IMAT 2017 Q30 [Carbohydrate Monomers]

The following structures are found in plant cells:

  1. cell wall
  2. cell membrane
  3. nucleolus
  4. mitochondrion

Which cell structures might contain molecules containing carbohydrate monomers?

A. 1 only
B. 1 and 4 only
C. 2 and 3 only
D. 1, 2, 3 and 4
E. 4 only

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The correct answer is D.

Before we start analyzing each item individually, we need to understand what a carbohydrate monomer is :

  • A monomer of carbohydrates is more commonly known as a monosaccharide

  • Monosaccharide units are the constituents of all sugars and starches

  • The most important monosaccharides are the following 3 sugars :
    1. glucose
    2. fructose
    3. galactose

Now let’s review what a plant cell is made of : the different organelles, such as the chloroplast, the mitochondrion and the vacuole are encased in a cell wall and a cell membrane.

  • The cell wall is the external layer of the cell and is a non-living component. Its purpose is to protect the cell from osmotic and mechanical stress, and to give support to the cell. It is absent in animal cells.

    Its main components are proteins, water and cellulose, which is a polysaccharide composed of β-1,4 linked glucose units, and thus making item 1 correct. Cellulose is not digested by humans, and is instead used as fiber, which helps to have adequate bowel movements.

  • The cell membrane (or plasma membrane) is located between the cytoplasm and the cell wall. Its role is to regulate the transport of molecules entering and exiting the cell. Similarly, it is made of proteins, phospholipids and carbohydrates, and thus making item 2 correct as well. The carbohydrates are involved in cell recognition and adhesion.

  • The nucleolus is where the genetic information is stored. It is made of proteins, DNA and RNA. Both DNA and RNA are made of ribose (or deoxyribose in the case of DNA), which is a type of sugar, making item 3 correct as well.

  • Knowing that at least these 3 items are correct, we can already determine that the right answer is D. But why is 4 also correct ? The mitochondria is an organelle that produces ATP from pyruvate (the bio-product of glycolysis), but also glycoproteins. Those can also be found in the structures of the different transport channels on the mitochondrial membrane.