What Happens to the Dal Bhat You Eat?

In What Does a Molecule Look Like? you saw that a molecule is a group of atoms held together. Water has 3 atoms. The molecules in food are much bigger: many of them are long chains, built from small units joined end to end, like the beads on a mala.

• Starch, in rice, wheat, maize and potatoes, is a chain of glucose units. Glucose is a sugar. One starch molecule can have thousands of glucose units, some chains with branches.
• Protein, in dal, eggs, milk, meat and soybeans, is a chain of units called amino acids. There are 20 kinds of amino acid, and a protein usually has hundreds of them, in a special order, folded up into a shape.
• Fat, in ghee, oil and nuts, is made of a small molecule called glycerol with three long fatty acids joined to it.
• Fiber, in saag, fruit, the husk of grains and the skins of beans, is the part of plant food your body can’t digest. Much of it is cellulose: a chain of glucose too, like starch, but its glucose units are joined by a different kind of link.

A big molecule like starch can’t pass through the wall of your gut, and your cells can’t use it as it is. So the first job is to take the chains apart into their units. Taking food apart into small molecules that can pass into the blood is called digestion.

You learned in Grade 6 the parts of the digestive system: the mouth, the food pipe, the stomach, the small and large intestines, and the glands that help. In Grade 8 you saw how muscle in the walls of the gut squeezes food along, and how glands make juices with enzymes in them Grade 8: Cell, tissue and organ. Here is what those enzymes do to the molecules.

An enzyme is a protein that makes one particular chemical reaction happen fast. Each digestive enzyme cuts one kind of link. It holds a chain, and a water molecule joins in at the link, splitting it. One link cut uses up one molecule of water. Then the enzyme lets go, unchanged, and cuts the next link, over and over again.

Different enzymes work in different parts of the gut:

Where	What happens
mouth	Teeth grind food into small lumps. Saliva wets it, and its enzyme, amylase, starts cutting starch into shorter chains.
stomach	The stomach churns the food with acid and the enzyme pepsin, which starts cutting proteins into shorter pieces. The acid also kills most germs in food.
small intestine	Juice from the pancreas has enzymes for starch, protein and fat. Bile from the liver breaks fat into tiny droplets, as soap does to grease Grade 10: Soap, so the fat enzymes can reach it. Enzymes on the gut wall finish cutting starch and protein. At the end there are single glucose units, single amino acids, and fatty acids and glycerol.
large intestine	Only fiber and what can’t be digested arrive here. Bacteria living in it feed on some of the fiber, and the wall takes back water. What is left leaves as feces.

Once a meal has been cut into small molecules, they have to get out of the gut and into the blood. This happens in the small intestine, through its wall. Taking digested food through the gut wall into the blood is called absorption.

The wall of the small intestine isn’t smooth. Its inside is covered in millions of tiny finger-like bumps called villi (one is a villus), each about half a millimeter to a millimeter tall. Each villus has tiny blood vessels inside it, very close to the surface. The small molecules pass through the thin wall of the villus into the blood. (Most of the fat’s pieces go first into tiny vessels of another liquid, the lymph, which later empties into the blood.) The big molecules can’t pass, which is why they must be digested first.

From the small intestine, the blood carries the glucose, amino acids and other small molecules round the whole body, to every cell, as you will learn in Grade 10 Grade 10: Blood circulation in the human body. A meal takes about 6 to 8 hours to pass through the stomach and small intestine, and then spends a day or two more in the large intestine.

The blood brings small molecules to every cell. Now comes the surprising part: the cell joins them back together, into new big molecules, but not the same ones you ate. It builds the molecules it needs.

Take protein. The protein in dal is a dal plant’s protein, made for a seed. Your body can’t use it as it is. But once it is cut into amino acids, your cells can join those same amino acids in a new order, to make your own proteins: the proteins in your muscles, your skin, your hair and your nails, and enzymes like amylase. The ribosomes in your cells do the joining, following instructions from the cell’s DNA Grade 8: Ribosomes. It is like taking apart a toy house made of plastic bricks and using the same bricks to build a bus.

The same happens to the other pieces:

• Glucose units are joined into glycogen, a branched chain like starch, stored in your liver and muscles to use between meals.
• Fatty acids and glycerol are joined back into fat, stored under your skin and around your organs.
• Spare glucose can even be rearranged into fat. The atoms of the glucose are taken apart and put together into fatty acids, a different kind of molecule. It is one way a person who eats much more rice and sweets than they burn puts on fat.

Making new big molecules from the small ones is how you grow, and how you repair yourself: replacing worn skin, healing a cut, mending a muscle after hard work. Even a grown-up who stops growing keeps replacing molecules and cells every day, so everyone needs food to build with.

Most of the food you eat is not built into you. It is used for energy: to keep your heart beating and your brain thinking, to move, to keep you warm at about 37 °C, and to run the building work itself (joining molecules takes energy too).

You learned in Grade 7 and Grade 8 where this energy comes from: in the mitochondria of each cell, glucose reacts with oxygen and gives carbon dioxide and water, and energy is released Grade 8: Mitochondria. This is respiration, and it happens in every living cell, all the time.

$$\text{glucose} + \text{oxygen} \longrightarrow \text{carbon dioxide} + \text{water} + \text{energy}$$

Now look at it as a chemist would. The glucose and oxygen molecules are taken apart into their atoms, and the atoms are put back together in a new way, as carbon dioxide and water. No atom is lost and none is made. But in the new arrangement, the atoms hold on to each other more tightly than before. In Can You Make a Hand Warmer? you saw that pulling atoms apart takes in energy, and joining them gives it out. Here, joining up gives out more than pulling apart took in, and that extra is the energy you get. Respiration is an exothermic change, like burning Grade 9: Exothermic reaction.

Burning slowly

If you dry rice and set fire to it, it burns: it reacts with oxygen, gives carbon dioxide and water, and gives out all its energy at once, as heat and light. In your cells the overall change is the same, and so is the total energy, but it happens in many small steps, each helped by its own enzyme. So the energy comes out a little at a time, and much of it is caught and stored in small molecules called ATP, which carry it to wherever in the cell it is needed. The rest comes out as heat, which keeps you warm.

How much energy is in food?

Food packets in Nepal list the energy in a food in kilojoules (kJ), often with kilocalories (kcal, or Calories) beside them; 1 kcal is about 4.2 kJ.

Now you can put the whole story together, in one model.

In every step, the molecules are taken apart and their pieces put together in new ways. Nothing is made from nothing, and nothing disappears: the atoms in your dal bhat end up either in you, or back in the air, the water and the soil.

So how much of the food you eat becomes you? You can work it out.

A plant does the same two things with its food, but it makes the food itself. In Why Do Two Plants From the Same Field Grow So Differently? you saw that a green plant joins carbon dioxide and water into sugar by photosynthesis, using the energy of sunlight. Then, like you:

• it builds the sugar into new molecules: cellulose for the walls of new cells, starch to store, and, with nitrogen from the soil, proteins;
• it burns some of the sugar in respiration, in its own mitochondria, day and night, to get energy to live and grow.

The starch in a grain of rice was stored by the rice plant for its baby plant, the seed’s embryo. When a seed sprouts, its own enzymes cut that starch into glucose, which the seedling uses for energy and to build its first root and shoot. When you eat rice, you take that store for yourself. And because the plant made its sugar with the energy of sunlight, the energy you get from your dal bhat came, in the end, from the Sun Grade 9: The Sun: the ultimate source of energy.

• How Does a Touch Reach Your Brain? How your senses send messages along nerves to your brain, on the energy your cells get from food.
• Why Are There So Few Tigers? Follow energy and matter beyond one body: from plants to deer to tigers, and back to the soil and air.
• Why Do Two Plants From the Same Field Grow So Differently? Read it here: how genes and surroundings decide how much a plant or animal grows.
• Going further still: How Does Food Become Energy, and Become You? in Going Further 9–12: where the energy in food really comes from, how ATP catches it, and how sugar and nitrogen from the soil become your muscles, fat and DNA.