The incredible journey

The first bite

Digestive tract

Let's start by taking a bite from a burger.

When you take a bite of a burger you take in bread, tomato, lettuce, cheese and meat. The first thing you do is of course, chew. Technically known as masticating, this is a form of mechanical digestion. Mechanical digestion is simply breaking down larger bits of food into smaller pieces. This process results in the food pieces having a larger surface area to volume ratio, and this aids in chemical digestion.

Chewing breaks up your food and also stimulates the release of saliva. Saliva contains an enzyme called amylase. Enzymes are biological catalysts that speed up chemical reactions, which break down your food into simpler molecules. Chemical digestion when enzymes chemically modify the food.

Amylase breaks down starch into sugar. Starch, in this case our bread, is basically a long chain of sugar molecules bonded together and the amylase helps to break these bonds leaving individual sugar molecules. You can prove this by taking a piece of white bread and chewing it. Keep chewing and don't swallow, and eventually you will taste a slight sweetness, which is sugar due to your saliva breaking down the starch. Saliva also acts as a lubricant to the food, making it easier to swallow. After chewing, you force the food into a bolus at the back of your mouth, and using the special muscles of your tongue, you swallow.

So far, only the bread has been chemically digested but the remaining chewed-up starch, lettuce, tomato, cheese and meat head towards the stomach. They do this via the esophagus; a tube behind the trachea (windpipe). Here, waves of muscular contraction known as peristalsis force the food into the stomach. These muscular waves are why food doesn't just "fall" into the stomach, and why you can still swallow while hanging upside down!

Acid bath

Food generally stays in the stomach for three to six hours where it is subjected to more mechanical and chemical digestion. The stomach muscles contract and relax in what we know as churning, which further helps to grind up the food. The stomach also produces acid, which creates the correct pH conditions for more enzymes to chemically digest the food. These enzymes are known as proteases. They break down proteins into amino acids, and lipases, which break down fats (lipids) into fatty acids. This is where the cheese and meat are chemically digested, since they are fat and protein. The stomach can cope with such strong acid (pH 0-1) as the stomach wall produces mucous to stop the wall being damaged.

After the stomach, the food moves into the first part of the small intestine, known as the duodenum. Here, an organ known as the pancreas adds alkaline and more enzymes to the duodenum. The alkaline neutralizes any excess stomach acid creating the perfect pH for the enzymes, which continue to break down the meat, cheese, bread, lettuce and tomato.

The liver produces a substance called bile, which is stored in the gall bladder. This bile is released into the duodenum to emulsify fat. Here, mechanical digestion breaks down large pieces of fat into smaller pieces. No new chemicals are produced. The effect is similar to using a dishwashing liquid on greasy dishes.

After the duodenum, food continues down the small intestine where more digestion takes place over the next 24-48 hours. The small intestine is up to seven meters long in adults. It has millions of folds on its interior surface called villi, and on these villi it has millions of tiny folds called microvilli. These are blood rich areas that, due to their many folds, increase the surface area of the small intestine to enormous proportions. Hypothetically, if you could flatten out your small intestine the area would easily cover a football field!

These blood-rich folds are where nutrient absorption occurs. Food molecules that have been sufficiently broken down are now small enough to pass through the gut wall into the blood, where sugars (from the broken down bread) are taken to every cell of the body for respiration, which is where we get our energy from. Amino acids (from the protein in the cheese and beef) are used for many different purposes, including cell functions, cell repair, and growth. Lipids (also from the cheese and beef) are used for cell membrane development, insulation and sometimes energy. This break-down and absorption is basically the whole point of eating, and without it we could not survive.

What a waste

After the long journey down the small intestine, indigestible food (fiber), vitamins, minerals and water pass into the large intestine. No more digestion occurs now, but water, vitamins and minerals are absorbed into the bloodstream and transported around the body to where they are needed. Here, the vitamins and minerals contained in the tomato and lettuce are now utilized, leaving the fibrous parts behind. This fibrous material is forced down the colon by peristalsis until it reaches the rectum. Here, it is stored until we get that urge to go to the toilet. Finally, after all this time, and after its nine-meter journey from the mouth, undigested food is egested (not excreted) as feces.

The energy you just used to read this article was gained from the food you recently ate and that your body digested after chewing, churning, emulsifying and after applying countless chemical reactions.

David Canavan has an MSc in Behavioral Ecology and teaches science, math and ICT at Garden International School. David is fascinated by science and loves animals, especially the dangerous kind; the more dangerous the better. You may contact David at davidc@gardenbangkok.com .