Building and breaking down carbohydrates, proteins and lipids

If you drink four protein shakes a day, where does all that extra protein go? The tempting answer is muscle: more protein in, more muscle built. But the body has no store for protein, the way it stores fat. Protein beyond what it needs cannot be kept for later. Part of it ends up leaving in your urine, as something that is not protein at all. Following that protein means learning the two directions food molecules travel in the body.

A monomer is a small unit, small enough to be absorbed and carried round the body. A polymer is many monomers joined into a chain, big enough to be stored or to hold a shape. Glucose is a monomer, a simple sugar. Join many glucose units and you get starch, glycogen or cellulose: three carbohydrates, three different polymers from the same unit. Starch is how a plant stores glucose. Glycogen is how an animal stores it. Cellulose builds the walls of plant cells. Now predict: why store glucose as starch or glycogen, and not as loose glucose? A big chain barely dissolves, so it stays where it is put. Loose glucose is small and soluble, so it would dissolve and drift away. Eight substances: glucose, starch, an amino acid, a protein, cellulose, glycerol, a fatty acid and glycogen. Sort them into small units and chains, then join each unit to every chain it builds. Which of the eight are monomers, and which polymers does each monomer build? The monomers are glucose and an amino acid; glycerol and a fatty acid are small units too. The polymers are starch, a protein, cellulose and glycogen. Glucose builds three of them: starch, glycogen and cellulose. Amino acids build proteins. Glycerol and fatty acids build no chain at all; one glycerol with three fatty acids makes a lipid. A protein chain uses many different kinds of amino acid, joined in a set order. One examiner's report, on a Foundation paper, describes a question of this kind. This question was the most challenging in the multiple choice Section A. Most candidates did not recognise that simple sugars make up complex carbohydrates. Fatty acids and amino acids were the most incorrectly chosen answers. Learn each pair as a direction instead: glucose builds the carbohydrates, amino acids build proteins, and fatty acids with glycerol build lipids.

Enzymes run these reactions in both directions. They join monomers into polymers, and they break polymers back down into monomers. Both directions need enzymes, because both reactions would be far too slow on their own at body temperature. Enzymes are specific, so each of these jobs needs its own enzyme. Now a check: an enzyme breaks a protein into amino acids. What has happened to the enzyme? Nothing. It is unchanged and free to work again. Only the substrate protein became amino acids. The enzyme is the tool, not the material. It sits on the arrow, it is not used up, and it is never one of the things being joined or broken. That gets muddled because an enzyme is itself a protein. But the reaction it runs never breaks the enzyme into amino acids. Building also needs energy, and that energy is released by respiration. The video on respiration, aerobic and anaerobic, explains where it comes from. Digestion is the breaking-down direction. The products of digestion are the monomers, and they are what gets absorbed. The body then builds its own polymers from those monomers. That is why eating muscle does not give you muscle: it is broken down to amino acids first. Now try this: a cow eats grass and builds muscle protein. Where did its amino acids come from? From the protein in the grass. Digestion turned it into amino acids, and the cow built its own protein from them. Keep two phrases apart. Breaking bread in half breaks it up, and it is still bread. An enzyme breaking starch into glucose breaks it down, and it is no longer starch. Testing a food for each of these three groups is a required practical, with a video of its own.

It is the products of digestion that are absorbed, never the polymers themselves. The precise sentence is 'the glucose from starch is absorbed', not 'starch is absorbed'. A starch chain is far too big to cross into the blood. Now rewrite 'protein is absorbed' so it is precise. The amino acids from the protein are absorbed, not the protein itself. One examiner's report, on a Higher paper, records both sides of this. It notes that knowledge of the enzymes and their actions was generally good. But it adds this. Many students' answers lacked precision, describing the lack of absorption of starch, fat or protein, rather than of the products of the digestion of these large molecules. The habit that earns it back: name the product. The glucose from starch, the amino acids from protein, the glycerol and fatty acids from fat. Four sentences from one student's answer about a meal being digested and used. The first: the starch is absorbed into the blood. The second: enzymes are broken down into amino acids. The third: fats are broken down into glycerol. The last: the amino acids are used to make new proteins. Which of the four is right as it stands, and what is wrong with each of the others? Only the last. The first should say the glucose from starch is absorbed. The second should say proteins, not enzymes, are broken down into amino acids. The third needs both products, glycerol and fatty acids. A report on a Foundation paper names the second and third slips in its own words. Most candidates mixed up the role of enzymes with amino acids and also could not correctly identify fats are broken down into glycerol and fatty acids. The fix for both: keep the enzyme on the arrow, and say a lipid's two products together, every time.

Back to the four protein shakes. Their protein is digested into amino acids, and amino acids have three possible fates. First, they can be built into the body's own proteins, as many as it needs. Second, they can be used in respiration, to release energy. Now predict the third fate: the body cannot store the extra. What must happen to it? It is broken down. Excess amino acids are broken down to urea, and the urea is excreted, leaving the body in urine. There is no protein store. Fat has a store and glucose has glycogen, but extra protein cannot be kept. Now answer the shakes: why does almost none of a fourth shake become muscle? The body only builds the protein it needs, and earlier meals already supplied that. The extra amino acids are used for energy, or broken down to urea and excreted.

To finish, test yourself on a seed, a lipid and those protein shakes. A seed stores energy as starch. Which monomer will the seedling break it down into? Glucose. Starch is a polymer of glucose, so breaking it down gives glucose back. Now a lipid is digested. Name both products, and how many of each one lipid gives. One glycerol and three fatty acids, from every lipid. One more, on the shakes: why can't today's extra protein be saved for tomorrow? The body keeps no store of protein, so excess amino acids are broken down to urea and excreted.

You're solid on this if monomer and polymer come out without a pause; if so, a thumbs-up, and move on. If the lipid's numbers keep slipping, give it a day and look again; it often settles overnight.

Next in the chapter: Measuring the energy in food by calorimetry. Hopefully that's next in your feed, but if not, search for ScholaFly B I oh three, oh five.

Every other video is on scholafly.com too, easy to search by video code or by keyword. Whatever you're revising, go there and have a look, or, watch the next video.