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Carbohydrates, Lipids, and Proteins (2.1/2.3/2.4)

Biological Molecules

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2.1 Molecules to metabolism

Nature of science:

Falsification of theories—the artificial synthesis of urea helped to falsify vitalism. (1.9)


Molecular biology explains living processes in terms of the chemical substances involved.

Carbon atoms can form four covalent bonds allowing a diversity of stable compounds to exist.

Life is based on carbon compounds including carbohydrates, lipids, proteins and nucleic acids.

Metabolism is the web of all the enzyme-catalysed reactions in a cell or organism.

Anabolism is the synthesis of complex molecules from simpler molecules including the formation of macromolecules from monomers by condensation reactions.

Catabolism is the breakdown of complex molecules into simpler molecules including the hydrolysis of macromolecules into monomers.

Applications and skills:

Application: Urea as an example of a compound that is produced by living organisms but can also be artificially synthesized.

Skill: Drawing molecular diagrams of glucose, ribose, a saturated fatty acid and a generalized amino acid.

Skill: Identification of biochemicals such as sugars, lipids or amino acids from molecular diagrams.


Only the ring forms of D-ribose, alpha–D-glucose and beta-D-glucose are expected in drawings.

Sugars include monosaccharides and disaccharides.

Only one saturated fat is expected and its specific name is not necessary.

The variable radical of amino acids can be shown as R. The structure of individual R-groups does not need to be memorized.

Students should be able to recognize from molecular diagrams that triglycerides, phospholipids and steroids are lipids. Drawings of steroids are not expected.

Proteins or parts of polypeptides should be recognized from molecular diagrams showing amino acids linked by peptide bonds.


Aim 7: ICT can be used for molecular visualization of carbohydrates, lipids and proteins in this sub-topic and in 2.3 and 2.4.

Aim 6: Food tests such as the use of iodine to identify starch or Benedict’s reagent to identify reducing sugars could be carried out.

Essential idea: Water is the medium of life.

2.3 Carbohydrates and lipids

Nature of science:

Evaluating claims—health claims made about lipids in diets need to be assessed. (5.2)


Monosaccharide monomers are linked together by condensation reactions to form disaccharides and polysaccharide polymers.

Fatty acids can be saturated, monounsaturated or polyunsaturated.

Unsaturated fatty acids can be cis or trans isomers.

Triglycerides are formed by condensation from three fatty acids and one glycerol.

Applications and skills:

Application: Structure and function of cellulose and starch in plants and glycogen in humans.

Application: Scientific evidence for health risks of trans fats and saturated fatty acids.

Application: Lipids are more suitable for long-term energy storage in humans than carbohydrates.

Application: Evaluation of evidence and the methods used to obtain the evidence for health claims made about lipids.

Skill: Use of molecular visualization software to compare cellulose, starch and glycogen.

Skill: Determination of body mass index by calculation or use of a nomogram.


The structure of starch should include amylose and amylopectin.

Named examples of fatty acids are not required.

Sucrose, lactose and maltose should be included as examples of disaccharides produced by combining monosaccharides.


Variation in the prevalence of different health problems around the world could be discussed including obesity, dietary energy deficiency, kwashiorkor, anorexia nervosa and coronary heart disease.

Theory of knowledge:

There are conflicting views as to the harms and benefits of fats in diets. How do we decide between competing views?


Potatoes have been genetically modified to reduce the level of amylose to produce a more effective adhesive.


Aim 8: There are social implications of obesity.

Essential idea: Proteins have a very wide range of functions in living organisms.


2.4 Proteins

Nature of science:

Looking for patterns, trends and discrepancies—most but not all organisms assemble proteins from the same amino acids. (3.1)


Amino acids are linked together by condensation to form polypeptides.

There are 20 different amino acids in polypeptides synthesized on ribosomes.

Amino acids can be linked together in any sequence giving a huge range of possible polypeptides.

The amino acid sequence of polypeptides is coded for by genes.

A protein may consist of a single polypeptide or more than one polypeptide linked together.

The amino acid sequence determines the three-dimensional conformation of a protein.

Living organisms synthesize many different proteins with a wide range of functions.

Every individual has a unique proteome.

Applications and skills:

Application: Rubisco, insulin, immunoglobulins, rhodopsin, collagen and spider silk as examples of the range of protein functions.

Application: Denaturation of proteins by heat or by deviation of pH from the optimum.

Skill: Drawing molecular diagrams to show the formation of a peptide bond.


The detailed structure of the six proteins selected to illustrate the functions of proteins is not needed.

Egg white or albumin solutions can be used in denaturation experiments.

Students should know that most organisms use the same 20 amino acids in the same genetic code although there are some exceptions. Specific examples could be used for illustration.


Proteomics and the production of proteins by cells cultured in fermenters offer many opportunities for the food, pharmaceutical and other industries.


Aim 7: ICT can be used for molecular visualization of the structure of proteins.

Aim 8: Obtaining samples of human blood for immunological, pharmaceutical and anthropological studies is an international endeavour with many ethical issues.

Essential idea: Enzymes control the metabolism of the cell.

Jessica Clark,
Oct 15, 2013, 8:19 AM
Jessica Clark,
Oct 15, 2013, 8:18 AM
Jessica Clark,
Oct 15, 2013, 8:18 AM
Jessica Clark,
Oct 15, 2013, 8:19 AM