Polysaccharides

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Think of a plate of rice and a cotton shirt. Rice is a cereal, full of starch, and cotton is mainly cellulose. Now break both down completely with water, and each one gives only one sugar, glucose. Same building block in both. Yet your body gets energy from rice, and nothing at all from cotton. How is that possible? The answer hides in one small Greek letter, and JEE questions on it come up often. So, let's begin.

0:33Three polymers of glucose

So, look at the figure on screen. Each hexagon, the six-sided shape, is one glucose unit. The top chain and the branched one below it are the two parts of starch, and the orange chain at the bottom is cellulose. Starch, cellulose and glycogen are all polymers of D-glucose. The letter D gives the configuration of the glucose. Okay? Their general formula is C six H ten O five, in brackets, with a small n outside. The small lowered numbers count the atoms, and the n outside the bracket tells us this unit repeats many, many times. Right? Now, glucose itself reduces Fehling's solution, and these chains are made only of glucose units. So tell me, will starch reduce Fehling's solution? No, starch will not reduce it, and in fact all three polysaccharides are non-reducing. The anomeric carbons are used up in the glycosidic linkages, the links between the units. There is only one free hemiacetal group, at the end of a very long chain, and that is too few to give the test. Clear?

1:37Starch: amylose and amylopectin

Let us start with starch. Starch is the storage polysaccharide of plants; you find it in cereals, roots and tubers. It is a polymer of alpha glucose. The Greek letter alpha tells us which anomer of glucose is used. With me? Starch has two components. The first is amylose, the top chain in the figure, and it is only fifteen to twenty percent of starch; the dash between the numbers means to. Amylose is unbranched, one long chain, and its units are joined C one to C four. That means carbon one of one glucose is linked to carbon four of the next. Okay? One amylose chain has two hundred to one thousand alpha D-glucose units, and amylose is soluble in water. Got it? Now look at the second chain in the figure, the one just below amylose. Compare it with the top chain. What is different about it? Yes, exactly, it has branches! This is amylopectin, the second component. And it is the major part, eighty to eighty five percent of starch. So be careful here: the larger share of starch is amylopectin, not amylose. Do not swap these two numbers. Right? Its chains are still joined C one to C four, but at each branch point the link is C one to C six. And unlike amylose, amylopectin is insoluble in water. Clear?

2:56Cellulose: the beta linkage

Now, the orange chain at the bottom of the figure is cellulose. Cellulose is the structural material of plant cell walls. The cotton of our puzzle is mainly cellulose. Okay? And it is the most abundant organic substance of all, right? Cellulose is made of straight chains of beta D-glucose units, joined C one to C four. Beta, the other Greek letter, stands for the other anomer of glucose. With me? Wait a moment. Amylose is also unbranched, and its units are also joined C one to C four. So what is the real difference between starch and cellulose? The difference is the anomer, exactly. Starch is built from alpha D-glucose, so it has alpha glycosidic linkages. Cellulose is built from beta D-glucose, so it has beta glycosidic linkages. That one Greek letter is the whole difference. Clear? Now back to our puzzle. Rice and cotton both give only glucose on hydrolysis. So why can we digest one and not the other? The reason is that our digestive enzymes, such as amylase, break the alpha glycosidic linkages of starch. But we have no cellulase, the enzyme that would break the beta linkages of cellulose, so humans cannot digest cellulose. And that solves our puzzle. Got it?

4:17Glycogen: animal starch

Finally, glycogen, also called animal starch. It is the storage carbohydrate of animals. Okay? It is stored in the liver, the muscles and the brain, and it is also found in yeast and fungi. Right? Now look at the figure again. The label next to the branched chain says glycogen has the same kind of structure as amylopectin, only more highly branched. So glycogen is like amylopectin, a branched polymer, but it is even more highly branched. Got it? Now, glycogen is a store of glucose. So when your body needs glucose, what must happen to this glycogen? Yes, exactly, enzymes break glycogen down to glucose whenever the body needs it.

4:59Example 1: glucose units in amylose of molar mass 81000

Clear? Now, Example one. Read it with me. We have a sample of amylose with an average molar mass of eight point one times ten to the power four grams per mole; the g mol to the power minus one on screen means grams per mole. Find the average number of glucose units in one molecule. Pause the video and try it first. Are you done with it? Okay, let's check. Many students divide straight away by one hundred and eighty, the molar mass of glucose, and get four hundred and fifty. Is that correct? No, that is the trap, so be careful here. Inside the chain, each unit is C six H ten O five, not C six H twelve O six; it is glucose minus one water molecule. So its mass is one hundred and eighty minus eighteen, which gives one hundred and sixty two grams per mole. Remember this number; it works for starch, cellulose and glycogen alike. Okay? Now divide. Eight point one times ten to the power four is eighty one thousand. Eighty one thousand over one hundred and sixty two gives five hundred. So, on average, five hundred glucose units per molecule. Right? And a quick check: five hundred lies within two hundred to one thousand, the range we saw for amylose. That is exactly what we expect. With me?

6:23Example 2: mass of maltose from 162 g of starch

Now a harder one, Example two. Read it with me. Starch is hydrolysed by diastase, an enzyme, only as far as maltose. Find the mass of maltose obtained from one hundred and sixty two grams of starch. Pause the video and try it first. Are you done with it? Okay, let's check. We have one hundred and sixty two grams, and one unit is one hundred and sixty two grams per mole. So we have exactly one mole of glucose units; the small g means grams and mol means moles. Okay? So one mole of units gives one mole of maltose, and the answer is three hundred and forty two grams, right? That is exactly the trap, so slow down here. Maltose is a disaccharide, made of two glucose units. So two units, plus one water molecule, give one maltose, C twelve H twenty two O eleven; the arrow means gives, and its molar mass, three hundred and forty two, is in brackets. Clear? So one mole of units gives only half a mole of maltose. Zero point five times three hundred and forty two gives one hundred and seventy one grams. Right? Notice that we got more mass than we started with, and that is fine. Hydrolysis adds water. Half a mole of water, zero point five times eighteen, is nine grams, and one hundred and sixty two plus nine gives one hundred and seventy one. The mass balances perfectly. Got it?

7:49Quick revision: starch, cellulose and glycogen compared

So, quick revision. Starch is made of alpha glucose: amylose is unbranched, and amylopectin is branched. Cellulose is made of beta glucose, in straight chains, and humans cannot digest it. Glycogen is animal starch, like amylopectin but more highly branched. Now try the practice problems on this page. All the best!