The Digestive System

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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?


 respiration

 protein synthesis

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.


Food tests

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



Fussy enzymes?


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.

 substrate concentration



 enzyme concentration

enzyme:substrate complex

enzyme substrate


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?

gall bladder

pancreas hepatic