Carbohydrates

In the full chemical name of a disaccharide, the glycosidic bond is specified, and the configuration at the anomeric carbon of the non-reducing sugar is named last. Here, "β-D-fructofuranoside" indicates that the fructose unit is in the β-configuration at its anomeric carbon (C-2).

Glycogen phosphorylase cannot cleave near a branch point. The debranching enzyme has two activities: first, transferase activity moves a short α-1,4-linked chain to a nearby non-reducing end; second, α-1,6-glucosidase activity hydrolyzes the remaining α-1,6 bond, releasing a free glucose molecule.

Branching frequency is measured by the ratio of α-1,6 branch points to α-1,4 linkages in the linear chain. Glycogen has a higher ratio than amylopectin, with branches occurring roughly every 8-12 glucose units compared to every 24-30 in amylopectin.

The vitrification (glass formation) hypothesis suggests that trehalose forms a stable glassy matrix. Its many -OH groups replace the hydrogen bonds normally provided by water to the polar head groups of membrane phospholipids and to the protein surface, maintaining their native conformation during dry periods.

The D/L nomenclature is a convention based on glyceraldehyde. A sugar is D if the -OH group on the chiral carbon farthest from the carbonyl group (the highest numbered chiral center) is drawn on the right side in a standard Fischer projection.

Chitin is the primary structural polysaccharide in the cell walls of fungi, the exoskeletons of arthropods, and the beaks of cephalopods. It provides rigidity and strength, analogous to the role of cellulose in plants.

Pancreatic α-amylase, like salivary amylase, is an endoglycosidase that hydrolyzes internal α-1,4 bonds. It cannot cleave α-1,6 bonds at branch points. Therefore, the products are the disaccharide maltose, the trisaccharide maltotriose, and oligosaccharides containing branch points called α-limit dextrins.

The enzyme glucose (xylose) isomerase catalyzes the reversible isomerization of glucose to the sweeter fructose. This process is used to convert a portion of the glucose from corn starch into fructose, creating high-fructose corn syrup (HFCS).

Sugars are polar and partition between an organic mobile phase and a water stationary phase held by the paper. Slight differences in structure (e.g., number of -OH groups) cause them to partition differently and thus migrate at different rates, allowing for separation and identification.

In alkaline conditions, monosaccharides undergo keto-enol tautomerism (Lobry de Bruyn–Alberta van Ekenstein transformation). For example, glucose can form an enediol intermediate that can then convert to either glucose, fructose, or mannose. This results in the epimerization of glucose to mannose at C-2.

A reducing end of a polysaccharide is the terminal monosaccharide with a free anomeric carbon that can undergo ring-opening to expose a free aldehyde or ketone group. All other residues are locked in glycosidic bonds and are non-reducing.

While capillary action in the lumen plays a minor role, the primary mechanism is the strong hydrogen bonding of water to the abundant -OH groups on the glucose units. This is especially effective in the less-ordered, amorphous regions of the cellulose microfibril where -OH groups are not already engaged in inter-chain H-bonds.

Glycogen is primarily stored in the liver (for maintaining blood glucose levels) and skeletal muscles (as a local fuel reserve for contraction). The brain does not store significant glycogen and relies on blood glucose. Adipose tissue stores energy as triglycerides.

In plants, ADP-glucose is the activated form used by starch synthase. In animals, UDP-glucose is the glucosyl donor for glycogen synthesis. This is a fundamental biochemical distinction between the kingdoms.

Concentrated H₂SO₄ dehydrates pentoses to furfural and hexoses to hydroxymethylfurfural. These compounds react with α-naphthol (in Molisch's reagent) to form a purple/violet ring. This is a general test for all carbohydrates.

A five-membered ring (furanose) forms when the keto group on C-2 of a ketose reacts with the hydroxyl group on C-5. This is the predominant ring form for fructose in solution, creating a fructofuranose structure.

The α-1,4 linkage in starch produces a helical chain that is flexible and digestible. The β-1,4 linkage in cellulose produces a straight, rigid chain that can form strong inter-chain hydrogen bonds, making it structural and indigestible to organisms lacking cellulase.

Dextran is a branched bacterial polysaccharide of D-glucose, with a backbone of α-1,6 glycosidic linkages and α-1,3 branch points. Its high molecular weight and colloidal osmotic properties make it useful for drawing fluid into the circulatory system.

Starch is synthesized in plastids. Transitory starch is synthesized in chloroplasts during photosynthesis and broken down at night. Storage starch is synthesized and stored in amyloplasts (non-pigmented plastids) in tissues like tubers, seeds, and roots.