1. Introduction to Bil 255 - Exams, reading, and grading policy.

2. The discovery of cells. Robert Hooke, in his 1665 book, Micrographia, was perhaps the first to describe biological cells. Look at the figure below from his book. It shows a longitudinal section of cork on the left and a transverse section on the right. He observes the number of cells as "in a Cubick Inch, above twelve hundred Millions, or 1259712000, a thing almost incredible, did not our Microscope assure us of it by ocular demonstration". Can you calculate the dimensions of a cell in microns, assuming that each cell is an identical perfect cube? An easier version of this question, designed to be solved without a calculator, will appear on the first exam.

Solution: There are 1,259,712,000 cells in a cubic inch. If all are tightly packed cubes, we can take the cube root of the reciprocal of 1,259,712,000 to obtain the width or length of one of the cubic faces, in inches. (There are approximately 109 cells per cubic inch. Therefore, there are about 1000 cells across the one inch face of this cube. Thus, each cell has a width of roughly 1/1000 inch.) We can convert inches to microns (micrometers) by first multiplying by 25.4 (mm/inch) and then by 1000 (microns/mm). I calculated that each cell would be about 23.5 microns in width, length or depth.

3. Origin and continuation of life.

• Energy is required to overcome the natural tendency of large, complex molecules to break down into smaller ones. The energy used to make large biomolecules, for the most part, comes from the sun.
• The energy source of the sun, under reducing conditions, may have driven the formation of the first macromolecules by literally cooking up the a mixture of smaller molecules.
• Changed atmospheric conditions and a cooling Earth then prevented this "cooking process" from continuing. However, the appearance of photoautotropic organisms permitted these cells to use the less energetic sunlight which could pass through the atmosphere of Earth.
• The first photoautotrophs were probably purple bacteria, similar to some present-day bacteria living in sulfur-rich hot springs and volcanic vents.

4. Cells are prokaryotic or eukaryotic.

• Endosymbiosis was an important contribution to the evolution of eukaryotes. (Possible Steps in Evolution of Eukaryotic Cells)

  The endosymbiont hypothesis found some powerful support in the results of an article in the scientific journal Nature. This article showed striking similarities between the genome of Rickettsia prowazekii, which causes a form of typhus, and that of the mitochondrial DNA from some eukaryotes. Here is a link showing the evolutionary relationships between some organisms and mitochondria. (Reprinted by permission from Nature 396, 109-110, copyright 1998, Macmillan Magazines Ltd.)

5. Prokaryotes are either Eubacteria or Archaea. The Archea form a distinct group of organisms, more similar to Eukaryotes than to Eubacteria. (fig. 1-3, Lodish)

• The Archea seem to be very primitive bacteria-like organisms which may be similar to the first cells.
• The Eubacteria include the
  • The world's smallest cells - Mycoplasmas, which cause a type of pneumonia in humans and the disease of coconut trees, Lethal Yellowing.
  • The largest bacteria - The Cyanobacteria (plankton, nitrogen-fixers, components of lichens, makers of stromatolites. Here are some links to information about stromatolites - Most commonly we think of ancient stromatolites. Click for a great site about stromatolites. The Petrified Sea Gardens in Saratoga, NY (now closed) sits on top of a rock base formed from stromatolite fossils:
    

    But, stromatolites are still being laid down in some very unusual situations, such as in shallow pools in hot dry climates. Look at this photo of stromatolites in Shark Bay, Western Australia. Also, cyanobacteria live inside the hairs of some polar bears, giving these animals a greenish color.

    

  6. The lower limit of cell size:
  • The smallest living cells are mycoplasmas, the smallest of which have a diameter of 100 nm. The largest living cells with a single nucleus include a frog egg, with a diameter of 2.5mm. But, a frog egg has a lot of yoke, a source of stored food! So, does the frog egg really count as the largest cell? What about certain types of slime molds, which have single cells which may be a meter long? These types of slime molds have many, many nuclei, each controlling a region of the cell. So, is this really one cell? Look at this figure to put some of these sizes in perspective.
  • Let's think about the small size of a mycoplasmal cell. A cell membrane is about 7 to 10 nm. in thickness. If a cell were much smaller than than a 100 nm. diameter mycoplasmal cell, then that cell would either have little interior space left after the membrane was put in place or it would have to have some different type of membrane structure. It is very difficult to imagine how to make such a thinner membrane.
  • Not only would the interior space of a cell smaller than 100 nm. in diameter be small, but the membrane would have to curve so sharply that the cell would tend to split open. The hydrophobic interactions holding together the lipid molecules comprising the membrane would be stretched, water would creep in between the lipid molecules, and the membrane would break.
  • A very small interior would also give rise to huge concentration fluctuations of all solutes within the cell. In class, we will discuss the "square root of N" law. Make sure that you understand the "square root of N" law and how to apply it to analyzing concentration fluctuations in cells.

  7. Cell interiors are crowded.

  • The interior of a cell is very, very crowded and packed with proteins and other components. The interior is NOT a dilute solution as you can see from these interpretations of David Goodsell:
  
  
  

  All text and images, not attributed to others, including course examinations and sample questions, are Copyright, 2008, Thomas J. Herbert and may not be used for any commercial purpose without the express written permission of Thomas J. Herbert.