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.
Correct — C, Starch is present in plant cells and glycogen is present in animal cells. Both are polymers of glucose and both are storage forms of it, but each kingdom uses its own. NCERT states it plainly in the Class 10 chapter on life processes: the carbohydrates a plant makes and does not use at once 'are stored in the form of starch, which serves as the internal energy reserve', and then, of ourselves, 'some of the energy derived from the food we eat is stored in our body in the form of glycogen'. Starch accumulates in leaves, seeds and tubers, which is why the iodine test on a leaf is the standard classroom proof of photosynthesis. Glycogen is stored in liver and muscle, and its branching is far heavier than starch's, so a great many chain ends are exposed at once and glucose can be released quickly when an animal needs it.
- (a)Both starch and glycogen are found in plant cells. — Puts glycogen in the wrong kingdom. A plant already has starch for storage; glycogen is the animal, and fungal, equivalent.
- (b)Both starch and glycogen are found in animal cells. — The mirror error. Animals do not synthesise starch — we eat it, digest it to glucose with amylase, and store what we do not burn as glycogen.
- (d)Both starch and glycogen are present in plant cells as well as animal cells. — The tempting all-of-them pick, and it erases the very distinction being tested. If both occurred everywhere, the iodine test on a leaf would prove nothing about photosynthesis.
Polysaccharides are long chains of monosaccharide units. Three of them are built entirely from glucose and differ only in how the units are joined and how much the chain branches. Starch is the plant's store, a mixture of the largely unbranched amylose and the branched amylopectin. Glycogen is the animal store, branched more densely still. Cellulose is the plant's structural material, a straight chain whose different linkage human enzymes cannot break — which is why we digest starch but not the cellulose in the same vegetable.
The item looks like a memory question and is really a test of whether the two words have been kept apart. A quick way to fix them is by where you meet them: starch in rice, potato and wheat, all plant products; glycogen in liver, which is meat. The iodine test is the other anchor — starch turns blue-black with iodine, and NCERT's activity of destarching a plant in the dark and then testing a leaf depends on the fact that a plant stores starch and only starch. One nuance for later study is that glycogen is not exclusive to animals; fungi and bacteria store it too. Nothing in this question turns on that, since the four options only offer plants and animals.
- Starch and glycogen are both polymers of glucose, and both are energy stores.
- Plants store surplus carbohydrate as starch, in leaves, seeds, tubers and roots.
- Animals store it as glycogen, chiefly in the liver and in skeletal muscle.
- Glycogen is more highly branched than starch, so glucose can be mobilised from it faster.
- Cellulose is also a glucose polymer, but a structural one, and human enzymes cannot digest it.
- Marking the all-of-them option because both molecules are made of glucose; the storage forms are kingdom-specific.
- Assuming animals can make starch because they can digest it.
- Confusing cellulose with starch — same monomer, different linkage, and only one of them is food for us.
As a which-statement-is-correct item on storage molecules, as a match of biomolecule to type, or through the iodine test in a photosynthesis question.
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 third member of the same trio. UPSC sets starch against cellulose where CDS sets it against glycogen, and both turn on the fact that identical glucose units can be assembled into molecules with quite different behaviour.
- practice — not a real PYQ
Glycogen is stored in the human body mainly in
- (a)the liver and skeletal muscle
- (b)the kidneys and spleen
- (c)adipose tissue
- (d)the pancreas
Answer(a) the liver and skeletal muscle — liver glycogen maintains blood glucose, while muscle glycogen fuels the muscle itself.
- practice — not a real PYQ
Which one of the following polysaccharides cannot be digested by human beings?
- (a)Starch
- (b)Glycogen
- (c)Cellulose
- (d)Maltose
Answer(c) Cellulose — humans have no enzyme for its linkage between glucose units, so it passes through as dietary fibre.