The Digestive System
The digestive system can function in two forms: mechanical, in which
physical actions break down food particles, and chemical, where enzymes
break down molecules.
The mouth
When food is ingested it is broken down by chewing, this forms what’s
known as a bolus. The mouth contains three types of salivary glands
to aid this: parotid, submandibular and sublingual. When anticipating
ingestion, they produce saliva which contains enzymes that kill
pathogens and break down carbohydrates. (Lysozymes and
amylase respectively). Amylase in the saliva acts as a catalyst to
starch hydrolysis in the mouth, hydrolysis meaning when water
assists in the breakdown of molecules. On its own it’s too slow however,
so it works alongside enzymes.
Image via wikipedia
Oesophagus
Once the bolus is swallowed, permitted by the pharynx, it enters the oesophagus. The
Epiglottis is a tissue flap that prevents any food from entering the windpipe by closing the
trachea. Once the bolus is in the oesophagus peristalsis begins. This is the muscles
contracting to move food down until it enters the stomach.
Stomach
The stomach wall churns the food to mix it with gastric juices, resulting in chyme. Specific
cells in the lining produce an enzyme known as pepsin, which digests proteins. Pepsin
works best in an acidic environment, so the stomach also secretes hydrochloric acid to
provide it with its optimum pH and kill off microbes. When enzymes are not in their
optimum conditions they denature, causing them to lose their shape and stop functioning
properly. This will be explained in more detail later. Hydrolysis also plays a role in the
stomach.
, Small intestine
This is where absorption takes place. The small intestine is made up of three parts: the
duodenum, then the jejunum and finally the Ileum, which is the longest. This is where most
chemical digestion takes place, as well as some hydrolysis. The pancreas secretes the
amylase, protease and lipase enzymes into the small intestine. These break down
carbohydrates, proteins and lipids respectively. Because the bolus has just passed through
the stomach, it is too acidic for these enzymes to work
effectively. So the liver secretes bile into the small
intestine, which neutralises the acid. Bile also breaks down
fat into tiny droplets, called an emulsion, making it easier
for enzymes to break them down further. The lumen of the
small intestine is lined with villi, which are then lined
with microvilli. They increase the surface area of the
lumen for an increased rate of absorption. Glucose,
amino acids, vitamins and minerals pass through the
villi to get to the capillaries, which take them to the liver
to be sorted. In the centre of each villus is a lacteal
which absorbs fat, which is then carried by the
lymphatic system to the liver too.
Image via socratic
Pancreas
The pancreas has two parts, the endocrine and exocrine. The endocrine secretes
hormones like insulin to control the blood glucose levels and the exocrine secretes
pancreatic juice into the small intestine.
Liver
The liver is where bile is produced, bile is stored in the gallbladder until it’s released. It’s
also where the important metabolic chemicals are sorted for assimilation, which is a
catabolic process where they enter the correct tissues to be used. Catabolism is the
process of breaking down molecules to be assimilated.
Large Intestine
Any food still undigested at this point will enter the large intestine where it is moved along by
slow peristalsis. Its walls absorb water, how much is dependant on the body’s hydration,
leaving faeces. ‘Friendly’ bacteria live in the large intestine to feed on its contents, in
turn they produce vitamin K and Folic acid.
The rectum and anus
Waste matter is compacted and stored in the rectum before defecation from the anus. The
anal sphincter is a ring of muscle acting as a guard to the anus.
Enzymes
Enzymes are biochemical catalysts usually composed of proteins. Meaning they speed up
the rate of a reaction without being used up themselves. The proteins of an enzyme are
composed of long chains of amino acids, the combination of which determine the shape of
the enzyme’s active site.
The digestive system can function in two forms: mechanical, in which
physical actions break down food particles, and chemical, where enzymes
break down molecules.
The mouth
When food is ingested it is broken down by chewing, this forms what’s
known as a bolus. The mouth contains three types of salivary glands
to aid this: parotid, submandibular and sublingual. When anticipating
ingestion, they produce saliva which contains enzymes that kill
pathogens and break down carbohydrates. (Lysozymes and
amylase respectively). Amylase in the saliva acts as a catalyst to
starch hydrolysis in the mouth, hydrolysis meaning when water
assists in the breakdown of molecules. On its own it’s too slow however,
so it works alongside enzymes.
Image via wikipedia
Oesophagus
Once the bolus is swallowed, permitted by the pharynx, it enters the oesophagus. The
Epiglottis is a tissue flap that prevents any food from entering the windpipe by closing the
trachea. Once the bolus is in the oesophagus peristalsis begins. This is the muscles
contracting to move food down until it enters the stomach.
Stomach
The stomach wall churns the food to mix it with gastric juices, resulting in chyme. Specific
cells in the lining produce an enzyme known as pepsin, which digests proteins. Pepsin
works best in an acidic environment, so the stomach also secretes hydrochloric acid to
provide it with its optimum pH and kill off microbes. When enzymes are not in their
optimum conditions they denature, causing them to lose their shape and stop functioning
properly. This will be explained in more detail later. Hydrolysis also plays a role in the
stomach.
, Small intestine
This is where absorption takes place. The small intestine is made up of three parts: the
duodenum, then the jejunum and finally the Ileum, which is the longest. This is where most
chemical digestion takes place, as well as some hydrolysis. The pancreas secretes the
amylase, protease and lipase enzymes into the small intestine. These break down
carbohydrates, proteins and lipids respectively. Because the bolus has just passed through
the stomach, it is too acidic for these enzymes to work
effectively. So the liver secretes bile into the small
intestine, which neutralises the acid. Bile also breaks down
fat into tiny droplets, called an emulsion, making it easier
for enzymes to break them down further. The lumen of the
small intestine is lined with villi, which are then lined
with microvilli. They increase the surface area of the
lumen for an increased rate of absorption. Glucose,
amino acids, vitamins and minerals pass through the
villi to get to the capillaries, which take them to the liver
to be sorted. In the centre of each villus is a lacteal
which absorbs fat, which is then carried by the
lymphatic system to the liver too.
Image via socratic
Pancreas
The pancreas has two parts, the endocrine and exocrine. The endocrine secretes
hormones like insulin to control the blood glucose levels and the exocrine secretes
pancreatic juice into the small intestine.
Liver
The liver is where bile is produced, bile is stored in the gallbladder until it’s released. It’s
also where the important metabolic chemicals are sorted for assimilation, which is a
catabolic process where they enter the correct tissues to be used. Catabolism is the
process of breaking down molecules to be assimilated.
Large Intestine
Any food still undigested at this point will enter the large intestine where it is moved along by
slow peristalsis. Its walls absorb water, how much is dependant on the body’s hydration,
leaving faeces. ‘Friendly’ bacteria live in the large intestine to feed on its contents, in
turn they produce vitamin K and Folic acid.
The rectum and anus
Waste matter is compacted and stored in the rectum before defecation from the anus. The
anal sphincter is a ring of muscle acting as a guard to the anus.
Enzymes
Enzymes are biochemical catalysts usually composed of proteins. Meaning they speed up
the rate of a reaction without being used up themselves. The proteins of an enzyme are
composed of long chains of amino acids, the combination of which determine the shape of
the enzyme’s active site.
