Chapter 6 – Digestion and Nutrition：6章–消化和营养.doc

Chapter 6 Digestion and NutritionSection 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 trees 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 plants 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 37C. As temperature increases above 37C, 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 earthworms 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 foo