Carbohydrates Overview

Carbohydrates

• Carbohydrates are a group of carbonyl compounds (aldehydes or ketones) that also contain several hydroxyl groups.
• Carbohydrates have the general formula (CH₂O)ₙ.
• Isomers have the same formula but different structures. Fructose and glucose have the same formula (C₆H₁₂O₆).
• Carbohydrates are classified by the number of units: monosaccharides, disaccharides, oligosaccharides, and polysaccharides.
• The "-ose" suffix is added to the name of carbohydrates.
• Monosaccharides are classified as either aldoses (aldehyde group) or ketoses (ketone group) based on the carbonyl group.
• The number of carbons determines the specific type of monosaccharide (triose, tetrose, pentose, hexose, heptose, nonose). Examples include glyceraldehyde, erythrose, ribose, glucose, sedoheptulose, and neuraminic acid.
• Ketoses contain a ketone group in their structure. Examples include ribulose and fructose.
• Sometimes, carbohydrates' oxygen content can change depending on the conditions.
• In certain conditions, carbohydrates can have more or less oxygen and nitrogen.
• Polysaccharides are large polymers made up of many monosaccharides linked together. Common examples include starch, glycogen, and cellulose.
• Starch and glycogen are storage polysaccharides, with glycogen being branched and starch having a linear structure.
• Cellulose is a structural polysaccharide with a linear structure. It's composed of beta-glycosidic bonds.
• Chitin is a structural polysaccharide containing N-acetylglucosamine.
• Disaccharides are formed when two monosaccharides combine through glycosidic bonds.
• Examples of disaccharides include maltose, lactose, and sucrose. Maltose contains two glucose units. Lactose contains galactose and glucose. Sucrose contains glucose and fructose.
• Glycogenin is the protein central to glycogen synthesis. Glucose is added to the chain, and degradation starts from the ends for faster energy production.
• Starch is made up of two components: amylose (linear) and amylopectin (branched). Amylose does not appear different than glycogen.
• Amylopectin has α(1→6) glycosidic bonds linking the chains of glucose.
• Carbohydrates can be part of other molecules.
• Carbohydrates can form glycosaminoglycans, which have negatively charged heteropolysaccharide chains. Glycoproteins have proteins with attached oligosaccharides. Proteoglycans are proteins with more carbohydrate content than protein.
• Glycoproteins and proteoglycans play important roles in various biological processes and are found in the extracellular matrix(ECM).
• Glycoproteins are important in cell-cell communication and antigenicity (blood types), with specific carbohydrates dictating blood type.
• These structures aid in lubrication and are part of the ECM.

Digestion

• Digestion of carbohydrates begins in the mouth with the breakdown of glycosidic bonds by enzymes like alpha-amylase, which is an enzyme that breaks down starch into smaller sugars.
• Disaccharidases break down the disaccharides into monosaccharides.
• Cellulose has beta linkages and is thus not digestible by humans.

Absorption

• Monosaccharides are absorbed into the intestinal mucosal cells through specific transporters.
• Some monosaccharides use sodium-dependent transport mechanisms.
• Others are absorbed via facilitated transport.

Carbohydrate Metabolism

• Blood glucose concentration is tightly controlled (~5 mM).
• Carbohydrates are essential for energy production and brain function.
• Low blood glucose can cause confusion, coma; high blood glucose can cause vascular damage.