The main nutrients (things that we need) in our food are carbohydrates (sugars and starch), proteins, fats, vitamins and minerals. Before we can make use of the nutrients in our food they must get (be absorbed) into the bloodstream so they can be carried round the body to where they are needed. They can only be absorbed into the bloodstream if they are in the form of very small soluble molecules.
Many nutrients such as starch and proteins consist of very large molecules which are insoluble in water. We know that proteins do not dissolve in water because our bodies are made mainly of protein and we do not dissolve every time we go swimming. Digestion is the process by which our bodies convert very large insoluble molecules into the much smaller soluble molecules which can be absorbed into the bloodstream.
In order to digest our food special chemicals (biological catalysts called enzymes) are added to it.
Sugars and starch are carbohydrates, compounds containing carbon, hydrogen and oxygen. For humans they are our main energy source. They are chemically very similar but sugars are very small soluble molecules and starch is much larger insoluble molecules. We can absorb sugars into the bloodstream straight away but we must digest starch.
This is carried out by converting it into a sugar called glucose. Enzymes catalyse the conversion of starch
into glucose (and also, for example in photosynthesising leaves, glucose into starch). If you want to know more about enzymes, catalysts
and catalysis please link to the sheet on them 
.
There are many other sugars, for example sucrose, lactose, maltose, fructose etc, and enzymes also catalyse the conversion of one sugar into another. The sugar you buy in bags in the shops is sucrose.
Proteins are the main building blocks of all living things. They are very complex molecules of carbon, hydrogen, oxygen, nitrogen and other elements. There are thousands of different proteins but they are all made up from much simpler molecules called amino-acids. Proteins are very large insoluble molecules but amino-acids are much smaller soluble molecules. We digest proteins by converting them into amino-acids and this process is catalysed by other enzymes.
Fats are digested by converting them into fatty acids by means of still other enzymes.
The gut is a long tube passing right through our body. Most other animals also have a gut passing through their bodies. In humans the parts of the gut are
We often use the word mouth to mean an opening, for example the mouth of a cave. Almost all animals have a gut, a long tube inside the body with an opening at each end. Of course the gut of an earthworm is nothing like the gut of an octopus or a boa constrictor or a ladybird or an cow. When talking about animals other than Man scientists usually use the word mouth to mean the opening at the beginning of the gut, and the anus to mean the opening at the other end. So correctly our mouth is just the hole in the front of our face; if we are using mouth to mean just this then correctly the space where we chew our food is called the buccal cavity. But in everyday speech most people refer to the buccal cavity as the mouth, and few people would regard this as wrong.
Digestion takes place mainly in the stomach and small intestine, while absorption takes place mainly in the small intestine. The large intestine is where water is removed.
The stomach is strongly acidic and has an acid-resistant lining, but the intestine and oesophagus do not. The liver produces a special alkaline liquid called bile which is stored in the gall-bladder and released into the small intestine to neutralise the stomach acid. Bile also emulsifies fats (breaks them down into very tiny droplets) to help them to be digested by the enzymes. Bile is a brown liquid, which is why our faeces are brown.
For a really good account of the gut look here. This is an American web page so uses American spellings and examples, and non-metric units, but I don’t think that matters.
Just after
we have eaten something the blood supply to the stomach and intestine
is increased and so of course it is reduced to other
parts of the body. We should not run around after we
have just eaten a big meal, or we may get cramp.
If we do run around while there is food in the
stomach the blood supply to the stomach may be reduced and
it will not work properly, so food will stay in it
longer than usual. The dangers of running around after a meal
are discussed in the sheet on respiration - to see it
click here
The amino-acids produced by digesting the proteins we eat are absorbed into the bloodstream and then reformed into the proteins our body needs. There are thousands of different proteins but only about twenty amino-acids.
Think of proteins as words on a Scrabble board and amino-acids as the letter tiles. After we have finished one game we put all the letters back into the bag and then in the next game use the same letters to make completely different words - the same bag of letters can make thousands of different words. So we can eat the proteins in a chicken drumstick, convert them into amino-acids and then rebuild the amino-acids into the proteins in human brain cells.
Proteins and amino-acids are sugars with added nitrogen and other elements. Our bodies are continuously building them up and breaking them down, and breaking them down may convert them into sugars (which give us energy) and a substance containing nitrogen (nitrogen waste). This is very poisonous and we must get rid of it. It is converted by the liver into a substance called urea, and then this is removed from the bloodstream by the kidneys in the form of a solution of urea in water called urine (“wee”). The urine is stored in the bladder until it is released through the urethra (and penis for boys) when we go to the lavatory.
There are thousands of different proteins but only about twenty amino-acids. We get the amino-acids we need from the proteins in our food, and build them up into the different proteins we need. Not all foods contain all the proteins which will form every amino-acid but we can make some of the amino-acids we need by breaking down others. But there are some amino-acids which we cannot make: we must get them from our food. People who eat meat get most of the amino-acids they need from meat but vegetarians, and particularly vegans, need to take care to ensure that they eat a wide variety of different types of foods to make certain they have all the amino-acids they cannot themselves make.
Different animals can make different amino-acids, for example dogs can make certain amino-acids which cats cannot. These they must get from their food. This is why you can feed a dog on cat food but you cannot feed a cat on dog food.
One method of treating a certain type of childhood cancer uses this idea: the child can make an amino-acid which the cancer cell needs but cannot make: it must get it from the child’s blood. The child is treated with an enzyme which changes this amino-acid in the child’s blood. The cancer cells die because they cannot get this amino-acid, but the child can make it so is unharmed.
Meat is very rich in protein. Carnivores (animals which eat mainly meat) get most of their energy from the sugars obtained by converting amino-acids into sugars and nitrogen waste. Hence the urine of carnivores is much stronger than that of herbivores or omnivores who get their energy from foods containing carbohydrates. This is why dog urine kills plants but cow urine does not. For more about this see the Page on Nutrition in Plants.
Substances which cannot be digested, that is, food which we have eaten but which cannot be converted into small soluble molecules, pass through the gut unchanged and eventually pass out through the anus (the hole in our bottom) in the form of a semi-solid brown mass called faeces (“poo”).
All plants are made up of plant cells, with cell walls. The cell walls are made of a special carbohydrate called cellulose. Our bodies cannot digest cellulose. Cellulose is therefore one of the main substances present in our faeces. Although cellulose (sometimes called vegetable fibre or just fibre) is not a nutrient for us we do need to eat foods containing it in order to keep our digestive system healthy. Leaves and stems are very rich in fibre. Whole cereal grains such as wheat, maize and barley are also very rich in fibre and this is called bran; some “breakfast cereals” contain lots of bran.
Humans, and many other animals, cannot digest things like tomato seeds and apple pips. These pass through the gut and come out in the faeces unchanged, sometimes several kilometres from where the fruit containing the seed was eaten. This is one of the most important ways the seeds of many plants are dispersed - some seeds will not germinate until they have passed through an animal’s gut! Of course, the seeds must have an acid-resistant coating so they are not digested as they pass through the gut, which is why we cannot digest them in the first place...
It
is often said that faeces are our waste products but this
is not correct: we do not produce them, they are
merely substances which we have swallowed and have passed through our
gut. Our body's two waste products, the substances we have actually
produced, by our own metabolism, and must get rid of, are urea, and carbon dioxide
which is produced by our respiration. To find out more about
respiration click here
The substances that come out of our anus are the substances which we have ingested (swallowed) but not digested (converted into very small soluble molecules). Even though we sometimes call them excrement the correct word is egesta - the word excrement (or more correctly excreta) is reserved for the substances our body has actually produced, that is, carbon dioxide and urea. (We also need to excrete the heat produced by respiration to keep ourselves cool.) All animals need to ingest, digest, egest and excrete in some way.
We cannot digest cellulose, but leaves contain lots of cellulose, so animals which eat mainly leaves (particularly grass which is a special sort of leaf) must be able to do so. They have special digestive systems, for example cows break down cellulose by “chewing the cud”.
Many millions of years ago our early ancestors also ate leaves and were capable of digesting cellulose. There was a special and very large part of the gut called the caecum (pronounced like Julius Caesar, and spelled cecum in America) where cellulose was broken down by special bacteria. Once our ancestors stopped eating leaves the caecum was no longer used, and gradually got smaller. Today it is tiny: we call it our appendix. It is close to the beginning of the large intestine. The appendix is termed a vestigial organ, an organ we still have although it serves no purpose.
Earthworms live in the soil and eat little bits of organic material (little bits of things which were once alive, such as pieces of rotting leaf). They cannot of course take their food back to their kitchen to wash all the soil off before they cook it: they have to eat it just as they find it. And this means they have to swallow not only their food but also quite a lot of the soil stuck to it. They cannot digest the soil so it passes through their body and eventually comes out of their anus, in the form of very tiny particles. We call their faeces worm casts - you often see worm casts on lawns. Worms may swallow lots of soil but they do not eat it.
Earthworms make long tunnels under the ground which help drain the soil and allow the air to get into it, and also, through their casts, keep bringing fresh soil to the surface. No wonder they are called Nature's Gardeners!
Our faeces contains the substances which we have eaten but cannot use. But just because we cannot use these substances it does not mean that no other organisms can. In fact lots of animals, bacteria and fungi feed on nothing but faeces, that is, the substances that other animals have not been able to digest. We may not fancy eating cowpat but for the larvae of the dung beetle it is their favourite food! From our point of view a cow is an animal which is useful because it converts grass into beef and milk; from the dung beetle's point of view a cow is an animal which is useful because it converts grass into cowdung.
Elephants eat enormous quantities of food but actually digest very little of it. So they also make enormous quantities of droppings. These provide the only food for the larvae of the world's largest beetle, the goliath beetle.
Which is just as well - imagine what Africa would be like if there were no goliath beetles!
Animal droppings (egesta) contain the bits of the food the animal has eaten but not been able to digest, so by looking at an animal's droppings we can often learn a lot about what it eats.
For example, otters live in rivers and eat fish. An otter cannot digest fish scales so these are in its droppings. By looking at the fish scales in an otter's droppings (spraint) we can tell what sort of fish it has eaten.
At one time anglers often killed otters because they ate salmon and trout and other fish which fishermen like to catch - the anglers thought that if they killed otters there would be more fish. But there was not, there was actually less. When scientists looked at the otters' spraint they found that although otters did eat some salmon and trout they ate mostly eels, and eels eat other fishes. So killing the otters meant there were more eels and so less other fishes. Now most anglers do not want to kill otters. (It is illegal to kill them in any case.) Most species of otter live in rivers but there are also some which live in the sea.
We have even found and been able to analyse fossilised dinosaur droppings! Old faeces are called coprolites. Studying faeces and coprolites hundreds, thousands or even millions of years old may not be your idea of fun but it is very important because it is often the only way we can find out what the animals living at the time were eating. But as I said at the beginning, do not try it at home!
The faeces of herbivores such as cows and sheep and camels contain lots of undigested plant material, and also many quite harmless bacteria, and very few parasites. If they are left in the open air for a time they quickly dry and lose their smell. In hot dry places such as deserts camel dung is used as a fuel; in the Middle Ages cowdung was (and in some countries still is) an important ingredient of the plaster used on walls. But the faeces of carnivores contain almost no undigested plant material, but many very harmful bacteria and parasites, and are very dangerous for a long time, and you should never touch them without wearing throw-away gloves or using a pooper-scooper. It is also very important to wash your hands when you have been to the lavatory. Every year, even in countries like Britain, children are going blind because of a parasite they have picked up from dog or cat poo.
Owls, and many other birds of prey, swallow their food whole. They cannot digest the bones, feathers and fur, but these do not pass through the intestine, instead they are regurgitated from the animal's mouth in the form of a pellet. You can often find owl pellets under the trees where owls are roosting. Studying owl pellets gives us lots of information about what the owl has eaten - and is a lot more pleasant and safer than studying faeces and coprolites!
Barn owl pellets are particularly interesting for young people to study, because a meal for a barn owl is usually a small mammal such as a vole or shrew, swallowed whole and then regurgitated later as a single pellet. So each pellet will contain all the bones of just one animal, which can be separated from the fur and then laid out to reconstruct the complete skeleton.
©Barry Gray April 2014
