Dr. Mohit Gupta

Plant Physiology 
Question asked in Section - C of  CSIR-NET December 2014.

Q102. Following are certain statements regarding secondary metabolites found in plants:
A. All terpenes are derived from a six carbon element.
B. Alkaloids are nitrogen containing compounds.
C. Pyrethroids, a monoterpene ester found in the leaves and flower of chrysanthemum species, show insecticidal activity.
D. Limonoids are groups of alkaloids and have antiherbivoral activity.

Which one of the following combinations of above statements is correct?
1.  A  and B              2.  A  and D             3. B and C               4. C and D

Ans (3)  Secondary Metabolites......one of the widest topic in Plant Biochemistry and Physiology and extremely important too........90% of colleges and universities in India do not do justice to this topic......students are either not taught or they are just handed photocopies.......coz teachers do not want to take pain of preparing something so wide in ambit. 

Prof. C.R Babu from Botany Dept., Delhi University produced dozens of Ph.Ds in this topic as sec. metabolites are extremely important for Molecular plant classification; Prof. C.R Babu has also written the flora of Dehradun.. 

If we analyze GATE-Biotech syllabus.........nearly 70% topics in Plant Biotech Topic are from Sec. Metabolites...and every year we get questions from here.

In CSIR-NET every time we get few questions from here....which students normally dont attempt....I mean how can they attempt as the topic is so wide and difficult. At Grassroots it takes nearly 18 hours to justify this topic alone........only then one can attempt this topic.

Secondary metab. are also known as natural products, secondary products........and there are 1000's of different sec. metab. in differet plants........but all have been clubbed under three categories:
1. Terpenoids
2. Phenolocs
3. Nitrogen containing compounds, which includes
a. Alkaloids
b. Cyanogenic glycosides
c. Glucosinolates

I certainly cannot justify this topic writing here.............but for students knowledge let me tell that we call them as sec. metab. as they have no direct role in growth and reproduction of plants........but then many phytohormones like Cytokinin, Gibberellin, Abscassic acid, Brassino-steroids are sec. metab........even pigments like carotenoids and phytol chain of chlorophyll are sec. metab..and they have direct role in plant growth....isn't that amazing.

Now coming to the question.......
Choice A says "All terpenes are derived from a six carbon element" this is a wrong choice......as all terpenoids are synthesized from Isopentenyl di phosphate (IPP) which is a 5-carbon compound.

Infact terpenoids are synthesized at dual locations in the plant cell i.e. cytoplasm and plastids. Cytoplasm has MVA i.e. Mevalonic Acid Pathway & plastids have DXP i.e. Deoxy Xylulose phosphate Pathway.

Choice B says "Alkaloids are nitrogen containing compounds"....as we saw in classification of sec. metab. that alkaloids contain nitrogen....so it is a correct choice

Choice C says "Pyrethroids, a monoterpene ester found in the leaves and flower of Chrysanthemum sp, show insecticidal activity"........is a correct option as pyrethroids are extracted from Chrysanthemum cinerariaefolium and have insecticidal properties............even Rotenoids have insecticidal properties is but are Phenolics and obtained from Derris elliptica.

Choice D says "Limonoids are groups of alkaloids and have antiherbivoral activity."......this is a wrong option.......as Limonoids are terpenoids and not alkaloids. Neem plant i.e. Azadirachta indica has azadirachtin which is a limonoid and has insecticidal properties.

So the correct choice is.............. 3 i.e. B and C

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