Digestion

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Biology Form Four Digestion

Digestion is the process by which food is broken down from complex insoluble substances into simple soluble substances. There are two types of digestion – chemical digestion and mechanical digestion. Mechanical digestion involves breaking up large pieces of food into smaller pieces. This provides a large surface area for the action of enzymes on food. There is no change to the chemical composition of food during mechanical digestion. Mechanical digestion occurs in the mouth (mastication) and in the stomach (churning). Chemical digestion involves the action of enzymes. Large insoluble food molecules are broken down into smaller soluble molecules to be absorbed into the blood stream.

The Role of Teeth
Teeth are found embedded in the upper and lower jaws within the buccal cavity. They cut and grind food into smaller pieces before it is swallowed. The grinding is called mastication. Prolonged chewing of the food ensures that a larger surface area is exposed for the activity of enzymes. Draw a longitudinal section through an canine or molar tooth. Ensure that the drawing is labeled correctly.

How do you take care of your teeth? Why is dental care important?

Chemical Digestion
Buccal Cavity As the food is being chewed/masticated by the teeth in the buccal cavity, it is softened by the secretions from three pairs of salivary glands which pour their contents through ducts into the buccal cavity. The secretion, saliva, contains the enzyme amylase. This enzyme begins the digestion of starch, converting it to maltose. Saliva also contains mucus, a sticky fluid which helps to soften and lubricate the food.

Starch

amylase

Maltose

Stomach The lining of the stomach secretes gastric juice which contains the enzymes pepsinogen and hydrochloric acid (HCl). Pespsinogen is activated to pepsin by the hydrochloric acid.

Pepsin begins the digestion of protein into polypeptides.
Hydrochloric acid is secreted to kill bacteria in the stomach and it provides the optimum pH for the activity of pepsin. pepsin

Protein

Polypeptides

The food is churned in the stomach by the muscular activity of the stomach walls for approximately three hours. The food leaves the stomach in a semi-liquid state called chyme.

The small intestine

Duodenum
The duodenum receives pancreatic juice from the pancreas and bile produced by the liver.

Bile is an alkaline watery fluid secreted from the liver through the bile duct. It is stored temporarily in the gall bladder. Bile contains salts which emulsify fats, that is, these salts reduce the surface tension of fats breaking them up into small globules. Pancreatic juice is produced in the pancreas and travels to the duodenum via the pancreatic duct. Pancreatic juice contains several enzymes:

Pancreatic amylase continues the digestion of starch to maltose. amylase

Starch

Maltose

Trypsin breaks down protein into polypeptides and polypeptides into amino acids. trypsin

Protein

Polypeptides

Polypeptides

trypsin

Amino Acids

Lipase splits fat molecules into fatty acids and glycerol.
Lipase

Fat Molecule

3 fatty acids + glycerol

Pancreatic juice and bile are alkaline. They therefore neutralize the chyme from the stomach and create the optimum pH for these enzymes.

Ileum Final digestion occurs here. The walls in the ileum secrete intestinal juices. The juice contains enzymes which break down the remaining partially digested food into soluble products. Polypeptides are broken down into amino acids by peptidases. polypeptidase

Polypeptides

Amino Acids

Maltase converts maltose to glucose.
maltase

Maltose

Glucose

Sucrase converts sucrose to glucose and fructose.
sucrase

sucrose

glucose + fructose

Lactase converts lactose to glucose and galactose.
lactose

lactose

glucose + galactose

Lipase converts fats to fatty acids and glycerol.
lipase

Fat Molecule

3 fatty acids + glycerol...
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