A Trip Through the Digestive System
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Let's imagine that you are riding a molecule-sized submarine through the digestive system of someone who has just eaten a cheese sandwich with lettuce and tomato on whole wheat. The food contains sugars and starches, proteins, fats, water-soluble and fat soluble vitamins, minerals, and fibre.
The mouth will chew (masticate) the sandwich into smaller pieces so the rest of the system can work on it more efficiently.
The salivary glands add saliva consisting of water, slippery mucus proteins, and the enzyme amylase. Saliva moistens and lubricates the food. The amylase also begins breaking starch molecules into disaccharides (double sugars) called maltose.
When the moist, slippery food, now called a bolus gets pushed to the pharynx at the back of the mouth, it triggers the swallowing reflex, and swallowing then becomes automatic. The epiglottis is pulled down over the opening to the lungs, and the food is pushed down the oesophagus by waves of muscular contraction called peristalsis. Your submarine ride here will be smooth.
For a more detailed description of peristalsis, click on the icon At the bottom of the oesophagus, the bolus pushes on the cardiac, sphincter which opens, letting food into the stomach. This sphincter is a one way gate unless something goes seriously wrong.
In the stomach, muscular churning smashes up the remaining food solids. At the same time, large quantities of hydrochloric acid (HCl) and the enzyme pepsin are pumped into the lumen (hollow) of the stomach. The HCl helps to break down cell membranes and causes large molecules to unravel so enzymes can work on them. Your submarine is going to need strong defences against chemical attack in the stomach and your ride is going to be rough.
The HCl stimulates the enzyme pepsin, which chops proteins into shorter polypeptides. It finds a particular amino acid in a cheese protein and breaks the bond between it and the adjacent amino acid. Pepsin also attacks any foreign proteins from contaminating bacteria, and any other enzymes present. The stomach contents are reduced to a very acidic liquid referred to as chyme.
Meanwhile, the fats, which are in a watery medium, tend to clump together as tiny droplets. These droplets make the chyme opaque, something like milk.
The bottom end of the stomach is kept closed by the very powerful pyloric sphincter, a circular muscle. When food has been in the stomach long enough for the pepsin to have done its job, the pyloric sphincter releases it one squirt at a time into the duodenum. The duodenum is the first of three parts of the small intestine - the other two are the jejunum and the ileum.
The duodenum is the digestive system's great mixing pot. Secretions pour into it from the pancreas, liver, and gall bladder via the common bile duct. The liver produces bile which is stored in the gall bladder. Bile includes amphiphilic molecules called bile salts, whose main job is to emulsify the fat droplets. The bile salts disperse the fats as small suspended droplets, making it easier for enzymes to attack them.
The bile also includes bile pigments, waste products from the destruction of red blood cells. It is these pigments that colour faeces.
The secretions from the pancreas are far more complex. The pancreatic fluid is rich in the base NaHCO3 (sodium bicarbonate) which neutralises the HCl from the stomach, leaving the duodenal fluid slightly basic.
The pancreas also secretes numerous enzymes that are responsible for most of the rest of the chemical digestion.
Pancreatic amylase continues nibbling starch into maltose. The bread will be almost entirely digested to disaccharides within about half an hour of its release from the stomach.Lipase begins to work on the microdroplets of fat, slowly breaking them down into glycerol and fatty acids. The milk fats from the cheese and butter will be partly digested and partly still in droplets when it is time to absorb them.
Nucleases break DNA and RNA down into their basic molecules for absorption.
Numerous protease enzymesare released by the pancreas, including the "choppers" trypsin and chymotrypsin, and the nibblers aminopeptidase and carboxypeptidase, which begin snipping off individual amino acids from these peptides. By the time the cheese proteins are ready to be absorbed, they will consist of dissolved individual amino acids and very short chains. Your submarine's sensors and analysers could be kept busy for days just trying to figure out what was pouring into the duodenum and how it was working on the food.
The mixture of chyme, bile, sodium bicarbonate, and enzymes from the duodenum is then passed along to the next part of the small intestine, the jejunum. From here on, your ride will be smooth and you can concentrate on what is happening around you.
As the intestinal fluid flows slowly along the three metres of jejunum, the enzymes have time to work on the food. The indigestible fibre helps to keep the insoluble large molecules separated so the enzymes can work on them. It also increases the rate of peristalsis in the intestine, preventing stagnation.
The jejunum grades into the last part of the small intestine, the ileum. Both do the same jobs, but chemical breakdown is dominant in the jejunum and absorption is dominant in the ileum. The surface of small intestine is covered with villi and microvilli, giving it a huge surface area, and a large blood supply carries food away efficiently.
More enzymes are embedded in the membranes of the intestinal wall. Sucrase, maltase, and lactase break double sugars into simple sugars. These simple sugars ( monosaccharides) can be absorbed by facilitated diffusion through protein channels into the cells of the intestine and passed to the blood. Dipeptidases do the same thing with the short chain peptides. All the amino acids can then be absorbed.
Soluble fatty acids and glycerol from the cheese and butter are absorbed in the same way, but only part of the lipid has been digested. The rest remains in tiny droplets which are taken into cells by endocytosis. These can later be broken down by intracellular enzymes as needed. At the same time substances dissolved in lipids such as fat soluble vitamins and cholesterol can be absorbed at the same time.
Weakly soluble minerals are a problem to absorb. Iron is hard to take in, but it can combine with small acids in the gut to allow it to pass more easily. Vitamin C is one substance that helps bind iron.
The nutrient value of the intestinal fluid is now low, but a wide variety of bacteria (Enterobacteriaceae) in the intestines contribute to your nutrition while feeding themselves. We give them a suitable environment, and they synthesize some nutrients, such as vitamins, that we cannot make for ourselves.
As your submarine passes out of the small intestine into the colon, you will pass through the caecum, where you can see the opening to the dead-end appendix, a degenerate part of the caecum. In humans the caecum and appendix have no important function, but many herbivores have much larger caeca that house cellulose digesting bacteria.
By the end of the ileum, most of the useful nutrients will have been removed from the food. Waste material will be left, consisting of undigested fibre, mucus, dead cells, intestinal bacteria, bile pigments, and a large amount of water. The many litres of water that are dumped into the digestive tract every day must be recovered or the body would rapidly dehydrate.
The colon is specialised to recover water about 90% of all the water released into the intestine. Your submarine will now be immobilised, and you can only wait.
The remaining waste material is combined with a thick mucus from the colon to form faeces. The faeces are passed on to the rectum, where they are stored until they are eliminated.
The rectum ends in the very strong anal sphincter muscle, which is under voluntary control. This muscle is considerably more powerful than the circular muscles moving the faeces forward by peristalsis, so that, except under extreme circumstances, we can decide consciously when to eliminate the faeces.
Your journey is now complete, but human faeces still contain substantial energy, and large amounts of useful raw materials that may be exploited by other organisms with different digestive strategies. You should don your anti-contamination suit and escape your now stranded vehicle before the attacks by the dung beetles, fly maggots, bacteria and fungi begin!