Biology Essay on Carbohydrates | Organic Molecules | Biology

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Essay # 1. Introduction to Carbohydrates:

Chemically, carbohydrates are organic molecules in which carbon, hydrogen, and oxy­gen bond together in the ratio C_x (H₂O)_y, where x and y are whole numbers that differ de­pending on the specific carbohydrate. They are reduced compounds having large quantities of hydroxyl groups. The presence of the hydroxyl groups allows carbohydrates to interact with the aqueous environment and to participate in hydrogen bond formation, both within and between chains.

The simplest carbohydrates also contain either an aldehyde moiety (termed polyhydroxyaldehydes) or a ketone moiety (polyhydroxyketones). Derivatives of the carbohydrates can contain nitrogen/s, phosphates and sulfur compounds. Carbohydrates can also combine with lipid to form glycolipids or with protein to form glycoproteins.

The aldehyde and ketone moieties of the carbohydrates with five and six carbons will spontaneously react with alcohol groups present in neighboring carbons to produce in­tramolecular hemiacetals or hemiketals, respectively. This results in the formation of five- or six-membered rings.

As the five-membered ring structure resembles the organic molecule furan, the derivatives with this structure are termed as furanoses. Those with six-membered rings, resemble the organic molecule pyran are termed pyranoses and are depicted by either Fischer or Haworth style diagrams. The numbering of the carbons in carbohydrates proceeds from the carbonyl carbon, for aldoses, or the carbon nearest the carbonyl, for ketoses.

The rings can open and re-close, allowing rotation to occur about the carbon bearing the reactive carbonyl, yielding two distinct configurations (α and β) of the hemiacetals and hemiketals. The carbon about which this rotation occurs is the anomeric carbon and these two forms are termed anomers. Carbohydrates can change spontaneously between α and β configurations- a process known as mutarotation. In the Fischer projection, α configuration places the hydroxyl attached to the anomeric carbon to the right, towards the ring, while in the Haworth projection, α configuration places the hydroxyl downward.

Carbohydrates can exist in either of two conformations, as determined by the orientation of the hydroxyl group about the asymmetric carbon farthest from the carbonyl. With a few exceptions, those carbohydrates that are of physiological significance exist in the D- conformation. Carbohydrates are the main energy source for the human body. Animals (including humans) break down carbohydrates during the process of metabolism to release energy.

For example, the chemical metabolism of the sugar (glucose) is shown below:

C₆H₁₂O₆ + 6O2→6 CO₂ + 6 H₂O + energy

Carbohydrates are manufactured by plants during the process of photosynthesis. Plants har­vest energy from sunlight and stores in carbohydrate moieties.

6 CO₂ + 6 H₂O + energy (from sunlight) →C₆H₁₂O₆ + 6 O₂

All carbohydrates can be classified as monosaccharides, oligosaccharides or polysaccha­rides. Two to ten monosaccharide units, linked by glycosidic bonds, make up an oligosac­charide. Polysaccharides are much larger and contain hundreds of monosaccharide units.

Essay # 2. Classification of Carbohydrates:

(a) Monosaccharide:

Monosaccharides are simple sugars, having 3 to 7 carbon atoms. They can be bonded together to form polysaccharides. Cells also use simple sugars to store energy and construct other kinds of organic molecules. The names of most sugars end with the letters ‘ose’. Glucose and other kinds of sugars (fructose, and galactose) may be linear molecules (C₆H₁₂O₆) but in aqueous solution they take ring form.

There are two isomers of the ring form of glucose. They differ in the location of the OH group on the number 1 carbon atom. The number 1 carbon atom of the linear form of glu­cose is attached to the oxygen on the number 5 carbon atom.

(b) Disaccharides:

Disaccharides are composed of 2 monosaccharides joined together by a condensation reaction.

There are three common disaccharides:

i. Maltose (or malt sugar) consists of glucose monomers. Amylase enzyme digests starch molecules to produce maltose.

ii. Sucrose (or cane sugar) composed of glucose and fructose. Plants synthesize sucrose to transport to non-photosynthetic parts of the plant, because it is less reactive than glucose.

iii. Lactose (or milk sugar) is made up of galactose and glucose. It is found only in mam­malian milk.  

(c) Polysaccharides:

Monosaccharides may be bonded together to form long chain compounds called polysaccha­rides. The monomeric building blocks used to generate polysaccharides can be varied; in all cases, however, the predominant monosaccharide found in polysaccharides is D-glucose. Polysaccharides that are composed of a single monosaccharide building block are termed as homopolysaccharides, while polysaccharides composed of more than one type of monosac­charide, they are termed as heteropolysaccharides.

For examples, starch and glycogen are composed of glucose monomers bonded together, producing long chains. They serve the function as stored food, starch in plants and glycogen in animal, in the liver and muscles. Glycogen is poly (1-4) glucose with 9% (1-6) branches (Fig. 3.5).

Starch is a long (100s) polymer of glucose molecules, where all the sugars are oriented in the same direction. Unbranched starch is called amylose, while branched starch is known as amylopectin. Amylose is simply poly-(1-4) glucose units in a straight chain. In fact the chain is floppy, and it tends to coil up into a helix. Amylopectin is poly (1-4) glucose with about 4% (1-6) branches. This gives it a more open molecular structure than amylose.

As it has more ends, it can be broken more quickly than amylose by amylase enzymes. Amy­lopectin is a form of starch that is very similar to glycogen except for a much lower degree of branching (about every 20-30 residues). Another example of polysaccharide is cellulose. Cellulose is a long (100’s) polymer of glucose molecules. However, the orientation of the sugars is little different. In Cellulose, every other sugar molecule is “upside-down”. Glyco­gen is different from both, starch and cellulose in that the glucose chain is branched or “forked” (Fig. 3.6).