Chemical Digestion

Section 1 of 9Why chemical digestion is needed

Chemical digestion

• The breakdown of large insoluble molecules into small soluble molecules.

Reason

• Large insoluble molecules cannot be absorbed through the wall of the small intestine.

Result

• Small soluble molecules are easily absorbed into the capillaries in the intestine wall.

Enzymes

• Enzymes are required for chemical digestion.

Section 2 of 9Digestive enzymes

Enzyme	Substrate	Product	Made in
Amylase	Starch	Smaller sugars (maltose)	Salivary glands and pancreas
Protease	Protein	Amino acids	Stomach and pancreas
Lipase	Lipids (fats and oils)	Fatty acids and glycerol	Pancreas, secreted into the duodenum

Section 3 of 9Hydrochloric acid in gastric juice

Gastric juice

• The stomach produces several fluids that together are called gastric juice, including hydrochloric acid.

Kills bacteria

• The low pH denatures the enzymes in bacterial cells so they cannot carry out cell reactions.

Optimum pH

• Pepsin has an optimum of about pH 2, and the acid keeps the stomach within this range.

Section 4 of 9Digestion of starch (Supplement)

Digestion of starch (Supplement)

Amylase is secreted in the mouth and, from the pancreas, into the duodenum.

Amylase digests starch to maltose, a disaccharide.

Maltase digests maltose to glucose on the membranes of the epithelium lining the small intestine.

Section 5 of 9Digestion of protein (Supplement)

Sites

• Protein digestion takes place in the stomach and the duodenum.

Pepsin

• Produced in the stomach, it breaks down protein in acidic conditions.

Trypsin

• Produced in the pancreas and secreted into the duodenum, it breaks down protein in alkaline conditions.

Section 6 of 9Role of bile (Supplement)

Made and stored

• Bile is made by liver cells and stored in the gall bladder.

Alkaline

• It neutralises the hydrochloric acid arriving from the stomach.
• Small intestine enzymes have a higher, more alkaline, optimum pH than stomach enzymes.

Emulsification

• It breaks large fat drops into smaller ones, giving lipase a larger surface area.

Section 7 of 9More from the specification

More from the specification

Nutrient assimilation by vital organs

• Nutrient assimilation by vital organs dietary iron, vitamins,
  • and amino acids are transported via systemic circulation to support the metabolism of internal organs.

Hydrolytic Enzymes of Chemical Digestion

• Specific biological catalysts cleave insoluble large polymers into soluble, permeable monomers capable of crossing intestinal epithelial membranes into the bloodstream.
  • (carbohydrases, proteases, and lipases)

Mechanical Preparations by Teeth and Stomach

• Physical mastication by specialized dentition and gastric muscular churning break food down physically, preventing digestive tract obstruction and maximizing substrate exposure to digestive juices.

Water and Mineral Absorption

• Water, mineral ions,
  • and water-soluble vitamins are absorbed across the intestinal epithelium down osmotic and electrochemical gradients without requiring prior chemical cleavage.

Scurvy

• A deficiency disease caused by lack of vitamin C.

Adaptation

• The process by which populations become better suited to their environment.

Growth

• A permanent increase in size and dry mass.

Digestive system

• Organ system that breaks down and absorbs food.

Physical digestion

• Breaking food into smaller pieces without chemical change.

Dietary fibre

• Indigestible plant material giving bulk to the intestine contents.

Vitamin

• A nutrient needed in small amounts to stay healthy.

Absorption

• Movement of small food molecules into the blood.

Organ

• A structure of different tissues working together for specific functions.

Section 8 of 9Key terms

Term	Meaning
Chemical digestion	Breakdown of large insoluble molecules into small soluble molecules by enzymes.
Amylase	Enzyme that breaks starch into maltose.
Maltase	Enzyme that breaks maltose into glucose.
Protease	Enzyme that breaks protein into amino acids.
Pepsin	Protease made in the stomach with an optimum near pH 2.
Trypsin	Protease made by the pancreas that works in alkaline conditions.
Lipase	Enzyme that breaks fats and oils into fatty acids and glycerol.
Gastric juice	The fluids produced by the stomach, including hydrochloric acid.

Section 9 of 9Exam technique

1

Saying salivary amylase works in the stomach — the acid there denatures it.

2

Saying bile digests fat chemically — bile only neutralises acid and emulsifies fat, and lipase digests it.

3

Giving every digestive enzyme pH 7 — pepsin works best at about pH 2 and the others in the small intestine are alkaline.

4

Thinking protease makes only one product — it breaks protein down into amino acids.