A balanced diet will contain many different fats and oils. The commonest type of molecule in these lipids is called a triglyceride. They are made from a small molecule of glycerol attached to three fatty acids.
These triglycerides are too large to be absorbed in the small intestine (ileum) and so need to be broken down into their constituent parts. In the digestive system there are enzymes called lipases that can catalyse the digestion of lipids into fatty acids and glycerol.
Most digestion of lipids happens in the duodenum. The pancreas produces a lipase enzyme that mixes with the food in the duodenum. Although bile does not contain any digestive enzymes, it does have bile salts that play an important role in the digestion of lipids. Bile salts cause large fat droplets in the duodenum to change into many smaller lipid droplets – a process called emulsification. (Read my post on bile if you want more details here….)
Fatty acids and glycerol molecules are small enough to cross the epithelium in the villus in the ileum. This absorption of fatty acids and glycerol is slightly different to the other products of digestion as they do not pass immediately into the blood. They are assembled immediately into structures called chylomicrons and these move into the single, blind-ended tube in the villus called a lacteal. The lacteals merge together into lymph vessels that eventually empty into the blood in the neck. (Please read my post on absorption in the small intestine to read more about this)
Proteins are large insoluble molecules made up of many hundreds of amino acids joined together in a long chain. So in order to obtain these molecules from our diet, the large protein must be digested (broken down) into the smaller amino acid subunits. Amino acids can be absorbed into the blood stream in the ileum, part of the small intestine.
The family of enzymes that can catalyst the digestion of proteins are called proteases.
Protein digestion happens in a two-stage process. In the first stage the large protein molecules are broken down into smaller proteins (often called polypeptides) by a protease enzyme. Pepsin is one such protease and acts in the stomach.
Remember that the food in the stomach is mixed with hydrochloric acid. This results in a very acidic liquid in the stomach (chyme). Pepsin works in the stomach and so rather unusually for a digestive enzyme, it has an optimum pH of pH 1.5 – pH2.
The second protease enzyme that you should know about is trypsin. Trypsin is made in the pancreas and so enters the duodenum soon after the stomach contents pass the pyloric sphincter (see diagram above). The acidic chyme that enters the duodenum is rapidly neutralised by hydrogencarbonate ions (an alkali) secreted in the bile and in pancreatic juice. Trypsin has an optimum pH of around pH 7.5.
As shown in the diagram above, there is a final stage to protein digestion. The actions of pepsin in the stomach and trypsin the duodenum result in small protein fragments called peptides. Many peptides are still too large to be absorbed into the blood in the ileum and so need digesting further into their constituent amino acids. Peptidase enzymes are embedded in the epithelial cell membranes in the small intestine and this final reaction completes the digestion of proteins.
Amino acids are absorbed by active transport into the blood capillaries in the villi in the small intestine.
You must remember that “Digestion” has a specific meaning in Biology. It is the term used for the process that involves the chemical breakdown of large, insoluble food molecules into smaller, simpler molecules that can be absorbed into the blood. Many of the molecules in food are polymers – that is macromolecules made from long chains of repeating subunits. Examples of dietary macromolecules include proteins, polysaccharides and fats. These molecules are too large to be able to pass into the blood in the villi of the small intestine and so the body has evolved to chemically break them down into their constituent monomers or building blocks. Digestion is the process in the alimentary canal that achieves this.
Digestion reactions are also known as hydrolysis reactions because a molecule of water is required in the reaction to break the covalent bond holding the monomers together. These reactions are all catalysed (sped up) by specific molecules called digestive enzymes.
Why do different food types need different digestive enzymes to speed up their breakdown in the digestive system?
(If you are unsure, you need to revise the way enzymes work to catalyse reactions by a “lock and key” theory?)
Digestion of Carbohydrates
Many simple carbohydrates (e.g. glucose) do not need digesting. This is because they are already small enough to be absorbed into the blood directly in the ileum (small intestine). But larger disaccharide sugars (e.g. maltose and sucrose) do need to be broken down, as do all polysaccharides (e.g starch).
The family of enzymes that break down carbohydrates are called carbohydrases.
Starch is a large polysaccharide made up of many hundreds of glucose residues linked together. It is way too big to be able to cross the epithelial lining of the small intestine and so needs to be digested. This happens in a two-stage process. Firstly there is an enzyme amylase that can catalyse the following reaction:
starch + water ——-> maltose
Amylase is made in the salivary glands and so works in the mouth. But the main region for the digestion of starch is in the duodenum. This is because amylase is also made in the pancreas.
Maltose is a disaccharide molecule made of two glucose residues joined together. Maltose itself requires digesting to its constituent glucose molecules in order to be absorbed. So the second stage in the digestion of starch involves a second enzyme, maltase that is found embedded into the epithelial lining of the ileum. Maltase catalyses the breakdown of a molecule of maltose into two molecules of glucose which can be absorbed into the blood.
maltose + water ——> glucose
The liver is the largest internal organ and plays over 500 different roles in the body. Many functions are to do with the processing of various chemicals such as carbohydrates, amino acids and lipids. The liver also removes alcohol and other drugs from the bloodstream: this is why alcoholics often suffer from liver disease.
But one function of the liver that you need to understand in detail concerns its role in the digestive system. The liver cells produce a green liquid called bile which is stored in a sac underneath the liver called the gall bladder. Bile can pass from the gall bladder down the bile duct and as shown in the diagram below, it then mixes with the contents of the duodenum (small intestine) soon after the acidic chyme leaves the stomach.
What is in bile?
Bile contains a mixture of chemicals. It has an alkaline substance (hydrogencarbonate ions) which helps to neutralise the chyme as it leaves the stomach. Remember the pH of the stomach contents is around pH1-2 and in the duodenum, there is a pH of around pH7.5. The difference in pH is due to the alkali present in bile and pancreatic juice. Bile also contains excretory molecules called bile pigments. These are waste molecules from the liver that have been made from the breakdown of haemoglobin. And finally there are the bile salts. These play an important role in the digestion of lipids in the duodenum.
How do bile salts improve digestion of lipids in the duodenum?
The duodenum is where many digestive reactions happen in the body. This is because the pancreatic juice contains many enzymes, all of which catalyse a specific digestive reaction. One such enzyme is lipase and this enzyme catalyses the following reaction:
lipids + water ——> glycerol and fatty acids
Fatty acids and glycerol are small enough molecules to be absorbed into the villi in the ileum. They pass into the lacteal in the centre of each villus to be carried around the body in the lymphatic system.
But the problem is that in the duodenum, the lipid molecules will exist as large droplets. Large droplets of fat/oil will have a reduced surface area for lipase to bind to and so the rate of digestion of the lipid would be slow. But bile salts interact with the lipid droplet causing a few large droplets to be broken down into dozens of tiny droplets. This is called emulsification and while it does not chemically alter the lipid, it does make it easier for lipase to break it down. Lipase and bile salts together break down lipids much faster than lipase alone.
Final key point: there are no digestive enzymes in bile. But in spite of this, bile plays a crucial role in the digestion of lipid droplets in the duodenum.
The first part of the small intestine, called the duodenum is principally involved in digestion. Large insoluble food molecules such as proteins, lipids and starch are chemically broken down into smaller molecules in reactions catalysed by digestive enzymes.
This post will look at the longer regions of the small intestine, the jejunum and ileum. There are some digestive reactions that happen here but the main function of these parts of the intestine is the absorption of the smaller products of digestion into the body.
You should understand already which molecules are produced as products of digestion: glucose from the digestion of carbohydrates, amino acids from the breakdown of proteins and fatty acids and glycerol from the digestion of triglyceride lipids. (see my post on digestion) These then are the molecules that diffuse from the intestine into the body in the small intestine.
How is the structure of the small intestine adapted for absorption?
The main idea here is that the lining of these parts of the small intestine has a very large surface area. The intestine is long, the wall is ridged and the lining (called the epithelium) has many thousands of tiny projections called villi. Each villus is 1-3mm long but the effect is to increase the surface area for absorption by many hundreds of times.
These epithelial cells have a cell membrane which is folded into many thousands of tiny structures called microvilli. Microvilli (or a brush border) can only be seem with an electron microscope and act to increase the surface area still further.
The cells that line the villus are called epithelial cells and are found in a layer that is just one cell thick. This reduces the distance the products of digestion have to move across to be absorbed.
Each villus contains a dense network of blood capillaries. This means that glucose and amino acids can easily diffuse into the blood and then be taken away from the small intestine to the liver in the hepatic portal vein. There is also a blind-ended single tube called a lacteal in each villus. This tube forms part of the lymphatic system and is used to transport fatty acids and glycerol away from the small intestine.
Most animals including humans feed by a process called holozoic nutrition. This means that the animal has a gut tube (alimentary canal) that runs through its body and the animal has a mouth at one end (this is where the food goes in), an anus at the other end where undigested food (faeces) passes out.
Ingestion: the first stage of feeding involves food being taken up into the mouth (not too complicated to understand I hope…)
Most food is made up of very large molecules (macromolecules) such as starch, proteins and lipids. These molecules are too large to be absorbed from the gut tube into the blood. So the second stage of feeding involves chemically breaking down these large food molecules into smaller solubles molecules that can be absorbed.
Digestion: the chemical breakdown of large insoluble food molecules (e.g. proteins/starch/lipids) into smaller soluble molecules (amino acids/sugars/fatty acids and glycerol) that can be absorbed. Digestion is a chemical process and every digestive reaction is catalysed by a specific enzyme. Almost all these digestive enzymes are secreted into the gut tube and mixed in with the food. Please see a later post on Digestion to get a full understanding of these reactions.
Absorption: the small products of digestion are then absorbed into the blood stream. This process occurs almost exclusively in a region of the small intestine called the ileum. The structure of the ileum is beautifully adapted for efficient absorption – see a later post on Absorption for full details!
Assimilation: this is the stage of processing food that is sometimes left out. This is because it does not happen in the alimentary canal but instead in the body cells of the animal. The small soluble products of digestion (glucose, amino acids, fatty acids etc.) are taken up into cells and used to build the animal’s own macromolecules – proteins/lipids/glycogen etc. Assimilation is a term for how small molecules are built up and used in the cells of the animal.
Egestion: this is the final stage and involves undigested food, mostly cellulose in humans, being passed out of the anus as faeces at the end of the alimentary canal.
Don’t confuse egestion with excretion….. Egestion is simply passing out undigested food from the end of the large intestine. If you think of your body like a tube of polos (you know I do…) then the stuff that comes out of the large intestine has never actually been inside your cells. It is mostly cellulose and other plant fibres that went into the mouth a day or two ago, haven’t been digested nor absorbed and so come out the other end. Excretion on the other hand is the process of removing waste molecules that have been made inside cells. So the lungs removing carbon dioxide, the kidneys taking urea out of the blood to make urine – these are examples of excretion.
A slightly lavatorial way to end the post but important nonetheless: if you are talking about urine production, this is excretion, but “number twos” or “Richards” (look up cockney-rhyming slang for EAL speakers) are not excretion, they are egestion….