Hi everyone, welcome to IGCSE Study Buddy where you can revise biology topics from the Cambridge IGCSE syllabus. If you're enjoying these videos so far, please don't forget to hit the like button and subscribe to my channel. This video summarizes part 2 of topic 7, Human Nutrition.
In the previous video, we learned about physical digestion which is the breakdown of food into smaller pieces without chemical change to the food molecules. Now let's dive into chemical digestion. Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules. So what is the role of chemical digestion? It is to produce small soluble molecules that can be absorbed.
First, we must learn the functions of enzymes in the digestive system. The enzyme amylase breaks down starch to simple reducing sugars. Amylase first breaks down starch to maltose and then maltase breaks down maltose to glucose.
Amylase acts in the mouth and the duodenum. Maltase does its job on the lining of the small intestine. The enzyme protease breaks down protein to amino acids.
In the stomach, where the conditions are acidic, the protease pepsin breaks down protein to amino acids. In the small intestine, where the conditions are alkaline, the protease trypsin breaks down proteins to amino acids. Lipase is the enzyme that breaks down fats and oils to fatty acids and glycerol.
This takes place in the duodenum. This table summarizes where in the digestive system amylase, protease and lipase are secreted and where they act. So as you can see amylase is produced by the salivary glands as well as the pancreas and is released into the mouth and duodenum respectively. Amylase breaks down starch to maltose which is later broken down into glucose. The protease pepsin is produced in the stomach and acts in the stomach.
The protease trypsin is secreted by the pancreas into the duodenum. The proteases break down protein into amino acids. The enzyme lipase is secreted by the pancreas and acts in the duodenum, digesting fats and oils into fatty acids and glycerol.
Now let's learn the functions of hydrochloric acid in gastric juice. Gastric juices are fluids produced by the stomach. One of the fluids produced is hydrochloric acid.
The functions of hydrochloric acid in gastric juice are Hydrochloric acid kills harmful microorganisms in food. It provides an acidic pH for optimum enzyme activity. So pepsin, which is produced in the stomach, has a low optimum pH, which is acidic. So the hydrochloric acid provides the ideal acidic environment for pepsin to work at its fastest rate.
Let's look at the function of bile in chemical digestion. We already came across the role of bile in physical digestion in the previous video, which is to emulsify fats. Bile is an alkaline mixture that neutralizes the acidic mixture of food and gastric juices entering the duodenum, from the stomach to provide a suitable pH for enzyme action. The enzymes in the small intestine have a higher or more alkaline optimum pH than those in the stomach. So the bile gives the food mixture a more suitable pH for those enzymes to work best in the small intestine.
Finally, let's learn about absorption. Absorption is the movement of digested food molecules from the digestive system into the blood. The small intestine is the region where nutrients are absorbed.
Most water is absorbed from the small intestine but some water is also absorbed from the colon. The ileum is adapted for absorption as it has a highly folded surface with millions of villi and microvilli which are tiny finger-like projections. These greatly increase the surface area of the ileum, allowing absorption to take place faster and more efficiently.
Let's take a look at the structure of a villus. These finger-like projections are called villi which is the plural of villus. These are capillaries. This is a thin surface layer.
On top of this layer are microvilli. These are blood vessels. And these are the capillaries.
are lacteals. Capillaries absorb glucose and amino acids and carry it away from the small intestine in the blood. Lacteals absorb fatty acids and glycerol and transport them away from the small intestine in the lymph.
That concludes part 2 of chapter 7. human nutrition. I hope you found this video useful. Thank you for watching and please don't forget to subscribe to IGCSE Study Buddy for more biology revision videos. Bye-bye.