Chapter 27: Bacteria and Archaea

Among the six statements below, which two best account for the results obtained by the researchers (see Figure 27.1)? 1. Low-glucose conditions caused mutations that made individual E. coli cells better suited to these conditions. 2. Daughter cells acquired the ability to tolerate low-glucose conditions as they received the enzymes and membrane components that had been modified by their mother cell. 3. The initial E. coli population may have included some cells whose genes favored their survival in low-glucose conditions-OR-such genetic variants arose by chance early in the experiment. 4. The first few generations of E. coli in low-glucose conditions responded to the challenge by increasing the use of certain enzymes and ion pumps, while decreasing the use of others. This behavior was recorded in their gene sequences, which were later transmitted to daughter cells. 5. From generation to generation, there was an increase in the proportion of the experimental populations adapted to low-glucose conditions, because such bacteria produced relatively more offspring than did ancestral bacteria under low-glucose conditions. 6. During each generation, individual cells evolved to increase their survival in low-glucose conditions. A) 3 and 5 B) 1 and 5 C) 2 and 4 D) 1 and 6 E) 1 and 3

A

Assuming that each of these possesses a cell wall, which prokaryotes should be expected to be most strongly resistant to plasmolysis in hypertonic environments? A) extreme halophiles B) extreme thermophiles C) methanogens D) cyanobacteria E) nitrogen-fixing bacteria that live in root nodules

A

If a bacterium regenerates from an endospore that did not possess any of the plasmids that were contained in its original parent cell, the regenerated bacterium will probably A) lack antibiotic-resistant genes. B) lack a cell wall. C) lack a chromosome. D) lose base pairs from its chromosome. E) be unable to survive in its normal environment.

A

In this 8-year experiment, 12 populations of E. coli, each begun from a single cell, were grown in low-glucose conditions for 20,000 generations. Each culture was introduced to fresh growth medium every 24 hours. Occasionally, samples were removed from the populations, and their fitness in low-glucose conditions was tested against that of members sampled from the ancestral (common ancestor) E. coli population. REFER TO FIGURE 27.1 Which term best describes what has occurred among the experimental populations of cells over this 8-year period? A) microevolution B) speciation C) adaptive radiation D) sexual selection E) stabilizing selection

A

Jams, jellies, preserves, honey, and other foodstuffs with a high sugar content hardly ever become contaminated by bacteria, even when the food containers are left open at room temperature. This is because bacteria that encounter such an environment A) undergo death by plasmolysis. B) are unable to metabolize the glucose or fructose, and thus starve to death. C) undergo death by lysis. D) are obligate anaerobes. E) are unable to swim through these thick and viscous materials.

A

Mitochondria are thought to be the descendants of certain alpha -proteobacteria. They are, however, no longer able to lead independent lives because most genes originally present on their chromosome have moved to the nuclear genome. Which phenomenon accounts for the movement of these genes? A) horizontal gene transfer B) binary fission C) alternative gene splicing D) meiosis E) plasmolysis

A

Prokaryotic ribosomes differ from those present in eukaryotic cytosol. Because of this, which of the following is correct? A) Some selective antibiotics can block protein synthesis of bacteria without effects on protein synthesis in the eukaryotic host. B) Eukaryotes did not evolve from prokaryotes. C) Translation can occur at the same time as transcription in eukaryotes but not in prokaryotes. D) Some antibiotics can block the synthesis of peptidoglycan in the walls of bacteria. E) Prokaryotes are able to use a much greater variety of molecules as food sources than can eukaryotes.

A

The data were collected from the heterocytes of a nitrogen-fixing cyanobacterium inhabiting equatorial ponds. Study the graph below and choose the most likely explanation for the shape of the curve. (REFER TO PHONE) A) Enough O2 enters heterocytes during hours of peak photosynthesis to have a somewhat-inhibitory affect on nitrogen fixation. B) Light-dependent reaction rates must be highest between 1800 hours and 0600 hours. C) Atmospheric N2 levels increase at night because plants are no longer metabolizing this gas, so are not absorbing this gas through their stomata. D) Heterocyte walls become less permeable to N2 influx during darkness. E) The amount of fixed nitrogen that is dissolved in the pond water in which the cyanobacteria are growing peaks at the close of the photosynthetic day (1800 h).

A

The following questions refer to structures found in a gram-positive prokaryotic cell. Not present in all bacteria, this structure enables those that possess it to germinate after exposure to harsh conditions, such as boiling: A) endospore B) sex pilus C) flagellum D) cell wall E) capsule

A

Which is the bacterial structure that acts as a selective barrier, allowing nutrients to enter the cell and wastes to leave the cell? A) plasma membrane B) capsule C) cell wall D) nucleoid region E) pili

A

Which of the following are responsible for high levels of O2 in Earthʹs atmosphere? A) photoautotrophs B) photoheterotrophs C) chemoautotrophs D) chemoheterotrophs that perform decomposition E) parasitic chemoheterotrophs

A

Which of the following contains a copy of the chromosome, along with a small amount of dehydrated cytoplasm, within a tough wall? A) endospore B) sex pilus C) flagellum D) cell wall E) capsule

A

Which of the following use light energy to synthesize organic compounds from CO2? A) photoautotrophs B) photoheterotrophs C) chemoautotrophs D) chemoheterotrophs that perform decomposition E) parasitic chemoheterotrophs

A

Which question, arising from the results depicted in Figure 27.2, is most interesting from a genetic perspective, and has the greatest potential to increase our knowledge base? A) If reciprocal crossing-over could occur even if the piece of donated Hfr DNA is identical to the homologous portion of the recipientʹs chromosome, what prevents this from occurring? B) Why do geneticists refer to the same structure by at least three different names: sex pilus, mating bridge, and conjugation tube? Why all the jargon? C) What forces are generally responsible for disrupting the mating bridge? D) How is it that a recipient cell does not necessarily become an Hfr cell as the result of conjugation with an Hfr cell? E) What makes a cell an ʺHfr cellʺ?

A

If new genetic variation in the experimental populations arose solely by spontaneous mutations, then the most effective process for subsequently increasing the prevalence of the beneficial mutations in the population over the course of generations is A) transduction. B) binary fission. C) conjugation. D) transformation. E) meiosis.

B

In a hypothetical situation, the genes for sex pilus construction and for tetracycline resistance are located together on the same plasmid within a particular bacterium. If this bacterium readily performs conjugation involving a copy of this plasmid, then the result should be A) a transformed bacterium. B) the rapid spread of tetracycline resistance to other bacteria in that habitat. C) the subsequent loss of tetracycline resistance from this bacterium. D) the production of endospores among the bacteriumʹs progeny. E) the temporary possession by this bacterium of a completely diploid genome.

B

Match the numbered terms to the descriptions that follow. For each item, choose all appropriate terms, but only appropriate terms. 1. autotroph 2. heterotroph 3. phototroph 4. chemotroph an organism that relies on photons to excite electrons within its membranes A) 1 only B) 3 only C) 1 and 3 D) 2 and 4 E) 1, 3, and 4

B

Mycoplasmas are bacteria that lack cell walls. On the basis of this structural feature, which statement concerning mycoplasmas should be true? A) They are gram-negative. B) They are subject to lysis in hypotonic conditions. C) They lack a cell membrane as well. D) They undergo ready fossilization in sedimentary rock. E) They possess typical prokaryotic flagella.

B

Penicillin is an antibiotic that inhibits enzymes from catalyzing the synthesis of peptidoglycan, so which prokaryotes should be most vulnerable to inhibition by penicillin? A) mycoplasmas B) gram-positive bacteria C) archaea D) gram-negative bacteria E) endospore-bearing bacteria

B

The following questions refer to structures found in a gram-positive prokaryotic cell. If this structure connects the cytoplasm of two bacteria, one of these cells may gain new genetic material: A) endospore B) sex pilus C) flagellum D) cell wall E) capsule

B

The following questions refer to structures found in a gram-positive prokaryotic cell. Which of the following is a structure that permits conjugation to occur? A) endospore B) sex pilus C) flagellum D) cell wall E) capsule

B

Though plants, fungi, and prokaryotes all have cell walls, we place them in different taxa. Which of these observations comes closest to explaining the basis for placing these organisms in different taxa, well before relevant data from molecular systematics became available? A) Some closely resemble animals, which lack cell walls. B) Their cell walls are composed of very different biochemicals. C) Some have cell walls only for support. D) Some have cell walls only for protection from herbivores. E) Some have cell walls only to control osmotic balance.

B

What is the primary ecological role of prokaryotes? A) parasitizing eukaryotes, thus causing diseases B) breaking down organic matter C) metabolizing materials in extreme environments D) adding methane to the atmosphere E) serving as primary producers in terrestrial environments

B

Which of the following use light energy to generate ATP, but do not release oxygen? A) photoautotrophs B) photoheterotrophs C) chemoautotrophs D) chemoheterotrophs that perform decomposition E) parasitic chemoheterotrophs

B

Which two processes are responsible for the shape of the curve at Time B? 1. transduction 2. entry of single-stranded Hfr DNA 3. rolling circle replication of single-stranded Hfr DNA 4. activation of DNA pumps in plasma membrane 5. ʺtoilet paperʺ replication of recipient cellʹs plasmids A) 1 and 4 B) 2 and 3 C) 3 and 5 D) 1 and 3 E) 4 and 5

B

Which two structures play direct roles in permitting bacteria to adhere to each other, or to other surfaces? 1. capsules 2. endospores 3. fimbriae 4. plasmids 5. flagella A) 1 and 2 B) 1 and 3 C) 2 and 3 D) 3 and 4 E) 3 and 5

B

How is the recipient cell different at Time D than it was at Time A? A) It has a greater number of genes. B) It has a greater mass of DNA. C) It has a different sequence of base pairs. D) It contains bacteriophage DNA. E) It has a greater number of introns.

C

E. coli cells typically make most of their ATP by metabolizing glucose. Under the conditions of this experiment, what should be true of E. coliʹs generation time (especially early in the course of the experiment, but less so later on)? A) Generation time should be the same as in the typical environment. B) Generation time should be faster than in the typical environment. C) Generation time should be slower than in the typical environment. D) It is theoretically impossible to make any predictions about generation time, under these conditions.

C

In a hypothetical situation, a bacterium lives on the surface of a leaf, where it obtains nutrition from the leafʹs nonliving, waxy covering, while inhibiting the growth of other microbes that are plant pathogens. If this bacterium gains access to the inside of a leaf, it causes a fatal disease in the plant. Once the plant dies, the bacterium and its offspring decompose the plant. What is the correct sequence of ecological roles played by the bacterium in the situation described here? Use only those that apply. 1. nutrient recycler 2. mutualist 3. commensal 4. parasite 5. primary producer A) 1, 3, 4 B) 2, 3, 4 C) 2, 4,1 D) 1, 2, 5 E) 1, 2, 3

C

Modes of obtaining nutrients, used by at least some bacteria, include all of the following except A) chemoautotrophy. B) photoautotrophy. C) heteroautotrophy. D) chemoheterotrophy. E) photoheterotrophy.

C

The following questions refer to structures found in a gram-positive prokaryotic cell. Which of the following requires ATP to function, and permits some species to respond to taxes (plural of taxis)? A) endospore B) sex pilus C) flagellum D) cell wall E) capsule

C

The thermoacidophile, Sulfolobus acidocaldarius lacks peptidoglycan. What is likely to be true of this species? 1. It is a bacterium. 2. It is an archaean. 3. The optimal pH of its enzymes will lie above pH 7. 4. The optimal pH of its enzymes will lie below pH 7. 5. It could inhabit certain hydrothermal springs. 6. It could inhabit alkaline hot springs. A) 1, 3, and 6 B) 2, 4, and 6 C) 2, 4, and 5 D) 1, 3, and 5 E) 1, 4, and 5

C

The typical prokaryotic flagellum features A) an internal 9 + 2 pattern of microtubules. B) an external covering provided by the plasma membrane. C) a complex ʺmotorʺ embedded in the cell wall and plasma membrane. D) a basal body that is similar in structure to the cellʹs centrioles.

C

Which of the following obtain energy by oxidizing inorganic substances; energy that is used, in part, to fix CO2? A) photoautotrophs B) photoheterotrophs C) chemoautotrophs D) chemoheterotrophs that perform decomposition E) parasitic chemoheterotrophs

C

Which of these can be inferred from Figure 27.1? A) Most of the genetic change that permitted adaptation to the new, low-glucose environment occurred toward the conclusion of the experiment. B) Rates of mitosis increased over the course of the experiment. C) The highest rate of genetic change occurred during the first quarter of the experiment. D) After 5,000 generations, the bacteria were 100% more fit than the original, ancestral bacteria.

C

Which of these statements about prokaryotes is correct? A) Bacterial cells conjugate to mutually exchange genetic material. B) Their genetic material is confined within a nuclear envelope. C) They divide by binary fission, without mitosis or meiosis. D) The persistence of bacteria throughout evolutionary time is due to their genetic homogeneity (i.e., sameness). E) Genetic variation in bacteria is not known to occur, nor should it occur, because of their asexual mode of reproduction.

C

Which statement about bacterial cell walls is false? A) Bacterial cell walls differ in molecular composition from plant cell walls. B) Cell walls prevent cells from bursting in hypotonic environments. C) Cell walls prevent cells from dying in hypertonic conditions. D) Bacterial cell walls are similar in function to the cell walls of many protists, fungi, and plants. E) Cell walls provide the cell with a degree of physical protection from the environment.

C

Which statement about the domain Archaea is false? A) Genetic prospecting has recently revealed the existence of many previously unknown archean species. B) Some archaeans can reduce CO2 to methane. C) The genomes of archaeans are unique, containing no genes that originated within bacteria. D) Some archaeans can inhabit solutions that are nearly 30% salt. E) Some archaeans are adapted to waters with temperatures above the boiling point.

C

Which of these is the most common compound in the cell walls of gram-positive bacteria? A) cellulose B) lipopolysaccharide C) lignin D) peptidoglycan E) protein

D

Carl Woese and collaborators identified two major branches of prokaryotic evolution. What was the basis for dividing prokaryotes into two domains? A) microscopic examination of staining characteristics of the cell wall B) metabolic characteristics such as the production of methane gas C) metabolic characteristics such as chemoautotrophy and photosynthesis D) genetic characteristics such as ribosomal RNA sequences E) ecological characteristics such as the ability to survive in extreme environments

D

If all prokaryotes on Earth suddenly vanished, which of the following would be the most likely and most direct result? A) The number of organisms on Earth would decrease by 10—20%. B) Human populations would thrive in the absence of disease. C) Bacteriophage numbers would dramatically increase. D) The recycling of nutrients would be greatly reduced, at least initially. E) There would be no more pathogens on Earth.

D

If archaeans are more closely related to eukaryotes than to bacteria, then which of the following is a reasonable prediction? A) Archaean DNA should have no introns. B) Archaean chromosomes should have no protein bonded to them. C) Archaean DNA should be single-stranded.D) Archaean ribosomes should be larger than typical prokaryotic ribosomes. E) Archaeans should lack cell walls.

D

If the experimental population of E. coli lacks an F factor or F plasmid, and if bacteriophage are excluded from the bacterial cultures, then which of these is a means by which beneficial mutations might be transmitted horizontally to other E. coli cells? A) via sex pili B) via transduction C) via conjugation D) via transformation E) both A and C above

D

In a bacterium that possesses antibiotic resistance and the potential to persist through very adverse conditions, such as freezing, drying, or high temperatures, DNA should be located within, or be part of, which structures? 1. nucleoid region 2. flagellum 3. endospore 4. fimbriae 5. plasmids A) 1 only B) 1 and 4 C) 1 and 5 D) 1, 3, and 5 E) 2, 4, and 5

D

Match the numbered terms to the descriptions that follow. For each item, choose all appropriate terms, but only appropriate terms. 1. autotroph 2. heterotroph 3. phototroph 4. chemotroph a prokaryote that obtains both energy and carbon as it decomposes dead organisms A) 1 only B) 4 only C) 1 and 3 D) 2 and 4 E) 1, 3, and 4

D

Match the numbered terms to the descriptions that follow. For each item, choose all appropriate terms, but only appropriate terms. 1. autotroph 2. heterotroph 3. phototroph 4. chemotroph an organism that obtains both carbon and energy by ingesting prey A) 1 only B) 4 only C) 1 and 3 D) 2 and 4 E) 1, 3, and 4

D

Match the numbered terms to the descriptions that follow. For each item, choose all appropriate terms, but only appropriate terms. 1. autotroph 2. heterotroph 3. phototroph 4. chemotroph an organism that obtains its energy from chemicals A) 1 only B) 2 only C) 3 only D) 4 only E) 1 and 4

D

REFER TO FIGURE 27.1 If it occurs in the absence of any other type of adaptation listed here, which of these is least reasonable in terms of promoting bacterial survival over evolutionary time in a low-glucose environment? A) increased efficiency at transporting glucose into the cell from the environment B) increased ability to survive on simple sugars, other than glucose C) increased ability to synthesize glucose from amino acid precursors D) increased reliance on glycolytic enzymes E) increased sensitivity to, and ability to move toward, whatever glucose is present in its habitat

D

REFER TO FIGURE 27.2 What is occurring at Time C that is decreasing the DNA content? A) crossing-over B) cytokinesis C) meiosis D) degradation of DNA that was not retained in the recipientʹs chromosome E) reversal of the direction of conjugation

D

Regarding prokaryotic reproduction, which statement is correct? A) Prokaryotes form gametes by meiosis. B) Prokaryotes feature the union of haploid gametes, as do eukaryotes. C) Prokaryotes exchange some of their genes by conjugation, the union of haploid gametes, and transduction. D) Mutation is a primary source of variation in prokaryote populations. E) Prokaryotes skip sexual life cycles because their life cycle is too short

D

The following questions refer to structures found in a gram-positive prokaryotic cell. Which of the following is an important source of endotoxin in gram-negative species? A) endospore B) sex pilus C) flagellum D) cell wall E) capsule

D

The following questions refer to structures found in a gram-positive prokaryotic cell. Which of the following is composed almost entirely of peptidoglycan? A) endospore B) sex pilus C) flagellum D) cell wall E) capsule

D

The predatory bacterium, Bdellovibrio bacteriophorus, drills into a prey bacterium and, once inside, digests it. In an attack upon a gram-negative bacterium that has a slimy cell covering which can inhibit phagocytosis, what is the correct sequence of structures penetrated by B. bacteriophorus on its way to the preyʹs cytoplasm? 1. membrane composed mostly of lipopolysaccharide 2. membrane composed mostly of phospholipids 3. peptidoglycan 4. capsule A) 2 → 4 → 3 →1 B) 1 → 3 → 4 → 2 C) 1 → 4 → 3 → 2 D) 4 → 1 → 3 → 2 E) 4 → 3 → 1 → 2

D

The termite gut protist, Mixotricha paradoxa, has at least two kinds of bacteria attached to its outer surface. One kind is a spirochete that propels its host through the termite gut. A second type of bacteria synthesizes ATP, some of which is used by the spirochetes. The locomotion provided by the spirochetes introduces the ATP-producing bacteria to new food sources. Which term(s) is (are) applicable to the relationship between the two kinds of bacteria? 1. mutualism 2. parasitism 3. symbiosis 4. metabolic cooperation A) 1 only B) 1 and 2 C) 2 and 3 D) 1, 3, and 4 E) all four terms

D

Which of the following traits do archaeans and bacteria share? 1. composition of the cell wall 2. presence of plasma membrane 3. lack of a nuclear envelope 4. identical rRNA sequences A) 1 only B) 3 only C) 1 and 3 D) 2 and 3 E) 2 and 4

D

Which statement about the genomes of prokaryotes is correct? A) Prokaryotic genomes are diploid throughout most of the cell cycle. B) Prokaryotic chromosomes are sometimes called plasmids. C) Prokaryotic cells have multiple chromosomes, ʺpackedʺ with a relatively large amount of protein. D) The prokaryotic chromosome is not contained within a nucleus but, rather, is found at the nucleoid region. E) Prokaryotic genomes are composed of linear DNA (that is, DNA existing in the form of a line with two ends).

D

A fish that had been salt-cured subsequently develops a reddish color. You suspect that the fish has been contaminated by the extreme halophile, Halobacterium. Which of these features of cells removed from the surface of the fish, if confirmed, would support your suspicion? 1. the presence of the same photosynthetic pigments found in cyanobacteria 2. cell walls that lack peptidoglycan3. cells that are isotonic to conditions on the surface of the fish4. its cells contain bacteriorhodopsin 5. the presence of very large numbers of ion pumps in its plasma membrane A) 2 and 5 B) 3 and 4 C) 1, 4, and 5 D) 3, 4, and 5 E) 2, 3, 4, and 5

E

Given that the enzymes that catalyze nitrogen fixation are inhibited by oxygen, what are two ʺstrategiesʺ that nitrogen-fixing prokaryotes might use to protect these enzymes from oxygen? 1. couple them with photosystem II (the photosystem that splits water molecules) 2. package them in membranes that are impermeable to all gases 3. be obligate anaerobes 4. be strict aerobes 5. package these enzymes in specialized cells or compartments that inhibit oxygen entry A) 1 and 4 B) 2 and 4 C) 2 and 5 D) 3 and 4 E) 3 and 5

E

Which term is least closely associated with the others? A) Hfr cells making use of a sex pilus B) rolling circle replication C) the ʺtoilet paperʺ model of replication D) conjugation involving an F factor E) recombination involving a bacteriophage

E

How can prokaryotes be considered to be more successful on Earth than humans? A) Prokaryotes are much more numerous and have more biomass. B) Prokaryotes occupy more diverse habitats. C) Prokaryotes are more diverse in metabolism. D) Only B and C are correct. E) A, B, and C are correct.

E

If the vertical axis of Figure 27.1 refers to ʺDarwinian fitness,ʺ then which of these is the most valid and accurate measure of fitness? A) number of daughter cells produced per mother cell per generation B) amount of ATP generated per cell per unit time C) average swimming speed of cells through the growth medium D) amount of glucose synthesized per unit time E) number of generations per unit time

E

Only certain prokaryotes can perform nitrogen fixation, but nitrogen-fixing prokaryotes are not known to live inside animals. Thus, how do animals gain access to fixed nitrogen? A) They may breathe it in from air that has experienced lightning discharges. B) They may ingest nitrogen fixers. C) They may ingest plants that harbor nitrogen fixers, or plants that absorbed fixed nitrogen from the soil. D) They may ingest other animals that had done either (B) or (C) above. E) Answers (B), (C), and (D) above are all possible.

E

The following questions refer to structures found in a gram-positive prokaryotic cell. Not present in all bacteria, this cell covering enables cells that possess it to resist the defenses of host organisms: A) endospore B) sex pilus C) flagellum D) cell wall E) capsule

E

Which of the following are responsible for many human diseases? A) photoautotrophs B) photoheterotrophs C) chemoautotrophs D) chemoheterotrophs that perform decomposition E) parasitic chemoheterotrophs

E

Consider the thermoacidophile, Sulfolobus acidocaldarius. Which graph most accurately depicts the expected temperature and pH profiles of its enzymes? (NOTE: the horizontal axes of these graphs are double, with pH above, and temperature below.) (REFER TO PHONE)

A

Cyanobacteria are A) photoautotrophs. B) photoheterotrophs. C) chemoautotrophs. D) chemoheterotrophs that perform decomposition. E) parasitic chemoheterotrophs.

A

During which two times can the recipient accurately be described as ʺrecombinantʺ due to the sequence of events portrayed in Figure 27.2? A) during Times C and D B) during Times A and C C) during Times B and C D) during Times A and B E) during Times B and D

A

Nitrogenase, the enzyme that catalyzes nitrogen fixation, is inhibited whenever free O2 reaches a critical concentration. Consequently, nitrogen fixation cannot occur in cells wherein photosynthesis produces free O2. Consider the colonial aquatic cyanobacterium, Anabaena, whose heterocytes are described as having ʺ...a thickened cell wall that restricts entry of O2 produced by neighboring cells. Intracellular connections allow heterocytes to transport fixed nitrogen to neighboring cells in exchange for carbohydrates.ʺ Which two questions below arise from a careful reading of this quotation, and are most important for understanding how N2 enters heterocytes, and how O2 is kept out of heterocytes? 1. If carbohydrates can enter the heterocytes from neighboring cells via the ʺintracellular connections,ʺ how is it that O2 doesnʹt also enter via this route? 2. If the cell walls of Anabaenaʹs photosynthetic cells are permeable to O2 and CO2, are they also permeable to N2? 3. If the nuclei of the photosynthetic cells contain the genes that code for nitrogen fixation, how can these cells fail to perform nitrogen fixation?4. If the nuclei of the heterocytes contain the genes that code for photosynthesis, how can these cells fail to perform photosynthesis? 5. If the cell walls of Anabaenaʹs heterocytes are permeable to N2, how is it that N2 doesnʹt diffuse out of the heterocytes before it can be fixed? 6. If the thick cell walls of the heterocytes exclude entry of oxygen gas, how is it that they donʹt also exclude the entry of nitrogen gas? A) 3 and 4 B) 2 and 5 C) 1 and 3 D) 4 and 6 E) 1 and 6

E