Both starch and glycogen are made up of glucose, however:

Correct — B, glycogen is made in animal cells, but starch is made in plant cells. Both are storage polymers built from the same monomer, alpha-glucose, joined by the same alpha-1,4 links with alpha-1,6 links at the branch points. What separates them is who makes them and how tightly they are branched. Plants store surplus glucose as starch, a mixture of unbranched amylose and moderately branched amylopectin, laid down as granules in seeds, tubers and roots. Animals store it as glycogen, which branches far more often — roughly every eight to twelve residues against every twenty-four to thirty in amylopectin — and is held mainly in liver and skeletal muscle. The heavy branching matters: it creates many free ends for enzymes to work on at once, so glycogen can be broken down quickly when muscle demands fuel, while the liver's store buffers blood glucose between meals.

• (a)starch is present in liver and glycogen is present in red blood cells. — Both halves are wrong. The liver stores glycogen, never starch, and mature mammalian red blood cells carry no such store — they have lost their nucleus and mitochondria and run on glucose taken from the plasma.
• (c)both starch and glycogen are present in animal and plant cells. — If both polymers were in both kinds of cell there would be nothing to distinguish them, and the word 'however' in the question would have nothing to introduce. No animal tissue synthesises starch.
• (d)both starch and glycogen are present in plant cells only. — This would leave animals with no carbohydrate reserve at all, which cannot be right — the muscle and liver stores of glycogen are what carry a body through exercise and through the hours between meals.

Polysaccharides made of glucose fall into two functional classes. Storage forms use alpha-glucose, which makes a coiled, easily attacked chain: starch in plants, glycogen in animals and fungi. Structural forms use beta-glucose, which makes flat, straight chains that stack into tough fibres: cellulose in plant cell walls, and chitin, a modified version, in fungal walls and arthropod exoskeletons. The difference between alpha and beta at a single carbon is the reason a human can digest a potato but not the paper it is wrapped in.

The word 'however' in the stem is doing the real work. It tells you the answer must state a contrast, so options (c) and (d), which assert sameness, are unlikely before you have thought about biology at all. Then the contrast has to be the right one: the two polymers differ by the organism that makes them, not by the tissue within one organism, which is what option (a) tries to sell. It is worth remembering that fungi also store glycogen, so 'animal' is a convenient shorthand rather than an exclusive claim; the exam's contrast is with plants, and against plants it holds.

• Starch is a mixture of amylose, which is unbranched, and amylopectin, which branches about every twenty-four to thirty glucose units.
• Glycogen branches about every eight to twelve units, giving many chain ends and a faster rate of mobilisation.
• The liver's glycogen maintains blood glucose between meals; muscle glycogen is used only by the muscle that stores it.
• Iodine gives a blue-black colour with starch and a red-brown colour with glycogen, which is a standard way to tell them apart.
• Cellulose is built from beta-glucose, which is why it is structural rather than a store, and why humans cannot digest it.

• Placing starch in the liver — the liver's store is glycogen, and the two are never interchangeable.
• Assuming mature red blood cells store fuel; they have neither nucleus nor mitochondria.
• Forgetting that fungi also make glycogen, so the animal-plant split is a contrast rather than an exclusive rule.

Asked as a completed sentence, where the stem grants the shared chemistry and the options compete to state the difference.

• practice — not a real PYQ

  Glycogen differs from amylopectin chiefly in that glycogen

  • (a)is made of beta-glucose
  • (b)is much more highly branched
  • (c)contains no alpha-1,4 links
  • (d)is a structural rather than a storage polymer

  Answer(b) is much more highly branched — branch points every eight to twelve residues against every twenty-four to thirty in amylopectin, which is what lets it be mobilised quickly.

• practice — not a real PYQ

  Which one of the following polysaccharides cannot be digested by human beings?

  • (a)Starch
  • (b)Glycogen
  • (c)Cellulose
  • (d)Amylopectin

  Answer(c) Cellulose — its beta-1,4 links need an enzyme humans do not make, so it passes through as dietary fibre.