The digestive system is responsible for converting food into a form that our bodies can use for energy and growth. Knowing how the digestive system works helps us understand how the body processes food and absorbs vital nutrients. Use this resource to explore the organs and functions that make up the digestive system, and how Indigenous peoples made cycad seeds safe to eat.
A lot of the nutrients we need to survive cannot be produced by the body, so we need to obtain them from food. But how does, say, the egg in our breakfast bagel get converted into proteins that we can use? This is where the digestive system comes in.
The digestive system:
takes in food
breaks down food into nutrients
brings nutrients from food into the bloodstream
removes undigested food from the body.
The digestive system consists of the digestive tract and accessory organs.
How does the digestive system help extract the proteins, lipids, carbohydrates, vitamins, minerals and fibre from a breakfast bagel? Image by Vicky Ng via Unsplash
The digestive tract
Did you know?
If you spread out your entire digestive tract, it would be about \(9\) metres long!
The digestive tract (also known as the alimentary or gastrointestinal (GI) tract) is a long tube that starts at the mouth and ends at the anus, and also includes the mouth, oesophagus, stomach, small intestine and large intestine. The teeth, tongue, salivary glands, liver, gallbladder and pancreas are known as accessory organs.
Let's explore what each of these organs does during digestion.
The digestive system starts with the mouth, where salivary glands and the tongue are found.
The epiglottis is the flap at the entry of the digestive tract.
The oesophagus is the channel from the mouth to the stomach.
The liver, pancreas and gallbladder sit around the stomach.
The first part of the small intestine is the duodenum.
The next part of the small intestine is the ileum.
At the start of the large intestine is the appendix and caecum.
The large intestine then leads to the rectum.
Finally, the anus is the exit point of the digestive system.
Digestion
Digestion is the process by which the body breaks down food into smaller components that can be absorbed and used for energy, growth and repair.
There are two ways we can do this: mechanically or chemically.
Mechanical digestion is the physical breakdown of food into smaller pieces. It occurs purely because of the forces created by motion.
Chemical digestion is when chemicals like enzymes or acids participate in chemical reactions that break down molecules into simpler substances.
Mechanical and chemical digestion happen at different stages of food digestion. Let's look at the steps involved.
Digestion starts with ingestion, where we take food into our mouth. Both mechanical digestion and chemical digestion occur at ingestion.
Mechanical digestion
Our teeth split, grind and crush our food into smaller pieces as we chew. This prepares the food for the next steps of digestion.
Chemical digestion
Our food mixes with saliva, a fluid produced by the salivary glands in the mouth to help moisten the food. It also contains enzymes that help to begin breaking down the food we eat, particularly carbohydrates.
By breaking the food into smaller pieces and forming a bolus (a mass of food and saliva), we make it easier for our body to pass the food through our digestive tract for further digestion and absorption.
Propulsion is the process of moving food through the digestive tract. Once we have chewed our food, we push it from our mouth into our oesophagus by swallowing.
Rhythmic muscle contractions called peristalsis then move the food down to the stomach and through the intestines. Peristalsis ensures that food keeps moving smoothly through the digestive tract so that we can digest and absorb nutrients.
A diagram showing peristalsis acting on a section of the oesophagus. On the left, the food is near the top of the oesophagus. The muscles directly above the food contract and the muscles directly under the food relax. A downward arrow shows the direction of food movement.
On the right, the food has moved further down the oesophagus. The muscles directly above the food contract and the muscles directly under the food relax. A downward arrow shows the food direction.
Mechanical digestion starts in the mouth where we use our teeth to chew food, and continues in the stomach. Food is churned to mix it with gastric juices, which is a complex, acidic mixture of:
water
hydrochloric acid
digestive enzymes.
Churning makes it easier for enzymes to further break down food molecules.
Did you know?
Stomach churning, also known as a tummy rumble or "borborygmi", happens when you are hungry and also while you're digesting food! You can just hear it more clearly when you're hungry because there's less food in your stomach to muffle the sounds.
Just like mechanical digestion, chemical digestion begins at ingestion. But things get more serious when we reach the stomach. This is where the bulk of chemical digestion occurs.
In the stomach, the gastric juices work to break down proteins into smaller pieces. This creates a semi-liquid mixture called chyme.
When chyme moves into the small intestine, digestive enzymes travel from the pancreas through the pancreatic duct and are also released into the small intestine. Bile, which is made in the liver and stored in the gallbladder, enters the small intestine through the bile ducts. These organs are known as the biliary tract. The bile and enzymes work together to continue breaking down carbohydrates, proteins and fats, allowing the body to absorb nutrients effectively.
Watch this video to learn about digestive enzymes.
We don't have chunks of cake or bits of burger floating around our bloodstream. So how do we get the energy from our food to our cells? Two things have to happen before we can make use of our lunch: it needs to be broken down into smaller pieces or digested and then it needs to be absorbed into the bloodstream.
Digestion takes place in lots of places along the alimentary canal, and absorption happens in the small intestine, which is found just after the stomach along the length of the digestive system. To learn more about the digestive system, watch this video [Digestion - What Is It?], and to learn more about absorption, watch this video [Small intestine and food absorption].
In this video, we're going to look at the enzymes involved in digestion in more detail. The large molecules which make up our food, like lipids, proteins and carbohydrates are too big to be moved into our blood. So, they need to be digested into smaller molecules by physical processes like chewing, and chemically by special proteins called enzymes.
Different types of enzymes digest the different types of food. So let's look at the food groups in turn. First, carbohydrates.
Foods like rice and pasta are made of carbohydrates. The simplest carbohydrates are sugars which can be joined into big chains to make complex carbohydrates like starch. Carbohydrates are digested by carbohydrase enzymes such as amylase. Amylase is a special type of carbohydrase which breaks down starch, a big carbohydrate, into smaller molecules. These can then be broken down further into glucose which is small enough to be moved into the blood. Amylase is found in your saliva which is where carbohydrates will first start to be broken down chemically, and another amylase is released into the small intestine from the pancreas, so much further down your digestive system.
Now, let's look at proteins. Proteins [are] found in meat, fish, beans and pulses. They are made up of amino acids and are digested by protease enzymes. The protease enzymes break down proteins into amino acids. Protease called pepsin breaks down proteins in the stomach, but this doesn't work once the food moves into the small intestine. In the small intestine, the conditions are different, so a different protease called trpysin is released into the small intestine from the pancreas for continued protein digestion.
What conditions do you think might be different in the small intestine compared to the stomach? Whilst the stomach is very acidic and has a pH of about 2, the small intestine has a higher pH of about 8. Protease enzymes which work well at pH 2 don't work at pH 8, which is why different protease enzymes are needed in the small intestine.
And the last group of food, lipids. Lipids digested by enzymes called lipases into glycerol and fatty acids, but it's not an easy job. It requires a process called emulsification to take place first. Emulsification breaks the lipids into smaller droplets. The smaller droplets have a larger surface area for the lipase enzymes to work on. To learn more about bile and emulsification, watch this video [Bile and Emulsification].
So, to recap. The main enzymes involved in digestion are:
carbohydrase called amylase is released in the saliva; this digests starch into smaller sugars
a protease called pepsin digest proteins in the stomach
then a protease called trypsin further digests proteins into amino acids in the small intestine
a carbohydrase called amylase and lipase are also released into the small intestine from the pancreas. They digest carbohydrates into sugars and lipids into fatty acids and glycerol.
These molecules are small enough to be absorbed into the blood.
Once food is broken down sufficiently, we are ready for absorption. Nutrients are taken into the bloodstream where they can then be transported to the cells that need them.
Absorption primarily occurs in the small intestine, which turns the chyme into a fluid that can be absorbed. About \(90\%\) of nutrient absorption occurs in the small intestine.
The small intestine consists of three main sections, each with a slightly different function:
duodenum – where most of the chemical digestion occurs, as digestive enzymes and bile are mixed with food
jejunum – where the majority of nutrients are absorbed, like glucose, amino acids, fatty acids, vitamins and minerals
ileum – where vitamin B12, bile salts and any remaining nutrients are absorbed.
Did you know?
The small intestine measures about \(7\) metres long. If you were to unfold it and lay it out flat, it would be about the size of a tennis court!
Absorption is facilitated by an intestinal lining of tiny finger-like projections called villi. Because of this structure, the small intestine has an enormous surface area for nutrient uptake.
Watch this video for more information on absorption and the function of the small intestine.
Can you believe your digestive system is as long as a bus? Yep, it's about nine metres long, all wrapped up inside your body. A lot of things go on in your digestive system, not just digestion. What else do you think happens?
Digestion goes hand in hand with absorption which happens in your small intestine, which is what we're going to look at in this video.
Digestion is the breakdown of food, from large insoluble molecules into small molecules which can be used by your body. Digestion involves many organs before it reaches the small intestine. For more information on the roles of these organs in the digestive system, watch this video [Digestion - What Is It?]. Enzymes and other chemical such as bile break down food; for more on the role of enzymes and bile in digestion, watch these videos [Digestive enzymes; Bile and Emulsification].
As they move through the digestive system, carbohydrates are broken down into sugars, proteins are broken down into amino acids and fats are broken down into fatty acids and glycerol. But the job of the digestive system doesn't stop at digestion. For our body to make use of these small, soluble molecules, we need to absorb them into our bloodstream. This happens in the small intestine.
The small intestine is made up of lots of tiny, finger-like projections called villi. The villi increase the surface area of the small intestine. The larger the surface area, the more absorption that can take place.
Each villus has a tiny blood vessel called a capillary and another vessel called a lacteal. The food molecules pass from the villi of the small intestine into either the blood capillary or the lacteal.
Once absorbed into the blood vessel in the small intestine, the digestive food molecules can then be transported around the body to where they are needed.
So why did those large molecules need to be broken down in the first place? The small intestine is kind of like a sieve; it has small holes in it. Small molecules are able to pass through those holes, whilst large molecules cannot. So carbohydrates, proteins and fats are all too large to pass through the holes in the small intestine, but sugars, amino acids, fatty acids and glycerol are all small enough to pass through.
Sugars and amino acids pass through the holes into the blood capillary, while fatty acids and glycerol pass into the lacteal of the villi. These small molecules move by diffusion, moving from a high to a low concentration. For example, the small intestine has a high concentration of sugar molecules, whereas the blood supplying the small intestine has a low concentration of sugar. So the sugar moves down the concentration gradient, from a high concentration in the small intestine, through the holes in the villi, into the blood capillary where it's in low concentration. Sugar, amino acids, fatty acids and glycerol all use diffusion to move into the blood within the small intestine.
So after digestion breaks down large molecules into small molecules, absorption of the small molecules happens inside our small intestine by diffusion into the blood vessels. What an amazing system!
The final stage of digestion is defecation, where waste is removed from the body. In the large intestine, water is absorbed from the remaining food material, turning it into solid waste known as faeces.
Our poo is stored in the rectum until it is ready for removal from the body through the anus. This process is important because it helps the body get rid of substances that it doesn’t need or cannot digest.
Indigenous knowledges in life science
Carbohydrates from cycad seeds
Plants are a great source of nutrients, but not all are safe to eat. One example is the seeds of cycads. They were a great source of carbohydrates for Indigenous communities in eastern tropical regions of the country. But they contain a toxic substance called cycasin that, when broken down inside the body, can cause nausea, abdominal pains and vomiting. In the longer term, cycasin can damage the nervous system and liver.
Cycasin chemical structure, image by RMIT, licensed under CC BY-NC 4.0
To make them safe to eat, cycad seeds were treated by a process called leaching. This involved running water through the open seeds over several days to remove cycasin. This left behind glucose, which could be used by cells for energy. Cycad seeds, image by Kylelovesyou via Wikimedia Commons, licensed under CC BY 3.0
Read the scenario and use the information provided to answer the questions in the quiz.
In 2022, a group of nutrition researchers in Denmark studied the amount of phylloquinone and menaquinone-4 (forms of vitamin K) released after the digestion of different foods, which is then available for absorption (bioaccessibility).
In separate experiments, they placed samples of broccoli, egg, canola oil or a phylloquinone supplement in a bench-top digestion model called INFOGEST 2.0, and measured the concentration of phylloquinone and menaquinone-4 in the final samples. Their results are shown in the table.
Data from Jensen MB, Biltoft-Jensen AP and Jakobsen J (2022) 'In vitro bioaccessibility of vitamin K (phylloquinone and menaquinones) in food and supplements assessed by INFOGEST 2.0 - vit K', Current Research in Food Science, 29;5:306-312, doi:10.1016/j.crfs.2022.01.018.
