Describe the molecular structure of starch (amylase), glycogen and cellulose, and relate these structures to their functions in living organisms.

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Rhodri Arnall                5/6/2007

Describe the molecular structure of starch (amylase), glycogen and cellulose, and relate these structures to their functions in living organisms.

 Carbohydrates are the main  source for the human body. Chemically, carbohydrates are  in which carbon, hydrogen and oxygen bond together in the ratio: Cx(H2O)y where x and y are whole numbers that differ depending on the specific carbohydrate to which we are referring.  Animals (including humans) break down carbohydrates during the process of  to release .  For example, the chemical  of the sugar glucose is shown below:

Animals obtain carbohydrates by eating foods that contain them, for example potatoes, rice, breads, etc.  These carbohydrates are manufactured by plants during the process of photosynthesis.  Plants harvest  from sunlight to run the reaction described above in reverse:

A potato, for example, is primarily a chemical storage system containing glucose  manufactured during photosynthesis. In a potato, however, those glucose  are bound together in a long chain. As it turns out, there are two types of carbohydrates, the simple sugars and those carbohydrates that are made of long chains of sugars - the complex carbohydrates.

                    In this essay I am going to attempt to describe the structure of starch, glycogen and cellulose and relate these structures to their functions in living organisms. Carbohydrates contain 3 elements which are Carbon, Hydrogen and Oxygen. The group includes monomers, dimers and polymers, as shown in this diagram:


The Isomerism which rotates the plane of polarized light to the right is called the Dextro (D or +) form. The one which rotates the plane of polarized light to the left is called the Laevo (L or -) form. The chemical and physical properties of the two isomerism are the same. Many enzymes will however only act on one type. Glucose easily forms stable ring structures and most molecules exist as rings rather than a chain. Carbon atom no 1 however may combine with oxygen atoms on carbon no 5 to form a ring (two further isomers).




                                                α D(+) Glucose                                  β D(+) Glucose

                                              Alphadextroglucose                          Betadextroglucose


        Monosaccharides are sugars. They dissolve easily in water to form sweet solutions (saccharide refers to sweet or sugar). Monosaccharides have the general formula (CH2O)n and consist of a single sugar molecule (mono). The main types of monosaccharides, if they are classified according to the number of carbon atoms in each molecule, are trioses (3C), pentoses (5C) and hexoses (6C). The names of all sugars end with –ose. Glucose forms a six-sided ring. The six carbon atoms are numbered as shown, so we can refer to individual carbon atoms in the structure. In animals glucose is the main transport sugar in the blood, and its concentration in the blood is carefully controlled. There are many monosaccharides, with the same chemical formula (C6H12O6), but different structural formulae. These include fructose and galactose. Common five-carbon sugars (where n = 5, C5H10O5) include ribose and deoxyribose (found in nucleic acids and ATP).The formula of a carbohydrate is always (CH2O)n. The n represents the number of times the basic CH2O unit is repeated, e.g. where n = 6 the molecular formula is C6H12O6. This is the formula shared by glucose and other simple sugars like fructose. These simple sugars are known as monosaccharides. The molecular formula, C6H12O6, does not indicate how the atoms bond together. Bonded to the Carbon atoms are a number of –H and –OH groups. Different positions of these groups on the carbon chain are responsible for different properties of the molecules. (CH2O)n where n is a number between 3 and 9. They are classified according to the number of carbon atoms. The monosaccharides fall into these categories:

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C = 3 = triose

C = 4 = tetrose

C = 5 = pentose

C = 6 = hexose

Trioses: (e.g. glyceraldehydes, Dihydroxyacetone), intermediates in respiration and photosynthesis.
Tetroses: rare.
Pentoses: (e.g. ribose, ribulose), used in the synthesis of nucleic acids (RNA and DNA), co-enzymes (NAD, NADP, FAD) and ATP.
Hexoses: (e.g. glucose, fructose), used as a source of energy in respiration and as building blocks for larger molecules.
          All but one carbon atom have an alcohol (OH) group attached. The remaining carbon atom has an aldehyde
 or ketone group attached.
         One important aspect of the structure of pentoses ...

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A good account of the structure and functions of starch, glycogen and cellulose. However it is very text heavy and would benefit from labelled diagrams when explaining harder concepts. 4 stars.