Answers to Mastering Concepts Questions

15.1
1. How were conditions on Earth before life began different from current conditions?

Before life began on Earth, the atmosphere contained a mixture of methane, ammonia, water, and hydrogen gases; volcanic eruptions, earthquakes, and meteor and asteroid strikes were common; no atmospheric layer of ozone shielded Earth’s surface from cosmic radiation; and Earth’s surface temperature and atmospheric pressure were dramatically higher than they are today.

2. What types of information can simulations of early Earth provide?

Simulations can demonstrate what might have happened on early Earth. They demonstrate that given the right environment, organic molecules can arise from simpler inorganic chemicals.

3. Why is RNA likely to have been pivotal in life’s beginnings?

RNA is an extremely versatile molecule that stores information encoding proteins, can act as an enzyme that can catalyze chemical reactions, and that can duplicate on its own.

4. What is a protocell?

A protocell is an aggregate of ancient RNA, DNA, proteins, and lipids housed within a bubble much like a cell. It is a simplified cell precursor.

5. When did the first cells probably originate?

The first cells probably originated between 3.8 and 3.7 billion years ago.

6. How did early life change Earth?

Early life introduced O2 into the atmosphere and reduced levels of CO2. Eventually, ozone formed in the upper atmosphere, shielding organisms from the sun’s UV radiation.

15.2
1. What are two domains that contain prokaryotes?

Domains Bacteria and Archaea contain prokaryotes.

2. Without looking at figure 15.7, sketch the internal and external features of a typical prokaryotic cell. What are the functions of each structure?

The nucleoid is the region where DNA is found.  The plasmids are circles of DNA outside the nucleoid.  The ribosomes are the site of protein synthesis.  The cell membrane determines what enters and leaves the cell. The cell wall is the external barrier. The pili and slime layer function in attachment. The flagellum moves the cell.

3. What terms do microbiologists use to describe carbon sources, energy sources, and oxygen requirements?

Autotrophs acquire carbon from inorganic sources and produce their own organic molecules.  Heterotrophs obtain their carbon from preexisting organic molecules.  Phototrophs get energy from the sun.  Chemotrophs get energy by oxidizing organic or inorganic materials. Organisms that require oxygen are aerobes.  Organisms that do not require oxygen are anaerobes.  Anaerobes can be obligate (oxygen is toxic) or facultative (oxygen can be used or not used).

4. In what ways are bacteria and archaea important to eukaryotic life in general and to human life in particular?

Microbes are decomposers, photosynthetic organisms, food sources, and nitrogen fixers. Microbes also cause many human diseases, and people harness microbial metabolism in many industrial processes.

5. What adaptations enable pathogenic bacteria to enter the body and cause disease?

Pathogenic bacteria enter the body with insect or tick bites, sexual activity, in food, air, or water, or by direct contact with cut or abraded skin surfaces. Once inside the body, pili help the bacteria to attach to host cells, and bacterial enzymes attack host tissues. Bacterial toxins can disable the host’s circulatory, digestive, or nervous system.

15.3
1. When did eukaryotic cells first appear?

Eukaryotic cells arose between 1.4 and 1.9 billion years ago.

2. How might the endoplasmic reticulum, nuclear envelope, and other internal membranes have arisen in eukaryotic cells?

The endoplasmic reticulum, nuclear membrane, and other membranes may have arisen from the infolding of the cell membrane of an early cell.

3. What is the evidence that mitochondria and chloroplasts descend from simpler cells engulfed long ago?

Mitochondria and chloroplasts have their own DNA, RNA, and ribosomes; they replicate by binary fission; their size, shape, and membrane structure are similar to those of prokaryotes; the pigments in chloroplasts are similar to the pigments of cyanobacteria; DNA sequences of mitochondria and certain bacteria are similar. 

4. What are two ways that multicellular organisms may have originated?

Multicellular organisms may have originated by individual cells merging to form a single organism, or by cells remaining stuck together after division.

5. How is multicellularity adaptive?

It allows for division of labor, new features, and the ability to form larger bodies. All of these features opened new possibilities for the evolution of new body forms.

15.4
1. What features define the protists?

Protists include all eukaryotes that do not belong to any other kingdom.

2. Describe examples illustrating why protists are important.

Protists are important economically and ecologically because they form the base of most aquatic food webs; they include parasites that cause disease in both plants and animals, including humans; they can help people find oil reserves; they are used to make paints reflective and make chocolate smooth and creamy.

3. What mode of nutrition do the algae, slime molds, water molds, and protozoa use?

Algae are photosynthetic autotrophs; slime molds and water molds are heterotrophic; protozoa can be autotrophs or heterotrophs.

4. List and describe the characteristics of the major groups of algae.

The algae groups are:

• Dinoflagellates: marine algae with cellulose plates and two flagella of different lengths
• Diatoms: ornate shapes and two-part silica shells
• Brown algae: marine algae with complex body forms; kelps are in this group
• Red algae: marine algae with reddish and bluish pigments other than chlorophyll a
• Green algae: chlorophylls a and b; starch; cellulose cell walls; alternation of generations; closest relatives of land plants

5. Compare and contrast the plasmodial slime molds, cellular slime molds, and water molds.

Both types of slime molds are heterotrophic protists living in damp habitats. Plasmodial slime molds, however, form a large mass with a single membrane, while cellular slime molds retain their cellular membranes even when they aggregate into a “slug.” Water molds are decomposers; they are also parasites of plants and animals.  

6. Describe each of the major groups of protozoa.

The major groups of protozoa and their characteristics are:

• Flagellated protozoans (trypanosomes): move by flagella
• Amoeboid protozoans (amoebas, forams, and radiolarians): move by pseudopodia
• Ciliates: propelled by movements of cilia
• Apicomplexans: nonmotile protozoans that parasitize animals

15.5
1. What combination of characteristics defines the fungi?

Fungi are eukaryotic and heterotrophic. They have cell walls made of chitin, and they store carbohydrates as glycogen.

2. Describe how fungi acquire food.

Microscopic hyphae grow within food and secrete enzymes that break down the organic molecules, which are absorbed at the tips of the hyphae.

3. Which features distinguish the five phyla of fungi?

The five phyla are distinguished by their sexual structures.  The basidiomycetes produce sexual spores on a club-shaped structure; the ascomycetes produce sexual spores in sacs; the glomeromycetes have large asexual spores and lack a sexual cycle; the zygomycetes produce thick-walled sexual zygospores; and the chytridiomycetes produce sexual and asexual spores with flagella.

4. Describe the life cycle of basidiomycetes.

Haploid basidiospores germinate, producing haploid hyphae. Two compatible hyphae fuse and form a dikaryotic mycelium, which grows within the food source.  When conditions are right, a mushroom is produced. Lining the gills are the basidia, where the two nuclei fuse to form the diploid zygote. The zygote undergoes meiosis, producing four haploid basidiospores.

5. How do fungi benefit humans?

Fungi are a source of food and many medicines. Antibiotics like penicillin and immune suppressants like cyclosporine are developed from fungi.

6. In what ways are fungi important in ecosystems?

Fungi are important in ecosystems as decomposers that break down dead organic matter, releasing inorganic nutrients. They are also parasites of plants and animals.

7. Compare and contrast endophytes, mycorrhizae, and lichens.

Endophytes are fungi that live in plants without causing disease.  Mycorrhizal fungi are associated with plant roots; they help the plant to absorb nutrients.  Lichens are a symbiotic relationship between a fungus and an alga.

15.6
1. How did the researchers test their hypothesis that endophytes help prevent plant disease?

Researchers used fungus-free plants that were placed in a protected greenhouse where different leaves could be sprayed with endophytes in a controlled manner.  Disease-causing water molds were then introduced to the sprayed and non-sprayed leaves, and the dead leaves were counted 15 days later.

2. This study used a mix of endophyte species. How would you design an experiment to determine whether one species of endophyte or some combination of multiple species protects the leaves against pathogens?

The test above could be repeated, but rather than simply having a mix of fungi sprayed on some leaves and no fungus sprayed on other leaves, the experimental group could be modified to include leaves sprayed with one species at a time, pairs of species, and mixtures of three or more species.

3. Endophytes were associated with a small but statistically significant reduction in cacao leaf disease.  In what ways might this seemingly small difference be important to the plant?

Reduced damage means a greater leaf area available for photosynthesis, which ultimately could lead to greater reproductive success.

 

Answers to Write It Out Questions

1. Suppose someone hands you a microscope and a slide on which you can see single-celled organisms.  Create a flowchart that you could use to identify the specimens as bacteria, algae, or yeasts.

Answers will vary, but should include a reference to the fact that bacteria will not have a nucleus since they are prokaryotic.  Both algae and yeasts are eukaryotes, but algae would contain chlorophyll and yeasts would not.

2. Distinguish between the following pairs of terms: (a) phototroph and chemotroph; (b) autotroph and heterotroph; (c) obligate anaerobe and facultative anaerobe; (d) yeast and mycelium; and (e) mycorrhiza and lichen.

a) Phototrophs get energy from sun, whereas chemotrophs use energy from the breakdown of organic or inorganic molecules.
b) Autotrophs make their own organic molecules from inorganic raw materials,  whereas heterotrophs must obtain organic molecules by eating other organisms.
c) Obligate anaerobes cannot survive in the presence of oxygen, whereas facultative anaerobes can survive with or without oxygen.
d) Both are related to fungi; a mycelium is a mass of hyphae, whereas a yeast is a single-celled type of fungus.
e) A mycorrhiza is a symbiotic relationship between fungi and plant roots; a lichen is a symbiotic relationship between a green alga and a fungus.

3. What adaptations in pathogenic bacteria enable them to cause disease?

Pathogenic bacteria enter the body with insect or tick bites, sexual activity, in food, air, or water, or by direct contact with cut or abraded skin surfaces. Once inside the body, pili help the bacteria to attach to host cells, and bacterial enzymes attack host tissues. Bacterial toxins can disable the host’s circulatory, digestive, or nervous system.

4. How have humans harnessed the metabolic diversity of bacteria and archaea for practical purposes?

Disease-causing bacteria can be used as weapons. Bacteria that use fermentation to generate ATP are used in the food industry in the production of wine, cheese, olives, yogurt, and other products. Enzymes from bacteria that live in extremely warm environments are used in PCR. Heterotrophic bacteria are important in water and waste treatment facilities and in bioremediation.

5. Use the Internet to learn how the federal government investigates outbreaks of foodborne illnesses such as E. coli O157:H7 and Salmonella. What can you do to protect yourself from food poisoning?

The federal government attempts to trace events backwards from the infection to the source where the food was actually grown. To prevent food poisoning, be sure that food from animal sources is fully cooked or pasteurized, and use a cooking thermometer. Keep raw poultry and shellfish from contaminating other foods or food preparation items, thoroughly wash hands and food preparation surfaces, wash vegetables prior to eating them, and defrost food in the refrigerator rather than at room temperature.

6. Describe the relationship between nutrient pollution and harmful algal blooms. Why might harmful algal blooms be more frequent in summer? What steps could coastal communities take to prevent nutrient pollution?

Because algae thrive in water with abundant light and nutrients, pollution with excess nutrients leads to an increase in harmful algal blooms. Such blooms are more frequent in summer, when temperatures are warmer and people fertilize their lawns. To prevent nutrient pollution, coastal communities could pass laws that limit the amounts or types of fertilizers that are used. They could also provide better drainage and runoff with proper water collection and treatment.

7. Explain why the fossil record for diatoms is much more complete than that of other protists, such as amoebae and slime molds.

The silica-rich shells of the diatoms are easily preserved in sediments.

8. Give examples of fungi that are important economically, ecologically, and as food for humans.

Economically important fungi – Yeast are used in the baking industry. Aspergillus is used in making soy sauce and other commercial products. Fungi such as Penicillium and others are used in making antibiotics.  Ecologically important fungi – Several species of fungi are decomposers in nature. Aspergillus can cause lung infections, and several species of fungi cause athlete’s foot and ringworm. Rusts, molds, blights, and smuts are common plant pathogens. Food for humans – Morels, some mushrooms, and truffles are valuable sources of food for humans.

9. Hyphae are highly branched structures. How does their extensive surface area contribute to their functions?

Extensive branching leads to high numbers of hyphal tips, which increases their ability to secrete digestive enzymes and absorb nutrients.

10. Use the Internet to find examples of fungi that cause diseases in plants or animals. How does each fungus infect a host and spread to new hosts? What can humans do to fight each disease? What are the costs and benefits of doing so?

[Answers will vary.]

 

Answers to Pull It Together Questions

1. Review the structures of nucleic acids and proteins in chapter 2. Which chemical elements had to have been in the chemical “soup” to generate these organic molecules on early Earth?

The elements that would have had to be available are carbon, hydrogen, oxygen, nitrogen, phosphorus, and (for some amino acids) sulfur.

2. List the evidence that supports the endosymbiont theory.

The mitochondria and chloroplasts are structurally similar to bacteria; both organelles reproduce by binary fission just like bacteria; both organelles contain DNA, RNA, and ribosomes like bacteria; and DNA analysis shows a close relationship between mitochondria and aerobic bacteria and between chloroplasts and cyanobacteria.

3. Which organisms in this concept map are heterotrophs, and which are autotrophs? Which groups contain both autotrophs and heterotrophs?

Fungi, animals, and some protists and prokaryotes are hetrotrophs. Plants and some protists and prokaryotes are autotrophs.

4. Add labeled arrows to this concept map that depict the relationships that connect (a) leaves and endophytes; (b) roots and mycorrhizal fungi; and (c) the fungal and photosynthetic partners in a lichen.

For (a) and (b), arrows would have to connect “Plants” and “Fungi”. For (c), arrows would have to connect “Fungi” with either “Protists” or “Bacteria”.