Which of the following category of biomolecules does Starch belong to?
- (a)Protein
- (b)Carbohydrate
- (c)Lipid
- (d)Nucleic acid
Correct — B, Carbohydrate. The quickest way to classify any biomolecule is to ask what it is built from. Starch is built from glucose units joined end to end, and a polymer of sugar units is a carbohydrate — specifically a polysaccharide. That is what makes starch the storage form of food in plants: a plant makes glucose by photosynthesis and stores the surplus as starch in seeds, tubers and roots, where it can be broken back down to glucose when needed. A protein would be a chain of amino acids, a lipid would be built on fatty acids and glycerol, and a nucleic acid would be a chain of nucleotides; starch contains none of these. Starch also carries the classic laboratory signature of its class — it turns blue-black with iodine solution.
- (a)Protein — Proteins are chains of amino acids and always contain nitrogen. Starch is made only of carbon, hydrogen and oxygen, has no nitrogen and no amino acids, so it cannot be a protein.
- (c)Lipid — Lipids are fats and oils, built from fatty acids and glycerol, and they do not dissolve in water. Plants do store food as oil in some seeds, but starch is not that store — it is the sugar polymer, not the fat.
- (d)Nucleic acid — Nucleic acids are DNA and RNA, chains of nucleotides that carry genetic information and contain both nitrogen and phosphorus. Starch carries no information and contains neither element.
Living matter is built from four great classes of large molecule: carbohydrates, proteins, lipids and nucleic acids. Each class has its own building block — sugars for carbohydrates, amino acids for proteins, fatty acids and glycerol for most lipids, nucleotides for nucleic acids. Carbohydrates themselves come in three sizes: single sugars such as glucose and fructose, double sugars such as sucrose, maltose and lactose, and polysaccharides such as starch, glycogen and cellulose.
Once the monomer test is in hand this item takes seconds, and the same test disposes of a whole family of questions. The one place candidates slip is in confusing the three common glucose polymers, because all three are carbohydrates made of the same unit but do different jobs: starch is the plant's food store, glycogen is the animal's food store in liver and muscle, and cellulose is the structural material of the plant cell wall. The difference lies in how the glucose units are linked, which is also why humans can digest starch but not cellulose, and why starch gives a colour with iodine and cellulose does not.
- Starch is a polysaccharide — a polymer of glucose — and therefore a carbohydrate.
- It is the storage carbohydrate of plants; glycogen plays the same storage role in animals.
- Cellulose is also a glucose polymer but a structural one, and the human gut cannot digest it.
- Starch gives a blue-black colour with iodine solution, which is the standard school test for it.
- Salivary amylase, called ptyalin, begins the digestion of starch in the mouth.
- Treating cellulose as a non-carbohydrate because it cannot be digested; it is a carbohydrate all the same.
- Mixing up starch and glycogen — both are glucose stores, but glycogen is the animal one.
- Assuming a plant food store must be starch; many seeds, such as groundnut and mustard, store oil instead.
Usually as a straight classification, or as a starch-versus-cellulose or starch-versus-glycogen comparison; the iodine test and the plant-store versus animal-store split are the two recurring hooks.
Which one of the following statements regarding starch and cellulose is NOT correct?
- (a) Both of them are of plant origin
- (b) Both of them are polymers
- (c) Both of them give colour with iodine
- (d) Both of them are made up of glucose molecules
Answer(c) Both of them give colour with iodine
The next question up from this one. Both starch and cellulose are plant-made glucose polymers, so both are carbohydrates, but only starch gives the blue-black colour with iodine. Classify by the monomer first, then separate the two by their linkage and their behaviour with iodine.
Which one of the following statements about starch and glycogen is correct?
- (a) Both starch and glycogen are found in plant cells.
- (b) Both starch and glycogen are found in animal cells.
- (c) Starch is present in plant cells and glycogen is present in animal cells.
- (d) Both starch and glycogen are present in plant cells as well as animal cells.
Answer(c) Starch is present in plant cells and glycogen is present in animal cells.
The same molecule tested on where it is found rather than what class it belongs to. Both are glucose polymers and both are carbohydrates; the division is that plants bank their surplus glucose as starch and animals bank theirs as glycogen.
- practice — not a real PYQ
The reagent used in the laboratory to test for the presence of starch in a food sample is
- (a)iodine solution
- (b)Benedict's solution
- (c)dilute hydrochloric acid
- (d)lime water
Answer(a) iodine solution — starch gives a blue-black colour with iodine, which is the standard school test for it.
- practice — not a real PYQ
Which one among the following is the storage carbohydrate of animals?
- (a)Starch
- (b)Cellulose
- (c)Glycogen
- (d)Sucrose
Answer(c) Glycogen — animals store surplus glucose as glycogen in the liver and muscles, the role starch plays in plants.