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What are Carbohydrates?

Carbohydrates (also called sugars or saccharides) are made from carbon and water.

They have the formula CnH2nOn.

"Most saccharides (Carbohydrates) are almost always more abundant in nature as the "D" form"

Five and six carbon carbohydrates (pentoses and hexoses) are the most common in nature.

The six carbon carbohydrate called glucose is the most commonly occurring six carbon carbohydrate.

All digested carbohydrates reaching body cells have been converted to glucose by the liver and enterocytes.

The ring form has two anomers.

It is a stereoisomer of a cyclic saccharide that differs only in its configuration at the anomeric carbon.

The anomeric carbon is important to the reactivity of carbohydrates because it is the site at which ring opening occurs, becoming the carbonyl group, the important functional group.

Anomers are identified as “α” or “β” based on the relation between the stereochemistry of the anomeric carbon and the furthest chiral centre in the ring.

The α anomer is the one in which these two positions have the same configuration; they are opposite in the β anomer.

Opposite carbon 1 and 6 = alpha; Same carbon 1 and 6 = beta

"The cell can oxidize glucose (Carbohydrates) transferring its chemical energy to a more readily useable form, ATP"

If the cell has sufficient ATP, glucose (a monosaccharide) is polymerized to the polysaccharide, glycogen or converted to fat.

Glycogen is a short term store of energy in animal cells and this process is called glycogenesis.

Glycogen contains alpha linkages and is found in all animal cells, but especially large amounts are found in the muscle and liver cells.

The liver regulates the blood glucose level, so liver cells are one of the few cell types capable of reforming glycogen back to glucose and releasing it to the bloodstream.

The breakdown of glycogen into glucose is called glycogenolysis.

Glucose → Glycogen = glycogenesis

Glycogen → Glucose = glycogenolysis

The method of glucose uptake differs throughout tissues depending on two factors; the metabolic needs of the tissue and availability of glucose.

The two ways in which glucose uptake can take place are facilitated diffusion (a passive process) and secondary active transport (an active process which indirectly requires the hydrolysis of ATP).

Only certain epithelial cells (enterocytes) in the digestive tract and the proximal tube of the kidney are capable of absorbing glucose against a concentration gradient.

This is done via a secondary active transport mechanism down the concentration gradient of sodium.

"Cells absorb glucose via facilitated diffusion"

Insulin increases the rate of facilitated diffusion for glucose and other monosaccharides.

In the absence of insulin, only neural and hepatic (liver) cells are capable of absorbing sufficient amounts of glucose via the facilitated transport system.

Plants form starch and cellulose from glucose.

a) Starch comes in two forms: amylose and amylopectin

1) Starch has alpha linkages

2) is a straight chain of glucose molecules

b) Cellulose has beta linkages Animals have the enzymes to digest alpha linkages but not beta linkages.

Some animals have bacteria in their digestive tracts that release an enzyme to digest the beta linkages in cellulose.

"Animals eat alpha linkages, bacteria break beta linkages"
Carbohydrates Citations


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