Digestion

Why is digestion needed?

Digestion is a process of break down food in the mouth, the stomach and the intestines into smaller substances so they can be absorbed and used for body. Some nutrients are too big to be absorbed. For example, in carbohydrates, polysaccharides and disaccharides are broken down into monosaccharides. However, some nutrients such as vitamins, minerals and water are small enough to be absorbed so they don't have to be digested.


Enzymes

There are some enzymes that can help digestion of food.

• Amylase

There are two kinds of amylase. One is salivary amylase, and another is pancreatic amylase. Salivary amylase is produced by salivary glands and helps digestion of starch  into disaccharides or monosaccharides in a mouth. Salivary glands are near the moouth. Salivary amylase is the most active in pH7.4 that is neutral, so it doesn't burn mouth. Pncreatic amylase helps digestion of carbohydrates into monosaccharides in small intestine, and pancreatic amylase is produced by pancreas.

• Protease

This enzyme helps digestion of protein (polypeptides) into amino acids. Protease is produced in stomach and the digestion of protein occurs in stomach too.

• Pancreatic lipase

This enzyme helps digestion of lipid into fatty acids and glycerols. Pancreatic lipase is produced in pancreas and the digestion happens in small intestine.

Types of digestion and how it works
There are two types of digestion, one is mechanical and another is chemical.

Mechanical digestion is by chewing, mashing, cutting, and pounding in mouth and stomach. Chemical digestion is with enzymes that speed up chemical reactions.

Mouth
In a mouth, mechanical digestion and chemical digestion are done by teeth and muscles and salivary amylase. The mecanical digestion is for breaking food intp smaller physically so that food can pass to the stomach through a throat and esophagus. Then salivary glands produce salivary amylase and it digests starch into dissacharides.  In the mouth there is epiglottis which prevents food from entering trachea.

Stomach
The sromach produces protease to help digestion of protein into amino acids. It is chemical digestion.  HCl which has pH around 2 is necessary in this digestion becasue it helps breaking down protein, and making the acidic pH necessary for the protease to be work efficiently. Since protease is strong acid stomach produce mucus to protect against HCl. Also the mechanical digestion occurs in the stomach as well. The muscles in the stomach mash and mix food. It is for fast digestion.

Liver
It stores nutrients and produces bile which helps digestion of lipid. Bile is stored in gallbladder.

Small intestine
In small intestie, lipids and carbohydrates are digested. Pancreas produce pancriatic lipase and helps digestion of lipid into fatty acids and glycerol. Lipase's optimum pH is about 8.0. Then lacteal which is lymphatic capillaries helps to absorb them in the villi of the small intestine. Also pancreas produce pancriatic amylase whose optimum pH is 6.7 to 7.0 and it helps digestion of carbohydrates into monosaccharides such as glucose and fructose. Samll intestine absorbs carbohydrates, amino acids, fatty acids and glycerol.
Samll intestine has villi which has a lot of advantages for fast and efficient absorption of nutrients, such as...
・villi contains capillaries bed close ti surface to allow fast absorption
・villi intestinal wall have many fold to increase surface area to increase the volume ratio
・villi locate close to blood vessels and lymph vessels
・villi wall consist of single layer of cells


Large intestine
Large intestine absorbs water, and minerals. Cellulose and glycogen which is made from glucose pass through the large intestine. Waste materials are compacted, stored in retum and released through the anus.

Simulation of Digestion
In biology class, we did simulation of digestion on the paper. We used scissors as enzymes and cut food. Small numtrients were written on small paper and they don't need to be cut. Large nutrients such as carbohydrates, amino acids and lipids are written big size, and they needed to be cut by each scissors(enzyme). This simulation was really helpful for me because I can imagine what is happening in each digestive organs and what kind of enzymes are used for nutrients.