THE DIGESTIVE SYSTEM - Animal Structure and Function - Cracking the AP Biology Exam

Cracking the AP Biology Exam


Animal Structure and Function

To carry on with the business of life, higher organisms must all contend with the same basic challenges: obtaining nutrients, distributing them throughout their bodies, voiding wastes, responding to their environments, and reproducing. To accomplish these basic tasks, nature has come up with solutions. However, for all their differences, most animals have remarkably similar ways of dealing with these challenges.

This chapter looks at the basic structures of animals and the ways in which they function. Since the AP Biology Exam includes many questions on human anatomy and physiology, we’ll focus primarily on how these systems have evolved in human beings.

The systems we’ll look at include:

  • The digestive system
  • The respiratory system
  • The circulatory system
  • The immune system
  • The excretory system
  • The nervous system
  • The musculoskeletal system
  • The endocrine system
  • The reproductive system
  • Morphogenesis, or “development”


All organisms need nutrients to survive. But where do the nutrients—the raw building blocks—come from? That depends on whether the organism is an autotroph or heterotroph. As you may recall, autotrophs make their own food through photosynthesis, and all of the building blocks—CO2, water, and sunlight—come from their immediate environment. Heterotrophs, on the other hand, can’t make their own food; they must acquire their energy from outside sources.

When we talk about digestion, we’re talking about the breakdown of large food molecules into simpler compounds. These molecules are then absorbed by the body to carry out cell activities. In fact, everything we’ll discuss in this section revolves around three simple questions:

1. What do organisms need from the outside world in order to survive?

2. How do they get those things?

3. What do they do with them once they get them?

Multicellular organisms have come up with a variety of ways of getting their nutrients. In simple animals, food is digested through intracellular digestion—that is, digestion occurs within food vacuoles. For example, a hydra encloses the food it captures in a food vacuole. Lysosomes containing digestive enzymes then fuse with the vacuole and break down the food. More complex animals have evolved a digestive tract and digest food through extracellular digestion. That is, the food is digested in a gastrovascular cavity. For example, in grasshoppers, food passes through specialized regions of the gut: the mouth, esophagus, crop (a storage organ), stomach, intestine, rectum, and anus.


The human digestive tract consists of the mouth, esophagus, stomach, small intestine, large intestine, and accessory organs (liver, pancreas, gall bladder, and salivary glands). Four groups of molecules must be broken down by the digestive tract: starch, proteins, fats, and nucleic acids.

The Mouth

The first stop in the digestive process is the mouth, or oral cavity.

When food enters the mouth, mechanical and chemical digestion begins. The chewing, softening and breaking up of food is called mastication (also known as mechanical digestion). The mouth also has saliva in it. Saliva, which is secreted by the salivary glands, contains an important enzyme known as salivary amylase. Salivary amylase begins the chemical breakdown of starch into maltose.

Once chewed, the food, now shaped into a ball called a bolus, moves through the pharynx and into the esophagus. Food moves through the esophagus in a wavelike motion known as peristalsis.

The waves of contraction push the food toward the stomach.

The Stomach

Once food has been chewed, it moves from the esophagus to the stomach. The stomach is a thick, muscular sac that has several functions:

  • It temporarily stores the ingested food.
  • It partially digests proteins.
  • It kills bacteria.

The stomach secretes gastric juices, which contain digestive enzymes and hydrochloric acid (HCl). One of the most important enzymes is pepsin, which breaks down proteins into smaller peptides. Pepsin works best in an acidic environment. When HCl is secreted, it lowers the pH of the stomach and activates pepsinogen into pepsin to digest proteins. The stomach also secretes mucus, which protects the stomach lining from the acidic juices. Finally, HCl kills most bacteria.

Food is also mechanically broken down by the churning action of the stomach. Once that’s complete, this partially digested food, now called chyme, is ready to enter the small intestine.

The Small Intestine

The small intestine has three regions: the duodenum, the jejunum, and the ileum. The chyme moves into the first part of the small intestine, the duodenum, through the pyloric sphincter. The small intestine is very long—about 23 feet in an average man. This is where all three food groups are completely digested. The walls of the small intestine secrete enzymes that break down proteins (peptidases) and carbohydrates (maltase, lactase, and sucrase).

The Pancreas

The pancreas secretes a number of enzymes into the small intestine: trypsin, chymotrypsin, pancreatic lipase, and pancreatic amylase. Trypsin and chymotrypsin break down proteins into dipeptides. Pancreatic lipase breaks down lipids into fatty acids and glycerol. Pancreatic amylase, on the other hand, breaks down starch into disaccharides. Ribonuclease and deoxyribonuclease break down nucleic acids into nucleotides.

These enzymes are secreted into the small intestine via the pancreatic duct.

Another substance that works in the small intestine is called bile. Bile is not a digestive enzyme. It’s an emulsifier, meaning that it mechanically breaks up fats into smaller fat droplets. This process makes the fat globules more accessible to pancreatic lipase. Bile enters the small intestine by the bile duct, which merges with the pancreatic duct.

Here’s something you should memorize:

Bile is made in the liver and stored in the gall bladder.

Once food is broken down, it is absorbed by tiny, fingerlike projections of the intestine called villi and microvilli. Villi and microvilli are folds that increase the surface area of the small intestine for food absorption. Within each of the villi is a capillary that absorbs the digested food and carries it into the bloodstream. Within each villus there are also lymph vessels, called lacteals, which absorb fatty acids.

Don’t forget that hormones are also involved in the digestive system: gastrin (which stimulates stomach cells to produce gastric juice), secretin (which stimulates the pancreas to produce bicarbonate and digestive enzymes), and cholecystokinin (which stimulates the secretion ofpancreatic enzymes and the release of bile).

Here’s a summary of the pancreatic enzymes.

The Large Intestine

The large intestine is much shorter and thicker than the small intestine. The large intestine has an easy job: It reabsorbs water and salts. The large intestine also harbors harmless bacteria that are actually quite useful. These bacteria break down undigested food and in the process provide us with certain essential vitamins, like Vitamin K. The leftover undigested food, called feces, then moves out of the large intestine and into the rectum.


Directions: Each of the questions or incomplete statements below is followed by five suggested answers or completions. Select the one that is best in each case. Answers can be found here.

1. Special structures for absorption of fats in the small intestine are called

(A) nephrons

(B) lacteals

(C) villi

(D) root hairs

(E) hormones

2. The enzyme that initially breaks down proteins in the human digestive system is called

(A) bile

(B) pepsin

(C) trypsin

(D) salivary amylase

(E) pancreatic amylase

3. The digestive enzyme that hydrolyzes molecules of fats into fatty acids is known as

(A) bile

(B) lipase

(C) amylase

(D) protease

(E) trypsin

Directions: Each group of questions consists of five lettered headings followed by a list of numbered phrases or sentences. For each numbered phrase or sentence, select the one heading that is most closely related to it and fill in the corresponding oval on the answer sheet. Each heading may be used once, more than once, or not at all in each group.

Questions 4–7

(A) Liver

(B) Small intestine

(C) Gall bladder

(D) Pancreatic duct

(E) Large intestine

4. Carries digestive enzymes to the small intestine

5. A storage organ for bile

6. Completely digests foodstuffs to simple substances

7. Reabsorbs water into the bloodstream