IMAT 2016 Q40 [Carbohydrates]

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In which of the following organelles do carbohydrates play a relevant role?

  1. Mitochondria
  2. Golgi apparatus
  3. chloroplasts

A. 1 and 3 only
B. 2 only
C. 1 and 2 only
D. 2 and 3 only
E. 1, 2 and 3

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The correct answer is E.

But first, what is a carbohydrate ?

It is a type of molecule that can be found in 3 different forms : sugar, starch or fiber. It is made of carbon, hydrogen and oxygen.

Carbs and mitochondrion

Carbohydrates play a very important role in the metabolism : for example glucose is one of the main carbohydrates. It helps give energy to the cells, after the process of glycolysis, in the form of ATP.

Those reactions happen in the mitochondria from a molecule of pyruvate (the bio product of glycolysis).

Carbs and the golgi apparatus

The golgi apparatus is an organelle that processes proteins and lipids. It is also very important for the addition of carbohydrates to protein molecules to synthesize glycoproteins. That is called glycosylation.

It does the following reactions :

  1. phosphorylises oligosaccharides present on the proteins
  2. removes 2 molecules of mannose (a carbohydrate)
  3. adds a molecule of N-Acetylglucosamine (GlcNAc)
  4. add a molecule of N-Acetylneuraminic acid (NANA)

Carbs and chloroplasts

It is a plant organelle that is composed of different structures :

  1. the stroma is the inside of the chloroplast
  2. it has 2 membranes : one outer and one inner
  3. thylakoids are compartiments inside the chloroplast
  4. a stack of thylakoids is a granum

The thylakoid is a structure in which light dependent reactions occur, also known as the electron transport chain. It results in the production of an ATP molecule.

Here is a diagram of the electron transport chain :

It is composed of 2 photosystems :

  1. PSI or P700
  2. PSII or P680

Both photosystems contain pigments ; their role is to collect light energy to ensure photophosphorylation (ADP is phosphorylated into ATP).

The resulting ATP, combined with NADPH, enters the Calvin cycle. It is a set of chemical reactions that result in the production of glucose.

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But Acetyl CoA is not exactly a carbohydrate, is it?