For the more scientifically motivated reader I now submit an interesting corollary to the essay on photosynthesis and respiration. Most everyone knows that our bodies (all living organisms) are made up of one or more discrete units called cells and that inside these cells reside the working parts (cell organelles). I will focus on just two of those organelles here, chloroplasts and mitochondria. Chloroplasts make plants green and are responsible for photosynthesis. Mitochondria, called the “powerhouse” of the cell are responsible for cellular respiration. This is not the same as the mechanical process of breathing. As stated in the previous essay, respiration is a catabolic (breaking down) process in which food is converted into energy with release of CO2 and H2O but more important is the energy that is released. This is an exergonic chemical reaction which means there is less energy in the final state (the CO2 and H2O) than in the initial state because energy was released from the food. This energy can be used to build up molecules of adenosine triphosphate (ATP). Adenosine triphosphate is like money in the bank which can be used when needed. It should come as no surprise that the cells that need the most energy like muscle cells have the most mitochondria. The process of building ATP is an endergonic reaction (one in which the final state, or products, have more energy). It is also an anabolic (building up) process since a more complex molecule is made from simpler ones. So, an exergonic reaction (the breakdown of glucose) provides the energy to make the endergonic reaction (ATP synthesis) go. Then ATP is broken down to ADP (adenosine diphosphate) and energy is released.
I previously alluded to aerobic respiration (with oxygen) and anaerobic respiration (without oxygen). It is interesting to note that during aerobic respiration 36 molecules of ATP are produced for every molecule of food (i.e. glucose) that is oxidized. On the other hand, for every molecule of glucose oxidized in anaerobic respiration only 2 molecules of ATP are produced. Clearly aerobic respiration is more efficient. Now let’s tie this in with early primitive forms of life eons ago to most present day forms. Remember, when life was first formed the atmosphere lacked free oxygen. This means that the first forms of life must have used anaerobic respiration as a means of obtaining energy. That was sufficient for primitive forms of life then but not for most forms today which are more complex and highly specialized. To put it another way, anaerobic respiration yields only 14.6 kilocalories of energy per molecule of glucose whereas aerobic respiration yields 686 kilocalories of energy. Dividing 14.6 by 686 and converting to a percent means that anaerobic respiration (also called fermentation) is only 2.1% as efficient as aerobic respiration. Interestingly though, when we do strenuous exercise, because our body can’t bring in oxygen fast enough to meet demand, it switches over to anaerobic respiration until we rest. This activity produces lactic acid as a waste product which causes fatigue.
Immediately below is a diagram of a cutaway view of a chloroplast and below is a similar diagram of a mitochondrion and also a photomicrograph (Sylvia S. Mader, Biology eighth ed., 2004)
Finally, let’s compare and contrast the chloroplast and mitochondrion.
CHLOROPLAST
- found in autotrophs only (i.e. plants, algae, etc.)
- associated with anabolism
- has its own DNA*
- associated with photosynthesis
- has double membrane
- endergonic Rx
Mitochondrion
- found in autotrophs & heterotrophs
- associated with catabolism
- has its own DNA*
- associated with cellular respiration
- has double membrane
- exergonic Rx
Taken from Diagrams Of The ATP Cycle – Image Results
Below is a photomicrograph provided by the author showing the upper epidermis (top layer of cells) and the photosynthetic palisade layer of cells below of a Syringa spp (lilac) leaf.
Virtually all the DNA of a cell is found in the nucleus. However, mitochondria have their own DNA which brings to mind an interesting hypothesis. From the above table we can see that chloroplasts in plants and mitochondria in all eukaryotes (organisms that have a membrane bound nucleus) have their own DNA and are the only cell organelles other than the nucleus supplied with DNA. This fact along with some other puzzling observations has led to an interesting hypothesis about their origin. As we learned in the last lesson, prokaryotes have a singular circular chromosome. This is also true of mitochondria. Furthermore, mitochondria contain ribosomes, which are responsible for protein synthesis that are about the same size as those found in bacteria. Also, mitochondria contain enzymes that are found in bacterial cell walls. Finally these bean shaped organelles, unlike other cell organelles, appear to be produced only by other mitochondria and not by cell division (mitosis and cytokinesis). But therein lies a problem: mitochondrial DNA cannot code for all the proteins found in them.
The Endosymbiont Hypothesis proposed by Lynn Margulis suggests that the first eukaryote cell was formed by a symbiosis among several prokaryotes. In pacman style one cell captured the other cell and both cells benefited. Both cells would have had DNA and ribosomes. In a similar fashion, chloroplasts may have formed when photosynthetic prokaryotes were ingested by larger nonphotosynthetic cells.
Biology is such a fascinating science and it truly builds upon the principles of chemistry, geology, and even physics.
Published By
Larry Baumer
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I graduated from Northern Illinois University in 1966 with a Bachelor of Science degree in Education and earned a Master of Science degree in Education also from NIU in 1973. I taught in the Harlem School District (5 years), a Chicago suburb (1 year), and the Rockford, IL School District for 27 years (26 at East High School). I culminated my teaching career at Kishwaukee College (8 years) Two important events occurred in 1988: I married my wife Angie and I received a summer teacher’s research fellowship through the University of Illinois School of Medicine at Rockford. My primary responsibility was light microscopy and Scanning electron miscroscopy of rabbit renal arteries (effect of high cholesterol diet). For 14 years I was a citizen scientist for the Illinois Department of Natural Resources in their RiverWatch program (monitoring water quality) My hobbies and activities include gardening, golfing, bowling, downhill and cross country skiing, photography, including photomicroscopy and time lapse photography, spending time with my wife and our dog, and in the winter playing around in my small home biology & chemistry lab.
Beyond what I have written in past profiles, in the early 1980’s I was an EMT with the Boone Volunteer Ambulance & Rescue Squad (BVARS) which fit in nicely with my science training and teaching. I also enjoy public speaking and made frequent scholarship presentations to graduating seniors and outstanding middle school students through the former Belvidere Y’ Men’s Club. I also made power point presentations of the RiverWatch program. But I most enjoyed making presentations at my high school reunions. Thanks guys for allowing me to do this.