PART 1: LIFE

1. We have now learned something about the basic physics of heat and temperature, as well as about chemistry. The next thing that we are going to talk about is life itself. While life is certainly more than chemical reactions, the chemical reactions are fundamental to what keeps us--and all known organisms--alive and healthy.

2. The next major topic that we need to learn about is cells. All living things are made up of one or more cells. Single celled organisms are things like bacteria and some algae--things that you really need a microscope to see well. Multicellular organisms can be much larger (like pine trees and whales).

3. Most basically, there are two major types of cells: prokaryotic cells and eukaryotic cells.

4. Prokaryotic cells lack a nucleus. Pretty much all prokaryotic cells are bacteria. Some cause disease (like strep throat/scarlet fever, some forms of pneumonia, tuberculosis, etc.), though many others are environmentally helpful (like the organisms that cause dead things to decay).

5. Eukaryotic cells all have a cell nucleus. These cells can be unicellular organisms (like the amoeba), or multicelluar (plants, animals, and fungi).

6. We will now discuss the parts of an average eukaryotic cell. (See p. 610 for a picture.) Note that these cells have small internal structures called organelles (which means "little organs"). Here are some of the more important ones:

a) The cell membrane. This is a very important part of the cell: it keeps stuff inside the cell. It also determines what can enter and what can leave the cell.

b) The endoplasmic reticulum (ER). This is a transport system within the cell. It is basically a system of tubes. There are two kinds: smooth ER is just tubes, while "rough ER" has small devices called ribosomes attached to it, making it appear rough. The ribosomes let the rough ER synthesize proteins for the cell (see below).

c) Ribosomes are small granules made of protein that are usually stuck to the surface of rough ER. They make proteins by indirectly reading the DNA in the cell nucleus.

d) The Golgi bodies are a series of five to twenty flat membrane sacs that look sort of like a stack of pancakes. These bodies produce chemicals for the cell. When a batch is finished, part of the body will be budded off and float away, taking the chemical where it is needed within the cell, or perhaps to the cell membrane to be dumped outside. For example, when we catch a cold and have a runny nose, the Golgi bodies in our lungs are going crazy producing mucous.

e) Vacuoles are basically storage regions within the cell that are surrounded by membranes. They can contain pretty much anything--food that is being digested by the cell, for example, or for simple storage.

f) The nuclear membrane. This keeps the nucleus of the cell separate from the rest. The nucleus is where most of the cell's the genetic material (DNA) is kept. This genetic material stores the blue prints for pretty much everything in the cell. If the cell is part of a larger organism (say, a skin cell) it contains the blueprints for the entire organism. Thus, most any cell in your body (except for red blood cells, which lack a nucleus) could be used to clone you! We will talk more about the nucleus later.

g) The mitochondria. These cells are called the "powerhouses of the cell), because this is where a lot of the energy is produced to run the cell. Basically, this is where the glucose (blood sugar) is "burned" to make energy. Inside the mitochondria is a second, wrinkled, surface. The individual wrinkles are called cristae. When the cell is active, the mitochondria swell up. When the cell is just sitting around, the mitochondria shink.

h) The chloroplasts. These are only found in plant cells. Chloroplasts are where chlorophyll (the chemical that makes plants green) is kept, and where photosynthesis occurs. Inside the chloroplasts are stacks of plates called grana. Chlorophyll molecules are attached to the grana.

7. Interestingly, evolutionary biologists believe that both mitochondria and chloroplasts began as independently-living bacteria. At some point millions of years ago, they invaded (or were eaten) by a third, larger, cell. However, the three cells lived better living together than they did apart, so the mitochondria and chloroplasts became permanent residents of the cell. Support for this evolutionary hypothesis comes from the fact that both the mitochondria and the chloroplasts have their own internal DNA.

8. As mentioned above, the cell nucleus houses one of the most important parts of the cell: the DNA. The DNA is what controls inheritance, both of the individual cell and (if the cell is part of a larger organism, like a turtle, a tree, or a human) the entire organism. In humans, the DNA codes for most everything: our bones, our muscles, our eye color, our skin color, etc.

9. Under normal circumstances, the DNA is usually kept tightly bundled up in objects called "chromosomes" (which means "colored bodies"). Humans normally have 23 pairs of chromosomes--each cell contains two copies of each chromosome, one from each parent. This is partly a back-up system: even if one of the chromosomes is slightly damaged, the other one may be able to pick up some or all of the slack. The chromosome pair is connected by a "centromere," which binds the two chromosomes together.

10.When cells divide they go through a process called mitosis. There are several steps to this process. Basically, the chromosomes unbundle, are replicated, and then re-bundled. Each set then goes to the opposite end of the cell, and the cell splits in half to form 2 cells.

11.Sometimes something goes wrong with the process, however. Something gets left out, or something is damaged, or something is reversed, or something is doubled that is not supposed to be doubled. This genetic change is known as a mutation. If the damage is great enough, the cell could simply die--in fact, cells contain self-destruct mechanisms for this purpose. Otherwise, the cell might become cancerous--it gets stuck in "grow" mode and cannot turn itself off.

12.Most mutations are somatic mutations--they only affect one particular cell, or one particular organism and cannot be inherited. If the genetic change takes place in the germ cells of an organism--in the female's egg cell, or in the male's sperm cell-- is it possible that the mutation could be a hereditary mutation that can be inherited by the children.

13. Examples of somatic mutations are things like cancer, or Down's Syndrome. Generally speaking, these diseases are not hereditary--a person with cancer will not pass cancer on to their children; the only thing that they might pass on is a genetic weakness of some kind that would make their children more likely to get cancer.

14. Examples of hereditary mutations are things like hemophilia and sickle-cell anemia. These diseases can be passed on to the children. Even if the children don't end up having the disease in an active form, they may be carriers. If two carriers have children, their children could have the disease.

15. Note that there are many, many genetic diseases--both somatic and hereditary. Most of these are, fortunately, rare. Also, the effects of diseases can vary from very mild to extreme. In fact, some are instantly fatal: a significant percentage of human pregnancies are believed to end so quickly that the woman never knew that she was pregnant; the fertilized egg containing damaged chromosomes either self-destructs or fails to implant, and is eventually expelled from the woman's body.