Hello, dear friend. Christmas is coming. It's time when everyone's summing up what's happened, planning the future, making wishes and dreaming. There's no Christmas without parties with friends, decorated streets, skating and family gatherings. This year was special, and Christmas should be celebrated in a special way. Black Science advises to stay home and take care of those around you.

Despite all the adversities of this year, there's at least one festive habit, that we won't have to kick — the habit of eating. Since we started talking about health, let's try to look at food from a scientific perspective. How does food help us stay energetic? What food really is for our body? Why is there hydrochloric acid in the stomach and why doesn't it digest itself?
For an organism to live on, it needs:
carbs, to receive energy quickly,
fats, to form cell membranes and accumulate energy,
proteins, to build cells, channels, other proteins and to get energy in case carbs and fats are used up.

An organism receives all these nutrients from food. And each nutrient helps cells to get energy to go on functioning effectively and healthily.

Energy for the organism is contained not in proteins, fats and carbs, but in ATP molecules. They phosphorylate and dephosphorylate — attach and lose molecules of phosphoric acid. When a phosphoric acid residue is attached to a molecule, a high-energy bond is formed, and energy is accumulated. When it's lost, energy is released. Each broken bond releases between 40 and 60 kJ of energy.

What is energy for our organism?
Of course nobody counts their ATP molecules, because there are too many of them: they play a role in almost every process of almost every cell in the organism. However, we can estimate how much energy we need for our organism to work as much as necessary. And to calculate the energy we get from food, there's the energy value of every product. It also helps estimate how much work our body should do so that energy from food isn't too little (we don't want to black out) or too much (we don't want to gain weight).

One calorie is an amount of warmth, which is necessary for heating up 1 ml of water by 1°C. And food calories are the amount of energy received after digesting all products.

That's why food calories are called energy value. There are two types of it: full and physiological energy value. To learn the former, food is burnt in a calorimeter and the released heat is measured. And for the latter, a person's energy expenditure while working with the jaws and the heat released during this process are measured in a special sealed chamber. It's also converted into calories.

Are all calories digested? It depends on the chemical composition of food, because the process of digestion isn't like you put everything into an oven and neither of Joules is lost. A part of the products will stay undigested. Let's deal with those, which will be digested.
What are calories and why does everyone count them?
Each food product contains proteins, fats and carbs in different ratios, and each component is digested by specific enzymes in different sections of a gastrointestinal tract. Everything begins in the mouth, where during chewing together with saliva a bit of amylase is secreted. It breaks down complex carbs into simpler ones. It lasts all the time the food is in the mouth (15−20 seconds), next through a food pipe it gets into the stomach, where it's awaited by the extremely aggressive gastric juice, which consists of:

hydrochloric acid, which participates in protein denaturation and creates the optimal acidity for enzymes, which are secreted by chief cells in an inactive state. Hydrochloric acid turns (activates) pepsinogens (these enzymes) into pepsins, has antiseptic effect, regulates evacuation of chyme (that's what food turned into) to the duodenum, and stimulates secretion of enterokinase and intestinal hormones in the duodenum.
pepsin, enzyme that breaks down complex proteins into peptides and then into amino acids,
stomach lipase, which catalyzes the hydrolysis of fats,
mucin, which has a protective function,
intrinsic factor, which helps vitamin B12 get absorbed.
How exactly are nutrients digested?
Wait a minute. In school we all were told that hydrochloric acid is a dangerous substance, which can lead to chemical burns. Then why doesn't the stomach digest itself? In fact, it does (otherwise there wouldn't have been a gastric ulcer), but it happens if you don't use your digestive system correctly. For example, if you starve yourself (periodically teasing yourself with appetizing flavors and thereby stimulating the production of gastric juice) or consume food with a chemically or thermally aggressive composition. However, if you treat yourself responsibly, the stomach has protection against hydrochloric acid: it's mucus, mucin, and bicarbonates. The bicarbonates react with hydrochloric acid, resulting in carbon dioxide, water and chlorine (which will get back into the cell, where hydrochloric acid will be formed again).
The most important digestion takes place farther, in duodenum, which secretes bile, pancreatic amylase (breaks down carbohydrates), trypsin (breaks down uncracked proteins), lipase, which hydrolyzes fats and bicarbonates (neutralize the acid stomach juice, as we said above).

Let's assume that everything has been digested at different stages, decomposed into monomers, glucoses, simpler proteins, amino acids, water, electrolytes, and so on, and we finally get to the main point — absorption of all these substances. Our gastrointestinal tract is lined with epithelium over its entire surface, the cells adhere to each other tightly, but still there are small spaces between them, through which some water, electrolytes (like sodium and chlorine from salt) are absorbed. Here we don't spend much energy, the transport occurs passively, according to the concentration gradient: the substance from the place where there's more of it gets to the place where there's less of it. There's also active transport (Na-K-pump), which transports against the concentration gradient across the cell membrane. In a nutshell, everything that's useful is transported in the form of amino acids, glucose and fructose, electrolytes, cholesterol, phospholipids, and so on into our cells. Water and ions are absorbed in the colon.

What will happen further? If we take carbos as the nutrients from which energy is most quickly obtained, then the following will happen with glucose: first, it'll enter the cell through channels. Then glycolysis, a process that produces two molecules of pyruvic acid and two molecules of ATP, will start. But that's not all. The pyruvic acid goes further into the mitochondrion, and from there into the citrate cycle or Krebs cycle. This is a sequential, looped process, resulting in pyruvic acid, which turns into fumarate, and 2 ATP molecules with the same high-energy bonds.

The final stage of energy exchange takes place in the intermembrane space of mitochondria, where 32 ATP molecules are formed during oxidative phosphorylation. (The process is complex, consisting in the transfer of electrons using special transporters and the creation of an electrochemical gradient due to proton accumulation).

All these processes are continuously going on in the organism, in every cell, in every mitochondrion. They consume the very energy that's released during the destruction (dephosphorylation) of ATP molecules, which goes to the digestion of the cookies, the work of the chewing muscles, the muscles holding a cookie, and so on. Calories are also spent on this, by the way. Such a difficult path has brought us to electrons and protons.
Merry Christmas, friends!
And bon appetit :)