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Grade 11F Science Related Reading/Biology
Name:____________________
Class:____________________
Diversity of Life
Biology Gr11F
Date:____________________
Read the passage below. Then answer the questions that follow.
Gut Instinct
Vocabulary
Development
Bacteria, a type of germ, are tiny organisms, each made of a single cell.
They’re far too small to see with the naked eye — but don’t let their tiny
size make you think they’re not important. You should get to know bacteria.
For every cell in your brain, bones, organs and blood, there are 10 bacteria.
They’re not going anywhere, so you might as well get used to them.
Copy the key vocabulary
written in bold in the
space provided below.
Try to write their
definitions in your
notebook
Bacteria, which outnumber all eukaryotes combined, differ from eukaryotes
in at least seven ways. Bacteria are prokaryotes. Unlike eukaryotes,
prokaryotes lack a cell nucleus and all other membrane-bound organelles.
They do contain ribosomes, granules of stored nutrients, enzymes, and
certain internal structures, however. Most bacterial cells are about 1 µm in
diameter; most eukaryotic cells are more than 10 times that size. They are
single cells. Bacterial chromosomes consist of a single circular piece of
DNA. Prokaryotes also have plasmids, small extra loops of DNA. They
reproduce by binary fission, a process in which one cell pinches into two
identical cells. Bacterial flagella are simple structures that spin like a
corkscrew to move the cell. Some also have short, thin, protein tubes called
pili that attach to surfaces or to other cells.
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Reading Check
How many chromosomes
does a bacterium have?
Scientists classify bacteria based on their structure using a technique called
Gram staining. Gram staining involves coloured dyes. Gram-positive
bacteria have a thick layer of peptidoglycan in their cell wall and no outer
membrane. They stain darkly with Gram staining dyes because their
peptidoglycan layer is very thick. Gram-negative bacteria have a thin layer
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Grade 11F Science Related Reading/Biology
of peptidoglycan in their cell walls covered by an outer membrane. The thin
peptidoglycan layer does not trap the dark dye, but does absorb the pink dye.
Their outer membrane makes them more resistant to medicines and the
body’s defenses than Gram-positive bacteria.
These tiny organisms have diverse metabolic abilities. For example, some
get their energy from sunlight through photosynthesis “photoautotrophs”.
Others get energy from inorganic molecules that contain sulfur or nitrogen
“chemoautotrophs”. Most bacteria get nutrients and energy by feeding on
other organisms “heterotrophs”. Bacteria can be helpful: Some live in the
intestines and aid with digestion. They can also be harmful: Some bacteria
cause serious disease. And, believe it or not, some bacteria can be both
helpful and harmful. For example, the kinds of E. coli bacteria that live in
your intestines are helpful. But the kinds of E. coli consumed through
contaminated food are a major cause of food poisoning.
In a recent and surprising study, scientists suggest that bacteria living in the
intestines can affect brain development and behavior. That connection may
seem strange, since the brain and bowels are far apart. On the other hand,
maybe the finding begins to explain why we talk about having a “gut
instinct.”
In the study, scientists from Sweden and Germany studied “germfree” mice
that had been raised in a sterile environment and kept away from bacteria
their whole lives. During various tests of mouse behavior, the germfree mice
were more adventurous than regular mice. The germfree mice were bolder
and more likely to take risks, like exploring new tunnels in mazes.
Those tests suggest that bacteria can influence mouse behavior. To further
explore the question of how bacteria acts on the brain, the scientists tried
restoring bacteria to the germfree mice — and found another interesting
twist.
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Looking closer
What type of bacteria
cannot make their own
food?
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Grade 11F Science Related Reading/Biology
After they received bacteria, newborn, germfree mice took fewer risks and
acted like the mice that had always been around bacteria. But when the
scientists exposed adult, germfree mice to bacteria, those mice didn’t change
their behaviors. They remained bold and adventurous. The researchers don’t
know how bacteria affect the brain. But the experiments show that bacteria
somehow affect behavior, and the influence happens very early in the
mouse’s life.
This is a microscopic closeup of E. coli, your single-cell frenemy. The kinds
of E. coli that live in your intestines help everything run smoothly. But when
E. coli is ingested through contaminated food, the bacteria can cause big
problems for humans. Credit: Rocky Mountain Laboratories, NIAID, NIH
The researchers hope they can build on the study to eventually learn
something about human behavior. But scientists know that the connection
between bacteria and behavior in mice isn’t a direct one. People with fewer
germs don’t necessarily put themselves in danger. And an unfortunate run-in
with E. coli food poisoning isn’t going to make a person less likely to take
risks — except, perhaps, in their choice of food.
Sven Pettersson worked on the study connecting bacteria and brain
development in mice. He is a microbiologist at the Karolinska Institute in
Stockholm and the Genome Institute of Singapore. Microbiologists study
life on the tiniest scales — like the single-cell existence of bacteria.
Pettersson says that the study hints at interesting relationships that may exist
between bacteria and other parts of the body. “It doesn’t take Einstein to
realize that if you can do that for the brain, then [these kinds of]
relationships could affect other organs in the body,” Pettersson told Science
News. “It’s tickling to think about.”
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Critical Thinking
Are the ribosomes of
E.coli surrounded by a
membrane? Explain
your answer.
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Grade 11F Science Related Reading/Biology
READING EFFECTIVELY
In the space provided, write the letter of the phrase that best completes the
statement.
_____ 1. Most bacterial cells are
a. more than 10 times larger than an average eukaryotic cell.
b. as large as an average eukaryotic cell.
c. one-tenth the size of an average eukaryotic cell.
d. double the size of an average eukaryotic cell.
_____ 2. Escherichia coli has long, thin, hairlike appendages called
___________, which are used for locomotion.
a. flagella
b. cilia
c. pili
d. ribosomes
_____ 3. One difference between bacteria and eukaryotes is that
a. bacteria have flagella whereas eukaryotes do not.
b. bacterial chromosomes are circular, whereas eukaryotic
chromosomes are linear.
c. bacterial cells are much larger than eukaryotic cells.
d. many bacteria are multicellular, whereas all eukaryotes are
unicellular.
_____ 4. Which of the following does not characterize the structure of
Escherichia coli?
a. a rigid cell wall
b. flagella
c. organelles
d. pili
_____ 5. Which of the following is an asexual form of reproduction in
bacteria?
a. transformation
b. conjugation
c. transduction
d. binary fission
_____ 6. Gram-positive bacteria stain purple because they have
a. a thin layer of peptidoglycan in their cell wall.
b. a thick layer of peptidoglycan in their cell wall.
c. no peptidoglycan in their cell wall.
d. no cell wall
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Grade 11F Science Related Reading/Biology
SKILL: READING EFFECTIVELY
Read each question, and write your answer in the space provided.
1.Describe two ways in which prokaryotes differ from eukaryotes.
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2. What are chemoautotrophs?
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3. If eight bacterial cells each undergo binary fission, how many
bacterial cells will result?
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4. In the study conducted by scientists in Sweden and Germany, what
was the difference in behavior between “germfree” mice and regular
mice? What does that indicate?
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5. Why do scientists believe that bacteria affect brain development and
behavior in mice only if introduced early in life?
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6.How could scientists in the study obtain “germfree” mice?
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Grade 11F Science Related Reading/Biology
7. Compare the structure of Gram-positive and Gram-negative bacteria.
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8. Why can infection by Gram-negative bacteria be more dangerous than
in infection by Gram-positive bacteria?
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Complete each statement by writing the correct term or phrase in the
space provided.
1. Bacteria often have small extra loops of DNA called
____________________.
2. The cell wall of bacteria contains a protein-carbohydrate compound
called _______________________.
3. Gram-negative bacteria are more resistant to medicines because they
have a(n) ______________________ ______________________.
In the space provided, write the letter of the description that best matches
the term or phrase.
_____ 1. plasmid
_____ 2. binary
fission
_____ 3. pili
_____ 4. E. coli
a. short outgrowths that enable bacteria to
attach to surfaces or to other cells
b. small extra loop of DNA
c. asexual reproduction in bacteria
d. helpful in intestines
e. gets energy from other organisms
_____ 5. heterotroph
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