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Digestion and absorption

Topic 6
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1

Biology SL

➢ Reactions taking place with an enzyme can occur with a lower input of energy than
the same reaction taking place without the presence of an enzyme
○

The input of energy is typically in the form of heat
o

➢ In humans 37 C is warm enough to maintain a good molecular movement and, with
the aid of enzymes, it provides enough activation energy for metabolic reactions to
occur
○

Warm-blooded organisms have an advantage over cold-blooded ones for
efficient digestion and other metabolic processes, due to their constantly
warm internal temperature

➢ Humans are incapable of digesting cellulose, which is extremely common organic
substance
○

Few living organisms can do, because most cannot produce the enzyme
cellulase

Human digestive system
➢ It is a long tube, called alimentary canal, with two accessory organs (pancreas and
liver) that are connected by ducts into the canal
➢ The alimentary canal begins with the mouth and ends with the anus
➢ Food does not move through the alimentary canal by gravity but by smooth muscles
controlled by the autonomic nervous system
○

Two layers of smooth muscles:
■

Circular
●

■

Longitudinal
●

○

Makes contractions inwards into the tube
Makes contractions along the tube

The muscle motion and food movement caused by the action of these two
muscle layers is called peristalsis
■

Relatively fast within the oesophagus and slows dramatically in the
intestines

➢ Peristalsis is also used in the stomach to mix food with digestive secretions,
including a protein-digesting enzyme
○

This movement is called churning

2

Digestion and absorption

Topic 6
...
1

Biology SL

➢ The actual absorption occurs through cells in an epithelial layer that is in direct
contact with the nutrients
○

The epithelial cells have tiny membrane projections called microvilli that
extend into the lumen of the intestine

➢ The villi and microvilli greatly increase the surface area for absorption within the
small intestine
➢ The interior of each villus contains a capillary bed for nutrient absorption and
transport of digested monomers by the bloodstream
➢ In addition, there’s a small vessel of the lymphatic system present, called lacteal,
that absorbs some of the nutrients
➢ After passing through the epithelial cells of villus most monomers are absorbed
into the inner capillary bed
○

However some of the larger monomers, such as fatty acids, are absorbed
first into the lacteal

➢ List of the substances absorbed through villi into the bloodstream or lymph fluid:
○

Water

○

Monosaccharides

○

Amino acids

○

Nucleotides

○

Glycerol

○

Fatty acids

○

Mineral ions

○

Vitamins

Digestion step-by-step
➢ The digestion of food starts already in the mouth
○

Teeth aid the mechanical digestion

○

Saliva contains the enzyme amylase which hydrolyses the starch
polysaccharide into the disaccharide maltose (chemical digestion)

➢ When swallowing, the food travels down through the oesophagus, due to to
muscles moving (peristalsis)
➢ In the stomach, hydrochloric acid lowers the pH, thus further aiding digestion
○

The low pH deactivates amylase from the saliva and therefore the starch
remains largely undigested

➢ From the stomach the food enters duodenum where enzymes from liver and
pancreas are emptied
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Digestion and absorption
○

Topic 6

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