Human Digestive System

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Secondary 1/2 Level

What you need to know for this topic:

  • Recognise the importance of food in providing us with nutrients such as carbohydrates, proteins and fats.
  • Describe the roles of the different nutrients in cellular processes such as respiration, growth and tissue repair.
  • Explain the importance of the digestive system in breaking down large, complex nutrient molecules into smaller, simpler molecules that can pass through cell membranes.
  • Identify the main parts of a digestive system and how they work together in the digestion of food.
  • Understand the two main types of digestion — chemical digestion and physical digestion.
  • Recognise the role of specific enzymes in digestion.
  • Appreciate the importance of hygiene habits and food handling practices in preventing food-borne diseases

Some parts of digestive system and their main functions

  • Mouth: Chewing to break up food into smaller pieces to increase the surface area to volume ratio for faster digestion by enzyme(amylase).
  • Esophagus: muscular tube connecting throat and stomach. Food is moved to the stomach by a series of contraction and relaxation of muscles that create a wave-like movement (the process is known as peristalsis)
  • Stomach: break down food by gastric enzymes (protease) and acid + breaking up of food by stomach muscle.
  • Small intestine: absorption of nutrients and minerals from food.
  • Large intestine: absorb water from the remaining indigestible food matter and transmit the useless waste materials from the body.
  • Rectum: hold the stool until it is ready to be evacuated.

Important terms and definitions:

Terms Definitions
Digestion large insoluble food molecules are broken down into small and soluble molecules that can be absorbed by the body cells
Absorption digested food substances are absorbed into bloodstream and body cells
Physical digestion breaks up food into smaller particles (still the same food substance but smaller size. purpose is to increase surface area to volume ratio to speed up rate of digestion by enzymes)
Chemical digestion breaks down large, complex molecules into small, simpler molecules that can be absorbed by the bloodstream. This reaction is usually speed up by an enzyme.

Practical phrasing for observation:

food test & what to add targeted nutrients observation for its presence observation for its absence

Benedict’s test

Add water and Benedict’s solution

 Reducing sugar

 

(glucose, fructose, galactose, maltose, lactose)

 Benedict’s solution turns from blue to brick red precipitate upon heating. Benedict’s solution remains blue in colour upon heating.

Iodine test

Add iodine solution

 Starch Iodine solution changes from brown to blue black in colour. Iodine solution remains brown in colour.

Ethanol emulsion test

Add ethanol and food sample into water

 Fat White emulsion is observed Clear solution is observed

Biuret test

Add sodium hydroxide and Copper (II) sulphate

 Protein Copper (II) sulphate solution turns from blue to purple / violet in colour Copper (II) sulphate solution remains blue in colour.

Difference between physical change and chemical change

Physical digestion breaks up food into smaller particles (still the same food substance but smaller size. purpose is to increase surface area to volume ratio to speed up rate of digestion by enzymes)
Chemical digestion breaks down large, complex molecules into small, simpler molecules that can be absorbed by the bloodstream. This reaction is usually speed up by an enzyme.

Enzymes, pH and where they are produced:

Mouth

  • Amylase : break down starch into maltose
  • pH: neutral or very slightly alkaline

Stomach

  • Protease : break down proteins into simpler protein molecules/shorter polypeptides (shorter protein chain). The next stage of digestion of polypeptide will occur in small intestine.
  • pH: hydrochloric acid (1.5 – 3.5). It helps to provide optimum condition for protease and to kill the bacteria present in the food as well.

Small intestine

  • Bile* (does not contain enzyme) is an alkaline fluid produced by liver, stored in gall bladder and released via bile duct into small intestine. It helps to break up the fat into smaller fat droplets, increasing surface area to volume ratio for faster digestion into fatty acids and glycerol by lipase.
  • Intestinal juice: contains maltase, protease and lipase and they are produced by small intestine.
  • Pancreatic juice: contain amylase, protease and lipase. The enzymes are produced by the pancreas and delivered into small intestine via pancreatic duct.
  • Protease at small intestine will break down polypeptides/simpler protein molecules (product from reaction between protein and stomach protease) into amino acids. Note: not 100% of protein is digested at stomach and hence this left over protein is also being digested at small intestine.
  • Lipase : break down fat (lipid) into fatty acids and glycerol (3 fatty acids to 1 glycerol)
  • pH in small intestine changes across the entire intestine but generally it is slightly alkaline overall as those enzymes work best in alkaline environment.

NOTE

Stomach protease and pancreatic protease work best at different pH; stomach protease works best in acidic environment whereas pancreatic protease works best in alkaline environment.

Secondary 3/4 Level

Functions of Proteins

https://ghr.nlm.nih.gov/primer/howgeneswork/protein

https://www.nature.com/scitable/topicpage/protein-function-14123348