Structure and Microscopy
Lab 4: Structure and Microscopy (100 points)
Student Name: Miguel Yanes
Student ID: 7070589
Course ID: BIO 202
-Each question on the lab worksheet must be answered completely, thoroughly, in complete
sentences and correctly in order to be considered for full credit
-If the question asks you to do research or find a source, a reputable, credible and/or scholarly
source citation must be included in order to be considered for full credit
-If a math formula is required to arrive to an answer, work must be shown otherwise, no credit
will be awarded
*Note-This lab is designed to be completed without the use of a microscope.
Pre-Lab Questions
1. Most bacteria have a cell wall made of peptidoglycan, the structure of which makes the cell
wall very strong. Explain the specific components that make up the bacterial cell wall which
give it strength and protection (10 points)
Most bacteria have a strong and protective cell wall primarily composed of peptidoglycan, a
unique macromolecule that provides structural support. Peptidoglycan is made of long chains of
two alternating sugars: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), which
are linked together by β-(1,4) glycosidic bonds to form a glycan backbone. Attached to each
NAM molecule is a tetrapeptide chain, and these peptide chains cross-link with those of
neighboring strands, creating a tightly interconnected mesh. This cross-linking gives the cell wall
its rigidity and allows it to resist osmotic pressure and mechanical stress. In Gram-positive
bacteria, the peptidoglycan layer is much thicker and is reinforced by teichoic acids, which
further strengthen the wall and help anchor it to the cell membrane. Together, these
components enable the bacterial cell wall to maintain the cell’s shape and protect it from
environmental damage and lysis.
2. What are the three key objectives for heat fixing a sample before beginning a staining
procedure? (5 points)
The three key objectives for heat fixing a bacterial smear before staining are:
A. To kill the bacteria – Heat fixing effectively kills the microbes on the slide, making them
safe to handle during the staining process.
, Structure and Microscopy
B. To adhere the bacteria to the slide – Heating causes the proteins in the cells to
coagulate and bind the cells firmly to the glass surface, preventing them from being
washed away during staining and rinsing.
C. To preserve cell morphology – Heat fixing helps maintain the basic shape and structural
features of the bacterial cells, allowing for accurate observation under the microscope.
3. Bacteria have many different shapes that often determine their class. Research and form a
hypothesis on the evolutionary reasons for so many different bacterial morphologies. (5
points)
The diversity in bacterial morphology evolved as an adaptive response to environmental
pressures, resource acquisition, and motility needs, with different shapes offering survival
advantages in distinct ecological niches.
4. Do a search online or look in your textbook for 2 antibiotics that affect Gram-positive
bacteria and list them. On what part of the cell do the antibiotics inhibit?
A. Penicillin
a. Target: Cell wall (peptidoglycan layer)
b. Mechanism: Penicillin inhibits the enzyme transpeptidase, which is responsible
for forming cross-links between peptidoglycan chains in the bacterial cell wall.
This weakens the wall, leading to cell lysis due to osmotic pressure. Gram-
positive bacteria are especially vulnerable because they have a thick
peptidoglycan layer and no outer membrane.
B. Vancomycin
a. Target: Cell wall (peptidoglycan synthesis)
b. Mechanism: Vancomycin binds directly to the D-Ala-D-Ala terminal of the
peptidoglycan precursors, blocking their incorporation into the cell wall. This
prevents proper cell wall synthesis, ultimately causing bacterial death. It is often
used against Gram-positive bacteria, especially those resistant to beta-lactams
like MRSA.
List 2 antibiotics that affect Gram-negative bacteria? On what part of the cell do the
antibiotics inhibit? (Be sure to cite your sources in your answer.) (10 points)
A. Polymyxins (e.g., Polymyxin B and Polymyxin E/Colistin)
a. Target: Outer membrane (lipopolysaccharide layer)
b. Mechanism: Polymyxins bind to lipopolysaccharides (LPS) and phospholipids in
the outer membrane of Gram-negative bacteria, disrupting membrane integrity.
Lab 4: Structure and Microscopy (100 points)
Student Name: Miguel Yanes
Student ID: 7070589
Course ID: BIO 202
-Each question on the lab worksheet must be answered completely, thoroughly, in complete
sentences and correctly in order to be considered for full credit
-If the question asks you to do research or find a source, a reputable, credible and/or scholarly
source citation must be included in order to be considered for full credit
-If a math formula is required to arrive to an answer, work must be shown otherwise, no credit
will be awarded
*Note-This lab is designed to be completed without the use of a microscope.
Pre-Lab Questions
1. Most bacteria have a cell wall made of peptidoglycan, the structure of which makes the cell
wall very strong. Explain the specific components that make up the bacterial cell wall which
give it strength and protection (10 points)
Most bacteria have a strong and protective cell wall primarily composed of peptidoglycan, a
unique macromolecule that provides structural support. Peptidoglycan is made of long chains of
two alternating sugars: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), which
are linked together by β-(1,4) glycosidic bonds to form a glycan backbone. Attached to each
NAM molecule is a tetrapeptide chain, and these peptide chains cross-link with those of
neighboring strands, creating a tightly interconnected mesh. This cross-linking gives the cell wall
its rigidity and allows it to resist osmotic pressure and mechanical stress. In Gram-positive
bacteria, the peptidoglycan layer is much thicker and is reinforced by teichoic acids, which
further strengthen the wall and help anchor it to the cell membrane. Together, these
components enable the bacterial cell wall to maintain the cell’s shape and protect it from
environmental damage and lysis.
2. What are the three key objectives for heat fixing a sample before beginning a staining
procedure? (5 points)
The three key objectives for heat fixing a bacterial smear before staining are:
A. To kill the bacteria – Heat fixing effectively kills the microbes on the slide, making them
safe to handle during the staining process.
, Structure and Microscopy
B. To adhere the bacteria to the slide – Heating causes the proteins in the cells to
coagulate and bind the cells firmly to the glass surface, preventing them from being
washed away during staining and rinsing.
C. To preserve cell morphology – Heat fixing helps maintain the basic shape and structural
features of the bacterial cells, allowing for accurate observation under the microscope.
3. Bacteria have many different shapes that often determine their class. Research and form a
hypothesis on the evolutionary reasons for so many different bacterial morphologies. (5
points)
The diversity in bacterial morphology evolved as an adaptive response to environmental
pressures, resource acquisition, and motility needs, with different shapes offering survival
advantages in distinct ecological niches.
4. Do a search online or look in your textbook for 2 antibiotics that affect Gram-positive
bacteria and list them. On what part of the cell do the antibiotics inhibit?
A. Penicillin
a. Target: Cell wall (peptidoglycan layer)
b. Mechanism: Penicillin inhibits the enzyme transpeptidase, which is responsible
for forming cross-links between peptidoglycan chains in the bacterial cell wall.
This weakens the wall, leading to cell lysis due to osmotic pressure. Gram-
positive bacteria are especially vulnerable because they have a thick
peptidoglycan layer and no outer membrane.
B. Vancomycin
a. Target: Cell wall (peptidoglycan synthesis)
b. Mechanism: Vancomycin binds directly to the D-Ala-D-Ala terminal of the
peptidoglycan precursors, blocking their incorporation into the cell wall. This
prevents proper cell wall synthesis, ultimately causing bacterial death. It is often
used against Gram-positive bacteria, especially those resistant to beta-lactams
like MRSA.
List 2 antibiotics that affect Gram-negative bacteria? On what part of the cell do the
antibiotics inhibit? (Be sure to cite your sources in your answer.) (10 points)
A. Polymyxins (e.g., Polymyxin B and Polymyxin E/Colistin)
a. Target: Outer membrane (lipopolysaccharide layer)
b. Mechanism: Polymyxins bind to lipopolysaccharides (LPS) and phospholipids in
the outer membrane of Gram-negative bacteria, disrupting membrane integrity.
