DP Chemistry: B.9 Biological pigments

B.9 Biological pigments

Written specifically for students to provide help and support for the IB Diploma chemistry programme this page provides full coverage of the syllabus content of Option B - sub topic B.9. It encourages you to think critically and provides many questions with full worked answers so that you can monitor and improve your knowledge and understanding.

Learning outcomes

After studying this topic you 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 your 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.

Relationships & vocabulary

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.

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.

Vocabulary

porphyrinhaemoglobin (or hemoglobin)foetal (or fetal)myoglobin
cytochromeanthocyanincooperative bindingthin-layer chromatography (TLC)

Learning slides

You can use this slide gallery for learning or for reviewing concepts and information. It covers all the key points in the syllabus for this sub-topic.

  

Something to think about

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.

Test your understanding of this topic

(Note that your teacher may have restricted your access to some or all of these questions and worked answers if they are going to use them as a class test or set them as an assignment.)

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.

More 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

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