7.5: Catabolism of food

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Learning Objectives

To describe how carbohydrates, fats, and proteins are broken down during digestion.

We have said that animals obtain chemical energy from the food—carbohydrates, fats, and proteins—they eat through reactions defined collectively as catabolism. We can think of catabolism as occurring in three stages (Figure \(\PageIndex{1}\)). In stage I, carbohydrates, fats, and proteins are broken down into their individual monomer units: carbohydrates into simple sugars, fats into fatty acids and glycerol, and proteins into amino acids. One part of stage I of catabolism is the breakdown of food molecules by hydrolysis reactions into the individual monomer units—which occurs in the mouth, stomach, and small intestine—and is referred to as digestion.

In stage II, these monomer units (or building blocks) are further broken down through different reaction pathways, one of which produces ATP, to form a common end product that can then be used in stage III to produce even more ATP. In this chapter, we will look at each stage of catabolism—as an overview and in detail.

Figure \(\PageIndex{1}\): Energy Conversions. The conversion of food into cellular energy (as ATP) occurs in three stages.

Summary

During digestion, carbohydrates are broken down into monosaccharides, proteins are broken down into amino acids, and triglycerides are broken down into glycerol and fatty acids. Most of the digestion reactions occur in the small intestine.

Concept Review Exercises

Answers

Exercises

Answers

Distinguish between each pair of compounds.
1. pepsin and pepsinogen
2. chymotrypsin and trypsin
3. aminopeptidase and carboxypeptidase
What are the primary end products of each form of digestion?
1. carbohydrate digestion
2. lipid digestion
3. protein digestion
In what section of the digestive tract does most of the carbohydrate, lipid, and protein digestion take place?
1. Pepsinogen is an inactive form of pepsin; pepsin is the active form of the enzyme.
2. Both enzymes catalyze the hydrolysis of peptide bonds. Chymotrypsin catalyzes the hydrolysis of peptide bonds following aromatic amino acids, while trypsin catalyzes the hydrolysis of peptide bonds following lysine and arginine.
3. Aminopeptidase catalyzes the hydrolysis of amino acids from the N-terminal end of a protein, while carboxypeptidase catalyzes the hydrolysis of amino acids from the C-terminal end of a protein.
1. glucose, fructose, and galactose
2. monoglycerides and fatty acids
3. amino acids
the small intestine
What are the products of digestion (or stage I of catabolism)?
What is the general type of reaction used in digestion?
Give the site of action and the function of each enzyme.
1. chymotrypsin
2. lactase
3. pepsin
4. maltase
Give the site of action and the function of each enzyme.
1. α-amylase
2. trypsin
3. sucrase
4. aminopeptidase
1. What is the meaning of the following statement? “Bile salts act to emulsify lipids in the small intestine.”
2. Why is emulsification important?
Using chemical equations, describe the chemical changes that triglycerides undergo during digestion.
What are the expected products from the enzymatic action of chymotrypsin on each amino acid segment?
1. gly-ala-phe-thr-leu
2. ala-ile-tyr-ser-arg
3. val-trp-arg-leu-cys
What are the expected products from the enzymatic action of trypsin on each amino acid segment?
1. leu-thr-glu-lys-ala
2. phe-arg-ala-leu-val
3. ala-arg-glu-trp-lys
proteins: amino acids; carbohydrates: monosaccharides; fats: fatty acids and glycerol
1. Chymotrypsin is found in the small intestine and catalyzes the hydrolysis of peptide bonds following aromatic amino acids.
2. Lactase is found in the small intestine and catalyzes the hydrolysis of lactose.
3. Pepsin is found in the stomach and catalyzes the hydrolysis of peptide bonds, primarily those that occur after aromatic amino acids.
4. Maltase is found in the small intestine and catalyzes the hydrolysis of maltose.
1. Bile salts aid in digestion by dispersing lipids throughout the aqueous solution in the small intestine.
2. Emulsification is important because lipids are not soluble in water; it breaks lipids up into smaller particles that can be more readily hydrolyzed by lipases.
1. gly-ala-phe and thr-leu
2. ala-ile-tyr and ser-arg
3. val-trp and arg-leu-cys