DP Chemistry: Biological pigments
Teacher only page

Biological pigments

B.9 Biological pigments (3 hours)

Pause for thought

One of the most beautiful sights in nature is the changing colour of autumn leaves (or fall leaves in the US). Other than haemoglobin and myoglobin, virtually all of the biological pigments covered in this sub-topic can be found in autumn leaves.

Image taken during an autumn visit to Westonbirt arboretum in the UK

The leaves on trees contain many plant pigments but the dominant one during the spring and summer is chlorophyll. It is the presence of chlorophyll, which acts to absorb energy from sunlight and enable photosynthesis to occur, that gives leaves their characteristic green colour. During the growing season the chlorophyll is replenished as it is used up but as the temperature falls during the autumn the amount of chlorophyll in the leaves decreases as it is converted into colourless tetrapyrroles. Now other pigments present in the leaves become unmasked. Carotenoids are present throughout the year. The extended conjugation systems of the carotenoids produce the well-known orange, yellow and brown colours and the hues in between of autumn leaves. Another group of biological pigment develop in the leaves during late summer. These are the anthocyanins and cause the fiery red colours in the leaves of many hardwood trees such as maple and oak.

Nature of science

The use of spectroscopy to measure absorbance quantitatively now provides a reliable means of communicating data based on colour. This was previously more subjective and difficult to replicate.

Learning outcomes

After studying this topic students should be able to:

Understand:

  • Biological pigments are coloured compounds that are produced by metabolism.
  • The colour of pigments is caused by highly conjugated systems with delocalized electrons, which have intense absorption bands in the visible region.
  • Porphyrin compounds, for example, haemoglobin, myoglobin, chlorophyll and many cytochromes, are metal chelates with large nitrogen-containing macrocyclic ligands.
  • Haemoglobin and myoglobin contain haem groups in which the porphyrin group is bound to an iron(II) ion.
  • The iron ion in the haem groups in cytochromes interconverts between iron(II) and iron(III) during redox reactions.
  • Anthocyanins are aromatic, water-soluble pigments widely distributed in plants. The specific colour of anthocyanins depends on metal ions and the pH.
  • Carotenoids are lipid-soluble pigments that absorb light energy for use in photosynthesis. They are susceptible to oxidation, catalysed by light.

Apply their knowledge to:

  • Explain the sigmoidal shape of haemoglobin’s oxygen dissociation curve in terms of the cooperative binding of haemoglobin to oxygen.
  • Discuss the factors (include temperature, pH and carbon dioxide) that influence oxygen saturation of haemoglobin.
  • Describe the greater affinity that fetal haemoglobin has for oxygen.
  • Explain the action of carbon monoxide as a competitive inhibitor of oxygen binding.
  • Outline the factors that affect the stabilities of anthocyanins, carotenoids and chlorophyll in relation to their structures.
  • Explain the ability of anthocyanins to act as acid-base indicators based on their sensitivity to pH.
  • Describe the function of photosynthetic pigments in trapping light energy during photosynthesis.
  • Investigate different pigments using paper and thin layer chromatography.

Clarification notes

The structures of chlorophyll, haem B and specific examples of anthocyanins and carotenoids can be found in Section 35 of the data booklet. Details of the names of other pigment and their structures are not required.

The explanation of cooperative binding in haemoglobin should be limited to conformational changes occurring in one polypeptide when it becomes oxygenated.

A knowledge of the specific colour changes that occur as the conditions change for biological pigments is not required.

International-mindedness

Artificial colourings are often added during the commercial preparation and processing of food. This can cause problems for international trade as the list of approved food colouring additives varies greatly by country and there is no universal standard.

Teaching tips

There is a lot of good chemistry in this sub-topic, much of which can build upon material already covered in the core/AHL and B.2 for Rf values and chromatography.

The important concept for students to understand is that double bonds normally absorb ultraviolet light as the pi electrons are promoted to a higher energy level (usually a non-bonding orbital) but when there is extensive conjugation the energy required decreases and falls within the visible region. The actual colour seen is the transmitted light, which is the complementary colour to the light absorbed. Anything that affects the amount of conjugation e.g. a change in pH will affect the precise colour observed.

Explain how cooperative binding occurs in haemoglobin. When one oxygen molecule binds to one of the four haem groups it causes a strain that is transmitted through the porphyrin complex to the other three groups. This alters the shape of the entire haemoglobin molecule making it easier for more oxygen atoms to bind so increasing further its affinity to bind to oxygen. This increases the efficiency of oxygen transport around the body. Distinguish between adult and fetal haemoglobin and explain how fetal haemoglobin must have a greater affinity for oxygen in order to allow oxygen to diffuse from the mother to the foetus across the placenta. Factors that affect the active site e.g. pH, temperature and carbon dioxide will affect haemoglobin’s ability to transport oxygen. Carbon monoxide irreversibly occupies the active site and so acts as a poison.

Pigments can easily be separated by chromatography (paper or thin-layer) as no dye or staining is necessary to see the spots after elution. Thin-layer has the advantage that individual component pigments can be obtained in a pure form by extracting each one separately from the silica layer in a solvent then simply evaporating the solvent.

Study Guide

Pages 137 & 138

Questions

For ten 'quiz' questions (for quick testing of knowledge and understanding with the answers explained) see MC test: Biological pigments.

For short-answer questions see Biological pigments questions together with the worked answers on a separate page Biological pigments answers.

Vocabulary list

porphyrin
haemoglobin (or hemoglobin)
foetal (or fetal)
myoglobin
cytochrome
anthocyanin
cooperative binding
thin-layer chromatography (TLC)

Teaching slides

Teachers may wish to share these slides with students for learning or for reviewing key concepts.

  

Other resources

1. A good description of haemoglobin and cooperative binding by Interactive biology which is also put into a biological context at about the correct level for this course.

  Haemoglobin & the oxygen dissociation curve

2. How to carry out thin-layer chromatography.

  Thin-layer chromatography

All materials on this website are for the exclusive use of teachers and students at subscribing schools for the period of their subscription. Any unauthorised copying or posting of materials on other websites is an infringement of our copyright and could result in your account being blocked and legal action being taken against you.