Carbohydrates: Glucose, Fructose, and Galactose

• Carbohydrates have a stoichiometric formula of (CH2O)n, with 'n' representing the number of carbon atoms.
• The ratio of carbon to hydrogen to oxygen in carbohydrates is 1:2:1.
• Carbohydrates are classified into monosaccharides, disaccharides, and polysaccharides.

Monosaccharides

• These are simple sugars like glucose.
• Monosaccharides typically contain three to seven carbon atoms.
• Most monosaccharide names end in "-ose".
• An aldose has an aldehyde group (R-CHO), while a ketose has a ketone group (RC(=O)R').
• Depending on carbon number, monosaccharides can be trioses (3 carbons), pentoses (5 carbons), or hexoses (6 carbons).
• Glucose is a vital energy source for humans with the chemical formula C6H12O6.
• During cellular respiration, glucose releases energy used to produce ATP.
• Plants create glucose from carbon dioxide and water to meet their energy needs.
• Humans and animals get glucose from starch catabolized from eating plants.
• Galactose (in lactose) and fructose (in sucrose) are common monosaccharides.
• Isomers of glucose (like galactose and fructose) have the same chemical formula but different structures.
• Monosaccharides can be linear or ring-shaped, and they usually form rings in aqueous solutions.
• Ring-form glucose has two hydroxyl group arrangements around the anomeric carbon (carbon 1).
• These arrangements are alpha (α) position when the hydroxyl group is below carbon 1, or beta (β) position when it's above.

Disaccharides

• These form when two monosaccharides undergo dehydration reaction.
• This process involves a hydroxyl group from one monosaccharide combining with a hydrogen from another, releasing water and forming a covalent (glycosidic) bond.
• Glycosidic bonds can be alpha or beta.
• An alpha bond forms if the OH group on carbon-1 of the first glucose is below the ring plane.
• A beta bond forms if the OH group on carbon-1 is above the ring plane.
• Common dissacharides are lactose (glucose + galactose), maltose (glucose + glucose), and sucrose (glucose + fructose).

Polysaccharides

• These are long chains of monosaccharides linked by glycosidic bonds.
• Chains may be branched or unbranched and consist of different monosaccharides.
• In polysaccharides, the molecular weight can reach 100,000 daltons or more.
• Starch, glycogen, cellulose, and chitin serve as primary examples of polysaccharides.

Starch

• Plants store sugars as starch, a mixture of amylose and amylopectin (both glucose polymers).
• Plants synthesize and store excess glucose as starch in roots, seeds, and other parts.
• Starch in seeds provides food for the plant embryo.
• Starch also serves as a food source for animals.
• Amylase in saliva catalyzes the breakdown of starch into smaller molecules, such as maltose and glucose.
• Glucose starch comprises monomers linked by α 1-4 or α 1-6 glycosidic bonds.
• Amylose forms from unbranched glucose monomer chains with α 1-4 linkages.
• Amylopectin is a branched polysaccharide with α 1-6 linkages at branch points.

Glycogen

• It stores glucose in humans and other vertebrates
• Glycogen consists of glucose monomers
• Glycogen is a highly branched molecule stored in liver and muscle cells
• Glycogen is the animal equivalent of starch.
• Glycogenolysis releases glucose when glucose levels drop.

Cellulose

• It is the most abundant natural biopolymer.
• It's a primary component of plant cell walls, providing structural support.
• Wood and paper primarily consist of cellulose.
• Glucose monomers are linked by β 1-4 glycosidic bonds to form cellulose.
• Every other glucose monomer in cellulose is flipped, allowing tight packing into extended chains which results in high tensile strength.
• Humans can't break down β 1-4 linkages in cellulose; however, herbivores (cows, koalas, buffalos) can digest cellulose with specialized gut flora.
• Rumen bacteria and protists of resident animals secrete cellulase to break down cellulose.
• Cellulases break down cellulose into glucose monomers for energy.
• Termites also have cellulase-secreting organisms in their bodies.

Chitin

• It is a nitrogen-containing polysaccharide
• It forms the exoskeleton of arthropods like insects and crustaceans.
• Chitin comprises repeating N-acetyl-β-d-glucosamine units.
• Chitin is a major component of fungal cell walls.

Benefits of Carbohydrates

• Provide 4.3 Kcal per gram
• Fiber supports bowel movement by adding bulk and regulates blood glucose consumption
• Fiber removes excess cholesterol by binding to it in the small intestine, preventing absorption into the bloodstream.
• Fiber-rich diets have a protective role in reducing the occurrence of colon cancer.
• Whole grains and vegetables create a feeling of fullness.
• Glucose breaks down during cellular respiration, making ATP
• Carbohydrates provide an immediate source of energy