4.2.4 Break It Down - Investing Enzyme Action

Science Background - Digestion is a necessary process in the human body because the foods we eat cannot benefit the body until they are broken down. Food consists of carbohydrate, protein, and lipid macromolecules that are much too large for the small intestine to absorb. Instead, these macromolecules must first be broken down into their components before they can be absorbed and their nutritional benefits conferred to the body. Digestion begins in the mouth with mastication, or chewing. This process of mechanical digestion physically transforms the food we eat by grinding it into smaller pieces. This increases the surface area of ingested food that can be exposed to digestive enzymes. As mastication is occurring, the food is mixed with saliva excreted from salivary glands in the mouth. Saliva contains the enzyme amylase, which catalyzes the breakdown of starch, a polysaccharide macromolecule, into its component sugars. The partially digested mass of food, called a bolus, moves through muscle action from the mouth, down the esophagus, to the stomach. Once food reaches the stomach, the process of gastric digestion can occur. During gastric digestion, glands in the stomach lining secrete enzymes and hydrochloric acid that mix with the bolus. One of the enzymes, pepsin, catalyzes the reduction of proteins into polypeptides and amino acids. Pepsin requires temperatures around 37°C (i.e., body temperature) and low pH to catalyze this reaction. Hydrochloric acid serves to lower the pH, which inactivates and kills some potentially harmful microorganisms in the food. Digestion of the bolus in the stomach results in a semi-liquid referred to as chyme. Exiting the stomach, the chyme passes through the pyloric sphincter, which separates the stomach and the small intestine. This first section of the small intestine is called the duodenum. Once there, the chyme stimulates the production of alkaline intestinal fluids that counteract the acidity of the gastric products. When the semiliquid becomes alkaline, gastric digestion ceases and intestinal digestion begins. Pancreatic juice is secreted by the pancreas into the duodenum. This liquid contains the enzymes amylase, trypsin, and lipase. Amylase breaks down starches into their component sugars, trypsin hydrolyzes polypeptides and converts them into amino acids, and lipase breaks down fats into glycerol and fatty acids. After the chemical breakdown of the food in the stomach and small intestine has been accomplished, absorption must occur for the needed nutrients to reach the cells. Absorption is the passage of substances through the intestinal mucosa into the blood or lymph. The majority of nutrient absorption occurs in the second and third sections of the small intestine, the jejunum and ileum, respectively. Water absorption and salt absorption occur in the large intestine, or colon. We can think of digestion much like a textbook. You are not handed a textbook on the first day of class and told to read it in one night to prepare for a comprehensive test. Instead, the textbook is divided into smaller sections, or chapters, and you are instead required to read and take tests on these smaller components. Reading the whole textbook at once makes it very difficult for you to “absorb” the complex material. Breaking it down makes the material much easier to learn. Similarly, unconsumed pieces of food are much too large and complex for the body to absorb nutrients from. The structure of the food itself must instead be broken down and large, complex macromolecules reduced to their smaller components before the body can absorb nutrients. This process of breaking down food, both mechanically and enzymatically, is called digestion. Consider the different foods you eat throughout a typical day. Food consists of three common classes of macromolecules: carbohydrates, proteins, and lipids. Almost every meal you eat contains all three of these types of macromolecules. Consider a cheeseburger; we don’t typically think of this as a complex meal, but it consists of a carbohydrate-rich bun and lipid- and protein-rich beef and cheese. In order to absorb the nutrients provided by this food, these macromolecules must be broken down into their smaller components. The chart below lists macromolecule classes, the function of those classes, and the components of the macromolecules (i.e., what the body breaks food down into so the nutrients can be absorbed). Macromolecule Class Function Components Carbohydrates (more specifically, polysaccharides energy, energy storage, structural components of cells monosaccharides (glucose, fructose), disaccharides (sucrose, maltose) Proteins wide variety of functions: some are enzymatic proteins that catalyze reactions, structural proteins that provide support, storage proteins, transport proteins, hormonal proteins, receptor proteins, contractile and motor proteins, and defensive proteins amino acids, polypeptides Lipids energy storage, cellular membrane construction, cell signaling glycerol and fatty acid chains Deoxyribonucleic acid (DNA), ribonucleic acid (RNA)* store and transmit genetic information nucleotides

In Activity 1, you crushed an effervescent tablet and dissolved it in water, then you used a half tablet that was not crushed, and compared the time to dissolve. This simulates mastication of food and digestion in the mouth by amylase.

What is the purpose of chewing food before it reaches the stomach?

Chewing food increases the amount of surface area in contact with amylase, and the enzyme can work quickly to assist in the breakdown of starch.

What served as the source of protein in Activity 2?

albumin

After heating the protein with various pepsin/HCl solutions, you added 6 drops of Biuret reagent. What is true about Biuret reagent? (select all that apply)

Biuret reagent is purple in the presence of protein macromolecules.

Biuret reagent turns pink-violet in the presence of short-chain polypeptides, indicative of digested protein.

Which is the enzyme, pepsin or hydrochloric acid? What does it break down?

pepsin is the enzyme; breaks down proteins into short chains of amino acids

What is pepsin's ideal pH?

1.5 to 2

In which test tube did protein digestion occur? 

2-3, containing pepsin and hydrochloric acid

Can you think of another function of stomach acid besides digestion?

Stomach acid assists in killing potentially harmful bacteria that may be present in ingested food.

Exiting the stomach, the resulting chyme moves through which sphincter into the small intestine?

pyloric sphincter

What pH does the intestine prefer?

alkaline

In Activity 3: Intestinal Digestion of Starch, what were we testing?

We wanted to determine whether temperature effects the digestion of starch.

What is pancreatin?

a mixture of pancreatic enzymes including amylase

What color does the indicator iodine turn when starch is broken down into smaller monosaccharides?

yellow

After performing Activity 3: Intestinal Digestion of Starch, what color did the starch + pancreatin powder turn in the 37-40 degrees Celsius water bath?

brownish yellow (negative for starch)

What can you conclude about amylase when tested at different temperatures?

Amylase functions best at normal body temperatures (37 degrees celsius) but becomes denatured at high temperatures (100 degrees celsius).

According to the information in Activity 3: Intestinal Digestion of Starch, pancreatic juice contains which enzymes? (select all that apply)

lipase

amylase

trypsin

trypsin

polypeptides into amino acids

amylase

starch into sugars

lipase

lipids/fats into glycerol and fatty acids

In Activity 4: Intestinal Digestion of Proteins and Lipids, litmus powder was added to the milk, so that it would change color based on the pH of the solution to which it is added. Which macromolecule, when broken down, will become acidic? (select all that apply)

proteins break down into amino acids, becoming more acidic

lipids break down into fatty acids, becoming more acidic

4-2 litmus milk, water

gray-purple (negative for acid production)

4-1 litmus milk, pancreatin powder

pink-purple (positive for acid production)

pepsin

produced in the stomach --> used in the stomach

trypsin

produced in the pancreas --> used in the small intestine

amylase

produced in the salivary glands & pancreas --> used in the mouth and small intestine

lipase

produced in the pancreas --> used in the small intestine

hydrochloric acid

produced in the stomach --> used in the stomach