Chapter 3 Remarks

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

This cell biology chapter, like the biochemistry chapter, should fit you like an old shoe. Again, Biology 115 (Principles of Biology) and Biology 221 (Cellular and Molecular Biology) covered this in a fair amount of detail. I will not be lecturing on this topic for very long. You can expect exam questions to deal with this chapter in some way and you should be ready.

As before, when you read this chapter, you might take special notice the plant-specific content. Remember, plant cells have just about everything found in animal cells plus three important structures: cell wall, chloroplast, and vacuole.

Here are a few things to notice:

The cell wall is described between pages 56 and 58. There is a nice section about cell walls, and gives you a new perspective about them. I hope you get the impression that the wall is really quite porous and permeable. It does not regulate ion movement into or out of the cell. Of course, once it is impregnated with waxes (cutin and suberin) it becomes waterproof. Think of the implications of that!
A very important feature of cell walls is the plasmodesma (pg 57-58). Your book tells you about these strange features of many plant species. The existence of these tells you that plant cells are sometimes not completely separated (cytoplasmically) from their relatives. This is likely an artifact of incomplete cytokinesis cause by the phragmoplastic method used in plants; furrowing found in other organisms is much less likely to result in a pit-field. It is worthy of note that mature guard cells in epidermal tissue lack plasmodesmata. That is one of several important points overlooked by Donald Kaplan in his explanation of the organismal concept (page 70; see below).
The vacuole is discussed on pages 62-63. You should remember that this is one of the important compartments in a cell. It is not empty as the name, vacuole, might imply. It is well described here as more than a container too. There is a lot of interesting biochemistry being discovered in vacuoles. It is also developmentally interesting as shown in Figure 3.22.
You are introduced to the chloroplast on Page 64. There will be more on that later in the Photosynthesis chapter. This organelle carries out photosynthesis with proteins originating from chloroplast DNA as well as proteins originating from nuclear DNA. What are the implications of that finding? Think about transport processes as well as the endosymbiont theory and evolutionary processes. How did it come about? What was the selection pressure to arrive at this arrangement?
The plant mitochondrion is described on pages 65-6...Yes, plants have mitochondria and carry out respiration! Remember plants have everything animal cells have plus three additional structures (which are?). To me, this says plants are more than animals...not less than animals as is supposed by most people.

The Faulty Diagram

The arrows on Figure 3.13 on page 56 are likely out of registration. I have a paperback and a hardbound copy of this book. In both, the arrows don't point to the right areas of the wall environment. Each book has problems with different arrows, so maybe I cannot help you with this. I'll try anyway. The middle lamella is a very dark band in the central region where the two cell's walls touch. This is the pectic glue holding the walls together. The primary wall is a similarly-thin but light stripe bordering the middle lamella. Since we have two cells in contact here, there is one light stripe on each side of the middle lamella. Thus each cell has its own primary wall. Between the primary wall and the "interior of the cell" are three layers of secondary cell wall. The middle layer of the secondary wall layers is the thickest in this view. Notice how the secondary wall is laid down inside the primary wall.

If you understand this, then perhaps you can answer questions such as:

What happens to the cell volume as the secondary wall is added?
If the secondary wall contains suberin, what happens next?
Can a cell avoid this fate? If so, how so?

How to Say Nothing

You can try to read the section called "How Cell Walls Grow" on pages 56 and 57. Maybe you can get something more out of it than I did. This looks like a lot of hand-waving to me; "maybe this is so but maybe this is not." I hope you can see that even professional writers backed-up by reviewers and editors, can write paragraphs meaning very little. I would delete this whole section or reduce it to this: "The correlation between the orientation of microtubules and cellulose microfibrils is not well understood. Any cause-effect relationship would be speculative at present."

Of course that would be bowing to "content" rather than peaking your curiousity and challenging your thinking skills. The part about colchicine doesn't help you understand the role of microtubules any better, but it does give you an example of how someone might try to test hypotheses for this question. It provides a launching pad for your creative thinking...if you let it!

The Organismal Theory

This theory is old, but has resurfaced recently (1991) in a widely-read journal. I am opening a discussion of this theory on the internet mail-list group on plant education. While I see the merits of the theory, I see some holes in it. First, there is the lack of plasmodesma connections in some cell contacts. Second, cells of plants demonstrate totipotency; that is, a single cell can be grown up into a complete organism...it doesn't just continue to develop from where it's position in the organism might have dictated. Third, certain cells in plants are programmed to die while adjacent cells thrive. Fourth, simple manipulation with plant hormones can cause a root to develop on just about any organ of a plant. Fifth, some plants, such as Kalanchoe, develop epiphyllous buds with no hormonal intervention; in this case a whole new organism is formed from a cell on the leaf margin. For this physiologist, there are plenty of reasons that the cell theory seems more plausible than the organismal theory. It is nice that the book included this theory, but I wish there were more discussion of it. Maybe you will study this in graduate school and make yourself famous!

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