Absorption of Fructose and Amino Acids with the Help of Carrier Ions like Na

- Fructose and amino acids are two types of nutrients that are absorbed in the small intestine.
- The intestinal mucosa contains carrier proteins that help transport these nutrients across the cell membrane and into the bloodstream.
- These carrier proteins require the presence of specific ions, such as sodium (Na+), to function properly.
- In the case of fructose absorption, a protein called GLUT5 acts as the carrier. It transports fructose across the membrane of the intestinal cells into the bloodstream with the help of Na+ ions.
- Similarly, amino acids are absorbed with the help of several different carrier proteins that require Na+ ions for transport. These carrier proteins are selective, meaning they only transport certain types of amino acids.
- Once absorbed, fructose and amino acids are transported to the liver, where they are either used for energy or converted into other compounds.

Chylomicrons as Lipoprotein Particles Transported from Intestine into Blood Capillaries

- Chylomicrons are large lipoprotein particles that are formed in the intestinal cells after the absorption of dietary fat.
- These particles contain a mixture of lipids, including triglycerides, cholesterol, and phospholipids.
- Chylomicrons are too large to enter the blood capillaries directly, so they are transported through the lymphatic system.
- The lymphatic vessels eventually merge with the blood vessels near the heart, allowing the chylomicrons to enter the bloodstream.
- Once in the bloodstream, the chylomicrons are acted upon by enzymes that break down the triglycerides into fatty acids and glycerol. These compounds can then be used by cells throughout the body for energy or stored as fat.

Salivary Amylase Hydrolyses Only a Small Percentage of Starch in the Mouth

- Starch is a complex carbohydrate that is broken down into simpler sugars, such as glucose, during digestion.
- Salivary amylase is an enzyme that is produced in the salivary glands and helps to break down starch in the mouth.
- However, salivary amylase is only active for a brief period of time before it is inactivated by the acidic environment of the stomach.
- Studies have shown that salivary amylase hydrolyses only a small percentage of starch in the mouth, typically less than 10%.
- The majority of starch digestion occurs in the small intestine, where pancreatic amylase and other enzymes continue the process of breaking down the complex carbohydrate into simpler sugars.

Oxyntic Cells in Stomach Secrete Pepsinogen, Not Pepsin

- Pepsin is an enzyme that helps to digest proteins in the stomach.
- However, pepsin is not actually secreted by the oxyntic cells in the stomach. Instead, these cells secrete an inactive precursor molecule called pepsinogen.
- Pepsinogen is activated by the acidic environment of the stomach, which causes it to undergo a conformational change and become the active enzyme pepsin.
- Pepsin then helps to break down proteins into smaller peptides and amino acids, which can be absorbed in the small intestine.
- The production and secretion of pepsinogen and pepsin is regulated by several hormones, including gastrin and secretin, which are released in response to the presence of food