Digestion, absorption and ileum structure

In a nutshell

Digestion is an important process that allows larger molecules to be broken down into smaller molecules that cells can use. This summary explores the enzymes involved in digestion and how the products are absorbed.

Digestion

Definition 

During digestion, large biological molecules in food are broken down by hydrolysis into smaller molecules that can be absorbed across cell membranes. As a result, they can be transported around the body for cells to use.

Examples

• Carbohydrates are broken down into disaccharides then further broken down to form monosaccharides.
  
• Lipids are broken down into fatty acids and monoglycerides.
  
• Proteins are broken down into their constituent amino acids. 

​

Enzymes

Enzymes are substrate specific therefore organisms will require many different enzymes to digest many different substrates. Digestive enzymes catalyse hydrolysis reactions to break down biological molecules in food. Hydrolysis reactions involve splitting molecules by adding water to break the bonds that hold them together.

Examples

• Carbohydrases catalyse the breakdown of carbohydrates. Amylase is a carbohydrase that breaks down starch to create maltose. 
  
• Proteases or peptidases break down proteins. Pepsin is a peptidase that breaks down proteins.
• Lipases catalyse the breakdown of fats.

Carbohydrate digestion

Carbohydrates, like starch, are biological molecules that contain carbon, hydrogen and oxygen. 

Starch is broken down by amylase. Amylase is produced by the salivary glands and secreted into the mouth where it hydrolyses starch into a disaccharide called maltose. Salivary amylase is denatured by the acidic pH of the stomach. However, amylase is also produced in the pancreas which releases the enzyme into the small intestine. As a result, the hydrolysis of starch continues. 

Membrane-bound disaccharidases are digestive enzymes that break down disaccharides into monosaccharides. They are found on the cell membrane of the epithelial cells that line the ileum. Some disaccharidases and their products are listed below.

Carbohydrate absorption

Carbohydrates are transported across the ileum epithelium and absorbed into the bloodstream. The table below describes the transport method through which monosaccharides are absorbed. 

Protein digestion 

Proteins are polymers made of subunits called amino acids. Amino acids are joined together by peptide bonds and these bonds are hydrolysed during protein digestion. Proteins are digested by endopeptidases and exopeptidases. These are explained in the table below. 

Protein absorption

The products of protein digestion are amino acids and these are transported through co-transport like glucose and galactose. Here, sodium-dependent transporter proteins are used to co-transport sodium ions and amino acids across a membrane. 

Lipid digestion

Most dietary fat is a type of lipid called triglyceride. Triglycerides are macromolecules that consist of three fatty acid molecules, which are each linked by an ester bond to one of the three $$-OH$$ groups of a glycerol molecule. 

Bile is a liquid made in the liver and it contains bile salts that are secreted into the small intestine. The bile salts binds hydrophobic fat droplets and emulsifies them. This means that they break them down into smaller droplets. This increases the surface area for the lipase to break down the lipids.

Lipases are made in the pancreas they are transported into the small intestine. There, they break down lipids into fatty acids and monoglycerides. These will then bind with bile salts to produce micelles.

Lipid absorption

Micelles constantly break apart and re-form so they are able to release monoglycerides and fatty acids near the epithelial cells. They can enter these cells directly by diffusion as they are soluble in lipids.

​​​​

Ileum structure

The ileum is the last part of the small intestine and it has an important role in absorbing the products of digestion. It has various adaptations to facilitate this function.