210 Chapter 6

Importance of Digestion

1. Digestive systems enable the absorption of nutrients needed for growth,maintenance, and repair. The digestive system and other organ systems areinterdependent. The four components of digestion are ingestion, digestion,absorption, and egestion.

2. In protozoa, digestion occurs in intracellular vacuoles.

3. Some simple organisms have a single gastrovascular cavity with oneopening, which both stores food and digests it.

4. Digestive tracts allow the separation of processes into different regions.

Practice

Understanding Concepts

1. Define ingestion, digestion, absorption, and egestion.

2. Compare the digestive systems of the amoeba, bird, and earthworm.

3. How does the presence of an alimentary canal alter the organizationof the digestive organs within the body?

6.3 IngestionThe digestive tract of adult humans, normally 6.5 m to 9 m long, stores and breaksdown organic molecules into simpler components. Physical digestion begins inthe mouth, where food is chewed and formed into a bolus (the Greek word forball) by the tongue. Figure 1 shows the entire digestive system.

Saliva

Saliva, the watery fluids produced by the salivary glands, contains amylase

enzymes, which break down starches (complex carbohydrates) to simpler carbo-hydrates. Saliva lubricates the food so it can be swallowed, dissolves food particles,and makes it possible to taste what is being eaten.

The way that we discern flavour is that food particles dissolved in solutionpenetrate the cells of the taste buds located on the tongue and cheeks. Differenttypes of receptors respond to specific flavours. For example, the taste buds are

amylase: an enzyme that breaks downcomplex carbohydrates

Figure 6

The digestive system of a bird(a) The esophagus carries food to the crop.(b) The crop stores food.(c) The gizzard has thick muscular walls. Birds

swallow small pebbles to help grind thefood.

(d) The stomach begins chemical digestion.(e) The intestine absorbs digested food.(f) Waste is removed through the anus.

(a)

(b)

(d)

(c)

(e)

(f)

Digestion and Nutrition 211

6.3

Figure 1

The human digestive system and accessoryorgans

stomach

Figure 2

Taste buds are located along the tongue.

papillae

mouth

pharynx

esophagus

small intestine

large intestine

rectum

anus

salivary glands

liver

gall bladder

pancreas

The tiny projections on your tongue that giveit a velvety appearance are called papillae.It is within the papillae that the majority oftaste buds on the tongue are found. Themost numerous papillae (which are downthe middle of your tongue) do not have tastebuds. These papillae are involved in tactilesensation.

DID YOU KNOW ?

equipped with receptors (Figure 2) that have a specific geometry that permit theidentification of sweet tastes from carbohydrates. Nerve cells for taste are stimu-lated when receptor sites are filled by chemical compounds with a complemen-tary shape. You can find out the significance of dissolving foods by drying yourtongue and then placing a few grains of sugar or salt on it. You will not detect anyflavour until the crystals dissolve.

Teeth

The teeth are important structures for physical digestion. Eight chisel-shapedteeth at the front of your mouth, called incisors, are specialized for cutting. Theincisors are bordered by sharp, dagger-shaped canine teeth that are specializedfor tearing. Next to the canine teeth are the premolars. These broad, flattenedteeth are specialized for grinding. The molars are found next to the premolars.These teeth tend to be even broader and have cusps that are even more flattened.They are designed for crushing food. The last set of molars are the wisdom teeth,so called because they usually do not emerge until about 16 to 20 years of age.Often these molars are troublesome and must be removed because there is notenough space for them to grow in.

Each tooth has two divisions: the root and the crown. An enamel crowncovers the tooth with calcium compounds and forms the hardest substance in thebody. Immediately inside the enamel is dentin, a bonelike substance, which is

tastebuds

taste poremicrovilli

connectivetissue

sensorynerve fibre

epitheliumof tongue

taste cell

periodontalmembrane

crownenamel

root canal

dentin

bone

pulp cavity(contains nerves andblood vessels)

root

gingiva (gum)

molars (12)

premolars (8)canines (4)

incisors (8) incisors (8)lower jaw upper jaw

Figure 3

(a) Human teeth and(b) a human molar. Each tooth has a root and

an enamel crown. Enamel, which is formedof calcium compounds, is the hardest sub-stance in the body.

(a)

(b)

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part of the root structure. The dentin encases the pulp cavity, which containsnerves and blood vessels (Figure 3).

Tooth decay is caused by bacteria living off nutrients that cling to the teeth.These harmful microbes produce corrosive acids that erode a tooth’s structure.Infections can also spread to the periodontal membrane, which anchors theteeth to the jawbone. As an infection progresses, the periodontal tissue is slowlydestroyed and the teeth loosen.

Esophagus

Once swallowed, food travels from the mouth to the stomach by way of theesophagus. The bolus of food stretches the walls of the esophagus, activatingsmooth muscles that set up waves of rhythmic contractions called peristalsis.Peristaltic contractions, which are involuntary, move food along the gastroin-testinal tract (Figure 4). The only points at which food is moved voluntarily alongthe tract is during swallowing and during the last phase, egestion. Peristalticaction will move food or fluids from the esophagus down to the stomach even ifyou stand on your head.

Ingestion

1. Saliva is important in digestion because it• contains amylase enzymes that initiate carbohydrate breakdown;• lubricates the food passage;• dissolves food particles;• activates the taste buds.

2. Teeth are necessary for biting, tearing, grinding, and crushing food intosmaller particles.

3. After food is swallowed, movement through the esophagus is regulated byperistalsis, contractions of smooth muscle.

Practice

Understanding Concepts

1. What are the functions of saliva?

2. How does chewing assist in the digestion of food?

3. What are amylase enzymes and why are they necessary?

peristalsis: rhythmic, wavelike contractionof smooth muscle that moves food along thegastrointestinal tract

phase 1 phase 2 phase 3

bolus

smooth muscleis stretched

contraction ofsmooth muscle

Figure 4

Rhythmic contractions of the smooth musclemove food along the digestive tract.