2.1.4.P7
|
Many cells rely on chemotaxis -- detecting differences in concentrations of chemicals on one side compared to the other -- in order to determine which way to go. Our neutrophils (white blood cells) use this to find and destroy invading bacteria as a part of our immune system. Chemical guidance of cells is also crucial during development, a process in which cells have to arrange into highly organized structures (organisms, people). Later in the class we will look at chemotaxis in terms of the laws of diffusion. At this point, let's just try to get an idea of the scales and number of molecules that are involved.
|
Neutrophils moving across a concentration gradient.
|
The strength of the concentration of a chemical is typically reported in terms of the number of moles present per liter. The number of moles per liter is called the molarity. (Note: molality is a ratio of the number of moles of the solute divided by the mass of the solvent. We will not use that in this class.)
The video shown at the upper right shows chemotaxis of amoeboid cells. In other words, it shows cells migrating toward a region with a high concentration of a chemical, in this case cAMP. Cells start in a large reservoir on top (outside of the field of view) and migrate toward a large reservoir filled with a high concentration of a chemical that attracts the migrating cells (chemoattractant) which is below the field of view. The field of view is horizontal. The cells are not "falling" but moving on their own to find the chemoattractant.
In the field of view, the channel is about 10 microns thick (a constant thickness is ensured by support "beams" that are visible as rectangles along the top and bottom rim). The length of the channel, top to bottom black line, is 300 microns. The video is a time-lapse film. The time until the label at the bottom of the movie turns red is 15 minutes, the overall movie shows a time interval 30 minutes long.
1. In order to get a benchmark idea of how the idea of molarity translates into molecular terms relevant for biology, consider a volume of one cubic micron (micron= micrometer = 10-6 m), about the size of a small bacterium. What is the molarity of a solute that has one molecule per cubic micron?
2. Estimate the typical speed of amoeboid cells. This speed is comparable to white blood cells and other fast moving cells. At such a speed, how long would a white blood cell take to get from your head to your toe to fight a bacterial invasion?
3. Estimate the length, surface area, and volume of an amoeboid cell.
4. The cell serves as a measurement device for the chemical signal that surrounds it. Which of the numbers you estimated in (3) should be relevant to the cell's ability to measure a chemical signal? Explain your assumption about how a cell senses a chemical signal.
5. Typical signal strengths for many biological signaling pathways are 10 nM (nanomoles per liter). Assuming that the average concentration of cAMP in the channel is 10nM, how many molecules of cAMP are within one micron distance from each cell?
W. Losert and J. Redish 8/31/12
Comments (0)
You don't have permission to comment on this page.