Photosynthetic Pigments - Chlorophyll, Carotenoids and Phycobilins - Practice Questions & MCQ

• Pigments are substances that have the  ability to absorb light at specific wavelengths
• Light is a form of electromagnetic energy that travels in waves.
• The energy of light is called Quantum energy contained in packets called photons.
• The light reflected by the pigments is the one they do not absorb
• There are three types of photosynthetic pigments:
  • Chlorophyll
  • Carotenoids
  • Phycobilins

Chlorophyll

• It is water-insoluble but soluble in organic solvents.
• It is synthesized from the raw materials succinyl co-A, glycine, nitrogen, magnesium, and iron.
• The precursor of chlorophyll is protochlorophyll.
• Protochlorophyll lacks two hydrogen atoms, one at the 7th carbon atom and the other at the 8th carbon atom of IVth pyrrole ring.
• In the presence of sunlight, these hydrogens are added to the protochlorophyll and it is converted to chlorophyll.
• Chlorophylls are magnesium-porphyrin compounds having a hydrophilic head and hydrophobic or lipophilic phytol tail.
• The chemical formula of the lipophilic tail is C55H39OH.
• The porphyrin ring consists of 4 pyrrole rings, a central magnesium ion, and 20A long phytol tail.
• Chlorophyll formation is a reduction process occurring in the presence of light.
• If plants are placed in the dark, they get yellowish due to a lack of chlorophyll. This is called etiolation.
• Etiolation is seen in angiosperms only.
• Gymnosperms have an enzyme called chlorophyllase which converts protochlorophyll to chlorophyll. Hence, etiolation does not occur in gymnosperms when kept in the dark.

Carotenoids:

• Carotenoids are plant pigments responsible for bright red, yellow, and orange hues.
• These act as shield pigments to prevent chlorophyll from photooxidation.
• These help plants absorb light energy for use in photosynthesis. 
• These are water-insoluble but soluble in organic solvents.
• There are two types of carotenoids: Carotenes and Xanthophylls.
• The difference between the two groups is chemical: xanthophylls (C40H56)2) contain oxygen, while carotenes (C40H56) are hydrocarbons and do not contain oxygen. 
• Also, the two absorb different wavelengths of light during a plant’s photosynthesis process, so xanthophylls are more yellow while carotenes are orange. 
• The common type of carotene is beta-carotene.
• The common type of xanthophyll is lutein.
• The ratio of xanthophyll to carotene in plants is 3:1 or 2:1.

Phycobilins:

• These are water-soluble pigments.
• They contain 4 pyrrole rings but lack magnesium ions and a phytol tail. 
• These occur in blue-green algae and red algae.
• There are two main types of phycobilins - phycocyanin (blue) and phycoerythrin (red)