#7.2 Biological molecules - Syllabus 2016 - 2018

2.1    Testing for biological molecules
2.2    Carbohydrates and lipids
2.3    Proteins and water


This section introduces carbohydrates, proteins and lipids: organic molecules that  are important in cells. Nucleic acids  are covered in a separate section. Biological molecules are based on the versatile element carbon. This section explains  how macromolecules, which have a great  diversity of function  in organisms, are assembled from smaller  organic molecules such  as glucose, amino  acids,  glycerol and fatty acids.

Life as we know it would not be possible without water. Understanding the properties of this extraordinary molecule is an essential part of any study  of biological molecules.

The emphasis in this section is on the relationship between molecular structures and their functions. Some of these ideas  are continued in other  sections, for example, the functions of haemoglobin in gas transport in Transport of mammals, phospholipids in membranes in Cell membranes and transport and antibodies in Immunity.

Learning Outcomes

Candidates should  be able to:

2.1    Testing for biological molecules

Tests for biological molecules can be used in a variety of contexts, such  as identifying the contents of mixtures of molecules and following the activity of digestive enzymes.

a)   carry out tests for reducing sugars and non-reducing sugars, the iodine in potassium iodide solution  test for starch, the emulsion test for lipids and the biuret test for proteins to identify the contents of solutions

b)   carry out a semi-quantitative Benedict’s test on a reducing sugar  using dilution, standardising the test and using the results (colour standards or time to first colour change) to estimate the concentration

2.2    Carbohydrates and lipids

Carbohydrates and lipids have important roles in the provision and storage of energy and for a variety of other  functions such as providing barriers  around cells: the phospholipid bilayer of all cell membranes and the cellulose cell walls of plant cells.

a)   describe the ring forms  of α-glucose and β-glucose

b)   define  the terms monomer, polymer,  macromolecule, monosaccharide, disaccharide and polysaccharide

c)   describe the formation of a glycosidic bond by condensation, with reference both to polysaccharides and to disaccharides, including sucrose

d)   describe the breakage of glycosidic bonds in polysaccharides and disaccharides by hydrolysis,  with reference to the non-reducing sugar  test

e)   describe the molecular structure of polysaccharides including starch (amylose and amylopectin), glycogen and cellulose and relate  these structures to their functions in living organisms

f) describe the molecular structure of a triglyceride  with reference to the formation of ester bonds and relate  the structure of triglycerides to their functions in living organisms

g)   describe the structure of a phospholipid and relate  the structure of phospholipids to their functions in living organisms

2.3    Proteins and water

An understanding of protein structure and how it is related to function  is central  to many aspects of biology, such  as enzymes, antibodies and muscle contraction.

Globular and fibrous proteins play important roles in biological processes such  as the transport of gases and providing support for tissues.

Water  is a special  molecule with extraordinary properties that  make  life possible on this planet  150 million kilometres from the Sun.

a)   describe the structure of an amino  acid and the formation and breakage of a peptide bond

b)   explain the meaning of the terms primary structure, secondary structure, tertiary structure and quaternary structure of proteins and describe the types of bonding  (hydrogen, ionic, disulfide and hydrophobic interactions) that  hold these molecules in shape

c)   describe the molecular structure of haemoglobin as an example of a globular protein, and of collagen  as an example of a fibrous protein  and relate  these structures to their functions (The importance of iron in the haemoglobin molecule should be emphasised. A haemoglobin molecule is composed of two alpha (α) chains  and two beta  (β) chains, although when describing the chains  the terms α-globin and β-globin may
be used. There should  be a distinction between collagen molecules and collagen  fibres)

d)   explain how hydrogen bonding  occurs between water molecules and relate  the properties of water to its roles in living organisms (limited to solvent action,  specific  heat capacity  and latent  heat  of vapourisation)