groups are used in the bond), hydrolyzed by sucrase → invert sugar.
・Lactose (milk sugar): galactose + glucose, β-1,4 bond, reducing, hydrolyzed by lactase.

Polysaccharides:
・Starch: polymer of α-D-glucose (mainly α-1,4 bonds with α-1,6 branches); amylose (linear, blue-violet in the iodine-starch reaction) + amylopectin (branched, reddish purple).
・Cellulose: polymer of β-D-glucose (β-1,4 bonds), the main component of plant cell walls.
・Glycogen: energy storage in animals; more highly branched than starch.

Further Study (Principles and Applications)

Cellulose and starch are both composed of glucose, but they have completely different properties because of the α/β difference in their bonds. Humans can digest α bonds (starch) but cannot digest β bonds (cellulose) because humans lack cellulase. Herbivores such as cows can digest cellulose using cellulase produced by intestinal bacteria.
Industrial uses of cellulose: pulp, paper, cotton, regenerated fibers (rayon) (viscose and cuprammonium methods), celluloid (nitrocellulose), and acetate fibers (diacetate and triacetate).
Sugar metabolism: glucose → glycolysis → pyruvate → acetyl-CoA → citric acid cycle → ATP. A maximum of 38 mol ATP is obtained from 1 mol glucose. The brain and red blood cells basically use only glucose as an energy source.
Diabetes: Insufficient insulin action causes excess blood glucose; the kidneys cannot reabsorb it completely, resulting in glucose in the urine. Diabetic ketoacidosis (accumulation of ketone bodies caused by increased fat metabolism), retinopathy, nephropathy, and neuropathy are complications.
Photosynthesis: 6CO2 + 6H2O → C6H12O6 + 6O2 (chlorophyll, chloroplasts, and the Calvin cycle). It is central to CO₂ fixation on Earth.

Points of Caution (Common Confusions and Frequent Errors)

① Sucrose is nonreducing; the other common disaccharides are reducing. ② The difference between starch and cellulose is whether the bond is α or β. Their properties are dramatically different. ③ Fructose is reducing even though it is a ketone because it is isomerized to an aldehyde in basic solution through its α-hydroxy ketone structure (enediol formation). ④ Galactose is a C4 stereoisomer (epimer) of glucose and a component of lactose.

Practice

1. State the structural reason why sucrose does not show reducing properties.
2. Starch and cellulose are both glucose polymers, but humans can digest only the former. State the reason.
3. Briefly explain the mechanism by which the iodine-starch reaction produces color.