BIOL 107 606 Fundamentals of Microbiology

Correct Answer A: The cell will shrink and shrivel up

Explanation:

The external solution (12% NaCl) is hypertonic compared to the inside of the cell (2% NaCl). In a hypertonic solution, water moves out of the cell by osmosis to balance the concentration gradient. As water exits the cell, it loses volume, causing it to shrink and shrivel—a process called crenation in animal cells.

Why the other options are incorrect:

B) There would be no effect on the cell:

Incorrect. There is a significant difference in solute concentration, so osmosis will occur, and water will leave the cell, affecting its shape and volume.

C) The cell will expand and likely burst:

Incorrect. This would happen in a hypotonic solution (where the outside solute concentration is lower than inside), causing water to move into the cell. Here, the solution is hypertonic, so the opposite occurs.

VCorrect Answer C: Tt

Detailed Explanation of the Correct Answer:

C. Tt is correct because Tay-Sachs is autosomal recessive, meaning that an individual must inherit two copies of the recessive allele (tt) to have the disease. If a child is affected (tt), then both parents must have passed on a recessive allele. This means each parent must be carriers—they do not have the disease themselves but carry one copy of the recessive allele. Therefore, their genotypes must both be Tt.

Explanation of Why the Other Options Are Incorrect:

A. TT – This represents two dominant alleles. A parent with this genotype cannot pass on the recessive allele (t), so they cannot produce a child with Tay-Sachs.

B. tt – This genotype would mean the parent actually has Tay-Sachs disease. The question clearly states that both parents do not have the disease, so this cannot be correct.

Summary:

If a child has Tay-Sachs (tt) and the parents do not show symptoms, they must both be carriers with the genotype Tt. Therefore, the correct answer is C. Tt.

Correct Answer B: Reduced

Explanation:

In this reaction, fluorine (F) gains an electron from lithium (Li) to form fluoride (F⁻). Gaining electrons is the definition of reduction.

Why the other options are incorrect:

A) Hydrolyzed:

Hydrolysis involves a reaction with water, which is not occurring in this reaction.

C) Hydrogenated:

Hydrogenation involves the addition of hydrogen atoms, usually to organic compounds—not relevant here.

D) Oxidized:

Oxidation is the loss of electrons. In this case, lithium is oxidized, not fluorine.

Correct answer D: glycocalyx, cell wall, plasma membrane

Detailed explanation:

The cellular envelope of bacteria consists of several layers that surround the bacterial cell. The order of these layers from outermost to innermost is as follows:

Glycocalyx (outermost layer)

The glycocalyx is a gel-like layer composed of polysaccharides, and sometimes proteins. It can exist in two forms:

Capsule: A well-organized and tightly bound layer that protects the bacterium from desiccation, helps in adhesion to surfaces, and can help bacteria evade the immune system by preventing phagocytosis.

Slime layer: A more loosely attached, disorganized layer that helps bacteria stick to surfaces and form biofilms.

The glycocalyx plays a role in adherence and protection, but it is not present in all bacteria.

Cell wall

The cell wall is a rigid structure located just beneath the glycocalyx (if present) and provides shape and structural support to the bacterial cell. It protects the cell from osmotic pressure and helps maintain cell integrity.

​​​​​​​In Gram-positive bacteria, the cell wall is thick and composed of peptidoglycan, while in Gram-negative bacteria, the cell wall is thinner, and there is an additional outer membrane outside the peptidoglycan layer.

Plasma membrane (innermost layer)

The plasma membrane (also known as the cytoplasmic membrane) is a lipid bilayer that surrounds the cytoplasm and regulates the movement of substances into and out of the cell. It is critical for processes such as energy generation, nutrient transport, and cell signaling.

​​​​​​​The plasma membrane also houses proteins involved in transport, metabolic processes, and communication with the environment.

Why the other options are incorrect:

A cell wall, plasma membrane, glycocalyx

This option incorrectly places the glycocalyx as the innermost layer. The glycocalyx is the outermost layer, not the innermost.

B plasma membrane, glycocalyx, cell wall

This option is incorrect because the plasma membrane is the innermost layer, not the outermost. The glycocalyx or capsule layer (if present) is the outermost, followed by the cell wall beneath it.

C glycocalyx, plasma membrane, cell wall

This order places the plasma membrane between the glycocalyx and cell wall, which is incorrect. The plasma membrane is the innermost structure, and the cell wall lies outside it (unless it’s a Gram-negative bacterium, where there is an outer membrane as well).

Summary:

The correct order of the layers of the bacterial cellular envelope from outermost to innermost is glycocalyx, cell wall, and plasma membrane. This arrangement helps bacteria with protection, structure, and interaction with their environment. Thus, the correct answer is D glycocalyx, cell wall, plasma membrane.

Correct answer D: DNA has thymine, while RNA replaces that with adenine

Detailed explanation of the correct answer:

Option D is correct because it accurately describes the fundamental difference in nitrogenous bases between DNA and RNA. Both DNA and RNA are made up of nucleotides, and each nucleotide contains a nitrogenous base. DNA contains four bases: adenine, thymine, cytosine, and guanine. RNA also contains four bases, but instead of thymine, RNA uses uracil. The rest of the bases remain the same in both molecules: adenine, cytosine, and guanine are shared between DNA and RNA. Therefore, the one base that is different is thymine in DNA and uracil in RNA. This is a consistent and important feature used to distinguish RNA from DNA. During transcription, when RNA is made from DNA, uracil in the RNA pairs with adenine in the DNA, replacing the role that thymine would play in DNA pairing.

Explanation of incorrect answers:

Option A: DNA has thymine, while RNA replaces that with adenine

 is incorrect because adenine is not a replacement for thymine in RNA. Adenine is present in both DNA and RNA. The base that replaces thymine in RNA is uracil, not adenine.

Option B:DNA has adenine, while RNA replaces that with uracil.

 is incorrect because adenine is not replaced in RNA. Adenine is one of the four nitrogenous bases found in both DNA and RNA. Uracil replaces thymine, not adenine.

Option C:DNA has uracil, while RNA replaces that with thymine.

is incorrect because DNA does not contain uracil. Uracil is only found in RNA. DNA always contains thymine instead of uracil. This option reverses the correct relationship between DNA and RNA.

Option E: DNA has adenine, while RNA replaces that with thymine>

is incorrect because adenine is never replaced in RNA. Adenine is present in both DNA and RNA. Thymine is the base that is replaced in RNA, and it is replaced by uracil, not by adenine.

Summary:

The correct base difference between DNA and RNA is that DNA contains thymine, and RNA contains uracil in its place. All other bases — adenine, cytosine, and guanine — remain the same between the two molecules. The correct answer is option D.

Correct Answer D: Net movement of particles from areas of high concentration to areas of low concentration

Simple diffusion is the process where molecules move passively down their concentration gradient, meaning they move from a region where they are more concentrated to where they are less concentrated. This process does not require cellular energy or assistance from transport proteins, and it occurs until equilibrium is reached.

Why Other Options are Incorrect

A)Net movement of particles between cells:  Movement between cells describes transport across cell junctions or extracellular spaces, not diffusion across a plasma membrane.

B) Net movement of particles from areas of low concentration to areas of high concentration:  Movement from low to high concentration requires energy input and is characteristic of active transport, not simple diffusion.

C) Net movement of particles into and out of the cell using vacuoles for bulk transport:  Movement involving vacuoles refers to bulk transport mechanisms such as endocytosis or exocytosis, which are energy-dependent and different from diffusion.

Summary:

Simple diffusion is the passive movement of particles directly through the plasma membrane from an area of higher concentration to lower concentration, without energy or specialized transport mechanisms.

Correct Answer B: They serve as high-energy electron carriers that transport electrons throughout the cell

Detailed Explanation of the Correct Answer:

B. They serve as high-energy electron carriers that transport electrons throughout the cell is correct because NAD⁺ and FAD are coenzymes that become NADH and FADH₂ when they accept electrons during glycolysis, the citric acid (Krebs) cycle, and pyruvate oxidation. These reduced forms carry high-energy electrons and hydrogen atoms to the electron transport chain (ETC) in the mitochondria. There, the electrons are used to generate a proton gradient, which ultimately drives the synthesis of ATP through oxidative phosphorylation.

Explanation of Why the Other Options Are Incorrect:

A. They serve to help produce co-enzyme A – This is incorrect. Coenzyme A is involved in the conversion of pyruvate to acetyl-CoA, but NAD⁺/FAD are not involved in its production.

C. They serve as the final electron acceptor for the electron transport chain – This is incorrect. The final electron acceptor in the ETC is oxygen, not NAD⁺ or FAD.

D. They serve to directly help catalyze the reaction of ADP to ATP – Incorrect. While NADH and FADH₂ contribute to the process that leads to ATP production, they do not directly catalyze the conversion of ADP to ATP. That function is performed by ATP synthase.

Summary:

NAD⁺/NADH and FAD/FADH₂ function as electron carriers, shuttling high-energy electrons to the electron transport chain, where most of the cell's ATP is ultimately generated. Therefore, the correct answer is B.

Correct answer A: 5' UAGCGU 3'

Detailed Explanation:

Transcription is the process of synthesizing RNA from a DNA template. During transcription:

The RNA polymerase reads the DNA template strand from 3' to 5' and synthesizes the mRNA strand in the 5' to 3' direction.

The RNA uses uracil (U) instead of thymine (T), which is found in DNA.

Given the DNA template strand:

3' ATCGCA 5'

Here’s how to transcribe it step-by-step:

The DNA base A (adenine) pairs with U (uracil) in RNA.

The DNA base T (thymine) pairs with A (adenine) in RNA.

The DNA base C (cytosine) pairs with G (guanine) in RNA.

The DNA base G (guanine) pairs with C (cytosine) in RNA.

The DNA base C (cytosine) pairs with G (guanine) in RNA.

The DNA base A (adenine) pairs with U (uracil) in RNA.

Therefore, the RNA transcript is:

5' UAGCGU 3'

Explanation of Incorrect Options:

B. 5' GCTATG 3'

This is incorrect because it contains the wrong bases for transcription. In transcription, RNA uses uracil (U), not thymine (T), and the base pairings are mismatched.

C. 5' TAGCGT 3'

This is incorrect because it uses T (thymine) instead of U (uracil) in the RNA transcript.

D. 5' GCUAUG 3'

This is incorrect because it does not correctly match the template strand bases. The base pairing is not consistent with the transcription process.

Summary:

The correct transcription for the template sequence 3' ATCGCA 5' is 5' UAGCGU 3', which matches option A.

Correct Answer A: Eukaryotic

Explanation:

Protists are eukaryotic organisms, meaning they have a true nucleus enclosed within a membrane and possess membrane-bound organelles. They can be unicellular or multicellular and include organisms such as amoebas, algae, and paramecia. Unlike prokaryotes (like bacteria), protists have complex cellular structures.