How Does Food Become Energy, and Become You?: Going Further 9–12

Two jobs for every meal

You know from Grade 10 how food reaches your cells: the digestive system breaks it into small, soluble molecules, such as glucose from starch, amino acids from proteins, and fatty acids from fats, and the blood carries them to every cell Grade 10: Blood circulation in the human body. Inside the cell, these small molecules have two jobs.

Fuel. Some are broken down with oxygen, to carbon dioxide and water, and the energy that comes out runs the cell. This is cellular respiration.
Building material. Others are put together into the big molecules a cell is made of: proteins, fats, DNA, and stores such as glycogen. Making the molecules of life from smaller ones is biosynthesis (bio, life, and synthesis, putting together).

The two jobs are linked. Building takes energy, and that energy comes from respiration. And the building material itself is mostly the same sugar: its carbon, hydrogen and oxygen atoms end up in all your molecules.

A brass plate with a mound of white rice, a papad, and small bowls of dal, green saag, vegetable curry, yoghurt and cut fruit, with slices of tomato and lime — **Figure 20.1** Dal bhat tarkari. The rice gives mostly starch (fuel, and carbon for building); the dal gives protein, and with it the nitrogen your body can’t get any other way. Photo: Subhashish Panigrahi, CC BY-SA 4.0

A growing teenager eats a plate of rice. What happens to the starch in it?

Pick an answer to see why.

Respiration: what goes in, what comes out

You met the word equation in Grade 8 Grade 8: Mitochondria and the balanced one in Grade 9, where it is an exothermic reaction Grade 9: Exothermic reaction:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \longrightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy}$

Treat this equation as a model: a simple picture that leaves out the details but gets the important things right. It says what goes into a cell and what comes out.

Figure 20.2 Respiration as inputs and outputs. Glucose and oxygen come into a cell from the blood; carbon dioxide and water go back to it. The energy comes out as ATP, which the cell spends on its work, and as heat. A mitochondrion is drawn cut open, much larger than the others (a real one is about 1–2 µm long, in a cell about 20 µm across): a smooth outer membrane and a folded inner one. Glucose itself never enters a mitochondrion: it is first split into two halves in the cytoplasm, and the halves are broken down the rest of the way inside the mitochondria, where most of the energy is caught. Not to scale.

Count the atoms on each side.

Atom	In (glucose + 6 O₂)	Out (6 CO₂ + 6 H₂O)
carbon	6	6
hydrogen	12	12
oxygen	6 + 12 = 18	12 + 6 = 18

No atom is made or lost: they only change partners. So the mass going in equals the mass coming out. That is the first thing the model tells us. The second is about energy, in the next section.

Worked example

Following the mass

Your cells respire 180 g of glucose (one mole: C = 12, H = 1, O = 16). What mass of oxygen do they use, and what masses of carbon dioxide and water do they make? Show that mass is conserved.

Answer

Molar masses: glucose, C₆H₁₂O₆ = 6 × 12 + 12 × 1 + 6 × 16 = 180 g; O₂ = 32 g; CO₂ = 12 + 32 = 44 g; H₂O = 2 + 16 = 18 g.

From the equation, 1 mole of glucose uses 6 moles of oxygen and makes 6 moles of each product.

In: 180 g of glucose + 6 × 32 = 192 g of oxygen = 372 g.

Out: 6 × 44 = 264 g of carbon dioxide + 6 × 18 = 108 g of water = 372 g.

The same. Notice that most of the mass leaves as carbon dioxide, a gas: 264 g of it, more than the glucose you started with, because each carbon atom has picked up two oxygen atoms.

Activity

Do you breathe out more carbon dioxide after exercise?

Make lime water from chuna, as in Grade 10 Grade 10: Lime water from the paan shop, and pour the same amount, about 2 cm deep, into two clean glasses. You also need a straw and a clock.

Sit quietly for five minutes. Then breathe in through your nose and out gently through the straw into the first glass, at your normal pace. Count how many breaths it takes for the lime water to turn clearly milky.
Now skip, or run up and down stairs, for two minutes. At once, do the same with the second glass, breathing out through the straw at your normal (now faster) pace. Count the breaths again, and time both.
Compare. Repeat on another day to see if you get the same result.

Safety: chuna is caustic: ask an adult to help make the lime water. Keep it away from your eyes, wash your hands, and always blow out through the straw, never suck in.

What you should see

After exercise the lime water usually turns milky in fewer breaths and less time. Your working muscles were respiring faster, so you were making more carbon dioxide each minute and breathing it out faster. (A fair test needs the same amount of lime water, the same straw and the same gentle breaths; you will see the difference in time more clearly than in breaths.)

Where does the energy come from?

The equation says energy comes out. But where from? Chemistry gives a clear answer, and you have the tools for it in Why Do Some Reactions Get Hot? (Breaking and making): breaking a bond always takes energy in; making a bond always gives energy out. In respiration, all the bonds in glucose and in the oxygen molecules are broken, and new bonds are made in carbon dioxide and water.

Figure 20.3 Respiration as structural formulas. Glucose is drawn as the ring it forms in water, seen tilted (the thick edges are nearer to you). Each line is a bond; a double line is a double bond.

Worked example

Adding up respiration’s bonds

Use these average bond energies (in kJ/mol) to estimate the energy change when one mole of glucose is respired: C–C 348, C–H 416, C–O 358, O–H 463, O=O 498, C=O (in CO₂) 804. Count the bonds in Figure 20.3.

Answer

Break (energy in): the bonds in glucose and in 6 O₂.
5 C–C = 5 × 348 = 1,740
7 C–H = 7 × 416 = 2,912
7 C–O = 7 × 358 = 2,506
5 O–H = 5 × 463 = 2,315
6 O=O = 6 × 498 = 2,988
Total in: 12,461 kJ.

Make (energy out): 12 C=O = 12 × 804 = 9,648, and 12 O–H = 12 × 463 = 5,556. Total out: 15,204 kJ.

Energy change: ΔH = 12,461 − 15,204 = −2,743 kJ for each mole (180 g) of glucose. More energy comes out of making the new bonds than went into breaking the old ones, so the reaction gives out energy. Burning glucose in a calorimeter gives 2,803 kJ per mole; the estimate from average bond energies is within about 2%.

So the energy doesn’t come out of glucose “breaking”. It comes from the difference: the bonds in carbon dioxide and water are, all together, stronger than the bonds in glucose and oxygen.

Which bonds make the difference? Count the shared pairs of electrons, the glue of every bond (a double bond has two). Before: glucose has 24 bonds, all single, so 24 pairs, and the six O=O bonds have 12 pairs: 36 pairs. After: 12 C=O bonds have 24 pairs, and 12 O–H bonds have 12: 36 pairs again. The same number of pairs; what changes is how tightly they are held.

	Pairs	Energy to break	Per pair (kJ/mol)
glucose	24	9,473	395
6 O₂	12	2,988	249
6 CO₂	24	9,648	402
6 H₂O	12	5,556	463

The weak pairs are in oxygen. The O=O double bond holds its two pairs far more loosely than any other bond here. Respiration takes those loosely held pairs and puts them into strong bonds, and the difference comes out as energy. That is why oxygen is so important to us: it is the half of the fuel that holds the most energy to give.

In respiration, energy is given out because:

Pick an answer to see why.

The energy follows the oxygen

If most of the energy comes from oxygen’s weak bond, then the energy a food releases should depend mostly on how much oxygen it uses, whatever the food. Test the idea.

Pick a food, then change how much of it is burned (in a flame or in your cells, the amounts are the same). The graph shows the oxygen it uses and the energy it releases. Compare the energy per gram of food with the energy per gram of oxygen.

Food burned: 20 g

Oxygen used: 21.3 g
Energy released: 311 kJ
Per gram of food: 15.6 kJ/g
Per gram of oxygen: 14.6 kJ/g

Glucose, the sugar in your blood: 15.6 kJ from each gram. Its molecule already holds 6 oxygen atoms, so it needs less oxygen from the air than a fat does.

Fat gives about 39 kJ per gram, sugar only about 16. Yet per gram of oxygen, all three give close to 14 kJ. A sugar molecule already holds plenty of oxygen of its own (look at all the O’s in glucose), so it needs less from the air. Fat holds very little oxygen, so each gram of it combines with much more, and gives out more energy.

The kilojoules on a food packet come from this. A food’s energy is measured by burning it in pure oxygen in a sealed steel can under water and measuring how much the water warms; then a little is taken off for what the body can’t use. The figures on labels are about 17 kJ per gram of carbohydrate or protein and 37 kJ per gram of fat. (Many packets also give kilocalories, kcal: 1 kcal = 4.184 kJ.)

Energy from a breath

Because the energy follows the oxygen, measuring the oxygen a person uses is a way to measure the energy they use, without burning anything. Every liter of oxygen (measured at 0 °C and normal air pressure) used in respiration releases about 20 kJ, within a few per cent whatever the food. Sports scientists measure an athlete’s energy use this way, with a mask that analyzes every breath.

Catching the energy: ATP

A flame releases all of glucose’s energy at once, as heat and light, at hundreds of degrees. A cell can’t work like that. Instead, respiration happens in dozens of small steps, each helped along by its own enzyme (a protein that speeds up one reaction). At several of the steps, the energy that comes out is used to make a small molecule, ATP (adenosine triphosphate), from ADP (adenosine diphosphate) and phosphate.

You met ATP in Grade 8 as the molecule that carries energy around the cell Grade 8: Mitochondria. Think of it as small change. Glucose is a big note; the cell can’t spend a 1,000-rupee note at a tea stall. So respiration changes each glucose into about 30 ATP, and the cell spends them one at a time, wherever energy is needed:

moving: muscle proteins use ATP to pull;
building: every join in a protein, fat or DNA chain is paid for with ATP (or a molecule like it);
pumping: the cell membrane uses ATP to pump substances in and out, as nerve cells do with sodium and potassium.

When ATP is used, it splits back into ADP and phosphate, which go back to the mitochondria to be joined again. Not all of glucose’s energy is caught: roughly a third to a half ends up in ATP (estimates differ, depending on how it is counted), and the rest becomes heat at once. That heat is not wasted: it is what keeps you warm.

Why does a cell make ATP, instead of using the energy from glucose directly?

Pick an answer to see why.

A black-and-white electron microscope image of two rounded mitochondria, cut across, each with a dark double outline and thin folds of membrane running across its inside — **Figure 20.4** Two mitochondria in a cell from a lung, seen with an electron microscope. The folds of the inner membrane run across them, as in Figure 20.2. Photo: Louisa Howard, Public domain

Food becomes you

Now the second job. You are made of the same few elements as your food, but not in the same proportions.

Element	Share of your body’s mass (roughly)	In sugar?
oxygen	65%	yes
carbon	18%	yes
hydrogen	10%	yes
nitrogen	3%	no
calcium	1.5%	no
phosphorus	1%	no
potassium, sulphur, sodium, chlorine, magnesium and others	about 1.5% together	no

Most of the oxygen and hydrogen are in your water, about 60% of you. Take the water away, and what is left is mostly protein (about 16% of your body), fat (varies, often 15–25%), and the minerals in bone. Very little is sugar.

Every one of these big molecules has a skeleton of carbon, and its carbon, hydrogen and oxygen can all come from sugar. A cell breaks glucose into small pieces, two or three carbon atoms long, and uses them as a kit. But some of the molecules need elements that sugar doesn’t have:

Starch (in plants), glycogen (in animals) and cellulose (the wall of plant cells) are glucose units joined in long chains: carbon, hydrogen and oxygen only.
Fats are glycerol with three fatty acids joined to it. The fatty acids are built two carbon atoms at a time from pieces of sugar: carbon, hydrogen and oxygen only. (The fats in cell membranes also carry phosphorus.)
Proteins are chains of amino acids. Every amino acid has an amino group, –NH₂, with nitrogen in it, and two of the twenty (cysteine and methionine) have sulphur. So proteins need nitrogen and sulphur.
DNA and RNA are chains of nucleotides. Each has a base rich in nitrogen and a phosphate group with phosphorus.

Figure 20.5 What a cell builds from sugar. Glucose gives the carbon, hydrogen and oxygen of every big molecule of life. Proteins also need nitrogen and a little sulphur, DNA and RNA nitrogen and phosphorus: those come from the soil. Respiration of glucose pays for the building, in ATP. Not to scale: a starch or protein chain is hundreds or thousands of units long.

Every big molecule is built the same way: small units joined into a chain. At each join, the chain ends up short of one water molecule, H₂O. (In the cell each join goes by way of ATP or a molecule like it, but that is the overall result.) Taking a chain apart in digestion puts the water back: your gut breaks a protein back into amino acids, and then your cells join them up again in a new order, to make your proteins. How a cell knows which order to use is in How Does a Gene Make a Protein? (A chain that folds).

Pick what to build, then join units one at a time. Watch the water given off at each join, and the atoms in the chain: blue boxes are elements sugar can’t supply.

Units joined: 3 of 8
Water given off: 2
The chain: C₁₈H₃₂O₁₆

Starch: glucose units joined in a chain, thousands long in a real grain of rice. Only carbon, hydrogen and oxygen: a plant builds it from its own sugar.

A cell has plenty of glucose but no nitrogen. Which of these can it not make?

Pick an answer to see why.

Where the nitrogen comes from

Nearly four-fifths of the air is nitrogen, but almost no living thing can use it: the two atoms of N₂ are held by a triple bond, one of the strongest there is. You learned in Grade 9 that plants take nitrogen from the soil instead, as nitrates (and ammonium), made by lightning and by bacteria, some living in the root nodules of beans and lentils Grade 9: Abiotic components. That is why farmers add nitrogen fertilizers such as urea Grade 9: Nitrogenous fertilizers.

Inside the plant, the nitrate is turned into ammonium and attached to small carbon pieces from its sugar. That makes amino acids. A plant can make all twenty this way, from sugar it made in sunlight and nitrogen from the soil.

You can’t. Animals can’t build nitrates into amino acids. Our nitrogen comes in proteins we eat, from plants or from animals that ate plants. Your cells can make about half of the twenty amino acids for themselves, moving nitrogen from one amino acid onto a carbon skeleton made from sugar. The other nine they can’t make at all: these essential amino acids must come in food.

Sugar into fat

Eat more than you use, and your cells keep the extra. A little goes into glycogen, but the body can hold only a few hundred grams of that. The rest becomes fat: the liver breaks the extra glucose into two-carbon pieces and builds fatty acids from them, then joins three to a glycerol. The fat is stored in adipose tissue under your skin and around your organs Grade 9: Adipose connective tissue. Fat is the better store: a gram of it holds more than twice the energy of a gram of sugar, and it is stored without water, while glycogen holds about three times its own mass of water.

Think about it

Where does a buffalo’s ghee come from?

A buffalo eats grass and straw and drinks water. Yet its milk is about 7% fat and 4% protein, and from the milk comes ghee. Where do the carbon atoms in the ghee come from? And the nitrogen in the milk’s protein?

Answer

The carbon comes from the cellulose of the grass: carbon, hydrogen and oxygen only. A buffalo can’t digest cellulose itself, but microbes in its stomach can. They break it down into small acids, two to four carbons long, and the buffalo builds fatty acids from these, two carbons at a time, then joins them to glycerol to make milk fat. The grass got its carbon from carbon dioxide in the air, by photosynthesis. The nitrogen in the milk protein came from the proteins of the grass and of the stomach microbes themselves, and before that from the soil the grass grew in.

The evidence: following the atoms

How do we know any of this? Nobody can watch a single carbon atom go from a grain of rice into a muscle. But scientists have found ways to follow atoms, and the evidence came in steps, each one correcting the last explanation.

A story from science

A willow in a pot

In the 1600s, Jan Baptist van Helmont, a Flemish doctor, wanted to know where a plant’s mass comes from. Most people thought: from the soil. He dried 200 pounds of earth (about 91 kg), put it in a pot, and planted a willow that weighed 5 pounds. For five years he gave it nothing but rainwater, and covered the pot so that no dust fell in. Then he weighed both again. The willow weighed 169 pounds and 3 ounces (about 77 kg), not counting the leaves it had dropped each autumn. The earth, dried again, had lost only about 2 ounces (57 g).

So the tree had not come from the soil. Van Helmont concluded that it came from the water. He was partly right: the hydrogen in a plant does come from water. But the same man had named a new “gas” given off by burning charcoal, his gas sylvestre, which we now call carbon dioxide, and he never guessed that the tree had been built mostly from it. That had to wait more than a century, for experiments that showed plants take in carbon dioxide in light. And the 57 g the soil did lose? Most likely much of it was exactly what this topic is about: the nitrogen, phosphorus, sulphur and other minerals the tree needed for its proteins and DNA.

In the 1930s and 40s, a new kind of evidence arrived: isotope tracers. An isotope of an element has the same chemistry but a different mass (you met isotopes in Grade 10: The trouble with atomic weight), so atoms of a rare isotope can be put into a molecule and then traced, by their extra mass or their radioactivity, wherever they go.

A story from science

Following carbon from the air

After the Second World War, radioactive carbon-14 could be made in nuclear reactors. At Berkeley, in the USA, Melvin Calvin, with Andrew Benson and James Bassham, gave carbon dioxide made with carbon-14 to a green alga, Chlorella, in the light. After a few seconds, or a minute, they killed the algae in hot alcohol, separated the molecules inside on paper, and laid photographic film on the paper: wherever there was carbon-14, the film went dark. After only a few seconds, almost all the carbon-14 was in one small molecule with three carbon atoms. After half a minute, it was in sugars, and also in amino acids and other molecules. The carbon of the air was going into sugar, and from sugar into the building blocks of proteins. Calvin received the Nobel Prize in Chemistry in 1961.

A black-and-white photograph of a smiling man in a tweed jacket, seated in a laboratory in front of racks of glass flasks and tubes, holding a test tube — **Figure 20.6** Melvin Calvin in his laboratory at Berkeley. Photo: "Photolab", Public domain

A story from science

The body that is always rebuilding

Rudolf Schoenheimer was a German biochemist who left Germany in 1933, when the Nazis came to power, and worked at Columbia University in New York. There, with David Rittenberg, he used heavy isotopes as tracers: hydrogen-2 and nitrogen-15, which aren’t radioactive but are heavier than the usual atoms. They fed rats ammonium salts made with nitrogen-15. Soon the heavy nitrogen turned up in the amino acids of the rats’ own proteins: in almost all of them, but not in lysine. So the body can build some amino acids from simple nitrogen, but not lysine (one of the nine essential ones).

They found something more surprising. It had been thought that a grown animal’s body was like a finished house, and that the food it ate was only fuel. But the tracers showed that, even in adult rats that weren’t growing, the body’s proteins and fats were being broken down and rebuilt all the time, with atoms from the food taking the place of old ones. Schoenheimer called it “the dynamic state of body constituents”. He died in 1941; his book with that title came out the next year.

The atoms in you now are not the ones you had a few years ago. You keep your shape, and your molecules keep their patterns, but the atoms in them come and go. The rice and dal you eat this week will be, for a while, part of you.

Think about it

Revise the explanation

Here are three explanations of where a plant’s mass comes from, in the order people held them. For each, say what evidence showed it was wrong or incomplete.

a.“From the soil.”
b.“From the water.” (van Helmont)
c.“From carbon dioxide and water, and nothing else.”

Answer

a.Van Helmont’s willow gained about 77 kg while the soil lost only 57 g: almost none of the mass came from the soil.
b.Later experiments showed that plants take in carbon dioxide in light, and tracers (Calvin’s carbon-14) followed the carbon of carbon dioxide into sugars and then amino acids. Water gives the hydrogen (and, in photosynthesis, the oxygen given off), but most of a dry plant’s mass comes from the carbon dioxide of the air (its carbon, and much of its oxygen).
c.Very nearly right for the mass, but incomplete: proteins need nitrogen and sulphur, DNA nitrogen and phosphorus, which come from the soil (the small mass van Helmont’s soil did lose). A plant given carbon dioxide and water but no nitrogen grows poorly, with yellow leaves, because it can’t make enough protein or chlorophyll.

In short

Food has two jobs in a cell: fuel, in cellular respiration, and building material, in biosynthesis.
Respiration: glucose + oxygen → carbon dioxide + water + energy. No atoms are made or lost, so the mass in equals the mass out; most of the mass leaves as carbon dioxide, breathed out.
The energy comes from breaking the bonds of glucose and oxygen (energy in) and making the stronger bonds of carbon dioxide and water (more energy out). Oxygen’s weak double bond holds most of it, so the energy follows the oxygen used: about 14 kJ per gram of oxygen, or 20 kJ per liter, whatever the food.
The energy is caught in small amounts, as ATP (about 30 for each glucose), and spent on moving, building and pumping; the rest becomes heat.
Big molecules are small units joined, with one water molecule given off at each join. Sugar supplies their carbon, hydrogen and oxygen; proteins also need nitrogen and sulphur, DNA nitrogen and phosphorus, from the soil.
Tracers (carbon-14, nitrogen-15) followed the atoms: from the air into sugar and amino acids, and from food into the body’s own molecules, which are always being rebuilt.

Questions

A student says: “In respiration, glucose is turned into energy.” What is wrong with this?

Pick an answer to see why.

Which pair of substances must a growing bean plant take from its surroundings to make its proteins, besides carbon dioxide and water?

Pick an answer to see why.

a.

Use a model to explain respiration as a chemical process. In your answer, say what goes in and what comes out, what happens to the bonds, and why there is a net release of energy.

Answer

In: glucose (from digested food, carried by the blood) and oxygen (from the lungs, carried by the blood). Out: carbon dioxide and water (carried away by the blood), and energy.
$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \longrightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$
The atoms are rearranged: 6 C, 12 H and 18 O on each side, so mass is conserved. All the bonds in glucose and in the oxygen molecules are broken, which takes energy in (about 12,500 kJ per mole of glucose, from average bond energies). New bonds are made in carbon dioxide and water, which gives energy out (about 15,200 kJ). More comes out than went in, so there is a net release of about 2,800 kJ per mole. Much of the difference comes from oxygen’s O=O bond, whose electron pairs are held much more loosely than those in the C=O and O–H bonds that replace them. In the cell, part of the released energy is caught in ATP and the rest becomes heat.

b.

A person respires 360 g of glucose in a day. a. What mass of carbon dioxide do they breathe out from it? b. What mass of oxygen do they need? c. How much energy is released (2,800 kJ per mole)?

Answer

360 g is 360 ÷ 180 = 2 moles of glucose.

a.2 × 6 = 12 moles of CO₂: 12 × 44 = 528 g.
b.12 moles of O₂: 12 × 32 = 384 g.
c.2 × 2,800 = 5,600 kJ.

c.

A gram of fat releases 39 kJ, a gram of sugar about 16 kJ. Explain why, using the idea that the energy follows the oxygen.

Answer

The energy released in respiration (or burning) comes mostly from the oxygen molecules used, about 14 kJ per gram of oxygen, whatever the food. Sugar molecules already contain a lot of oxygen (C₆H₁₂O₆: one oxygen for every carbon), so each gram of sugar combines with only about 1.1 g of oxygen. Fat contains very little oxygen (tripalmitin, C₅₁H₉₈O₆, has 6 oxygens for 51 carbons and 98 hydrogens), so each gram of fat combines with about 2.9 g of oxygen. More oxygen used means more strong bonds made, and so more energy: about 2.5 times as much.

d.

Explain, using evidence, how the carbon, hydrogen and oxygen of a sugar molecule can end up in an amino acid in your muscle, and where the amino acid’s nitrogen comes from.

Answer

A plant makes sugar from carbon dioxide and water by photosynthesis. Calvin’s experiments with carbon-14 showed the carbon from carbon dioxide appearing in sugars within seconds, and then in amino acids.
In the plant, small carbon pieces from the sugar get an amino group (nitrogen) from the nitrate or ammonium its roots took from the soil, making amino acids, which are joined into proteins.
You eat the plant (dal, for example). Digestion breaks its proteins into amino acids, which the blood carries to your muscle cells.
Your cells join the amino acids into your own proteins. They can also make about half the twenty amino acids themselves, from carbon skeletons made from sugar plus nitrogen taken from other amino acids. Schoenheimer’s nitrogen-15 experiments showed this: nitrogen fed to rats turned up in most of the amino acids of their proteins (but not lysine, an essential amino acid).

So the carbon, hydrogen and oxygen came from sugar (and so, first, from carbon dioxide and water), and the nitrogen from the soil.

e.

A strand of DNA and a molecule of starch are both long chains. List two ways they are alike and two ways they are different, in what they are made of.

Answer

Alike: both are made of small units joined into a chain, one water molecule short at each join; both contain a sugar (glucose units in starch, deoxyribose in each nucleotide of DNA), so both have carbon, hydrogen and oxygen.

Different: starch’s units are all the same (glucose), while DNA has four kinds of nucleotide (with bases A, T, G and C), and their order carries information. Starch contains only carbon, hydrogen and oxygen; DNA also contains nitrogen (in its bases) and phosphorus (in its phosphate groups), which come from the soil.

f.

An athlete uses 3 liters of oxygen a minute while running. How much energy does she release in 20 minutes? If her muscles turn a quarter of it into the work of running, where does the rest go?

Answer

Energy: 3 L × 20 kJ/L × 20 = 1,200 kJ. A quarter, 300 kJ, becomes the work of running; the other 900 kJ becomes heat in her body (some at once in respiration, some in the muscles themselves). She gets rid of it by sweating and by sending more blood to her skin. This is why running makes you hot.

Where to go next

How Does a Gene Make a Protein? You know where the amino acids come from. How does a cell choose their order, from the instructions in its DNA?
How Does Your Body Stay at 37 °C? The heat from respiration keeps you warm. How your body keeps its temperature steady, whether you are sitting in the sun or running in the cold.
Can Life Run Without Oxygen? Yeast making jaand, your muscles in a sprint, microbes in a flooded paddy: how life gets energy from food when there is no oxygen.