Carbohydrates

Nature of science:

Construct models/visualizations—understanding the stereochemistry of carbohydrates is essential to understanding their structural roles in cells. Haworth projections help focus on the nature and position of attached groups by making carbon and hydrogen implicit. (1.10) Obtaining evidence for scientific theories—consider the structural role of carbohydrates. (1.8)

• Carbohydrates have the general formula C_x(H₂O)_y.
• Haworth projections represent the cyclic structures of monosaccharides.
• Monosaccharides contain either an aldehyde group (aldose) or a ketone group (ketose) and several –OH groups.
• Straight chain forms of sugars cyclize in solution to form ring structures containing an ether linkage.
• Glycosidic bonds form between monosaccharides forming disaccharides and polysaccharides.
• Carbohydrates are used as energy sources and energy reserves.

Applications and skills:

• Deduction of the structural formulas of disaccharides and polysaccharides from given monosaccharides.
• Relationship of the properties and functions of monosaccharides and polysaccharides to their chemical structures.

• Sugar is a major international commodity and is produced in about 130 different countries. Approximately three-quarters of production comes from sugar cane in tropical and subtropical regions and the remainder comes from sugar beet which is cultivated in temperate climates.
• Diabetes is a chronic disease that occurs when the body cannot effectively regulate blood sugar, due to a failure in the production or functioning of insulin. The World Health Organization projects that deaths from diabetes will double between 2005 and 2030.
• Lactose intolerance is a condition in which the individual is not able to digest lactose, the sugar found in milk and dairy products. It is due to a failure to produce sufficient levels of lactase, the enzyme that hydrolyses lactose into glucose and galactose. Globally lactose intolerance is the norm. It is an example of a Western perspective invading science.

Theory of knowledge:

• The use of aspartame as an artificial sweetener has been controversial for many years as the side effects are not fully investigated. Should scientists be held morally responsible for the adverse consequences of their work?

• The straight chain and α-ring forms of glucose and fructose are given in the data booklet in section 34.
• The component monosaccharides of specific disaccharides and the linkage details of polysaccharides are not required.
• The distinction between α- and β- forms and the structure of cellulose are not required.

• Carbohydrates are used in the pharmaceutical industry to bind preparations into tablets.
• Ethanol is produced as a biofuel from the fermentation of carbohydrates in crops such as corn or sugar cane.

Syllabus and cross-curricular links:
Topics 10.1 and 10.2—organic functional groups
Topic 20.1—organic reactions
Topic 20.3—stereoisomerism Option C.4—biofuels
Biology topic 2.3—carbohydrates
Aims:

• Aim 6: Experiments could include using Benedict’s or Fehling’s solution tests to distinguish between reducing sugars and non-reducing sugars or using iodine solution to test for the presence of starch.
• Aim 8: The production of biofuels from crops raises many questions about related issues such as deforestation, soil erosion and sustainability. The “food vs fuel” debate refers to the controversies arising from developments that divert agricultural crops into biofuel production.