The digestive system in context
In mammals, several systems help to exchange substances with the
environment. This includes the
exchange of gases in the respiratory system and the exchange of food molecules in the digestive system. The more complex an organism is, the more organized its cells are.
Similar cells are often grouped together to form a tissue, and several of these tissues may form an organ.
Breaking down substances
The mechanical breakdown of food into smaller pieces is called physical digestion.
This process occurs:
by chewing food in the mouth
by squeezing in the stomach.
What is the purpose of physical digestion?
Smaller pieces of food will pass more easily through the alimentary canal.
From the small intestine to the blood
Physical digestion continues in the stomach as it squeezes its contents into the small intestine.
Here, the digested
particles are absorbed into the blood to be taken
elsewhere in the body.
Only smaller molecules, such as glucose, are able to diffuse from the small intestine,
through the villi, and into the blood, moving from higher to lower concentration.
Enzymes in the body
Enzymes catalyze thousands of reactions that need to take place in order to maintain life. What are some of these reactions?
Digestive enzymes break down large food molecules into smaller molecules so they can be
What are digestive enzymes?
Not all enzymes work inside cells. In what process do enzymes work outside cells?
Here the enzymes help to break down large food molecules into smaller molecules that are more easily absorbed.
Digestive enzymes are produced by specialized cells in the pancreas and digestive tract.
Simple food tests can be used to investigate the presence of
biological molecules within a food sample, including:
They are qualitative tests to investigate the type of molecule present, rather
than how much of it is present. starch
Factors affecting enzymes
The rate at which the digestive enzymes work in the digestive
system depends on several factors.
Each enzyme works best at only one particular
temperature and pH: this is called their optimum.
Digestion in the stomach
Hydrochloric (HCl) acid is secreted from the stomach wall when food enters, increasing the acidity of the stomach to about pH2. This is the optimum pH for protease enzymes.
proteins enter the stomach
amino acids enter the small intestine the presence
of proteins stimulates the secretion of HCl from parietal cells
Not the right pH?
If the pH changes sufficiently beyond an enzyme’s
optimum, the shape of the enzyme changes irreversibly. When this happens, the substrate will no longer fit and the enzyme is denatured.
optimum conditions pH is too low/too high
Digestion in the small intestine
The liver produces bile which is stored in the gall bladder and released into the small intestine.
Bile, which is alkaline, neutralizes the acidic contents coming from the stomach, creating the environment that the intestinal enzymes need to work.
Digestive enzymes found in the small intestine are damaged by a strongly acidic pH.
How does the body avoid this problem?