WARM UP #1: Theories, species, and cells
Selected student responses, N100 Spring 2003

Q1 = QUESTION 1: What is the difference between a theory and a belief? You may want to look these terms up before answering. Be as specific as you can, and give an example of each.

From Jess: The difference between the two is that theories are based on facts and data that are collected, while beliefs are things that people hold to be true without needing actual facts to support them. For example, the Cell Theory is a theory because many scientists through time collected information to support the theory that the cell is the smallest unit of life, and all living organisms are composed of one or more cells. One's belief in God, on the other hand, is a belief because rather than collecting data through time, people believe and have strong faith that God exists.

From taz: Q1 = theory- has been rigorously tested but is still open to tests, revision , and tentative acceptance or rejection. belief- Confidence in the existence of something not immediately susceptible to rigorous proof. by using the definitions we can say that a theory has been tested and still may leave room for doubt. A belief has not been tested, but we feel that it will hold true to what we believe.

From Enola: Q1 = First of all, the scientific definition of the word theory is much more specific than the term "theory" used in common conversation. In science, a theory can be defined as, "an explanation of the cause or causes of range of related phenomena. It has been rigorously tested but is still open to tests, revision, and tentative acceptance or rejection." On the other hand, a belief is based on personal opinion, faith, or cultural acceptance but has not been scientifically proven. In some cases a belief may be impossible to test.

Note from Dr. Marrs: Good answers! Theories are explanations based on testable hypothesess - they best explain the available facts and data. Theories cannot be “proven”, and are always open to revision if new knowledge comes to light that better that better fits the existing evidence. Beliefs are very strong and deeply held feelings, based on internal conviction. There is no need to have facts or data that support beliefs!

QUESTION 2: What characteristic determines whether organisms belong to the same species? Why are, for instance, are Rottweilers, bulldogs, and poodles - phenotypically (ie physical appearance) very different - considered to be members of the same species – dogs (Canis domesticus)? Why are all humans, despite our numerous phenotypic differences, considered to be one species (Homo sapiens)?

From Brookelyn: Q2 = According to Webster's Dictionary, a species is defined by similar characteristics and behaivors. To me that really doesn't make sense because dog has four legs and is a mammal and on the same token so is a tiger. But a tiger is defined as a cat.

From Scott: Q2 = I am not too clear on why this is true. I have read the material in the textbook, and still can not come to a clear understanding. I found myself going back to the reading where it talked about the different kingdoms. I am not sure if the way they are classified is by what kingdom the belong to or what family they are from. I am also not sure if either one of these two apects have anything to do with the main characteristic the determines what species they belong to.

From SillyPutty Q2 = Reproduction. A rottweiler and a bulldog can mate and reproduce another dog. All humans, unless there are things that prevent this from happening (healthwise), can reproduce and have other humans...,

From Josh Q2 = There are many reasons that organisms are defined as they are. For sexually reproducing species one reason is the bulldog and poodle can interbreed under natural conditions and produce fertile offspring just as one male human can with a female human despite thier phenotypic differences. The same species will also have a shared genetic history, they maintain genetic contact over periods of time.

Note from Dr. Marrs: Good answers to a difficult concept. When scientists classify organisms, being able to reproduce and produce offspring are usually the main characteristics that allow groupings of organisms into species. This ability is based on the similarity of the DNA and chromosomes, so the concept of DNA unity is in there as well. BUT...the species concept is very dynamic - many times very similar species (maybe they are really 'sub-species'?) can interbreed under the right conditions...and what about asexual organisms like bacteria?!? Bottom line: The concept of a "species" is a human idea - if populations of organisms are unable to interbreed and produce fertile offspring, they are most likely different species...but there are many exceptions!

QUESTION 3: Cells are generally microscopic, ranging in size from about 50 microns for animal cells to as small as 1 micron for bacterial cells. Why do you think that cells - animal, plant, or bacteria - don't get any bigger than this? Why do you think we are made of 75 trillion very small cells, rather than just a bunch of very large cells?

From borephyll Q3 = The reason our cells are so small is a direct relation between volume and surface area. For instance, looking at a basketball and a tennisball, the basketball seems like it would be more efficient, when in fact, it is more inefficient. This is because there is a larger volume to keep homeostasis maintained in. In the basketball, there is more volume to feed, and excreete the waste from...

From Killebrew Q3 = Cell size is determined by the surface-to-volume ratio. If cells were to grow too large, nutrients could not be processed quickly. Small cells are able to move materials and distribute them effectively. As the volume increases the ratio of plasma membrane to cytoplasm decreases. Nutrients are unable to flow in or out of the cell efficiently.

From Dave Q3 = The smaller the cell, the more efficiently materials cross its surface and become distributed through the interior. A large, round cell also would have trouble moving mareials through its cytoplasm. In small cells, tiny motions of molecules easily distrute materials.

From mommy Q3 = I am not really sure why cells don't get any bigger but I will give it a try. It might have something to do with the surface to area ratio which states that the volume of a cell increases faster than the surface area. I think the reason we are made up of trillions of smaller cells is because the smaller the cell the more efficiently materials corss the surface and are distribed throughout. Could you please explain better during lecture?

Note from Dr. Marrs: A lot of people also answered something like this: 'Q3 = I look at this as if you have more you can afford to lose more of them????? If we only had 5 cells and one died, it would be bad news... but if we lost one out of 75 trillion, we would still have 74.99999999 trillion left to take their place! ' This is a good point as well.

However, the bottom line is that our cells are so small due to the surface-to-volume requirements - the bigger the cell gets, the more difficult it is to exchange oxygen and carbon dioxide (and other waste products) efficiently. See your CD for an interactive exercise on this! Living organisms use all kinds of ways to increase the surface area of their cells and organ systems...for instance: Surface area of human...

• Skin: ~2 square meters, and the thickness of a paper towel.
• Lungs: About the size of your textbook, but divided up into 300 million alveoli cells to result in a surface area of ~ 160 square meters (the size of a tennis court!) - 80 times the surface area of our skin! All that surface area is needed to exchange O2 and CO2!
• Digestive system: 15 feet long, but cells lining the intestines are highly folded and have "ruffled" edges to result in a surface area of ~ 300 square meters!! All that surface area is needed to absorb the water and digested food we eat!

QUESTION 4: Thanks for the thoughtful answers . I know that this can be a difficult issue...science and religion are two different ways of trying to understand how the world works, neither of which excludes the other! In fact, they have many complementary aspects. They have fundamentally different foundations, though - the criteria we use to determine whether something is scientifically sound has nothing to do with religion, and vice versa.

On the lighter side: Some N100 Nicknames....(As of Tuesday night, January 21st)

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