Coaching Centre
The digestive system
How the digestive system breaks food into nutrients: mechanical and chemical digestion, teeth, enzymes, the organs of the digestive tract in order, how villi absorb nutrients, and what the large intestine and gut bacteria do.
Your digestive tract is a tube about 8 to 9 metres long that runs from your lips to your anus. Strictly speaking, food in that tube is still outside your body, in the same way the hole in a doughnut isn't part of the doughnut. Food only gets inside you when it is broken into molecules small enough to cross the gut wall into your blood.
Why we need to digest food
Every cell in your body needs a supply of nutrients: glucose for energy, amino acids to build proteins, fats for cell membranes, plus vitamins, minerals and water. The trouble is that food arrives as big lumps made of big molecules. A piece of bread is mostly starch, a long chain of thousands of glucose units joined together. Starch is far too big to pass through a cell membrane.
Digestion is the breakdown of food into small, soluble molecules that can be absorbed into the blood. It happens in two ways:
- Mechanical digestion breaks food into smaller pieces without changing the chemicals. Chewing, the churning of the stomach and bile breaking fat into droplets are all mechanical. Smaller pieces have a much bigger surface area for enzymes to work on.
- Chemical digestion uses enzymes to break large molecules into small ones. Enzymes are proteins that speed up particular reactions without being used up.
| Food molecule | Enzyme that digests it | Broken down into |
|---|---|---|
| Starch (carbohydrate) | Amylase (saliva, pancreas) | Sugars, then glucose |
| Protein | Protease, e.g. pepsin (stomach, pancreas) | Amino acids |
| Fat (lipid) | Lipase (pancreas) | Fatty acids and glycerol |
Each enzyme has a shape that fits only its own type of molecule, a bit like a key in a lock. That's why amylase can't digest protein. Enzymes also work best at a particular temperature and acidity. Your enzymes work best near body temperature, 37 °C, and pepsin works best in strong acid.
The journey through the digestive tract
The digestive tract (also called the gastrointestinal tract or gut) is one continuous tube. Other organs, the salivary glands, liver, gall bladder and pancreas, make juices that are piped into it.
| Organ | What happens there |
|---|---|
| Mouth | Teeth chew food into small pieces. Saliva from the salivary glands moistens it and contains amylase, which starts digesting starch. The tongue rolls the food into a soft ball called a bolus and pushes it to the back of the throat. |
| Oesophagus | A muscular tube about 25 cm long. Waves of muscle contraction called peristalsis squeeze the bolus down to the stomach in about 5 to 10 seconds. That's why you can swallow while upside down. |
| Stomach | A stretchy muscular bag that holds food for about 2 to 4 hours. It churns food with gastric juice, which contains hydrochloric acid and the protease pepsin. The acid kills most bacteria in the food. A thick layer of mucus stops the stomach digesting itself. The result is a soupy liquid called chyme. |
| Small intestine | About 6 metres long. Juice from the pancreas brings amylase, protease and lipase. Bile from the liver (stored in the gall bladder) breaks fat into tiny droplets. Digestion is finished here, and nutrients are absorbed through the villi into the blood. |
| Large intestine | About 1.5 metres long and wider. It absorbs most of the remaining water and some salts. Trillions of bacteria live here, breaking down fibre and making some vitamins (such as vitamin K). |
| Rectum and anus | The rectum stores faeces (undigested food, bacteria and dead gut cells) until it is pushed out through the anus. Getting rid of faeces is called egestion. |
- Step through all six stops. At which stop does starch start to be digested? At which stop is it finished?
- Watch the protein bar. Which two organs digest protein?
- Where does most of the water get absorbed? Why does that make sense, given what faeces are like?
- Add up the times for each stop. Roughly how long does a meal take to pass right through you?
Teeth: the first tools
Adults have 32 teeth, in four types, each shaped for a different job.
| Type | Number (adult) | Shape and job |
|---|---|---|
| Incisors | 8 | Flat and chisel-shaped, at the front, for biting and cutting. |
| Canines | 4 | Pointed, for tearing and gripping. |
| Premolars | 8 | Two bumps (cusps), for tearing and crushing. |
| Molars | 12 (including 4 wisdom teeth) | Broad with several cusps, at the back, for grinding. |
Animals' teeth match their diets. A koala has sharp front teeth to snip eucalyptus leaves and broad ridged molars to grind them to a paste. A dingo has long canines for gripping prey and blade-like back teeth that slice meat like scissors.
Villi: a huge surface for absorbing
Once food has been digested in the small intestine, the nutrients have to get into the blood. The inside wall of the small intestine is covered with millions of tiny finger-like folds called villi (one is a villus), each about 1 mm long. The cells on the villi have even smaller folds of their own, called microvilli.
Villi are built for absorption in three ways:
- Huge surface area. All the folds give the small intestine an absorbing surface of roughly 30 square metres, about the size of a badminton half-court. (Older textbooks say 250 m², the size of a tennis court, but more careful measurements in 2014 found it is closer to 30 m².)
- Thin walls, only one cell thick, so nutrients don't have far to go.
- A rich blood supply that carries nutrients away as soon as they arrive, so more can keep moving in.
The blood from the intestines goes straight to the liver, which processes the nutrients, stores spare glucose and removes toxins before the blood travels on to the rest of the body.
Four test tubes each get a 1 cm cube of cooked egg white (protein). Tube A: water only. Tube B: pepsin solution. Tube C: dilute hydrochloric acid. Tube D: pepsin and acid. All four sit in a water bath at 37 °C for 2 days.
The egg white in A, B and C looks unchanged. In D, the cube has shrunk, gone see-through at the edges and the liquid has turned cloudy.
Pepsin digests protein, but only in acid conditions, just like inside the stomach. Tube A is the control: it shows that water alone does nothing. Tubes B and C show it isn't the enzyme alone or the acid alone that does the job.
The independent variable is what's added to each tube. Keep the size of the egg cube, volume of liquid, temperature and time the same in every tube.
Test yourself
Which is an example of chemical digestion?
Chemical digestion uses enzymes to break bonds and make new, smaller molecules.
Proteins are digested into
Proteases break proteins into amino acids, which cells use to build their own proteins.
Where are most nutrients absorbed into the blood?
The villi of the small intestine absorb digested nutrients.
What moves food down the oesophagus?
Peristalsis works even if you're upside down.
Which feature does NOT help villi absorb nutrients?
A thick layer would slow absorption down. Villi need short distances.
The main job of the large intestine is to
It absorbs most of the water left in the undigested material, turning it into solid faeces.
Label the digestive system.
Put the parts of the digestive tract in the order food passes through them.
Mechanical or chemical digestion?
Enzymes are that speed up chemical reactions. breaks down starch, breaks down protein and breaks down fat. The stomach contains , which kills bacteria and helps pepsin work. Digested food is absorbed through the of the small intestine.
Coeliac disease damages the villi, leaving the small intestine wall much flatter. Explain why people with untreated coeliac disease can lose weight and become tired even though they eat normally.
Villi are tiny folds that give the small intestine a very large surface area for absorbing digested nutrients into the blood. If the villi are damaged and flattened, the surface area is much smaller, so far fewer nutrients such as glucose, amino acids and fatty acids are absorbed, even though the person eats normally. The unabsorbed nutrients simply pass out in the faeces.
With less glucose reaching the cells, less respiration can happen, so less energy is released and the person feels tired. With less food absorbed overall, the body uses up its fat and other stores to make up the difference, so the person loses weight. They may also miss out on vitamins and minerals such as iron, adding to the tiredness.