Starch (Carbohydrate) Digestion and Absorption

Summary

This video delves into the digestion of starch, highlighting its structure composed of amylose and amylopectin. It explains the partial breakdown by salivary amylase in the mouth, the inactivation of this enzyme in the stomach, and the complete digestion in the small intestine by pancreatic amylase and brush border enzymes. The process results in glucose absorption via sodium-glucose linked transporters and its use as energy or storage as glycogen. Undigested starch, known as resistant starch, ferments in the colon, producing short-chain fatty acids, while undigested portions are excreted, illustrating the complex journey of starch in the human body.

Takeaways

• 🍞 Starch is a key component of many foods, such as bread, and is made up of two types of glucose polymers: amylose and amylopectin.
• 🔗 Amylose is a linear chain of glucose molecules linked by α-1,4 glycosidic bonds, while amylopectin has a tree-like structure with branches linked by α-1,6 glycosidic bonds.
• 🦷 The digestion of starch begins in the mouth, where physical actions like chewing and chemical actions from salivary amylase start breaking down starch into smaller units.
• 🌊 Salivary amylase specifically targets α-1,4 glycosidic bonds, but its activity is limited as it becomes inactivated in the acidic environment of the stomach.
• 🔄 Starch reaches the small intestine in a partially hydrolyzed form, where most of the digestion occurs with the help of pancreatic amylase and brush border enzymes.
• 🧬 Pancreatic amylase continues the breakdown of α-1,4 glycosidic bonds, further hydrolyzing starch into smaller components.
• 🛠 Brush border enzymes, such as maltase and isomaltase, play crucial roles in breaking down the final bonds of starch, including the branch points.
• 🚰 The small intestine is equipped with sodium-glucose linked transporters (SGLTs) that facilitate the absorption of glucose into the body.
• 🔄 Glucose absorption is coupled with the transport of sodium ions, and once inside the cells, glucose is reabsorbed into the bloodstream through GLUT2 transporters.
• 🚀 The absorbed glucose can be used by the body for energy or stored in the liver as glycogen for future use.
• 🌱 Some starch, known as resistant starch, escapes digestion in the small intestine and reaches the colon, where it is fermented by gut microbiota, producing short-chain fatty acids.
• 🚮 Undigested and unabsorbed starch is ultimately excreted from the body as waste.

Q & A

What is starch made up of?

Starch is made up of two forms of glucose polymers: the linear amylose and the branched amylopectin.

How are the glucose molecules in amylose linked together?

In amylose, glucose molecules are linked together by alpha 1-4 glycosidic bonds to form a linear chain.

What is unique about the structure of amylopectin compared to amylose?

Amylopectin has a tree-like structure with linear chains of glucose and branch points, which are created by alpha 1-6 glycosidic bonds.

What is the role of salivary amylase in starch digestion?

Salivary amylase, secreted by the salivary glands, begins the digestion of starch by breaking down alpha 1-4 glycosidic bonds in the mouth.

Why does starch digestion not occur within the stomach?

Starch digestion does not occur in the stomach because the acidic environment inactivates the salivary amylase.

Where does most of the starch digestion and absorption take place?

Most of the starch digestion and absorption takes place in the small intestine.

What is the role of pancreatic amylase in starch digestion?

Pancreatic amylase, secreted by the pancreas, further breaks down the alpha 1-4 glycosidic bonds in the small intestine, continuing the digestion of starch.

What are brush border enzymes and how do they participate in starch digestion?

Brush border enzymes are enzymes found on the surface of the intestinal cells (enterocytes) that participate in the digestion of starch, including enzymes like maltase and isomaltase that hydrolyze different types of glycosidic bonds.

How is glucose absorbed into the body from the small intestine?

Glucose is absorbed into the body through sodium-glucose linked transporters (SGLTs) on the apical surface of enterocytes, which function as co-transporters for both sodium and glucose.

What happens to the portion of starch that is resistant to digestion in the small intestine?

The portion of starch that is resistant to digestion in the small intestine, known as resistant starch, reaches the colon where it undergoes fermentation by the gut microbiota.

What is the final fate of the starch that is not fermented or absorbed in the colon?

The starch that is not fermented or absorbed in the colon is excreted as waste by the human body.