CHAPTER 5 MEMBRANE TRANSPORT AND CELL SIGNALING
Scientific Skills Exercise
Teaching objective: The main objective of this exercise is to give students practice in
interpreting a graph, especially in regard to recognizing and comparing the major trends in two
sets of data. Secondary objectives include encouraging students to deal with a real-life activity of
one of the membrane transport proteins they read about in this chapter and, as they’re thinking
about hypothesis formulation and testing, to realize that what they’re learning in this chapter
connects to topics in other chapters.
Teaching tips: A version of this Scientific Skills Exercise can be assigned in MasteringBiology.
The instructor may want to point out that the rate of glucose “uptake” was not being measured
directly; rather, the experimenters were measuring the concentration of radioactive glucose in the
cells at successive time points, with the results reflecting the differing rates of glucose uptake. In
answering question 3, some students may observe that, for both data sets, the uptake of glucose
have the background to design an experiment in detail, but the allusions to studying specific
(purified) proteins in Concept 3.5 and the mentions of labeling in Chapter 2 (radioactive tracers
in Concept 2.2) and Chapter 4 (fluorescent labeling of molecules for microscopy; see Figure 4.3)
should enable them to come up with a general answer.
Answers:
1. (a) Incubation time (in minutes) is the independent variable. (b) Concentration of radioactive
3. The increasing concentrations of radioactive glucose at successive time points indicate that
5. Answers will vary. One way to test the first hypothesis mentioned in answer 4 would be to
Suggested Answers for End-of-Chapter Essay Questions
See the general information on grading short-answer essays and a suggested rubric at the
beginning of this document.
7. Scientific Inquiry
(a) Evaluating the data reveals that the proton pump builds up an H+ gradient across the
membrane by pumping protons out of the cell, represented by the initial decrease in pH seen in
the solution. Once the gradient is sufficient, H+ can move down its electrochemical gradient into
the cell, and sucrose appears to move with it. The proton pump, using ATP, continues to pump
8. Science, Technology, and Society
If a plant cell is placed in a hypertonic solution such as that found in wet saline soils, water will
be lost by osmosis from the cytoplasm and central vacuole of the cell. The cell may plasmolyze
9. Focus on Evolution
One might expect unicellular eukaryotes that live in hypertonic environments to have the
10. Focus on Interactions
Sample key points:
The plasma membrane is selectively permeable and regulates the passage of materials.
Nonpolar molecules diffuse through the lipid bilayer.
Polar molecules require transport proteins.
Sample top-scoring answer:
The selectively permeable plasma membrane mediates the passage of substances into and out of
the cell via several mechanisms. Small nonpolar molecules, such as O2 and CO2, can easily
diffuse through the hydrophobic interior of the lipid bilayer, which interferes with entry of polar
substances, even very small ones. Larger polar molecules, such as glucose and amino acids,
11. Synthesize Your Knowledge
The water is hypotonic to the plant cells, so the plant cells take up water. Thus, the cells of the