Notes to Instructors
Chapter 27 Bacteria and Archaea
What is the focus of these activities?
Activity 27.1 How diverse are the Archaea?
Relatively recent studies indicate that prokaryotes are incredibly diverse, so diverse that
Activity 27.2 How has small size affected prokaryotic diversity?
This activity is designed to help students understand how small size has limited
What misconceptions or difficulties can these activities reveal?
Activity 27.2
We often state that the prokaryotes are more diverse metabolically and the eukaryotes are
more diverse morphologically. Then we go on to indicate that a wide variety of metabolic
Answers
Activity 27.1 How diverse are the Archaea?
1. The Archaea are divided into two major groups: the Euryarchaeota and the
Crenarchaeota.
Notes to Instructors 195
b. Two additional groups of the Archaea have been proposed.
What new
groups are these?
What characteristics do members of these groups have?
Korarcheota Based on comparison of DNA sequences, these species found
in hot springs in Yellowstone Park appear to be the oldest
Archaean group Characteristics
Euryarchaeota This group includes the methanogens, many extreme
halophiles, and some extreme thermophiles. The methanogens
are strictly anaerobic. The extreme halophiles are aerobic
and require high environmental salt concentrations (>20%).
196 Activity 27.1
a. What characteristics are used to place organisms into each of these groups?
2. There is great diversity in the ways different species of microbe
• obtain energy for metabolic functions, and
• obtain carbon for building the macromolecules of life.
Fill in the chart to indicate how these characteristics are used to describe the
nutritional type or nutritional classification of organisms. (Refer also to Table 27.1
on page 565 in Campbell Biology, 9th edition.)
1. Bacteria first appear in the fossil record about 3.5 billion years ago. Humans first
appear in the fossil record only a few million years ago. Given this, which group
would you say is more highly evolved?
Answers to this question depend a great deal on how one defines evolved. Many
a. What kinds of arguments or evidence would you use to support the idea that
bacteria are more highly evolved?
If we consider organisms that are closely fit (in their metabolism, for example) to
b. How would you support the idea that humans are more highly evolved?
If we use the term evolved to indicate major changes in morphology that allow
Plants Animals Bacteria and/or Archaea
Energy
source
Light Organic
compounds
Light Inorganic
chemicals
Light Organic
compounds
Activity 27.1 197
27.1 Test Your Understanding
2. Given what you know about the origin of life on Earth, you want to look for life on
other planets.
a. What characteristics of the planet’s environment would you look for? Explain
your reasoning.
Although we know that life can exist in the absence of oxygen, to the best of our
b. What kind(s) of life would you look for? Explain your reasoning.
To the best of our knowledge, free oxygen is not present on other planets. Therefore,
c. What tests or probes would you use to find the kind of life you proposed in
question b? Explain your reasoning.
To answer this question, you need to make the assumption that life on other planets
may have evolved similarly to life on Earth. This assumption is supported by
Activity 27.2 How has small size affected prokaryotic diversity?
It is often said that bacteria tend to be more diverse biochemically and eukaryotes tend to
be more diverse morphologically.
Answer questions 1–7. Then write a summary argument (question 8) to support the state-
ment above. In other words, write an argument describing how small size has limited
morphological diversity but promoted biochemical (metabolic) diversity among the
prokaryotes.
1. All living organisms must maintain a relatively constant internal environment.
Maintaining this environment means that a certain concentration of each substance
must be maintained per unit volume of the cell. The ability of the cell to maintain a
specific concentration of a substance is affected by
198 Activity 27.2
a. How do surface-area-to-volume (SA/V) ratios change as the size and shape of
cells and organisms change? To answer this, calculate the SA and Vof a cube
1 mm on a side. Then do the same for cubes that are 2 mm and 4 mm on a side
and compare their SA/V ratios.
b. In general, how does surface area change as linear dimensions increase twofold?
Surface area increases as a function of the square of the increase in linear
dimension. For example, the surface area of a 1-mm square is 6 mm2. The surface
c. In general, how does volume change as linear dimensions increase twofold?
Volume changes as the cube of the difference in linear dimension. If linear
Cubes: 1-mm square 2-mm square 4-mm square
Linear dimension 1 mm 2 mm 4 mm
Activity 27.2 199
d. In general, how do SA/V ratios change as linear dimensions increase twofold?
If you compare the SA/V ratios among the cubes, you can see that as linear
2. Assume a bacterium is 10 m in linear dimension. Fill in the chart.
a. If modeled as a cube, what would its SA, V, and SA/V ratio be?
b. If modeled as a sphere, what would its SA, V, and SA/V ratio be?
c. What are the SA and Vvalues and the SA/V ratios for a cube-shaped eukaryotic
cell that is 100 m in linear dimension?
3. Assume that every cell requires a minimum of 1 unit of oxygen per m3per second
to stay alive. Fill in the chart.
a. How much oxygen must cross each m2of surface area per second in the 10-m
bacterium versus the 100-m eukaryote to keep each alive?
b. What effects might this difference have on metabolic rates in these organisms?
10-m bacterium 100-m eukaryote
a. Oxygen/m2of SA/second 10 units/6 m2/second 100 units/6 m2/second
a. 10-m bacterium as
a cube
b. 10-m bacterium
as a sphere
c. 100-m eukaryote,
cube-shaped
200 Activity 27.2
4. Given what you know about cell membranes, is there likely to be a maximum upper
limit on the number of molecules of a substance that can cross a given area of
membrane per unit time? If so, what factors would be involved in determining the
maximum upper limit?
Because oxygen can diffuse through the phospholipid bilayer, its movement into the
cell would be limited only by its rate of diffusion and the total surface area available.
As you answer questions 5, 6, and 7, fill in the chart on the next page.
5. a. On average, how large is a prokaryotic genome?
b. On average, how many times larger is a eukaryotic genome?
c. Are these genomes haploid or diploid?
a. Prokaryotic genomes can contain a thousand to several thousand genes.
6. Under ideal conditions, many bacterial cells can reproduce or duplicate themselves
within an hour. Some species (for example, E. coli) can reproduce every 20 minutes
under ideal conditions. If you inoculate 1 liter of culture medium with one bacterium
per milliliter (of medium) at time t= 0, how many bacteria will be present in
1 milliliter of the culture medium after 10 hours? (To do this calculation, assume
that in these culture conditions the bacteria duplicate—or double in number—every
hour.)
In the first hour, the one bacterium will become two. The number will double every
Activity 27.2 201
7. Assume that the mutation rate of bacterial cells in culture is 10⫺6to 10⫺8. This
means that in every 1 million to 100 million cells produced from a single original
cell, you would expect to find at least one mutation. How many mutations would
you be likely to find in 1 liter of the 10-hour culture of the cells you grew in question
6?
8. Using all of the information in this activity, write an argument entitled: “How small
size in prokaryotes played a role in limiting their morphological diversity and
promoting their biochemical (metabolic) diversity.”
There are many possible ways to make this argument. All arguments should mention
Prokaryote Eukaryote
Genome size 5a. 1,700 genes 5b. 30,000 genes
5c. Haploid versus
diploid
Haploid Diploid