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Chapter 6 – Digestion and Nutrition
Section 6.2 – Importance of Digestion (pages 208-210)
1. Define the term nutrients.
2. Autotrophs need organic substances; however, they do not eat them.
Explain how an apply tree obtains its needed organic substances.
3. What is digestion and what four phases does it include?
4. What two factors influence the effectiveness of an enzyme?
5. What is the complication with consuming chunks of food?
6. How does the presence of an alimentary canal alter the organization of the
digestive organs within the body?
7. How do protozoa absorb, digest and eliminate their food? Include a
diagram to illustrate your answer.
8. How does the hydra obtain nutrients?
9. What is a gastrovascular cavity?
10.What is meant by one-way traffic in digestion?
11. What is meant by two-way traffic in digestion?
12.The earthworm is an example of an organism that uses one-way traffic in
digestion. Explain the digestive process in the earthworm, including
chemical digestion and absorption.
Section 6.2 – Answers
1. Nutrients can be defines as complex organic molecules and simpler
substances that are needed by living organisms to carry out such life
processes as respiration, protein synthesis, and reproduction.
2. Autotrophs build the complex organic substances they require by using
water, carbon dioxide, minerals absorbed from the soil, and energy from
the sun. An apple tree will absorb water and minerals from the soil. These
molecules are small enough to pass directly into the cells of the apple
tree’s roots and from the roots, the water and minerals are transported to
the leaves. There are openings in the leaves that allow carbon dioxide to
enter and photosynthesis to occur. This process converts the water and
carbon dioxide into simple sugars that can be used as an energy source for
the plant’s cells. Some of the simple sugars are used to build complex
molecules needed for growth and other life functions such as starch,
cellulose, lipids and amino acids.
3. Digestion is a very complex chemical process, which involves many enzymes
that carry out a series of hydrolysis reactions. These reactions use water
to convert complex organic molecules into simpler ones that can easily
diffuse across a cell membrane. The four components of digestion include;
ingestion – taking in of nutrients, digestion – the breakdown of complex
organic molecules into smaller components by enzymes, absorption – the
transport of digested nutrients to the tissues of the body and egestion –
the removal of waste food materials from the body.
4. The rate at which an enzyme functions best to break down complex
molecules is affected by both temperature and pH. As temperature
increases, more energy is added and the more active the enzyme becomes.
In humans, enzymes are most efficient at 37°C. As temperature increases
above 37°C, enzyme effectiveness drops. Enzymes also work best within
certain pH ranges. Pepsin, a protein-digesting enzyme produced by the
stomach, works best at a low pH of 2.5. The enzyme amylase, functions to
break down complex carbohydrates and works best in neutral pH
environments (pH 7). Lastly, trypsin a protein-digesting enzyme in the
small intestine, works best in basic environments of pH 9.
5. Many heterotrophs consume large chunks of food. The complication with
this process is that in order to make use of the nutrients in the large
chunks of food it must first be broken down into much smaller molecules.
Each molecule must eventually be small enough to pass through a plasma
membrane.
6. Food passes along a single tube in one direction. This allows for the
specialization of cells along various areas of the canal, which makes
digestion more efficient.
7. A protozoa feeds on microorganisms, which it consumes through the
process of phagocytosis. The protozoa slowly engulfs food particles by
extending its pseudopods. The membrane around the food forms a food
vacuole, which fuses with lysosomes. The lysosome releases strong
digestive enzymes, which convert complex molecules into smaller molecules
that can diffuse into the cytoplasm to be used in cell metabolism. While
this digestive process is taking place, the food vacuole is slowly moving
through the cytoplasm ensuring that nutrients are evenly distributed
throughout the cell. Once digestion is completed the food vacuole, which
now contains the indigestible materials, will fuse with an opening called the
anal pore and wastes will be eliminated.
8. The hydra is a chunk feeder with only one body opening, the mouth. The
mature hydra is a sessile heterotroph and it cannot chase its prey.
Instead, a hydra uses its tentacles, which are sensitive to touch to trap
prey that passes close to the organism. Once a hydra detects prey close
by it extends its tentacles which eject stinging cells toward the prey. The
“harpoon-like” tips of the stinging cells pierce the prey and inject a poison,
which paralyses or kills the prey. The hydra will then use its tentacles to
push the prey into and through its mouth. Inside the gastrovascular cavity
cells release digestive enzymes that break down the food. Other cells with
flagella keep the contents of the cavity moving while other cells engulf the
food particles, similar to that of an amoeba.
9. A gastrovascular cavity is a fluid-filled digestive cavity with only one body
opening that functions as both mouth and anus. The cavity is usually lined
with numerous gland cells that secrete digestive enzymes.
10. Most heterotrophic organisms have digestive systems, which are involved
in one-way traffic. In this process, food enters the organism through one
body opening, the mouth, and indigestible materials exit the organism
through a second body opening, the anus. The food and wastes travel in one
direction through the organism providing thorough, efficient digestion.
11. Two-way traffic in digestion occurs in organisms that have only one body
opening, the mouth. This means that the one opening has to be used to
take in food an also to eliminate waste materials.
12. The digestive process of the earthworm is as follows:
 When food enters the earthworm’s mouth it is first moistened and then
sucked into the pharynx by muscular contractions.
 Food then moves through the esophagus by peristalsis and into the crop
where it can be temporarily stored. The crop stores and moistens the
food.
 Connected to the crop is the thick-walled gizzard, which grinds food into
smaller particles with the help of coarse soil particles (sand and gravel),
which have been consumed along with the food.
 This achieves mechanical or physical digestion of the food, which breaks
food chunks down and increases their surface area. The increased surface
area makes chemical digestion of food by enzymes more efficient.
 The ground up food will now pass from the gizzard into the intestine where
the final stages of digestion are carried out.
 Enzymes released from the intestine chemically break down food particles
into simple molecules that can pass across a plasma membrane.
 Absorption of these nutrients along with water and dissolved materials
occurs in the intestine. The nutrients diffuse into the cells lining the
intestine and then pass into the bloodstream to be distributed to all body
cells.
 Any nutrients that are not absorbed, along with indigestible materials,
pass out of the earthworm through the anus.