Organs of the Digestive System (Cambridge O Level Biology)

The Human Digestive System Diagram

The human digestive system

Preparing Food Substances for Absorption

• The purpose of the complex series of organs and glands in the digestive system is to prepare food substances for absorption
• Large chunks of food have to be broken down into small, soluble molecules that can diffuse into the blood via the wall of the small intestine
• Food contains many large, complex molecules that cannot be absorbed directly
  • For example, proteins are large molecules made up of small, soluble amino acids
  • Proteins must be digested in the alimentary canal before the amino acids can be absorbed
• The only substances that pass into faeces (called roughage or dietary fibre) are large, indigestible molecules such as cellulose, and some water

Stages of food breakdown

• Food taken into the body goes through 5 different stages during its passage through the alimentary canal (the gut):
  • Ingestion - the taking of substances, e.g. food and drink, into the body through the mouth
  • Mechanical digestion - the breakdown of food into smaller pieces without chemical change to the food molecules
  • Chemical digestion - the breakdown of large, insoluble molecules into small, soluble molecules
  • Absorption - the movement of small food molecules and ions through the wall of the intestine into the blood
  • Assimilation - the movement of digested food molecules into the cells of the body where they are used, becoming part of the cells
  • Egestion - the passing out of food that has not been digested or absorbed, as faeces, through the anus

Functions of the Digestive Organs Table

Structure	Function
Mouth / salivary glands	Mechanical digestion: teeth chew food to break it into smaller pieces and increase its surface area to volume ratio. Chemical digestion: amylase enzymes in saliva start digesting starch into maltose. The food is shaped into a bolus (ball) by the tongue and lubricated in saliva so it can be swallowed easily.
Oesophagus	The tube that connects the mouth to the stomach where the food bolus goes after being swallowed. Wave-like contractions will take place to push the food bolus down without relying on gravity.
Stomach	Food is mechanically digested by churning actions while protease enzymes start to chemically digest proteins. Hydrochloric acid is present to kill bacteria in food and provide the optimum pH for protease enzymes to work.
Small intestine	The first section is called the duodenum and is where the food coming out of the stomach finishes being digested by enzymes produced here and also secreted from the pancreas. pH of the small intestine is slightly alkaline - around pH 8-9. The second section is called the ileum and is where the absorption of digested food molecules takes place. The ileum is long and lined with villi to increase the surface area over which absorption can take place.
Large intestine	Water is absorbed from the remaining material in the colon to produce faeces. Faeces is stored in the rectum and removed through the anus.
Pancreas	Produces all three types of digestive enzymes: amylase, protease and lipase. Secretes enzymes in an alkaline fluid into the duodenum for digestion to raise the pH of fluid coming out of the stomach. Produces bile to emulsify fats (break large droplets into smaller droplets), an example of mechanical digestion.
Liver	Amino acids that are not used to make proteins are broken down here (deamination), producing urea. Stores bile to release into the duodenum as required.
Gall bladder	Stores bile to release into the duodenum as required.