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Unit 1:
Ch. 1
Ch. 2
Ch. 3
Interlude A
Unit 2:
Ch. 4
Ch. 5
Ch. 6
Ch. 7
Ch. 8
Ch. 9
Interlude B
Unit 3:
Ch. 10
Ch. 11
Ch. 12
Ch. 13
Ch. 14
Ch. 15
Interlude C
Unit 4:
Ch. 16
Ch. 17
Ch. 18
Ch. 19
Interlude D
Unit 5:
Ch. 20
Ch. 21
Ch. 22
Ch. 23
Ch. 24
Ch. 25
Ch. 26
Ch. 27
Ch. 28
Ch. 29
Ch. 30
Interlude E
Unit 6:
Ch. 31
Ch. 32
Interlude F
Unit 7:
Ch. 33
Ch. 34
Ch. 35
Ch. 36
Ch. 37
Ch. 38
Interlude G

» Getting Started » A Guide to the Reading » Tying it all together

Getting Started

Below are a few questions to consider prior to reading Chapter 21. These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.

  1. How do scientists use body mass and height to get a picture of the health of individuals?
  2. Where do we get the nutrients we need to survive? 
  3. What are the major nutrients that we need to survive?
  4. Why must our diet contain vitamins?
  5. What are the different parts of your digestive system and how do they function?
  6. Do all animals have the same types of digestive systems?  Do all animals eat the same types of food?
  7. What role do enzymes play in the digestion and uptake of nutrients?

A Guide to the Reading

When exploring the content in Chapter 21 for the first time, the following concepts typically give students the most difficulty. For each concept, one or more references have been identified which may help you gain a better understanding of these potentially problematic areas.

Nutrient Requirements for Survival

Organisms depend on a small number of elements to sustain life. The elements carbon, hydrogen, and oxygen make up 93 percent of the weight of an organism.  The other essential elements are needed only in small quantities.  There are a number of essential nutrients that humans cannot produce but must obtain from their diet.  Of the 20 required amino acids, humans can produce only 12 and must rely on their diet for the other 8.  Vitamins are essential for many of the metabolic processes carried out by the body and must be obtained from our diet.

For more information on this concept, be sure to focus on:

  • In Section 21.1, Certain nutrients can be obtained only from food
  • Table 21.2, Vitamins Needed in the Human Diet
  • Figure 21.5, Essential Amino Acids in the Human Diet
  • Figure 21.6, Insufficient Vitamin Intake Can Produce a Deficiency Disease

The Human Digestive System as a Model

The goal of the digestive system is to break down the major components of our diet, carbohydrates, fats, and proteins, into their smallest subunits that can be readily absorbed into the body.  The digestive systems of animals are diverse, but show common structural features.  This makes the human digestive system a good representative model.  Most digestive systems have a one-way flow of food that allows for specialized activities such as chewing, storage, digestion, absorption, and the elimination of wastes at different portions of the digestive tract.  Food enters the body at the mouth or oral cavity where the initial steps involved in starch breakdown occur with the help of mechanical chewing.  Food moves down the esophagus to the stomach where the addition of acid and pepsin continues to further break down the food with the aid of rhythmic contractions of the stomach.  The small intestines with their vili or brush border are the major site of nutrient absorption into the blood capillaries and lymph vessels.  The colon functions in absorbing a majority of the remaining water and minerals, leaving behind the solid feces to be excreted.

For more information on this concept, be sure to focus on:

  • Section 21.2, The Human Digestive System
  • Figure 21.5, The Human Digestive System Converts Food into Absorbable Nutrients
  • Figure 21.6, The Lining of the Small Intestine is Complexly Folded

Enzymes are the Key to Digestion

Digestion and absorption in the small intestine require that nutrients be broken down to their smallest subunit.  This is aided in both the stomach and small intestine by the addition of enzymes.  Humans produce a host of enzymes that facilitate the breakdown of nutrients into smaller subunits, with each enzyme acting on a specific class of chemical bond.  The first enzymes encountered by food in the mouth are amylases that break down starches.  Proteins are broken into amino acids by pepsin in the stomach and the pancreatic-derived enzymes trypsin and chymotrypsin in the small intestine.   Fats are broken down to fatty acids and monoglycerides in the small intestine by lipases secreted by the pancreas.  These enzymes break starches, proteins, and fats down into the smallest subunits, monosaccharides, amino acids, fatty acids, and monoglycerides that can be readily absorbed at the brush border of the small intestine.  Animals can only digest and absorb specific nutrients if they possess the enzymes specific to a nutrient.  Many human adults lack the enzyme lactase that digests lactose found in dairy products and all humans lack enzymes to digest cellulose.

For more information on this concept, be sure to focus on:

  • In Section 21.3, Animals convert complex molecules into simpler ones for absorption
  • In Section 21.3, Missing enzymes can prevent the uptake of nutrients

Evolutionary Changes in Digestive Systems

While the basic plan for the digestive system is similar among animals, there has been an evolution of digestive specialization that has increased the efficiency of nutrient acquisition.  Most animals lack the enzymes necessary for the breakdown and digestion of cellulose, a complex carbohydrate found abundantly in plants.  The diet of herbivores is composed of plant material that contains significantly less protein than the diet of carnivores or omnivores.  Herbivores have evolved specializations to deal with the daunting task of trying to digest enough cellulose and take up enough protein to meet their energy and nutrient needs.  First, the absorptive surface area that organisms devote to nutrient acquisition along the small intestines is related to the type of nutrients they ingest.  Many herbivores have evolved small intestines with large surface areas to increase the rate of nutrient uptake.  Second, herbivores tend to have larger stomachs to accommodate the large quantities of food that must be ingested to obtain the necessary nutrients.  Finally, their digestive system contains a healthy population of bacteria, fungi, and single-celled eukaryotes that possess the enzyme to break down cellulose.

For more information on this concept, be sure to focus on:

  • In Section 21.3, The capacity for absorbing nutrients depends on surface area
  • In Section 21.3, Herbivore digestive systems are modified for digesting plants
  • Figure 21.12, Herbivores and Carnivores Compared

Tying it all together

Several concepts presented in this chapter build upon concepts presented in previous chapters and are also revisited and discussed in greater detail in subsequent chapters, including:

Nutrients That Animals Need

  • Chapter 4 – Section 4.5, The Chemical Building Blocks of Living Systems

The Human Digestive System

  • Chapter 6 - Section  6.3, Cell Membrane as Transport Luggage

Comparing Animal Digestive Systems

  • Chapter 4 – Section 4.5, The Chemical Building Blocks of Living Systems
  • Chapter 7 – Section 7.3, How Cells Speed Up Chemical Reactions
  • Chapter 8 - Section 8.3,  Catabolism: Breaking Down Molecules for Energy

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