Chapter 4 Remarks

Remarks on: Randy Moore et al. 1995. Botany. Wm. C. Brown Publishers. Dubuque, IA.

Why a whole chapter on membranes? Check out the opening paragraph (page 73); 'nuff said. The paragraph is motivational and well-written.

This membrane chapter should be a total review of Biology 115 (Principles of Biology) and Biology 221 (Cellular and Molecular Biology). You can expect exam questions on this prerequisite information.

The Truth about Facts

The book nicely shows you an older hypothesis (Davson-Danielli) and some of its support but then goes on to show you modern evidence for the fluid-mosaic model. This is more than a history lesson. Think about how advances in technology expand and limitations in technology shrink our ability to know the truth. The phrase, seeing is believing, is taken as an absolute in our culture, yet, our vision limits us in some serious ways as shown in this section. Sometimes our current ideas sound like facts in books, just as the Davson-Danielli model may have sounded years ago. We sometimes cannot see how limited our vision is! This underscores why I chose a book that exposes our "warts."

Remember, the "facts" are likely to be somewhat erroneous when "future vision" is applied. Thus you may hear me use the word "factoid" on occasion. That word means: an idea that is taken as fact but could be suspect. Many of the ideas in biology fit into that category. The role of centrioles in cell division is a great example. Their position at the ends of spindle fibers indicated a role in organizing or anchoring the fibers. The idea became "factoid" in many books. Students lost points on exams if they did not give that idea on their exams. With the improvement of microscopy, ability to destroy centrioles and still have mitosis, and the clear fact that centrioles are missing from plant cells that nevertheless divide, have led us to remove that factoid from our books. Any book published recently will show this new uncertainty.

Factoid examples from this chapter

This chapter includes a presentation of diffusion (page 78). It has been pointed out recently that what is shown in Figure 4.7 and supporting text is really a function of turbulent flow rather than diffusion. If you think about the limitations of cell size (previous chapter) discussion, it would seem that a cell as large as the beaker shown is plausible if diffusion happens this quickly. You see, turbulent flow mixes the dye rapidly...the highly-compartmented cell cannot use that flow because of the endomembrane barriers to turbulent mixing.

The section on water potential gives the usual "factoid" about water concentration. The concentration differences (molarity of water) between distilled water and brine are insufficient to explain the rates of water movement. The water potential (an energy consideration) is the real driving force behind water movement (ie. this is the truer "factoid"). As the book moves into osmosis the presentation gets clearer about this. Yet Figure 4.8 has two variables in it! As water moves across the membrane, what new variable is introduced? That variable is important in living plant cells but not in animal cells. If you aren't sure what it is, please ask me, but I hope you will be able figure it out for yourself! (Hint: Figure 4.9).

As before, when you read this chapter, you might take special notice of the plant-specific content.

Here are a few things to notice:

The caption to figure 4.10 on page 80 talks about arrows, but there aren't any in the figure. How should they have been drawn?
The first full paragraph on page 81 should ask you to look at the photo on the unnumbered page 72. The inset shows the plant in its wilted state. You can see the profound effects of loss of turgor on a whole plant.
After reading about the facilitated diffusion of sucrose via co-transport of H+ and the section on active transport of H+, you should be able to write an essay on how sugar might be loaded into phloem cells of a leaf. Something like: defend/condemn the concept of sucrose loading into phloem as an active transport process might guide your thoughts.
I hope you enjoy the diagram Figure 4.18! This is the nicest diagram of exocytosis I have seen as it shows how the bilayer is involved. Even so, what is missing...particularly from the cell (plasma) membrane?
If you read the box on beer (page 84) you may have noticed that starch is not osmotically active as a solute, while glucose is. Think about why this might be so. If you read the biochemistry chapter the answer should be more obvious.
The text indicates that endocytosis is uncommon in plants (pg 84), particularly because of turgor pressure (pg 85). How might you induce a plant cell to do endocytosis? It is routinely observed in one kind of preparation. Would you expect this endocytosis to be phagocytosis or pinocytosis in the case of the induced plant cell?
Read carefully about ions and electical potential in membrane transport. This concept is important in cell energetics.
The section on pH activation of enzymes in various compartments (Figure 4.21) is interesting to me especially as it relates to auxin-induced cell growth (see text below figure and on facing page including Figure 4.24). You might not appreciate how recently these ideas have been developed. Your book here is "on the cutting edge."
Cell recognition by plants completes the chapter. I am fascinated by the section on pollen tube incompatibility (pg 88-89 and Fig. 4.26). This is an area that is a hotbed of research in New England (particularly U Mass) and Canada (U Toronto). Understanding pollen tube competition and success is an important advance in the study of plant reproduction. Considering how plants use animals as "pimps" to bring the pollen from many sources to the stigma, this recognition and competition process is very important to reproductive success.

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