Carbohydrates in Plants

Overview of Carbohydrates

Carbohydrates are produced in plants through photosynthesis, combining carbon dioxide and water. They are classified by the number of sugar units and molecular weight into monosaccharides, oligosaccharides, and polysaccharides.

Monosaccharides

Monosaccharides are simple sugars, including pentoses, hexoses, heptoses, sugar acids, and sugar alcohols. Glucose, a hexose with six carbon atoms, is the most abundant in nature, found in sweet fruits like grapes and in honey alongside fructose. Most glucose is bound in oligosaccharides or polysaccharides. Industrially, glucose is primarily derived from potato and corn starch.

Fructose, found in fruits, honey, and cane sugar, is industrially obtained from sugar cane and inulin. It converts to glycogen faster than glucose and is insulin-independent, making it a dietary option for diabetes management.

Invert sugar, a mixture of glucose and fructose, is a key component of honey (about 80%), formed by bee enzymes breaking down cane sugar. Honey also contains bactericidal substances, essential oils, and plant-derived compounds from bee foraging.

Sugar Alcohols

Sugar alcohols, such as mannitol, sorbitol, and xylitol, produce fewer acids that promote bacterial growth, reducing tooth decay risk compared to sugars. Xylitol further prevents acid formation post-sugar consumption and provides a cooling sensation when dissolved. Sugar alcohols serve as sugar substitutes in diabetes management.

1. Mannitol: Found in fungi, algae, and plant roots, leaves, and fruits, it is half as sweet as sucrose. Used as a mild laxative, it is not absorbed in the digestive tract and is present in ensiled corn and grass.
2. Sorbitol: Also half as sweet as sucrose, it has a pleasant taste and cooling effect. Found in apples, pears, plums, apricots, cherries, and rowan berries, it converts to fructose in the liver without raising blood sugar, making it suitable for diabetics.
3. Xylitol: Matches sucrose in sweetness and crystal structure, does not elevate blood sugar, and is ideal for diabetes and liver disease. It is sourced from fruits, vegetables, and industrially from birch wood.
4. Inositol: Present in plant and animal tissues, often bound to phosphoric acid or as calcium and magnesium salts, it is used in liver disease treatment.

Sugar acids, like glucuronic and galacturonic acid, form through monosaccharide oxidation and can polymerize into larger molecules.

Oligosaccharides

Oligosaccharides consist of two to eight sugar molecules. Sucrose (cane sugar), derived from sugar cane, sugar beet, and maple, is widely used. It is broken down in the intestines by glycosidase for absorption. Concentrated sucrose solutions have high osmotic pressure, preventing microbial growth, and are used in preserving jams, juices, and syrups.

Maltose, prevalent in germinating plant seeds, is formed during beer fermentation from barley starch. It is valuable in diets for young children with sensitive intestinal mucosa.

Lactose, a milk sugar, is broken down by lactase. Lactic fermentation bacteria convert it to lactic acid. Industrially sourced from whey, lactose normalizes intestinal flora and acts as a mild laxative, particularly for infants.

Polysaccharides

Polysaccharides comprise numerous monosaccharide units, forming chains or branched structures. They lack sweetness and have low water solubility. Key polysaccharides include starch, cellulose, pectins, algal and fungal polysaccharides, plant gums, and mucilages.

1. Starch: Found in cereal seeds, roots, tubers, and other plant storage organs, starch is easily digestible, protects digestive mucosa, and is used in powders due to its ability to absorb secretions and skin fats. Glucose and ethyl alcohol are industrially derived from starch.
2. Cellulose: A structural component of plant cell walls, composed solely of glucose units, it forms fibers used in textiles (e.g., flax, cotton, nettle).
3. Pectins: Filamentous polysaccharides of sugar acids, abundant in apples, citrus peels, red currants, figs, and beets, protect intestinal mucosa and are used in diarrhea treatment, such as with grated apples.

Other Polysaccharides

Seaweeds, particularly red and brown varieties, contain polysaccharides like agar-agar, carrageenan, and alginic acids, used as cough and diarrhea remedies, non-greasy gels in pharmaceuticals, and emulsifiers in cosmetics and food industries.

Plant gums, polysaccharides exuded upon mechanical damage (e.g., in cherry, plum, or acacia bark), harden in air and are used as adhesives (gum arabic) and in pastilles and sweets.

Mucilages, composed of various sugars, are found in plant cell sap or walls. Soluble in water, they have anti-inflammatory effects, are non-sticky, and are used externally for suppuration, gland inflammation, oral and pharyngeal issues, and skin conditions, and internally for digestive inflammation, diarrhea, and as laxatives.