Part C: Concept Checks

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Starch, from cereals like rice and wheat, and cellulose, from the cell walls of plants, are both long chains of the very same unit, D-glucose. Your body gets energy from starch every day. But humans cannot digest cellulose at all. Same building block, so why such different behaviour? Hold that thought, because Example two will answer it. This video is Part C of the unit problem bank, the concept checks: short questions where every answer needs a reason. Questions like these come up often in JEE, so let's begin.

0:38Check 1: why sugars reduce, and why sucrose does not

So, check number one has two questions on sugars. First, why do all monosaccharides, even ketoses, act as reducing sugars? Remember, a reducing sugar is one that reduces Tollens' reagent and Fehling's solution. Okay? Now, glucose is an aldose, so it has an aldehyde group. But fructose is a ketose, with a keto group and no aldehyde group at all. So fructose cannot reduce Tollens' reagent, can it? It can, and that is the trap. There are two reasons. First, the ring of every monosaccharide can open, to give a free carbonyl group, C double bond O, where the equals sign is the double bond. Right? Second, a ketose needs one more step. In alkali, as in the Tollens' and Fehling's reagents, fructose rearranges through an enediol into aldoses, glucose and mannose. And those aldoses do the reducing. With me? But careful here: fructose is not oxidised by bromine water, while glucose is, so bromine water can tell the two apart. Now the second question, on sucrose. Sucrose is made of glucose and fructose, and both of them reduce. So why does sucrose show neither reducing properties nor mutarotation? Look at the anomeric carbons, the ring carbons that let a ring open. They are carbon one of glucose and carbon two of fructose, written C dash one and C dash two on screen. In sucrose, both of them are part of the glycosidic linkage. Okay? So neither ring can open to give a free carbonyl group. Sucrose is non-reducing, and it shows no mutarotation either. Got it?

2:18Check 2: amino acids behave like salts

Now check number two, on amino acids. The figure shows the forms of an amino acid at different p H, with p H rising along the arrow at the bottom. The labels on the small arrows, O H minus and H plus, mean adding base or adding acid. An amino acid carries a carboxylic acid group, COOH, and an amino group, N H two. And yet it behaves like a salt, not like an acid or an amine. So why is that? Because the molecule reacts with itself. The COOH group gives a proton to the N H two group of the same molecule. Okay? This gives the zwitterion, H three N plus, CHR, COO minus, in the middle box of the figure. R is the side chain, the small plus and minus are the charges, and the dashes are bonds. So one end is positive and the other negative, a dipolar ion. Right? In the crystal, these dipolar ions hold one another by strong ionic attractions, just like the ions of a salt. Clear? So, like salts, amino acids are crystalline solids of high melting point, soluble in water. Now the second question. In neutral solution, the ordinary tests for the amino group and the acid group do not work. Which groups are really there? Yes, exactly, it is the zwitterion again. The groups are present as N H three plus and COO minus, not as free COOH and N H two. So their ordinary tests fail. Got it?

3:59Check 3: the sequence, and denaturation

Now check number three, on proteins. The figure shows the four levels of protein structure: primary, secondary, tertiary and quaternary. The primary structure is the sequence, the beads on the left, each bead an amino acid with a short name, like Gly for glycine. They are joined by peptide bonds, which are covalent. Now, why does this sequence matter so much? Because the sequence decides how the chain folds, into the alpha helix and beta sheet you see, and then into the overall shape. And the shape decides the function. The arrows on screen mean leads to. Okay? So even a single change in the sequence can destroy the activity of the protein. Right? Now the second question. Boil an egg, and the egg white coagulates. That is denaturation, and the protein loses its biological activity. So does boiling break the peptide bonds? It does not, and that is the trap. Heat, or a change of p H, breaks only the weak hydrogen bonds, written H-bonds, and other non-covalent forces. So the helices uncoil and the globules unfold. Clear? But the covalent peptide bonds need much harsher conditions. So the sequence stays, and the primary structure is unchanged. Got it?

5:21Example 1: why the two DNA strands are complementary

Now the two examples. Example one: why are the two strands of DNA complementary, and not identical? The figure shows a short piece of DNA, two strands joined across by pairs of bases. Pause the video and try it first. Done? Okay, let's check. The letters are the bases, adenine, thymine, guanine and cytosine, written A, T, G and C on screen. Pairing is specific: adenine pairs only with thymine, through two hydrogen bonds, the dashed lines, and guanine only with cytosine, through three. Okay? So the left strand reads A, G, T and C, from the top. Many students say the other strand is the same. Now read the right strand from the top. Is it really the same? No, it reads T, C, A and G, each base the partner of the one opposite it. Not the same sequence at all, so identical is the trap. Right? So the sequence of one strand fixes that of the other: complementary, not identical. And five prime and three prime at the ends, with the small dash read as prime, show that the strands run in opposite directions.

6:42Example 2: starch vs cellulose, alpha vs beta linkage

Clear? Now Example two, a step harder, and it is our opening puzzle. Starch and cellulose are both polymers of glucose. So why are their properties so different? In the figure each hexagon is a glucose unit: starch, as amylose and amylopectin, and glycogen are at the top, and cellulose is at the bottom. Pause the video and try it first. Done? Okay, let's check. Look at amylose and cellulose. Both are unbranched chains of glucose units, joined C one to C four. So where is the difference? The difference is alpha versus beta, the Greek letters on screen. Starch has alpha-linkages, and these give coiled or branched chains. In amylopectin, the branches join through C one to C six. Okay? Enzymes can attack these chains, so our body digests starch. Right? Now cellulose is made of beta-D-glucose units, and its beta-linkages give straight chains. With me? Straight chains pack side by side, with many hydrogen bonds between them, the H-bonds on screen. Clear? So cellulose forms strong, insoluble fibres, the structural material of plant cell walls. And humans have no enzyme to hydrolyse the beta-linkage. That solves our puzzle: the same glucose, but a different linkage, and so a completely different polymer. Got it?

8:08Quick revision: sugars, amino acids, proteins, DNA, starch

So, quick revision. Monosaccharides reduce because the ring opens, and ketoses change into aldoses in alkali. Sucrose is non-reducing because both anomeric carbons are in the linkage. Amino acids are zwitterions, so they behave like salts. Denaturation breaks only the weak forces, and the sequence stays. In DNA, adenine pairs with thymine and guanine with cytosine, so the strands are complementary. And starch has alpha-linkages, while cellulose has beta-linkages. Now go and try the practice problems on this page. All the best!