Do Plant Cells contain lysosomes? (from Loren, a teacher in Texas)
- Do Plant Cells contain lysosomes? (from Loren, a teacher in Texas)
- After our electricity went out, maggots appeared in meat that had been in our freezer. Where did they come from? (from Xasha, a student in Puerto Rico)
- How do muscle cells heal after being cut? (from Karen, a teacher in Oregon)
- If E. coli is normally found in the human gut, then why does food that is contaminated with E. coli make us sick? (from Mary, a teacher in Washington State)
- How many ATPs are actually made during the Krebs cycle? Is the real nummber 36, 38, or something else? (from Angelique, a teacher in Michigan)
- Why is it that hair exposed to the sun gets lighter, but skin exposed to the sun gets darker? (from Jen, a teacher in California)
- Why don't bacterial cells have nucleoli? (from Melissa, a teacher in Texas)
- How can you tell how old a plant root is? (from Caito, a student in California)
- How do ions flow across the Neuron cell membrane? (from Richard, a teacher in Michigan)
- What is the difference between radical and bilaterall symmetry?
- What techniques have improved crop yields during the past several decades? (from Wendy, a student in New York)
- How do plants balance their need for carbon dioxide with their need to prevent dehydration?
- Can you give any examples of cases in which knowledge of Ecology has helped the Environment? (from Anthony, in New York)
- My teacher said the chicken egg isn't one big cell, but that the little white membrane looking thing that is attached to the yolk is the CELL — not the whole thing. Is that right? (from a student in Kentucky)
- I was wondering if you knew how the Endoplasmic Reticulum got its name or who discovered it.
Ha! You’ve put your finger on a messy little squabble among plant biologists that has gone on for years.
Current evidence suggests that "YES," some plant cells do contain lysosomes.
I did not flatly state that plant cells contain lysosomes because many cell biologists claim that they do not, and it was possible that such a flat statement would be considered an "error" by many teachers. However, there is emerging evidence that, in fact, they do. Here's a paper from the Journal "Plant Cell," pointing out the existence of an organelle in a plant cell that meets every criterion of a lysosome:
"Barley Aleurone Cells Contain Two Types of Vacuoles: Characterization of Lytic Organelles by Use of Fluorescent Probes." Sarah J. Swansona, Paul C. Bethkea, and Russell L. Jones . Plant Cell, Vol. 10, 685-698, May 1998
Here's a passage from the paper:
" The existence of lysosomes in plants has long been debated (see, e.g., Moriyasu and Ohsumi 1996 ). Matile 1975 recognized that catabolic enzymes were essential for sustained biological activity and that these enzymes must be compartmentalized to prevent their indiscriminate hydrolysis of biopolymers. He proposed that plant proteases, nucleases, phosphatases, and other degradative enzymes were constituents of a "lytic compartment," a compartment that included the extracellular space, vacuoles, and other organelles containing lytic enzymes. With improved techniques for vacuole isolation, it became clear that many plant vacuoles contain enzymes found in animal lysosomes (Matile 1978 ; Wink 1993 ). Plant vacuoles were therefore seen as fulfilling the role of the animal lysosomal system (Boller and Wiemken 1986 ). "
This is just one paper of many reporting plant cell lysosomes. The first such report appeared more than 30 years ago: Matile (1968) "Lysosomes of root tip cells in corn seedlines." Planta 79: 181-196.
So, the answer to your question seems to be "yes," but it remains a controversial one.
Ken Miller (10/11/04)
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After our electricity went out, maggots appeared in meat that had been in our freezer. Where did they come from? (from Xasha, a student in Puerto Rico)
This is an easy one. Maggots are the larval forms of flies, and these larvae hatch from eggs laid by the flies. The eggs laid by flies (including fruit flies and house flies) are so small that they usually cannot be seen with the naked eye.
Maggots do NOT arise from bacteria or any other contamination in meat.
What this means is that at some point flies had enough contact with the surface of the meat to lay a few eggs on it. It wouldn't have taken more than a few seconds to lay dozens of eggs. The eggs would not have been visible unless the meat was examined by an expert using a high power magnifying glass.
The development of the eggs into larvae (maggots) would be slowed down by refrigerating the meat and stopped altogether by freezing. But they would not be killed.
When the power to the freezer went off, the meat warmed and the development of the eggs resumed. You opened the door and presto, there they were, in all their maggoty glory!
Ken Miller (9/19/04)
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How do muscle cells heal after being cut? (from Karen, a teacher in Oregon)
Thanks very much for your note! In general the information in that chart is correct. Skeletal muscle cells form by the fusion of smaller cells called myoblasts during embryonic development. When a muscle grows in size, the existing cells simply add new muscle fibers. The same is true as the heart increases in size during development into adulthood.
And, when the heart is injured (due to a heart attack, for example), unfortunately the remaining muscle cells cannot grow back to replace the injured ones. The same is true with skeletal muscle.
However (and here's where things get interesting) this does not mean that the muscle itself cannot grow back. Skeletal muscle contains another kind of cell called a "satellite cell," or "myogenic satellite cell." These seem to be stem cells for skeletal muscle, and they are able to develop into new myoblasts following muscle injury, and form new cells to help repair the damage. Whether the heart muscle contains similar cells is of very great interest, of course, and is still controversial.
You might take a look at the research done in Ron Allen's lab at the University of Arizona for some further information on these cells:
There's a great deal of interest in these cells in the pharmaceutical industry, of course.
Hope this helps to answer your question!
Ken Miller (March 29, 2004)
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If E. coli is normally found in the human gut, then why does food that is contaminated with E. coli make us sick? (from Mary, a teacher in Washington State)
Problems with E. coli are caused by two things: 1) Location, and Quantity.
E. coli is a common bacterium which is found just about everywhere. Small numbers of E. coli are found in most surface water, even the cleanest, clearest lakes and rivers. This and other bacteria are present on most surfaces in your house, kitchen, and cooking utensils, and therefore in most of the food we eat. That's how a newborn baby gets the E. coli (and other bacteria) that take up residence in the intestines. As you know, those bacteria carry out a number of beneficial functions, helping us to digest many types of food, and are seldom a cause of any harm.
If large numbers of E. coli are present in food or drinking water, however, the waste products and other toxins they normally produce can accumulate, and cause stomach disturbances that may be severe, so it is important to avoid food or water that is excessively contaminated with E. coli. There's something else to worry about, however.
There are many varieties, or "strains," of this bacterium which are quite different from the relatively harmless bacterium found in our intestines. Some of these strains release extremely toxic chemicals that can cause severe food poisoning and, in extreme cases, even death. You can find out more about these bacteria on this web site, produced by the Institute for Food Safety in Great Britain
These deadly bacteria are called E. coli, sure enough, but they are quite different from the "good guys" living in our intestines, as the web page should make clear.
Ken Miller (2/1/04)
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How many ATPs are actually made during the Krebs cycle? Is the real nummber 36, 38, or something else? (from Angelique, a teacher in Michigan)
Sure. In prokaryotes the usual figure is 38. Per glucose, you get 2 net ATPs from glycolysis. A total of 10 NADH's is made (in glucolysis and the Krebs cycle) per glucose, and 2 FADH2's. We generally say that you get 3 ATPs per NADH and 2 per FADH2, so that these carriers produce a total of 30+4 = 34 ATPs..... and the Krebs cycle itself produces 2 GTPs, which are equivalent to 2 more ATPs, for a total of 38.
Everything is the same in eukaryotes except that the 2 NADHs produced in glycolysis (in the cytoplasm) must be brought into the mitochondrion at a cost of some energy, usually estimated to be 1 ATP per NADH.... so in eukaryotes we usually say you get just 36 ATPs. Clear enough?
The fun part, however, is that none of this is as precise as textbooks make it seem. The 3 ATPs per NADH is an estimate made from lab experiments under carefully-defined conditions, and it's quite likely that the actual values vary considerably in the cell itself!
Hope this helps.
Ken Miller (1/31/04)
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Why is it that hair exposed to the sun gets lighter, but skin exposed to the sun gets darker? (from Jen, a teacher in California)
ANSWER: Beats me! I was puzzled, too, so I asked an expert.
Professor Walter Quevedo, a leading expert on the biology of the skin, explained it to me this way:
In the case of sun-induced bleaching of eumelanic (dark) black hair, hydrogen peroxide and superoxides (free radicals) induced by the UV component of sunlight,act to disrupt the melanin "granules" eventually to a degree where the products of disruption are no longer black. That lightens the shade of hair color. Remind your studentthat the hair is dead tissue, so those melanin granules, once bleached, cannot be replaced.
Unlike hair, the skin is alive, The melanocytes of the epidermis are stimulated to produce greater numbers of melanin "granules" (melanosomes that actually are much more complicated than granules). The melanosomes are transferred in increased numbers from the melanocytes to keratinocytes of the epidermis causing the darkening (tanning) of the skin.
The persistent darkness of sunburned ski, results from an inflammatory response (reddening )induced by UV light. It is a complicated process which ultimately results in the destruction of damaged components of the skin and their replacement. Various agents that produce the inflammation as well as growth factors produced by keratinocytes, act on melanocytes,as repair processes go on, to produce more melanin "granules" darkening the repaired epidermis. The elevation of melanin production may keep the skin dark for long periods following "healing" because the stimulating agents remain elevated for long periods.
I hope this helps to answer your student's excellent question!
Ken Miller (10/14/03)
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Why don't bacterial cells have nucleoli? (from Melissa, a teacher in Texas)
That's a great question, and it points up one of the big differences between eukaryotic and prokaryotic cells. Most prokaryotic cells have only a handful of genes for the ribosomal RNA molecules that make up ribosomes. They transcribe those genes and also synthesize the 70 or 80 proteins that make up ribosomes, and the ribosomes are assembled at dispersed sites throughout the cell that are too small to see in the light microscope.
Eukaryotic cells, however, are generally much bigger. Therefore, they need to produce many more ribosomes, and they have multiple copies of the ribosomal RNA genes.... sometimes several hundreds. The transcription of so many rRNA molecules in the same region draws in a host of ribosomal components, and this aggregation of protein and RNA crowds out other molecules, producing the dense spot in the nucleus that we call the nucleolus.
Interestingly, when some organisms (like frogs) form the huge cells that are their eggs (a frog egg may be several millimeters in diameter) they also produce thousands of extra copies of some of their ribosomal RNA genes.... these extra clusters of rRNA do the same thing, causing ribosome components to aggregate at several places in the cells large nuclei, and this forms what cell biologists call "accessory nucleoli."
So, it's all a matter of size. The sheer number of rRNA genes in eukaryotes is what produces the visible nucleolus.
I hope this helps to answer your student's excellent question!
Ken Miller (10/12/03)
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How can you tell how old a plant root is? (from Caito, a student in California)
Let's start with the obvious: if the plant is an annual, its roots are going to be less than a year old (since annuals only live through one growing season). Also, even on a perennial plant that is many years of age, new roots and root hairs are produced all of the time. Therefore, not all roots are indicative of a plant's age.
However, in most woody plants, new wood is added to existing roots in a pattern that varies with the growing season. In the summer growing season, the cells are large and have thin walls, which makes the wood appear light in color. In the winter, the cells are smaller with thick walls, and the wood appears darker. This produces a pattern of annual rings, as described on pages 592-593 of the Dragonfly book.
The important point here is that woody roots display the same pattern of annual rings as do the stems or trunk of a woody plant or tree. Properly interpreted, they allow one to estimate the age of the root.
For more information on how annual rings allow the age of a woody plant to be determined, visit the Laboratory of Tree Ring Research at the University of Arizona:
Ken Miller (6/12/02)
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How do ions flow across the Neuron cell membrane? (from Richard, a teacher in Michigan)
Richard, I think you are confusing what happens to produce the Resting Potential with the events that cause the Action Potential.
The Resting Potential is explained on page 898 of the Dragonfly Book. The neuron cell membrane contains a sodium/potassium ATPase, a protein that uses the energy of ATP to pump sodium ions out of the cell, and potassium ions into the cell. The pump works all of the time, like a bilge pump in a leaky boat, pumping K+ and Na+ in and out, respectively.
Now, when an action potential is fired, what happens? As described on page 899 (and just like you're used to teaching) "gates" in the membrane open, allowing Na+ ions to pour it. This inrush of positive ions "depolarizes" the membrane. A few milliseconds later, another series of gates opens, allowing potassium ions to rush out, which "repolarizes" the membrane. It's the gates that cause the repolarization, not "pumping out potassium ions." I believe that Figure 35-7 on page 899 shows this pretty well.
There is, however, one thing to keep in mind, and that is the work of the sodium/potassium ATPase pump.... while all of this is happening, just like that pump in the bottom of a leaky boat, it keeps working. Every time an action potential passes along an axon, a little more Na+ rushes in, and a little more K+ rushes out. What the pump does, sort of "in the background" is to pump that Na+ out and the K+ in, to make sure that the resting potential is maintained and the cell is always ready for another Action Potential.
I hope that this explanation (and pages 898-899) are clear on this point. But if they are not, please write again!
Ken Miller (3/7/02)
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What is the difference between radical and bilaterall symmetry?
Well, Ohio, ... I can't help you with "radical" symmetry, although I'm sure it's really "far out" (as we used to say in the 60s!).
"Radial" symmetry is another matter, however! Radial and bilateral symmetry are very well-explained on page 662 of the Dragonfly book:
As Figure 26-5 on that page shows, if an animal has radial symmetry, then its body plan is a little bit like that of a bicycle wheel. Any number of planes can be drawn through its center, each dividing the animal into matching halves. Starfish and sand dollars are good examples of animals that show radial symmetry.
Bilateral symmetry best describes our own body plan (as well as that of a lobster). Only a single plane, running the length of the body, can divide the animal into two halves that are mirror images of each other.
Ken Miller (2/14/02)
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What techniques have improved crop yields during the past several decades? (from Wendy, a student in New York)
Wendy, there are more answers to that question than I can possibly list. Remember that just about every state has an agricultural college whose missions include helping farmers to improve crop productivity. In your state, Cornell is that University. They have first-rate departments of plant science, and you might check with researchers at Cornell to see what they've been doing lately.
You might want to take a look at this web page (from Michigan State University) for a glimpse of some of the factors that influence crop productivity:
As you will see, what farmers try to do, with the aid of agricultural researchers, is to optimize everything from the nutrient content of the soil, moisture, spacing of plants, shade (or lack of shade), control of insect pests, and finally, the genetic capabilities of the plants themselves. Plant researchers and seed companies work every year to develop higher-yielding plants that produce more and more food per acre, and the improvements they have made are reflected in steadily rising crop yields.
Ken Miller (2/14/02)
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Can you give any examples of cases in which knowledge of Ecology has helped the Environment? (from Anthony, in New York)
There are many cases in which environmental knowledge has done a great deal of good on the local, national, and global levels. Sometimes, the environmental improvements have come when sound scientific information has been used to craft and pass well-informed, properly-implemented laws and international agreements. Other times, the improvements have come when the same kind of sound science has been used to influence the behavior of individuals, corporations, and governments without the use of legislation. I can only mention a few specific cases here, but there are many.
Before I talk about any specific cases, however, let me put in context for you.
When we talk about “improving the environment,” we mean the environment in the same way we define it in textbooks: the sum of all physical and biological factors surrounding organisms. That’s why, in some cases, ecological research concentrates on physical aspects of local or global environments, such as the chemical composition of the atmosphere or the presence of potentially dangerous compounds in water supplies. In other cases, research concentrates on biological aspects of the environment, such as protecting endangered species or ecosystems, or controlling the introduction of potentially dangerous organisms to new environments. Naturally, there are many cases of research that look at both physical and biological aspects of the environment, because they interact so strongly.
One of the most important international ecological agreements of the last half-century was the environmental treaty of 1987 in which 27 countries agreed upon the Montreal Protocol to Reduce Substances that Deplete the Ozone Layer. How did so many countries agree on a treaty that profoundly affected major businesses? The agreement was based on ecological knowledge that was absolutely sound. (The science behind the Montreal Protocol earned the 1995 Nobel Prize for Chemistry for Professors F. S. Rowland at University of California at Irvine, M. Molina at the Massachusetts Institute of Technology, Cambridge, and Paul Crutzen at the Max-Planck-Institute for Chemistry in Mainz, Germany.) Since the treaty was ratified, the increase in atmospheric chlorofluorocarbons has slowed significantly. If you are using our new dragonfly book, you can find a discussion of chlorofluorocarbons, the ozone layer and the effects of this treaty on pages 157-158.
Additional information on chlorofluorocarbons and their effect on the ozone layer can be found here.
More specific information on the Montreal Protocol and more references for even more information can be found here.
On a more local, but still national level, the Clean Air Act, passed 1970, had positive effects on both air and water supplies across North America. This legislation was also based on sound science that investigated the ecological effects of various pollutants on human health and on the health of both wild and domesticated animals and plants. Between the time the act was passed and 1998, there were dramatic decreases in emission of four of the six pollutants targeted: Carbon Monoxide, Volatile Organic Compounds, Sulfur Dioxide, and Lead. Largely because of the requirement that new automobiles use unleaded gasoline, lead emissions dropped an astonishing 98.2 percent! Soon thereafter, the amount of lead in rivers and streams across the country also dropped. Again, good science really works as the basis for good environmental policy!
For more information, you might want to check this out.
On the biological side of things, one of the most dramatic results of ecological studies on the plight of endangered species resulted in an international treaty usually called CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora) that was designed to encourage countries around the world to voluntarily stop trade in items obtained from endangered species of animals and plants.
More information on CITES.
One of the most recent issues involving the interactions of humans with the environment concerns the introduction of foreign (usually called “exotic”) species into new environments. Because introduced species often lack predators and parasites in new environments, they can often reproduce wildly out of control, becoming what are called “biological invaders” that cause serious problems for farmers and agricultural industries as well as natural ecosystems. Increased ecological knowledge is helping to improve this situation in at least two ways. On one hand, there are now tighter controls on the importation of organisms to many places. (You may be particularly aware of this if you live in a heavily agricultural state such as California, Texas, or Florida.) On the other hand, increased understanding of the biological factors that regulate plant and animal populations can sometimes help bring biological invaders under control by identifying and (CAREFULLY!) importing their natural predators or parasites. This is an area in which intensive research is underway. Many biologists see this phenomenon as far more important to the long-term future of life on the planet than any other single environmental issue.
For more information on this topic, you might want to check out this.
Check out a great list of links to ongoing research and issues in this area.
Hope this answers your question!
All The Best,
Joe Levine (2/11/02)
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How do plants balance their need for carbon dioxide with their need to prevent dehydration?
This is a great question, because it points out a problem that nearly all plants have to deal with. Most plants get carbon dioxide, of course, from the air, which enters the leaves through little openings on their undersides known as stomata (shown on pages 596 and 597 of the Dragonfly Book). So, when the sun is shining and conditions for photosynthesis are good, plants should keep those stomata open, right?
Well, not always. The interior of a leaf is moist, so on a hot day a plant will lose water — sometimes lots of it — due to evaporation through open stomata. So, how does a plant "decide" between the need to conserve water and the need to admit carbon dioxide for photosynthesis?
The answer seems to be built into to the mechanisms that open or close stomata. As I wrote on page 597, when water pressure drops in the guard cells on either side of the stomata, the opening closes, preventing water vapor from leaving. When water pressure is high, the stomata open. So, if the plant has plenty of water, it automatically tends to keep its stomata open. If it's short of water, it automatically closes them. Yes, that slows down photosynthesis, but it also prevents the plant from wilting and dying due to dehydration.
Most well-watered plants, by the way, tend to have their stomata open in the daytime and closed at nighttime. That's partly because most plants have an internal "clock" in their guard cells that opens and closes them on a daily basis, and also because photosynthesis in the guard cells during the daytime gives these cells the ATP they need to pump ions and keep their stomata open.
Ken Miller (2/7/02)
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My teacher said the chicken egg isn't one big cell, but that the little white membrane looking thing that is attached to the yolk is the CELL — not the whole thing. Is that right? (from a student in Kentucky)
Well, your teacher is essentially (but not totally) correct. In an unfertilized egg (the kind you buy at the grocery store), there is a small, whitish disk on one side of the yolk. This little structure is called the germinal disc, and it contains the nucleus and most of the cytoplasm of the egg cell. The yolk, however, is actually part of the cytoplasm of that cell, so that one could say that the yolk is actually a single huge cell.
You teacher is correct, however, in pointing out that the yolk itself doesn't contain the structures and organelles that we expect to find in living cells. In fact, it's little more than a mass of stored nutrition waiting for embryonic development.
Here's why your teacher's opinion is correct in an important way. If the cell is fertilized by a sperm, which happens inside the body of the hen before the shell is completely formed, the whole yolk doesn't divide. Instead, cell division is limited to the germinal disc, so that little cluster of cells known as the embryonic disc forms on top of the yolk. The embryo develops from this disk, and gradually sends blood vessels into the yolk to use it for nutrition as the embryo develops.
Q: Another student said they had always thought that white thing was the sperm.
That's not correct. No sperm are found in an egg, fertilized or not, once it has been encased in a shell and has left the body of the chicken
Q: I am confused if it is just one cell then how did the rest of the egg get there without cell division? And exactly how would that egg get fertilized since the shell can't be penetrated????? - perplexed in Kentucky
Well, "Perplexed," here's how it works: After a chicken mates with a rooster, the sperm cells that he deposits in her reproductive tract can live for more than a week. Egg cells develop in the chicken's ovary, which is where most of the proteins and lipids that make up the yolk are added to the egg. Once they are released from the ovary, egg cells are fertilized by these sperm left in the reproductive tract. The white of the egg and its shell are then added (after fertilization!) as the fertilized egg cell begins its first few rounds on cell division to produce the embryonic disk.
I hope this answers all of your questions!
Ken Miller (2/7/02)
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I was wondering if you knew how the Endoplasmic Reticulum got its name or who discovered it.
I sure do. It was discovered and named by Keith R. Porter, a scientist who worked at the Rockefeller University. Porter was probably the first person to figure out how to make pictures of the internal structures of cells using the newly-invented electron microscope. The endoplasmic reticulum was seen for the very first time in very thin tissue culture cells that Porter imaged in the EM. The very first published report was:
Porter, Claude, and Fullam (1945) Journal of Experimental Medicine, volume 81, page 233.
The name for this organelle was made up on the spot by Porter himself. He saw it as a lacy, delicate structure (a "reticulum") that was inside the cell ("endoplasmic"), so it made sense to call it an endoplasmic reticulum.
Keith Porter taught and did research for many years at Harvard, and left in 1970 to found the Department of Molecular, Cellular, and Developmental Biology at the University of Colorado in Boulder. I entered grad school there in 1970, and worked briefly in Porter's lab. I earned my Ph. D. in the lab next to his, and for three years was his chief teaching assistant in cell biology. He was a great, fun-loving, and creative scientist, and a brilliant teacher.
Ken Miller (2/7/02) Back to top