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Answers to Open-Ended Questions
Hoefnagels Essentials 2/e
Chapter 15
Answers to Mastering Concepts Questions
15.1
1. What can we learn from simulations of early Earth?
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.
2. 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.
3. About when did the first cells probably originate?
The first cells probably originated between 3.8 and 3.7 billion years ago.
4. 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 15.9, sketch the internal and external features of a typical
prokaryotic cell. What are the functions of each structure?
The nucleoid is the region where chromosomal 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 an external barrier
and maintains the cell’s shape. 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?
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Autotrophs are “self-feeders” that acquire carbon from inorganic sources; heterotrophs
are “other-eaters” that get carbon by consuming other organisms. Phototrophs acquire
energy from the sun; chemotrophs acquire energy from chemical sources. Obligate
aerobes require oxygen; obligate anaerobes are unable to exist in the presence of oxygen;
and facultative aerobes can survive with or without oxygen.
4. In what ways are bacteria and archaea essential to eukaryotic life?
Microbes are decomposers, nitrogen fixers, photosynthetic organisms, beneficial
residents of animal bodies, and food sources.
5. What adaptations enable pathogenic bacteria to enter the body and cause disease?
Pathogenic bacteria enter the body with animal bites, during sexual activity, in food, air,
or water, or by direct contact with wounded 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. What is the evidence that mitochondria and chloroplasts descend from cells?
Evidence of endosymbiosis includes the following: Mitochondria and chloroplasts have
their own DNA, RNA, and ribosomes; they have a double membrane; 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.
2. List a logical sequence of events that starts with an early prokaryote and ends with a
modern multicellular eukaryote.
A possible sequence of events might begin with membrane infolding in some lineages of
archaea; a membrane might have encircled the genetic material, forming a nucleus. Other
organelles also formed. Archaea engulfed aerobic bacteria, which lived inside the cells
(an event called endosymbiosis). These bacterial partners eventually became
mitochondria. Photosynthetic bacteria became chloroplasts by the same mechanism. With
all of the components of a eukaryotic cell in place, multicellular organisms may have
originated when individual eukaryotic cells merged to form a single organism, or when
cells remained stuck together after dividing.
15.4
1. What features define the protists?
Protists are eukaryotes that are not plants, fungi, or animals.
2. Describe examples illustrating why protists are important.
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Many examples are possible; this sample answer includes a few. Protists form the base of
many aquatic food webs; are parasites that cause disease in plants and animals, including
humans; can help people find oil reserves; and have multiple practical uses, including
making paints reflective and making chocolate smooth and creamy.
3. Compare and contrast the algae, slime molds, water molds, and protozoa.
Algae are photosynthetic autotrophs that live in the water. Slime molds and water molds
are both heterotrophic; slime molds have unusual life cycles and live in damp soils,
whereas water molds live as decomposers and as parasites in host plants and animals.
Unlike slime molds, water molds produce swimming spores. Protozoa are a diverse group
that can be autotrophs or heterotrophs and can live in water, in soil, or within a host
organism.
15.5
1. What combination of characteristics defines fungi?
Fungi are eukaryotic and heterotrophic; they digest their food externally. They have cell
walls made of chitin, and they store carbohydrates as glycogen. Most fungi are
multicellular, although the yeasts are unicellular. In most species, the only diploid cell is
the zygote.
2. Describe how fungi acquire food.
Microscopic hyphae grow within a food source and secrete enzymes that break down the
organic molecules, which are absorbed at the tips of the hyphae.
3. How do scientists classify the five phyla of fungi?
Scientists distinguish the five phyla based upon 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. How do fungi benefit humans?
Fungi benefit humans in many ways; this sample answer includes some examples. Fungi
are a source of food. Many mushrooms are edible (however, others are deadly), and
fermentation of yeasts is important in baking and brewing. Fungi are also useful in
medicine. Antibiotics like penicillin and immune suppressants like cyclosporine are
developed from fungi.
5. Compare and contrast endophytes, mycorrhizae, and lichens.
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Endophytes, mycorrhizae, and lichens are all fungal interactions with other organisms.
Endophytes are fungi that live within plants but do not produce disease symptoms.
Mycorrhizae are mutualistic fungal associations with plant roots. Lichens are mutualistic
interactions between fungi and green algae or cyanobacteria.
Write It Out
1. If you were developing a new “broad-spectrum” antibiotic to kill a wide variety of
bacteria, which cell structures and pathways would you target? Which of those targets
also occur in eukaryotic cells, and why is that important?
A broad-spectrum antibiotic would target structures that are similar in most or all types of
bacteria, including ribosomes, peptidoglycan in the cell wall, the chemicals of the cell
membrane, or certain metabolic enzymes that are unique to bacteria. New drugs that
target peptidoglycan or bacterial ribosomes would not harm eukaryotic cells, but drugs
that target membrane proteins and enzymes that are present in bacteria and eukaryotic
cells could have harmful or deadly side effects.
2. The amoeba Pelomyxa palustris is a single-celled eukaryote with no mitochondria, but
it contains symbiotic bacteria that can live in the presence of oxygen. How does this
observation support the endosymbiont theory?
The amoeba might be from a lineage that recently underwent an endosymbiosis event.
The bacterial symbiont is not yet fully dependent on its host, but the interaction likely
benefits both partners. This case illustrates an important step between endosymbiosis and
the origin of mitochondria.
3. Why might overwatering your plants make them more susceptible to infection by some
kinds of heterotrophic protists?
Water molds use swimming cells to spread from plant to plant or from one part of a plant
to another. Overwatering a plant increases the chance that a water mold will infect its
roots or leaves.
4. Review figure 15.32. Are fungi more closely related to animals or to plants? What
characteristics do fungi share with plants? What characteristics do fungi share with
animals?
Fungi are more closely related to animals. However, fungi share similarities with both
plants and animals. Like plants, fungi are not motile and their visible parts often spring
up from the soil or another substrate; fungi also have cell walls. Like animals, fungi are
heterotrophs and store carbohydrates as glycogen; fungi also share many metabolic
features with animals.
5. Hyphae are highly branched structures. How does their extensive surface area
contribute to their functions?
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Extensive branching leads to high numbers of hyphal tips, which increases their ability to
secrete digestive enzymes and absorb nutrients.
Pull It Together
1. To the side of the concept map, list groups within the Bacteria, Archaea, Protista, and
Fungi.
The bacteria are divided into 23 phyla; proteobacteria, cyanobacteria, and spirochaetes
are three examples. The archaea are typically divided into informal groups, such as the
thermophiles, acidophiles, and methanogens. The protists are informally classified into
algae, the funguslike water molds and slime molds, and the protozoa. Fungi are divided
into five phyla: the Chytridiomycota, Zygomycota, Glomeromycota, Ascomycota, and
Basidiomycota.
2. 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.
“Fungi” could connect with “live as endophytes within the leaves of” to “Plants.”
“Fungi” could connect with “that form mycorrhizae live in roots of” to “Plants.” “Fungi”
could connect with “form lichens when interacting with photosynthetic” to both
“Protists” and “Prokaryotes.”
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McGraw-Hill Education.