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Ch. 3 - Cell Structure and Function
Bauman - Microbiology with Diseases by Taxonomy 6th Edition
Bauman6th EditionMicrobiology with Diseases by TaxonomyISBN: 9780134832302Not the one you use?Change textbook
All textbooksBauman 6th EditionCh. 3 - Cell Structure and FunctionProblem 3
Chapter 3, Problem 3

A scientist who is studying passive movement of chemicals across the cytoplasmic membrane of Salmonella enterica serotype Typhi measures the rate at which two chemicals diffuse into a cell as a function of external concentration. The results are shown in the following figure. Chemical A diffuses into the cell more rapidly than does B at lower external concentrations, but the rate levels off as the external concentration increases. The rate of diffusion of chemical B continues to increase as the external concentration increases.
Graph showing Chemical A's diffusion rate plateaus with concentration, while Chemical B's rate increases linearly.


a. How can you explain the differences in the diffusion rates of chemicals A and B?
b. Why does the diffusion rate of chemical A taper off?
c. How could the cell increase the diffusion rate of chemical A?
d. How could the cell increase the diffusion rate of chemical B?

Verified step by step guidance
1
Step 1: Analyze the graph to understand the diffusion behavior of chemicals A and B. Chemical A shows a rapid increase in diffusion rate at low external concentrations but then plateaus, indicating a maximum rate. Chemical B shows a steady, linear increase in diffusion rate with increasing external concentration.
Step 2: Explain the difference in diffusion rates by considering the mechanisms involved. Chemical A likely uses facilitated diffusion through a carrier protein or channel that becomes saturated at higher concentrations, causing the rate to level off. Chemical B likely diffuses by simple diffusion, where the rate is directly proportional to the concentration gradient and does not saturate.
Step 3: Understand why the diffusion rate of chemical A tapers off. The plateau occurs because the carrier proteins or channels responsible for its transport become fully occupied or saturated, limiting the maximum rate of diffusion regardless of further increases in external concentration.
Step 4: To increase the diffusion rate of chemical A, the cell could increase the number of carrier proteins or channels in the membrane, thereby increasing the maximum transport capacity and allowing more molecules to be transported simultaneously.
Step 5: To increase the diffusion rate of chemical B, since it diffuses by simple diffusion, the cell could increase the concentration gradient by either increasing the external concentration or decreasing the internal concentration, enhancing the driving force for diffusion.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Passive Diffusion vs Facilitated Diffusion

Passive diffusion is the movement of molecules across a membrane without assistance, driven by concentration gradients, and typically shows a linear increase in rate with concentration. Facilitated diffusion involves specific carrier proteins or channels that help molecules cross the membrane, leading to a saturation point where all carriers are occupied, causing the rate to level off.
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Saturation Kinetics in Membrane Transport

Saturation kinetics occur when transport proteins become fully occupied by their substrate, limiting the rate of movement across the membrane. This results in a plateau in the rate of diffusion despite increasing external concentration, as seen with chemical A, indicating a carrier-mediated process with finite capacity.
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Mechanisms to Increase Diffusion Rate

Cells can increase diffusion rates by either increasing the number of transport proteins (for facilitated diffusion) or by altering membrane permeability. For chemical A, increasing carrier proteins would raise the maximum rate, while for chemical B, which diffuses passively, increasing membrane fluidity or concentration gradient can enhance diffusion.
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Related Practice
Textbook Question

Label each type of flagellar arrangement.

a. __________ <IMAGE> b. __________ <IMAGE>

c. __________ <IMAGE> d. __________ <IMAGE>

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Textbook Question

A "9 + 2" arrangement of microtubules is seen in __________ .


a. archaeal flagella

b. bacterial flagella

c. eukaryotic flagella

d. all prokaryotic flagella

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Textbook Question

Which of the following is not a function of a glycocalyx?


a. It forms pseudopods for faster mobility of an organism.

b. It can protect a bacterial cell from drying out.

c. It hides a bacterial cell from other cells.

d. It allows a bacterium to stick to a host.

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Textbook Question

Define cytosol.

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Textbook Question

The term fluid mosaic has been used in describing the cytoplasmic membrane. How does each word of that phrase accurately describe our current understanding of a cell membrane?

Textbook Question

Which of the following statements concerning growth and reproduction is false?

a. Growth and reproduction may occur simultaneously in living organisms

b. A living organism must reproduce to be considered alive

c. Living things may stop growing and reproducing yet still be alive

d. Normally, living organisms have the ability to grow and reproduce themselves

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