Question

Name the end products of digestion, and explain how each is absorbed in the small intestine. (pp. 370,382 )

Short answer

The end products of digestion are glucose, fructose, and galactose (from carbohydrates); amino acids (from proteins); and fatty acids and glycerol (from fats). Glucose, galactose, and amino acids are absorbed by the small intestine through active transport processes, while fructose is absorbed through facilitated diffusion. Fatty acids and glycerol are combined to form micelles, which diffuse across the intestinal cell membrane; they are then reassembled into triglycerides and packaged into chylomicrons, which enter lacteals and are transported through the lymphatic system before entering the bloodstream.

Step-by-step solution

Identify the major components of food and their end products

The three main components of food are carbohydrates, proteins, and fats. During digestion, these components are broken down into their simplest forms, which are known as the end products of digestion. Carbohydrates are broken down into simple sugars (monosaccharides) such as glucose, fructose, and galactose. Proteins are broken down into amino acids. And fats are broken down into fatty acids and glycerol.

Explain the absorption of glucose, fructose, and galactose

The absorption of monosaccharides (glucose, fructose, and galactose) occurs in the small intestine. Glucose and galactose are absorbed by the intestinal cells using a sodium-dependent active transport process. This means that these sugars are transported into the intestinal cells along with sodium ions, against their concentration gradient. This process requires energy. Once inside the intestinal cells, glucose and galactose are transported into the bloodstream for distribution throughout the body. Fructose, on the other hand, is absorbed through a different mechanism known as facilitated diffusion. In this process, fructose molecules bind to specific transport proteins in the cell membrane of the intestinal cells and are then transported into the cell. This process does not require energy. Once inside the intestinal cells, fructose is also transported into the bloodstream.

Explain the absorption of amino acids

Amino acids, the end products of protein digestion, are absorbed in the small intestine through an active transport process, similar to that of glucose and galactose absorption. There are specific transport proteins for each of the different amino acids, allowing them to be absorbed in the intestinal cells against their concentration gradient. This process requires energy. Once absorbed, amino acids enter the bloodstream and are transported to various parts of the body.

Explain the absorption of fatty acids and glycerol

The absorption of fatty acids and glycerol, the end products of fat digestion, occurs through a somewhat different process than the absorption of carbohydrates and proteins. Fatty acids and glycerol are firstly combined to form micelles with the help of bile salts. These micelles facilitate the diffusion of fatty acids and glycerol across the cell membrane of intestinal cells. Inside the intestinal cells, fatty acids and glycerol are reassembled into triglycerides and then packaged into structures called chylomicrons. These chylomicrons are then transported into lacteals, which are small lymphatic vessels located within the villi of the small intestine. From the lacteals, chylomicrons are transported through the lymphatic system and eventually enter the bloodstream for distribution throughout the body. In summary, the end products of digestion are monosaccharides (glucose, fructose, galactose), amino acids, fatty acids, and glycerol which are all absorbed in the small intestine through different mechanisms, allowing the body to effectively utilize these nutrients.

Key concepts

Monosaccharides Absorption

Monosaccharides are the simplest form of carbohydrates and include glucose, fructose, and galactose. These sugars are absorbed in the small intestine through unique mechanisms.

Glucose and galactose are absorbed using sodium-dependent active transport. This means these sugars hitch a ride with sodium ions to move into the intestinal cells. Since they are moving against their concentration gradient, this process requires energy, similar to using a battery power to move uphill. Once inside the cells, glucose and galactose are quickly transported into the bloodstream, ensuring they reach various body cells efficiently.

Fructose takes a different route called facilitated diffusion. It uses a special carrier protein to help pass through the cell membrane without needing any energy. It's as if fructose has a pass that allows it to enter the cell easily. Once inside, it too enters the bloodstream to be used by the body.

• Glucose and galactose: absorbed via active transport using energy.
• Fructose: absorbed via facilitated diffusion without energy.

Amino Acids Absorption

Amino acids, the building blocks of proteins, are absorbed similarly to glucose and galactose. They use active transport to move against a concentration gradient into the intestinal cells. This process requires energy because each amino acid needs to pair with specific transport proteins. Imagine them as keys fitting into particular locks to open the door for entry.

Once absorbed through the intestinal cells, amino acids swiftly enter the blood. They then travel throughout the body where they're used for various essential functions like building muscles and repairing tissues.

Here's a quick summary:

• Amino acids require specific transport proteins.
• Energy is required since they are absorbed actively.
• Once in the bloodstream, they are distributed to various body cells.

Fatty Acids and Glycerol Absorption

The digestion end products of fats, namely fatty acids and glycerol, undergo a fascinating absorption process. This process is notably different from carbohydrates and proteins. Before absorption, fatty acids and glycerol form small complexes called micelles along with bile salts.

Micelles help these fat molecules cross into the intestinal cells via simple diffusion. Once inside, they reunite to form triglycerides. Triglycerides are then enclosed within chylomicrons, which are special carriers resembling package cars.

These chylomicrons exit the intestinal cells and enter lacteals, which are small lymphatic vessels. From here, chylomicrons navigate through the lymphatic system and eventually merge into the bloodstream, distributing fat throughout the body.

Key points include:

• Micelles facilitate the initial absorption of fats in the intestine.
• Triglycerides are reformed and packaged into chylomicrons.
• Chylomicrons travel via the lymphatic system before entering the blood.