2.3 Carbohydrate Structure

2.3 Carbohydrate Structure

Carbohydrates are sugars and the polymers built from them. Their general formula approaches $CH_2O$. Glucose is $C_6H_{12}O_6$, six copies of that ratio. Not every related sugar matches the ratio exactly, so the careful phrase is approaching $CH_2O$. Carbohydrates serve as quick fuel, transport sugar, stored fuel, and structural material. The same glucose unit can do more than one of those jobs. The linkage decides which job it is.

Monosaccharides and disaccharides

Monosaccharides are single sugar units. Glucose, fructose, and galactose are the three six-carbon sugars to name here. Glucose and galactose keep the carbonyl at the end of the open chain. Fructose keeps a carbonyl within the chain. In water these sugars usually form rings. Glucose is the sugar cells oxidize most directly, and it is the repeating unit of starch, glycogen, and cellulose.

Disaccharides are two monosaccharides joined by a glycosidic linkage through dehydration synthesis. A hydrogen from one sugar and a hydroxyl from the other leave as $H_2O$, and an oxygen bridge remains. Maltose is glucose joined to glucose. Sucrose is glucose joined to fructose. Lactose is glucose joined to galactose. Sucrose is a common plant transport sugar. Lactose is milk sugar. Maltose appears when starch is cut into two-glucose pieces. Hydrolysis splits any of these dimers by adding water across the glycosidic linkage.

Counting linkages and water

Two glucose molecules joined into maltose form one glycosidic linkage and release one water molecule. A chain of 100 glucose units joined end to end contains 99 glycosidic linkages, so this dehydration count releases 99 water molecules. A connected polysaccharide without loops still has one fewer linkage than it has sugar units, branched or unbranched, because each linkage adds one more unit to the same molecule. Cells activate sugar before they add it to a growing chain. They do not simply press free glucose molecules together. The exam point is still the condensation logic: making the glycosidic bond releases water in that bookkeeping, and breaking the bond consumes water.

Starch and glycogen store glucose

Polysaccharides are long chains of monosaccharides. Starch is the plant storage polysaccharide, a polymer of glucose kept in plastids. Amylose is the largely unbranched form, a coil of glucose joined by alpha linkages. Amylopectin is the branched form of starch. Animals, including humans, hydrolyze starch with amylase, releasing glucose that can be used as fuel. Starch is an energy store, not a cell-wall fiber.

Glycogen is the animal storage polysaccharide, also alpha-linked glucose, and it is more highly branched than amylopectin. Vertebrates keep the main stores in liver and in muscle. Liver glycogen can be broken down to help maintain blood glucose. Muscle glycogen fuels the muscle that stores it. Extra branches mean more ends where enzymes can add or remove glucose quickly. An item that places a highly branched glucose store in liver and muscle is describing glycogen.

Alpha, beta, cellulose, and chitin

Cellulose is also a polymer of glucose, and it builds plant cell walls. The difference from starch is the ring orientation at carbon 1. In alpha glucose, the hydroxyl on carbon 1 lies below the plane of the ring. In beta glucose, that hydroxyl lies above the plane. Starch uses alpha linkages, and the chain tends to coil. Cellulose uses beta linkages. Each beta link flips the next glucose, the chain runs straight, and neighboring chains hydrogen-bond into strong fibers.

Most animals, including humans, lack an enzyme that hydrolyzes cellulose beta linkages. Human amylase hydrolyzes alpha linkages in starch. It does not digest cellulose. Cellulose is still glucose, and it is still a carbohydrate. It is not a lipid. Some herbivores house microbes that can split those beta linkages, so a cow can gain glucose from grass that human enzymes cannot release. In the human diet, cellulose is fiber, not a glucose source amylase unlocks.

Chitin is a structural polysaccharide that contains nitrogen. Its monomer is N-acetylglucosamine, a modified glucose with a nitrogen-containing group. Chitin uses beta linkages. It builds fungal cell walls and the exoskeletons of arthropods such as insects and crustaceans. Like cellulose, it is structure, not a quick fuel store in liver or muscle. Nitrogen plus a fungal wall or an arthropod shell points to chitin.

Energy storage points to starch in plants and glycogen in animals. Structure points to cellulose in plant walls and chitin where nitrogen is part of the sugar. Beta linkages that human amylase cannot split point to cellulose. Glucose plus fructose is sucrose, glucose plus galactose is lactose, and glucose plus glucose is maltose.