Question

The role of the liver in digestion is to: a. synthesize aminopeptidase and dipeptidase to digest polypeptides. b. synthesize lipase to form free fatty acids. c. secrete trypsin to break the bonds in polypeptides. d. secrete bicarbonate ions and bile salts to help emulsify fats. e. store bile between meals.

Short answer

The primary role of the liver in digestion is to secrete bicarbonate ions and bile salts to help emulsify fats, which corresponds to option (d).

Step-by-step solution

Understand the function of the liver in digestion

The liver performs various functions in the body including bile production, which aids in the process of fat digestion. Bile acts as an emulsifier that helps in breaking down dietary fats into smaller particles, making them more accessible to digestive enzymes.

Eliminate incorrect options

Eliminate options that do not describe the function of the liver in digestion. The liver does not synthesize aminopeptidase and dipeptidase (a), lipase (b), or trypsin (c), as these enzymes are associated with the small intestine and the pancreas. Instead, the liver does produce bile, which contains bile salts and is connected with emulsifying fats.

Choose the correct answer

Since the liver's major role in digestion is to produce bile, which contains bicarbonate ions and bile salts to help emulsify fats, the correct option is (d). Additionally, the liver stores bile between meals which is a secondary role mentioned in option (e). However, the primary role is described in option (d), making it the correct choice.

Key concepts

Bile Production

One of the liver's vital roles in the digestive system is to produce bile. Bile is a complex fluid containing bile salts, pigments, and cholesterol. Its primary function is to break down fats into smaller particles. These bile salts act as detergents, which essentially means they break apart fat globules, turning them into tiny droplets. This process is called emulsification, and it's crucial because it increases the surface area of lipids, making them easier for digestive enzymes to process.

Without adequate bile production, the digestion and absorption of fats would be significantly impaired, leading to malnutrition and deficiencies in fat-soluble vitamins (A, D, E, and K). Beyond its emulsifying properties, bile also helps to remove waste products from the liver, which are excreted into the intestine and eventually leave the body in feces.

Fat Digestion

Fat digestion begins in the stomach but primarily occurs in the small intestine with the aid of bile and pancreatic lipase. Once bile emulsifies fats into smaller droplets, pancreatic lipase can efficiently act on these droplets to break down triglycerides— the main constituents of dietary fat— into free fatty acids and monoglycerides. These are then absorbed by the small intestine's lining.

However, without bile's emulsifying action, the lipase wouldn't be able to access much of the fat, resulting in poorly digested fats passing through the digestive tract. Proper fat digestion is important not only for the absorption of essential nutrients but also for the maintenance of health, as fats contribute to energy, cell structure, and the transportation of vitamins.

Digestive Enzymes

Digestive enzymes are specialized proteins that speed up the chemical reactions necessary to turn the food we eat into substances that can be absorbed by our body. These enzymes work at various points in the digestive system. The liver itself doesn't produce digestive enzymes in the same way the pancreas does; however, it does contribute to the process indirectly through bile production.

Types and Functions of Digestive Enzymes

• Proteases and peptidases: These enzymes break down proteins into smaller peptides and amino acids.
• Lipases: They specialize in breaking down fats into fatty acids and glycerol.
• Carbohydrases: They break down carbohydrates like starch into simple sugars.
• Nucleases: They break down nucleic acids— which make up DNA and RNA— into nucleotides.

Each type of enzyme targets specific types of molecules, ensuring efficient digestion. By the time food reaches the small intestine, these enzymes have facilitated its breakdown into the smallest absorbable units.