Lesson Explainer: The Digestive Process Biology • Second Year of Secondary School

In this explainer, we will learn how to describe the processes of buccal, gastric, and intestinal digestion in the human body.

The human digestive system works tirelessly, whether we are awake or asleep, standing upright or even balancing upside down! It does this by using muscles and chemicals to digest the food we eat and break it down into smaller pieces.

The digestive system has many fantastic adaptations that allow it to carry out these functions. Did you know that the internal surface area of the small intestine is approximately a massive 250 square metres, the size of a tennis court? Did you know that stomach growling occurs when food, gases, and liquids are being moved through the digestive system, and this is simply louder when one is hungry as there is no food to muffle the sound?

In this explainer, we are going to look at the basic structure of the human digestive system, before going into more detail about the functions and adaptations of the mouth, esophagus, stomach, pancreas, and small intestine.

Digestion is a process in which large molecules are broken down into smaller molecules. This is essential, as we need to break down the large nutrients in the food we ingest into a form small enough to be absorbed into the bloodstream. These large molecules are broken down into their smaller constituent subunits by enzymes that are specific to the type of substrate they break down. For example, protein polymers are broken down by a group of enzymes called proteases. Once they are fully broken down, the monomers move out of the digestive system into surrounding capillaries. The blood then carries these smaller nutrients to the body cells that use them to build up a diverse range of molecules to be used by the body.

Example 1: Describing the Main Function of the Digestive System

What is the main function of the digestive system?

1. Collecting and processing information from external and internal stimuli
2. Breaking down large, insoluble molecules into smaller, soluble ones
3. Synthesizing large molecules from smaller ones
4. Regulating cellular metabolism and respiration
5. Maintaining a constant internal environment

Answer

Digestion is a process in which large insoluble macromolecules like fats are broken down into smaller subunits. This is essential, as we need to break down the large nutrients in the food we ingest into a form small enough to be absorbed into the bloodstream. These large molecules are broken down into their smaller constituent subunits by enzymes that are specific to the type of substrate they break down.

For example, fats (lipids) are large, insoluble molecules. They are broken down by a group of enzymes called lipases to form smaller, soluble subunits of fatty acids and glycerol.

Once they are broken down, the subunits move out of the digestive system into surrounding lacteals that connect the small intestine to the lymphatic system that then connects to the bloodstream at a larger junction. The blood then carries these soluble nutrients to the body cells that use them to build up a diverse range of molecules to be used by the body.

Therefore, the main function of the digestive system is breaking down large, insoluble molecules into smaller, soluble ones.

There are two main types of digestion based on how food is broken down that we will be discussing: chemical and mechanical digestion.

Chemical digestion refers to enzymes breaking down large nutrient molecules.

Mechanical digestion is the physical process of structures like teeth in the mouth and muscles in the stomach churning food into smaller pieces. This makes it easier for enzymes to digest food as it increases the surface area that enzymes can act on. You can see this process occurring in Figure 1 below.

You can see in Figure 1 that mechanical digestion, for example, by the teeth, increases the surface area of the food bolus compared to its volume. This means that there is a larger surface area upon which the enzymes, for example, in saliva, can act to break down the nutrients in the food bolus even further.

Let’s take a look at the route food takes when it enters the body and see how the digestive organs play important roles in digestion. The main organs that carry out digestive processes can be seen in Figure 2, and the direction food travels in is marked with light blue arrows.

You may have noticed that not all of the organs in Figure 1 had the food pass through them. Any organ that plays a role in digestion but is not part of the digestive tract as food does not pass through it is called an accessory organ. The main accessory organs in the human digestive system are the salivary glands, pancreas, liver, and gallbladder.

Let’s look at the role of each of these digestive organs in more detail, starting with buccal digestion.

The word buccal refers to the mouth, and the main structures involved are the teeth, tongue, and salivary glands. Human teeth, as you can see in Figure 3, are excellently adapted to chew different types of food, as humans are omnivores eating both animal and plant products. Our incisors at the front of the jaw cut food, and the adjacent sharp canines grip and tear food into smaller pieces. At the back of the jaw, premolars and molars grind down food to give it a larger surface area. These are all examples of mechanical digestion, using muscles in the jaw to break and grind food down.

We mostly think of the tongue as a tool to taste food, but it also plays an important role in moving food and helping the teeth in mechanical digestion. Three-paired salivary glands, which you can see in Figure 4, secrete saliva into the mouth. Saliva contains mucus to soften food by making it more fluid, and it contains the enzyme amylase. Amylase begins to catalyze the breakdown of the polysaccharide starch in food into the disaccharide maltose via a process called hydrolysis. Maltose will then be broken down by maltase enzymes into a monosaccharide called glucose when it reaches the small intestine. You can see this process occurring on the right in Figure 4, which shows a magnified diagram of saliva breaking down starch. Amylase works best in slightly alkaline conditions with a pH of approximately 7.4.

At the back of the mouth, in Figure 4, there are two tubes: the trachea that leads to the lungs and the esophagus that leads to the stomach. To prevent food entering the trachea and lungs, when the bolus reaches the pharynx at the back of the mouth, the trachea and larynx (voice box) rise. This causes a flap of tissue called the epiglottis at the back of the throat to close over the entrance to the trachea. Usually, the epiglottis is open, allowing gases to move down the trachea to the lungs. In Figure 4, the epiglottis is closed, so no food or drink should enter the trachea, only the esophagus. If the epiglottis did not close, food or drink may enter the lungs and cause choking.

Once food is in the esophagus, it must be moved down to the stomach. The esophagus is a long tube with glands that secrete more mucus onto the bolus and circular muscles in its lining. Figure 5 shows how these muscles rhythmically contract and relax to push the bolus through the esophagus to the stomach. This process is called peristalsis, and it occurs in many structures in the digestive system: the esophagus, the intestines, and the stomach. Peristalsis helps to move food and mix it with digestive juices.

Let’s look at gastric digestion next, which refers to processes occurring in the stomach.

When the food bolus approaches the end of the esophagus, it reaches a thick muscular ring called the cardiac sphincter. The cardiac sphincter is usually closed, but it opens when the bolus reaches it allowing peristaltic movements to push the bolus out of the esophagus and into the stomach.

The stomach is a muscular sac, which is able to move the food using peristalsis. Once the bolus reaches the stomach, it mixes with gastric juice and becomes chyme. Gastric juice, sometimes known as stomach acid, consists of strong hydrochloric acid and a substance called pepsinogen.

The hydrochloric acid helps in two ways. Firstly, it prevents any disease-causing microorganisms that may have been ingested from entering the rest of the digestive system or the bloodstream. It also creates acidic conditions that allow pepsinogen secreted by the stomach lining to be activated and converted into pepsin. This is shown in a magnified view at the bottom right of Figure 6.

Pepsin is a protease enzyme that catalyzes the hydrolysis of its protein substrate into smaller polypeptides. These polypeptides will eventually be broken down into amino acids in the small intestine. The optimum pH of pepsin is between 1.5 to 2.5, so it functions very effectively in the acidic stomach environment. The cells in the walls of the stomach itself are protected from being digested by pepsin by mucus that lines the inside of the stomach.

At the base of the stomach is another ring of muscle called the pyloric sphincter. The pyloric sphincter is also usually closed, but a peristaltic contraction causes it to open briefly, allowing some food to pass from the stomach and into the first section of the small intestine: the duodenum.

The final part of the digestive process is intestinal digestion in the small intestine. The small intestine is where the majority of chemical digestion will occur, as proteins, carbohydrates, and lipids will all be digested by enzymes in the small intestine.

Most digestive enzymes are secreted by the pancreas, which can be seen in Figure 7.

Pancreatic juice enters the upper intestine, called the duodenum, where it mixes with intestinal juice and bile. Bile is produced by the liver and stored in the gallbladder before being secreted into the duodenum via the bile duct, organs that are also shown in Figure 7. Bile emulsifies fats, breaking them down into smaller globules called emulsion droplets. It does this as lipids are insoluble in water, but the enzymes that break down fats are water soluble. Emulsification means that a larger surface area of fats is exposed to enzymes that can digest them. This increases the efficiency of lipid hydrolysis.

Example 3: Describing the Role of Bile in Intestinal Digestion

What is the primary purpose of bile in digestion?

1. To emulsify fats
2. To break down proteins
3. To activate trypsinogen
4. To catalyze the hydrolysis of carbohydrates

Answer

Bile is a substance produced by the liver and stored in the gallbladder before being secreted into the duodenum, the first section of the small intestine. Here, it combines with pancreatic juice and intestinal juice.

Bile’s main role is to emulsify fats (lipids), breaking them down into smaller globules called emulsion droplets. It does this as lipids are insoluble in water, but the enzymes that break down fats are water soluble. Emulsification means that a larger surface area of fats is exposed to enzymes that can digest them, as seen in the diagram below. This increases the efficiency of lipid hydrolysis, so the lipids are broken down faster and more efficiently.

Protease enzymes break down proteins, and carbohydrase enzymes break down carbohydrates like starch. Trypsinogen is an inactive enzyme that is activated by enterokinase enzyme in the duodenum into trypsin. Trypsin is an example of a protease enzyme.

Therefore, the primary purpose of bile in digestion is to emulsify fats.

Pancreatic juice contains sodium bicarbonate, an alkali that neutralizes the acidic gastric juice from the stomach as it enters the duodenum. This means the pH in the duodenum is about 6, but it increases as the small intestine continues to provide the optimum pH for many of the enzymes that will be acting there.

Pancreatic juice contains amylases that catalyze the hydrolysis of carbohydrates into simple sugars and lipases that catalyze the hydrolysis of lipids into fatty acids and glycerol. Pancreatic juice also contains trypsinogen. Trypsinogen is activated when it enters the duodenum and comes into contact with an enzyme called enterokinase. This converts trypsinogen to trypsin, a protease enzyme that catalyzes the hydrolysis of proteins into smaller polypeptides and eventually, amino acids.

The pancreatic juice and bile combine with intestinal juice secreted by cells in the small intestine wall. Intestinal juice also contains protease, lipase, and carbohydrase enzymes. There are many carbohydrases, such as maltase that breaks down maltose into glucose, sucrase that breaks down sucrose into glucose and fructose, and lactase that breaks down lactose into glucose and galactose. Furthermore, the breakdown of starch by amylases that began in the mouth will continue in the small intestine.

Once the nutrients have been broken down sufficiently by all of the enzymes, the simple sugars, amino acids, fatty acids, and glycerol are absorbed across the wall of the small intestine. Here, sugars and amino acids are absorbed by capillaries and into the blood to be delivered to body cells. As fatty acids and glycerol are not water soluble and are too large to pass into a capillary, they travel into lacteal vessels that transport them into the lymphatic system. They then enter the bloodstream at a larger junction.

Let’s summarize the processes that we have covered.

Food is placed into the mouth where it is mixed with saliva and mashed into a ball called a bolus. This is called buccal digestion. From the mouth, the bolus travels down the esophagus to the stomach where gastric digestion occurs and then into the small intestine where intestinal digestion, and the bulk of chemical digestion, occurs. Once it reaches the stomach, the bolus is reduced in size and is now known as chyme. The chyme then passes into the large intestine, which reabsorbs water, any remaining vitamins, and salts from the chyme to form solid feces. Feces are stored in the rectum before they are removed from the body by egestion through the anus.

Let’s recap some of the key points we have covered in this explainer.