Julianne Zedalis; John Eggebrecht

Learning Objectives

In this section, you will explore the following questions:

What are the differences between digestion and absorption?
What are different types of digestive systems in invertebrates and vertebrates?
What are the specialized functions of the organs involved in processing food in the human body?
How do organs work together to digest food and absorb nutrients?

Connection for AP^® Courses

Much information in this chapter is not within the scope of AP^®. However, the chapter provides us with the opportunity to review concepts we’ve explored previously, including structure and function, macromolecules, energy production, transport of substances across membranes, and enzyme activity. All living organisms require a source of energy and molecules needed to build cells, tissues, and organs. During digestion, food is broken down into smaller molecules for absorption and distribution to all cells of the body. Nutrients are required to carry out cellular processes and maintain homeostasis, and digestion and absorption require the participation of several organs. Different animals have evolved different types of digestive systems specialized to meet their dietary needs. You do not need to memorize details about the different types of animal digestive systems, but you might find it interesting to explore the evolution of the system through a few groups of animals, from intracellular digestion in simple invertebrates to a digestive tract and accessory organs in complex vertebrates. Using a human eating a turkey sandwich as an example, food is ingested through the mouth. The mouth is the location where both mechanical (chewing) and chemical breakdown of food begins via the enzyme amylase, which breaks down carbohydrates into simpler sugars. The food bolus then travels by peristalsis (alternating waves of contraction) down the pharynx and esophagus to the stomach. In the stomach, pepsinogen mixes with hydrochloric acid to form pepsin, which begins digesting proteins, such as turkey, into smaller chains of amino acids. Mucus in the stomach protects its lining from damage by acidity, and the tightening of a sphincter prevents stomach contents from regurgitating into the esophagus. Further digestion of the ingredients of the sandwich occurs in the small intestine aided by a variety of enzymes; for example, bile salts and pancreatic amylase dumped into the small intestine from the gallbladder and pancreas, respectively, help emulsify fats. Once the ingredients of the sandwich have been broken down into smaller nutrient molecules, including amino acids, glucose, and fatty acids, they are absorbed from the small intestine into the circulatory and lymphatic systems. The walls of the small intestine contain small, finger-like projections called villi and microvilli that increase surface area for absorption of nutrients by diffusion. The large intestine or colon does not produce digestive enzymes but functions to absorb water, salts, and some vitamins. Any nutrients from the sandwich are stored in the liver, and wastes are eliminated.

Information presented and the examples highlighted in the section support concepts outlined in Big Idea 2 and Big Idea 4 of the AP^® Biology Curriculum Framework. The AP^® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP^® Biology course, an inquiry-based laboratory experience, instructional activities, and AP^® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.

Big Idea 2	Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.A	Growth, reproduction and maintenance of living systems require free energy and matter.
Essential Knowledge	2.A.3 Organisms must exchange matter with the environment to grow, reproduce and maintain organization.
Science Practice	2.2 The student can apply mathematical routines to quantities that describe natural phenomena.
Learning Objective	2.6 The student is able to use calculated surface area-to-volume ratios to predict which cell(s) might eliminate wastes or procure nutrients faster by diffusion.
Essential Knowledge	2.A.3 Organisms must exchange matter with the environment to grow, reproduce and maintain organization.
Science Practice	6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.
Learning Objective	2.7 The student will be able to explain how cell size and shape affects the overall rate of nutrient intake and the rate of waste elimination.
Big Idea 4	Biological systems interact, and these systems and their interactions possess complex properties.
Enduring Understanding 4.A	Interactions within biological systems lead to complex properties.
Essential Knowledge	4.A.4 Organisms exhibit complex properties due to interactions between their constituent parts.
Science Practice	3.3 The student can evaluate scientific questions.
Learning Objective	4.8 The student is able to evaluate scientific questions concerning organisms that exhibit complex properties due to the interaction of their constituent parts.
Essential Knowledge	4.A.4 Organisms exhibit complex properties due to interactions between their constituent parts.
Science Practice	6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.
Learning Objective	4.9 The student is able to predict the effects of a change in a component(s) of a biological system on the functionality of an organism(s).
Essential Knowledge	4.A.4 Organisms exhibit complex properties due to interactions between their constituent parts.
Science Practice	1.3 The student can refine representations and models of natural or man-made phenomena and systems in the domain.
Learning Objective	4.10 The student is able to refine representations and models to illustrate biocomplexity due to interactions of the constituent parts.
Enduring Understanding 4.B	Competition and cooperation are important aspects of biological systems.
Essential Knowledge	4.B.2 Cooperative interactions within organisms promote efficiency in the use of energy and matter.
Science Practice	1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.
Learning Objective	4.18 The student is able to use representations and models to analyze how cooperative interactions within organisms promote efficiency in the use of matter and energy.

Animals obtain their nutrition by consuming other organisms. Depending on their diet, animals can be classified into the following categories: plant eaters (herbivores), meat eaters (carnivores), and those that eat both plants and animals (omnivores). The nutrients and macromolecules present in food are not immediately accessible to the cells. There are a number of processes that modify food within the animal body to make the nutrients and organic molecules accessible for cellular function. As animals evolved in complexity of form and function, their digestive systems have also evolved to accommodate their various dietary needs.

Herbivores, Omnivores, and Carnivores

Herbivores are animals whose primary food source is plant-based. Examples of herbivores, as shown in Figure 25.2 include vertebrates like deer, koalas, and some bird species, as well as invertebrates such as crickets and caterpillars. These animals have evolved digestive systems capable of handling large amounts of plant material. Herbivores can be further classified into frugivores (fruit-eaters), granivores (seed eaters), nectivores (nectar feeders), and folivores (leaf eaters).

Left photo shows a buck with antlers. Right photo shows a black, yellow, and white striped caterpillar eating a leaf.

Figure 25.2 Herbivores, like this (a) mule deer and (b) monarch caterpillar, eat primarily plant material. (credit a: modification of work by Bill Ebbesen; credit b: modification of work by Doug Bowman)

Carnivores are animals that eat other animals. The word carnivore is derived from Latin and literally means “meat eater.” Wild cats such as lions, shown in Figure 25.3a and tigers are examples of vertebrate carnivores, as are snakes and sharks, while invertebrate carnivores include sea stars, spiders, and ladybugs, shown in Figure 25.3b. Obligate carnivores are those that rely entirely on animal flesh to obtain their nutrients; examples of obligate carnivores are members of the cat family, such as lions and cheetahs. Facultative carnivores are those that also eat non-animal food in addition to animal food. Note that there is no clear line that differentiates facultative carnivores from omnivores; dogs would be considered facultative carnivores.

Top photo shows a lion. Bottom photo shows a ladybug.

Figure 25.3 Carnivores like the (a) lion eat primarily meat. The (b) ladybug is also a carnivore that consumes small insects called aphids. (credit a: modification of work by Kevin Pluck; credit b: modification of work by Jon Sullivan)

Omnivores are animals that eat both plant- and animal-derived food. In Latin, omnivore means to eat everything. Humans, bears (shown in Figure 25.4a), and chickens are example of vertebrate omnivores; invertebrate omnivores include cockroaches and crayfish (shown in Figure 25.4b).

Top photo shows a bear. Bottom photo shows a crayfish.

Figure 25.4 Omnivores like the (a) bear and (b) crayfish eat both plant and animal based food. (credit a: modification of work by Dave Menke; credit b: modification of work by Jon Sullivan)

Invertebrate Digestive Systems

Animals have evolved different types of digestive systems to aid in the digestion of the various foods they consume. The simplest example is that of a gastrovascular cavity and is found in organisms with only one opening for digestion. Platyhelminthes (flatworms), Ctenophora (comb jellies), and Cnidaria (coral, jelly fish, and sea anemones) use this type of digestion. Gastrovascular cavities, as shown in Figure 25.5a, are typically a blind tube or cavity with only one opening, the “mouth”, which also serves as an “anus”. Ingested material enters the mouth and passes through a hollow, tubular cavity. Cells within the cavity secrete digestive enzymes that break down the food. The food particles are engulfed by the cells lining the gastrovascular cavity.

The alimentary canal, shown in Figure 25.5b, is a more advanced system: it consists of one tube with a mouth at one end and an anus at the other. Earthworms are an example of an animal with an alimentary canal. Once the food is ingested through the mouth, it passes through the esophagus and is stored in an organ called the crop; then it passes into the gizzard where it is churned and digested. From the gizzard, the food passes through the intestine, the nutrients are absorbed, and the waste is eliminated as feces, called castings, through the anus.

Part A shows a hydra, which has a vase-shaped body with tentacles around the rim. The hydra’s mouth is located between the tentacles, at the top of the vase. Next to the hydra is a jellyfish medusa, which is bell shaped with tentacles hanging down from the edge of the bell. The mouth, in the lower middle part of the body, opens into the gastrovascular cavity. Part B shows a nematode, which has a long, tube-like body that is wide at one end and tapers down to a tail at the other. The mouth is in the center of the wide end. It opens into an esophagus, then a pharynx. The pharynx empties into a long intestine, which ends at the anus a short distance before the tail.

Figure 25.5 (a) A gastrovascular cavity has a single opening through which food is ingested and waste is excreted, as shown in this hydra and in this jellyfish medusa. (b) An alimentary canal has two openings: a mouth for ingesting food, and an anus for eliminating waste, as shown in this nematode.

Vertebrate Digestive Systems

Through evolution, vertebrate digestive systems have adapted to different diets. Some animals have a single stomach, while others have multi-chambered stomachs. Birds have developed a digestive system adapted to eating unmasticated food.

Monogastric: Single-chambered Stomach

As the word monogastric suggests, this type of digestive system consists of one (“mono”) stomach chamber (“gastric”). Humans and many animals have a monogastric digestive system as illustrated in Figure 25.6ab. The process of digestion begins with the mouth and the intake of food. The teeth play an important role in masticating (chewing) or physically breaking down food into smaller particles. The enzymes present in saliva also begin to chemically break down food. The esophagus is a long tube that connects the mouth to the stomach. Using peristalsis, or wave-like smooth muscle contractions, the muscles of the esophagus push the food towards the stomach. In order to speed up the actions of enzymes in the stomach, the stomach is an extremely acidic environment, with a pH between 1.5 and 2.5. The gastric juices, which include enzymes in the stomach, act on the food particles and continue the process of digestion. Further breakdown of food takes place in the small intestine where enzymes produced by the liver, the small intestine, and the pancreas continue the process of digestion. The nutrients are absorbed into the blood stream across the epithelial cells lining the walls of the small intestines. The waste material travels on to the large intestine where water is absorbed and the drier waste material is compacted into feces; it is stored until it is excreted through the rectum.

The basic components of the human and rabbit digestive system are the same: each begins at the mouth. Food is swallowed through the esophagus and into the kidney-shaped stomach. The liver is located on top of the stomach, and the pancreas is underneath. Food passes from the stomach to the long, winding small intestine. From there it enters the wide large intestine before passing out the anus. At the junction of the small and large intestine is a pouch called the cecum. The small and large intestines are much longer in rabbits than in humans, and the cecum is much longer as well.

Figure 25.6 (a) Humans and herbivores, such as the (b) rabbit, have a monogastric digestive system. However, in the rabbit the small intestine and cecum are enlarged to allow more time to digest plant material. The enlarged organ provides more surface area for absorption of nutrients. Rabbits digest their food twice: the first time food passes through the digestive system, it collects in the cecum, and then it passes through the anus as soft feces called cecotrophes. The rabbit re-ingests these cecotrophes to further digest them.

Avian

Birds face special challenges when it comes to obtaining nutrition from food. They do not have teeth and so their digestive system, shown in Figure 25.7, must be able to process un-masticated food. Birds have evolved a variety of beak types that reflect the vast variety in their diet, ranging from seeds and insects to fruits and nuts. Because most birds fly, their metabolic rates are high in order to efficiently process food and keep their body weight low. The stomach of birds has two chambers: the proventriculus, where gastric juices are produced to digest the food before it enters the stomach, and the gizzard, where the food is stored, soaked, and mechanically ground. The undigested material forms food pellets that are sometimes regurgitated. Most of the chemical digestion and absorption happens in the intestine and the waste is excreted through the cloaca.

Figure 25.7 The avian esophagus has a pouch, called a crop, which stores food. Food passes from the crop to the first of two stomachs, called the proventriculus, which contains digestive juices that break down food. From the proventriculus, the food enters the second stomach, called the gizzard, which grinds food. Some birds swallow stones or grit, which are stored in the gizzard, to aid the grinding process. Birds do not have separate openings to excrete urine and feces. Instead, uric acid from the kidneys is secreted into the large intestine and combined with waste from the digestive process. This waste is excreted through an opening called the cloaca.

Evolution Connection

Avian Adaptations

Birds have a highly efficient, simplified digestive system. Recent fossil evidence has shown that the evolutionary divergence of birds from other land animals was characterized by streamlining and simplifying the digestive system. Unlike many other animals, birds do not have teeth to chew their food. In place of lips, they have sharp pointy beaks. The horny beak, lack of jaws, and the smaller tongue of the birds can be traced back to their dinosaur ancestors. The emergence of these changes seems to coincide with the inclusion of seeds in the bird diet. Seed-eating birds have beaks that are shaped for grabbing seeds and the two-compartment stomach allows for delegation of tasks. Since birds need to remain light in order to fly, their metabolic rates are very high, which means they digest their food very quickly and need to eat often. Contrast this with the ruminants, where the digestion of plant matter takes a very long time.

Although both birds and humans are vertebrates, birds have a relatively higher metabolic rate than humans.
a. Why is the metabolic rate of birds relatively higher than that of humans?
b. How do birds compensate for such a high metabolism?

a. Birds have smaller surfaces to lose heat than humans, so their metabolic rate must be higher.
b. Birds need to eat greater amounts of food since they digest food quickly.
a. Birds need to be light to fly, so they need to digest their food faster than humans.
b. Birds need to eat greater amounts of food since they digest food quickly.
a. Birds have smaller surfaces to lose heat than humans, so their metabolic rate must be higher.
b. Birds need to eat often to maintain energy since they digest food quickly.
a. Birds need to be light to fly, so they need to digest their food faster than humans.
b. Birds need to eat often to maintain energy since they digest food quickly.

Ruminants

Ruminants are mainly herbivores like cows, sheep, and goats, whose entire diet consists of eating large amounts of roughage or fiber. They have evolved digestive systems that help them digest vast amounts of cellulose. An interesting feature of the ruminants’ mouth is that they do not have upper incisor teeth. They use their lower teeth, tongue and lips to tear and chew their food. From the mouth, the food travels to the esophagus and on to the stomach.

To help digest the large amount of plant material, the stomach of the ruminants is a multi-chambered organ, as illustrated in Figure 25.8. The four compartments of the stomach are called the rumen, reticulum, omasum, and abomasum. These chambers contain many microbes that break down cellulose and ferment ingested food. The abomasum is the “true” stomach and is the equivalent of the monogastric stomach chamber where gastric juices are secreted. The four-compartment gastric chamber provides larger space and the microbial support necessary to digest plant material in ruminants. The fermentation process produces large amounts of gas in the stomach chamber, which must be eliminated. As in other animals, the small intestine plays an important role in nutrient absorption, and the large intestine helps in the elimination of waste.

Illustration shows the digestive system of a goat. Food passes from the mouth, through the esophagus and into the rumen. It circulates through the rumen, then moves forward, and down into the small, pouch-shaped reticulum. From the reticulum the food, which is now cud, is regurgitated. The animal chews the cud, and then swallows it into the coiled omasum, which sits between the reticulum and the rumen. After circulating through the omasum the food passes onto the fourth stomach, the abomasum, where it is digested by enzymes. The food then enters the small intestine, then the large intestine. Waste is excreted through the anus.

Figure 25.8 Ruminant animals, such as goats and cows, have four stomachs. The first two stomachs, the rumen and the reticulum, contain prokaryotes and protists that are able to digest cellulose fiber. The ruminant regurgitates cud from the reticulum, chews it, and swallows it into a third stomach, the omasum, which removes water. The cud then passes onto the fourth stomach, the abomasum, where it is digested by enzymes produced by the ruminant.

Pseudo-ruminants

Some animals, such as camels and alpacas, are pseudo-ruminants. They eat a lot of plant material and roughage. Digesting plant material is not easy because plant cell walls contain the polymeric sugar molecule cellulose. The digestive enzymes of these animals cannot break down cellulose, but microorganisms present in the digestive system can. Therefore, the digestive system must be able to handle large amounts of roughage and break down the cellulose. Pseudo-ruminants have a three-chamber stomach in the digestive system. However, their cecum—a pouched organ at the beginning of the large intestine containing many microorganisms that are necessary for the digestion of plant materials—is large and is the site where the roughage is fermented and digested. These animals do not have a rumen but have an omasum, abomasum, and reticulum.

Parts of the Digestive System

The vertebrate digestive system is designed to facilitate the transformation of food matter into the nutrient components that sustain organisms.

Oral Cavity

The oral cavity, or mouth, is the point of entry of food into the digestive system, illustrated in Figure 25.9. The food consumed is broken into smaller particles by mastication, the chewing action of the teeth. All mammals have teeth and can chew their food.

The extensive chemical process of digestion begins in the mouth. As food is being chewed, saliva, produced by the salivary glands, mixes with the food. Saliva is a watery substance produced in the mouths of many animals. There are three major glands that secrete saliva—the parotid, the submandibular, and the sublingual. Saliva contains mucus that moistens food and buffers the pH of the food. Saliva also contains immunoglobulins and lysozymes, which have antibacterial action to reduce tooth decay by inhibiting growth of some bacteria. Saliva also contains an enzyme called salivary amylase that begins the process of converting starches in the food into a disaccharide called maltose. Another enzyme called lipase is produced by the cells in the tongue. Lipases are a class of enzymes that can break down triglycerides. The lingual lipase begins the breakdown of fat components in the food. The chewing and wetting action provided by the teeth and saliva prepare the food into a mass called the bolus for swallowing. The tongue helps in swallowing—moving the bolus from the mouth into the pharynx. The pharynx opens to two passageways: the trachea, which leads to the lungs, and the esophagus, which leads to the stomach. The trachea has an opening called the glottis, which is covered by a cartilaginous flap called the epiglottis. When swallowing, the epiglottis closes the glottis and food passes into the esophagus and not the trachea. This arrangement allows food to be kept out of the trachea.

Illustration A shows the parts of the human oral cavity. The tongue rests in the lower part of the mouth. The flap that hangs from the back of the mouth is the uvula. The airway behind the uvula, called the pharynx, extends up to the nostrils and down to the esophagus, which begins in the neck. Illustration B shows the two salivary glands, which are located beneath the tongue, the sublingual and the submandibular. A third salivary gland, the parotid, is located behind the pharynx.

Figure 25.9 Digestion of food begins in the (a) oral cavity. Food is masticated by teeth and moistened by saliva secreted from the (b) salivary glands. Enzymes in the saliva begin to digest starches and fats. With the help of the tongue, the resulting bolus is moved into the esophagus by swallowing. (credit: modification of work by the National Cancer Institute)

Esophagus

The esophagus is a tubular organ that connects the mouth to the stomach. The chewed and softened food passes through the esophagus after being swallowed. The smooth muscles of the esophagus undergo a series of wave like movements called peristalsis that push the food toward the stomach, as illustrated in Figure 25.10. The peristalsis wave is unidirectional—it moves food from the mouth to the stomach, and reverse movement is not possible. The peristaltic movement of the esophagus is an involuntary reflex; it takes place in response to the act of swallowing.

Figure 25.10 The esophagus transfers food from the mouth to the stomach through peristaltic movements.

A ring-like muscle called a sphincter forms valves in the digestive system. The gastro-esophageal sphincter is located at the stomach end of the esophagus. In response to swallowing and the pressure exerted by the bolus of food, this sphincter opens, and the bolus enters the stomach. When there is no swallowing action, this sphincter is shut and prevents the contents of the stomach from traveling up the esophagus. Many animals have a true sphincter; however, in humans, there is no true sphincter, but the esophagus remains closed when there is no swallowing action. Acid reflux or “heartburn” occurs when the acidic digestive juices escape into the esophagus.

Stomach

A large part of digestion occurs in the stomach, shown in Figure 25.11. The stomach is a saclike organ that secretes gastric digestive juices. The pH in the stomach is between 1.5 and 2.5. This highly acidic environment is required for the chemical breakdown of food and the extraction of nutrients. When empty, the stomach is a rather small organ; however, it can expand to up to 20 times its resting size when filled with food. This characteristic is particularly useful for animals that need to eat when food is available.

Visual Connection

Illustration shows the human lower digestive system, which begins with the stomach, a sac that lies above the large intestine. The stomach empties into the small intestine, which is a long, highly folded tube. The beginning of the small intestine is called the duodenum, the long middle part is called the jejunum, and the end is called the ileum. The ileum empties into the large intestine on the right side of the body. Beneath the junction of the small and large intestine is a small pouch called the cecum. The appendix is at the bottom end of the cecum. The large intestine travels up the left side of the body, across the top of the small intestine, then down the right side of the body. These parts of the large intestine are called the ascending colon, the transverse colon and the descending colon, respectively. The large intestine empties into the rectum, which is connected to the anus. The pancreas is sandwiched between the stomach and large intestine. The liver is a triangular organ that sits above and slightly to the right of the stomach. The gallbladder is a small bulb between the liver and stomach.

Figure 25.11 The human stomach has an extremely acidic environment where most of the protein gets digested. (credit: modification of work by Mariana Ruiz Villareal)

Which of the following statements about the digestive system is false?

Chyme is a mixture of food and digestive juices that is produced in the stomach.
Food enters the large intestine before the small intestine.
In the small intestine, chyme mixes with bile, which emulsifies fats.
The stomach is separated from the small intestine by the pyloric sphincter.

The stomach is the major site for protein digestion in animals other than ruminants. Protein digestion is mediated by an enzyme called pepsin in the stomach chamber. Pepsin is secreted by the chief cells in the stomach in an inactive form called pepsinogen. Pepsin breaks peptide bonds and cleaves proteins into smaller polypeptides; it also helps activate more pepsinogen, starting a positive feedback mechanism that generates more pepsin. Another cell type—parietal cells—secretes hydrogen and chloride ions, which combine in the lumen to form hydrochloric acid, the primary acidic component of the stomach juices. Hydrochloric acid helps to convert the inactive pepsinogen to pepsin. The highly acidic environment also kills many microorganisms in the food and, combined with the action of the enzyme pepsin, results in the hydrolysis of protein in the food. Chemical digestion is facilitated by the churning action of the stomach. Contraction and relaxation of smooth muscles mixes the stomach contents about every 20 minutes. The partially digested food and gastric juice mixture is called chyme. Chyme passes from the stomach to the small intestine. Further protein digestion takes place in the small intestine. Gastric emptying occurs within two to six hours after a meal. Only a small amount of chyme is released into the small intestine at a time. The movement of chyme from the stomach into the small intestine is regulated by the pyloric sphincter.

When digesting protein and some fats, the stomach lining must be protected from getting digested by pepsin. There are two points to consider when describing how the stomach lining is protected. First, as previously mentioned, the enzyme pepsin is synthesized in the inactive form. This protects the chief cells, because pepsinogen does not have the same enzyme functionality of pepsin. Second, the stomach has a thick mucus lining that protects the underlying tissue from the action of the digestive juices. When this mucus lining is ruptured, ulcers can form in the stomach. Ulcers are open wounds in or on an organ caused by bacteria (Helicobacter pylori) when the mucus lining is ruptured and fails to reform.

Small Intestine

Chyme moves from the stomach to the small intestine. The small intestine is the organ where the digestion of protein, fats, and carbohydrates is completed. The small intestine is a long tube-like organ with a highly folded surface containing finger-like projections called the villi. The apical surface of each villus has many microscopic projections called microvilli. These structures, illustrated in Figure 25.12, are lined with epithelial cells on the luminal side and allow for the nutrients to be absorbed from the digested food and absorbed into the blood stream on the other side. The villi and microvilli, with their many folds, increase the surface area of the intestine and increase absorption efficiency of the nutrients. Absorbed nutrients in the blood are carried into the hepatic portal vein, which leads to the liver. There, the liver regulates the distribution of nutrients to the rest of the body and removes toxic substances, including drugs, alcohol, and some pathogens.

Visual Connection

Illustration shows a cross section of the small intestine, the lumen, or inside of which has many fingerlike projections called villi. Muscle layers wrap around the outside of the intestine, and blood vessels interact with the muscle layer. A blowup shows that capillaries and lymphatic vessels travel up inside the villi. The surface of each villus is covered with hairlike microvilli.

Figure 25.12 Villi are folds on the small intestine lining that increase the surface area to facilitate the absorption of nutrients.

Which of the following statements about the small intestine is true?

Absorptive cells that line the small intestine have small projections that increase surface area and aid in the absorption of food.
The outside of the small intestine has many folds, called villi.
Microvilli are lined with blood vessels as well as lymphatic vessels.
The inside of the small intestine is called the lymphatic vessel.

The human small intestine is over 6m long and is divided into three parts: the duodenum, the jejunum, and the ileum. The “C-shaped,” fixed part of the small intestine is called the duodenum and is shown in Figure 25.11. The duodenum is separated from the stomach by the pyloric sphincter which opens to allow chyme to move from the stomach to the duodenum. In the duodenum, chyme is mixed with pancreatic juices in an alkaline solution rich in bicarbonate that neutralizes the acidity of chyme and acts as a buffer. Pancreatic juices also contain several digestive enzymes. Digestive juices from the pancreas, liver, and gallbladder, as well as from gland cells of the intestinal wall itself, enter the duodenum. Bile is produced in the liver and stored and concentrated in the gallbladder. Bile contains bile salts which emulsify lipids while the pancreas produces enzymes that catabolize starches, disaccharides, proteins, and fats. These digestive juices break down the food particles in the chyme into glucose, triglycerides, and amino acids. The bulk of chemical digestion of food takes place in the duodenum. Absorption of fatty acids also takes place in the duodenum.

The second part of the small intestine is called the jejunum, shown in Figure 25.11. Here, hydrolysis of nutrients is continued while most of the carbohydrates and amino acids are absorbed through the intestinal lining. Some chemical digestion and the bulk of nutrient absorption occurs in the jejunum.

The ileum, also illustrated in Figure 25.11 is the last part of the small intestine and here the bile salts and vitamins are absorbed into blood stream. The undigested food is sent to the colon from the ileum via peristaltic movements of the muscle. The ileum ends and the large intestine begins at the ileocecal valve. The vermiform, “worm-like,” appendix is located at the ileocecal valve. The appendix of humans secretes no enzymes and has an insignificant role in immunity.

Everyday Connection for AP® Courses

A transmission electron microscope image shows closely spaced, fine, finger-like projections called microvilli extending from the main part of the small intestine muscle toward the digestive tract.

Figure 25.13 Transmission electron microscope image of a thin section cut through an epithelial cell from a human jejunum (segment of the small intestine). The image shows the apical end of an absorptive cell with some of the densely packed microvilli that make up the striated border. Each microvillus is approximately 1 μm long by 0.1 μm in diameter and contains a core of actin microfilaments. (credit: “Microvilli”, Wikimedia Commons)

What is the role of microvilli in nutrient absorption?

Microvilli form the inner layer of epithelial tissue in the small intestine and increase the absorption of nutrients from chyme.
Microvilli are projections of absorptive cells that are involved in the absorption of bile salts and vitamin B12.
Microvilli increase the surface area of absorptive cells, thereby increasing the amount of nutrients that can be absorbed.
Microvilli use smooth muscle contractions to move the chyme, which contains nutrients, thereby increasing the rate of absorption.

Large Intestine

The large intestine, illustrated in Figure 25.14, reabsorbs the water from the undigested food material and processes the waste material. The human large intestine is much smaller in length compared to the small intestine but larger in diameter. It has three parts: the cecum, the colon, and the rectum. The cecum joins the ileum to the colon and is the receiving pouch for the waste matter. The colon is home to many bacteria or “intestinal flora” that aid in the digestive processes. The colon can be divided into four regions, the ascending colon, the transverse colon, the descending colon and the sigmoid colon. The main functions of the colon are to extract the water and mineral salts from undigested food, and to store waste material. Carnivorous mammals have a shorter large intestine compared to herbivorous mammals due to their diet.

Figure 25.14 The large intestine reabsorbs water from undigested food and stores waste material until it is eliminated.

Rectum and Anus

The rectum is the terminal end of the large intestine, as shown in Figure 25.14. The primary role of the rectum is to store the feces until defecation. The feces are propelled using peristaltic movements during elimination. The anus is an opening at the far-end of the digestive tract and is the exit point for the waste material. Two sphincters between the rectum and anus control elimination: the inner sphincter is involuntary and the outer sphincter is voluntary.

Accessory Organs

The organs discussed above are the organs of the digestive tract through which food passes. Accessory organs are organs that add secretions (enzymes) that catabolize food into nutrients. Accessory organs include salivary glands, the liver, the pancreas, and the gallbladder. The liver, pancreas, and gallbladder are regulated by hormones in response to the food consumed.

The liver is the largest internal organ in humans and it plays a very important role in digestion of fats and detoxifying blood. The liver produces bile, a digestive juice that is required for the breakdown of fatty components of the food in the duodenum. The liver also processes the vitamins and fats and synthesizes many plasma proteins.

The pancreas is another important gland that secretes digestive juices. The chyme produced from the stomach is highly acidic in nature; the pancreatic juices contain high levels of bicarbonate, an alkali that neutralizes the acidic chyme. Additionally, the pancreatic juices contain a large variety of enzymes that are required for the digestion of protein and carbohydrates.

The gallbladder is a small organ that aids the liver by storing bile and concentrating bile salts. When chyme containing fatty acids enters the duodenum, the bile is secreted from the gallbladder into the duodenum.

Science Practice Connection for AP® Courses

Activity

Create a mini-poster that shows the procurement, digestion, absorption, and distribution of nutrients through the digestive systems of one invertebrate animal and one vertebrate animal. Explain how the organs of the system promote efficiency in the use of matter and energy.

Think About It

Explain how the villi and microvilli aid in absorption of nutrients from the small intestine into the circulatory system.

Teacher Support

The activity is an application of AP^® Learning Objective 4.18 and Science Practice 1.4 because students are creating a representation of digestive systems to models how structure-function relationships among organs promote efficiency in the use of matter and energy.
The Think About It question is an application of AP^® Learning Objective 2.7 and Science Practice 6.2 because you are explaining how the shape and size of cells that line the small intestine promote nutrient uptake.

25.1 Digestive Systems