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Glucides - Organic Chemistry Tutor

Glucides or saccharides are ternary compounds that contain carbon, hydrogen and oxygen and that have an empiric general formula (CH2O)n.

Glucides have an energetic role, offering 50-70% total energy that is produced in the human body, meaning that they constitute the major element of the diet. Body's capacity to deposit glucides is limited, this function is possible only due to the liver (which can deposit up to 10% percent of its total mass) and the muscles (which can deposit up to 0,5% of their total mass). The total quantity of glucides from the deposits is enough to ensure energetic needs of the body only for half a day and only in repose condition. All body tissues can use glucose, the main and almost the exclusive circulating glucide in our blood.

Glucides offer the stock of carbon atoms needed for protein, nucleic acids and lipid biosynthesis. Glucides are also substances that have a structural role, as the proteoglycans from the fundamental substance of conjunctive tissue. Glucides are a part in the structure of other important biomolecular substances such as nucleic acids, coenzymes, complex lipids.

Considering their chemical structure they can be divided in: polyhydroxialdehides and polyhydroxiketones. Considering their structural complexity, glucides can be: oses (simple glucides, monosaccharides) or osides (complex glucides, polisaccharides). Considering the number of carbon atoms in the molecule, monosaccharides can be: trioses, tetroses, pentoses, hexoses, heptoses. The most important monosaccharides are glucose, fructose, galactose, ribose and deoxyribose. The most common polysaccharides are starch, cellulose and glycogen.

Monosaccharides - Physical properties

1) Monosaccharides are easily water soluble on behalf of their multiple -OH groups; they are hardly soluble in alcohol and insoluble in organic solvents (chloroform, hydrocarbonates).
2) They crystallize as white crystals.
3) Most of them have sweet taste, but the sweetness becomes more intense proportionally with the number of -OH groups in the molecule.
4) They are optically active substances.
5) They can have one or more optical isomers.

Monosaccharides - Chemical properties

Due to the fact that monosaccharides contain in their molecule carbonyl and hydroxyl groups, they have characteristic properties that belong to these functional groups.

Chemical properties belonging to the carbonyl group:
1) the reducer agents' action - in the reducing process, oses transform in the corresponding polyalcohols; hydrogen is bounding at the aldehide or ketone group and this is the reason why tetriols result from tretroses, pentols result from pentoses, hexols (hexitols) from hexoses.
2) the oxidant agents' action - in the carbonyl group's oxidation, aldoses transform in polyhydroxilic acids or aldonic acids.
3) the bases' action-strong alkaline solutions break oses in smaller molecules that do not have a glucide character.
4) reduction properties - oses have reduction capacity due to the existence of carbonyl group in their molecule.

Chemical properties belonging to the alcohol group: forming esters - oses can form esters when they react with anorganic or mineral acids; phosphoric esters have the properties of an acid, forming water soluble Na, K, Ca salts.

Glucose: Is a widely expanded hexose in the vegetal and animal life and it is the most important ose of the human body. It has an important energetic role, through its degradation it results energy that is stored in ATP. It is distributed to all cells and fluids in the body, except urine. In blood, glucose has constant limits: 80-120 mg/dl (glycemia).

Galactose: Is found in small quantities in blood, cerebrospinal fluid, urine. It forms with glucose lactose from milk.

Fructose: Is the most important and frequent ketose and can be found free - in many sweet fruits, honey, etc.- or in combined form - in diglucides, triglucides and polyglucides. In the human body, fructose is found in blood (small quantities), seminal plasma, in sperm along with citric acid.

Polysaccharides

Polysaccharides are natural macromolecular products that result in the polycondenssing process that implies several monosaccharides, as in the case of plant photosynthesis and through enzymatic or acid hydrolysis they lead to constitutive monosaccharides forming. The most important polysaccharides are starch, cellulose and glycogen.

Starch:
- Represents the main glucide deposit in plants and the result of chlorophyllian intake (photosynthesis).
- It is found especially in cereals and potatoe's tuberculum and has a significant role in human's nutrition.
- It is amorphous in water insoluble and in contact with warm water forms a paste like mass.
- The repetitive diglucidic unit is maltose.
- Through progressive enzymatic hydrolysis it is split to maltose and glucose.
- Starch granules are made of two major components: amylose (mainly interior) and amylopectin (exterior).

Glycogen:
- Is an animal provenance polyglucide and represents the deposit form of glucose in the body.
- Represents an important source of energy for the human body.
- It is located in the liver and muscles.
- The repetitive diglucidic unit is maltose.
- It has a similar structure to starch's amylopectin containing glucose units attached by alpha-1, 4 and alpha- 1, 6 connections.
- Enzymatic separation of glycogen can take place in the digestive tube (alimentary glycogen) or in the tissues (endogen glycogen).

Cellulose:
- Is the most frequent polysaccharide in the nature.
- Corresponds to the chemical formula: (-C6H10O5-)n in which n can be a number between 700-800 and 2500-3000.
- It is a solid, amorphous, white-colored substance, insoluble in water and organic solvents, soluble in Schweizer reactives. It doesn't have the characteristic sweetness of saccharides.
- Through enzymatic hydrolysis cellulose forms beta-glucose.


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