If all the animal species are broken up into groups, the light
blue section includes insects, and the rest of the
circle colors represent every other animal on Earth!
Bacteria outnumber us by orders of magnitude more than insects do; they live everywhere, in every environment. They have been found in 0.5 million year-old permafrost as well as 40 miles up in the atmosphere. There are approximately 100 million to 1 billion bacteria in every teaspoon of dirt, so in total there are currently 5 x1030 bacteria carrying out their daily routines. That means there are about 5 x 1019 living bacteria (that is 50,000,000,000,000,000,000) for every person who has EVER LIVED. Another way of visualizing this might be to imagine that each bacterium is a penny being stacked. The column would be a trillion light years high. That’s about five times the diameter of the observable universe.
Nanobacteria are still controversial, the 0.2 µm diameter is
close to the smallest size that could still hold DNA.
For comparison, the white line in panel A is 1 µm long,
and in Panel C the line is just 0.1 µm.
While the redwoods might be slightly taller than the sequoias,
the mass of the sequoias is much greater because the
trunks have such a large diameter.
On a smaller scale, the difference in size between bacteria and nucleated cells (eukaryotic cells) is still pretty stunning. A single macrophage cell of your immune system can ingest more than 100 bacteria without flinching, and macrophages are nowhere near the biggest eukaryotic cells. These different sizes demand some distinctions in how cells conduct their business; for example, how they move molecules into and within themselves.
A macrophage reaching out and ingesting bacteria.
The bacteria are the small, connected rods.
The cytoskeleton of the eukaryotic cell stretch out like fibers.
They help it move, can convey molecules from place to place,
and holds the cells shape.
Unfortunately, bacteria only have diffusion to move molecules around their insides. This makes things doubly hard on them because bacteria have limited access to resources; most often they meet up with few molecules that are important to them (being a small cell in a big environment). Therefore, they need to get as many of these resources into their cell as possible and move throughout their entire volume quickly.
Diffusion is the movement of from where there are
many to where there are few. If it is water
molecules that are moving, then call it osmosis.
There is also the mixing rate; this refers to the time it takes for a molecule that enters the cell to have an equal probability of being found in any part of the cell. A 1µm (1/1,000,000 of a meter) bacterium has a mixing time of roughly 1 millisecond. But since the volume increases by a factor of eight as the size doubles, it would not take much growth for the mixing time to become problematic if a cell was to rely on diffusion alone.
Finally, there is the issue of traffic time. Every reaction that takes place in a cell involves two or more molecules finding one another and then interacting. In both prokaryotic and eukaryotic cells there are some systems designed to help bring molecules together, but in the end, it is basically luck – they have to run into one another. The number of molecules can affect this time; say you want molecule A to meet molecule B. If the cell contained only one of each molecule, this could take a while, but if there are 1000A’s and 1000B’s, then the traffic time will be decreased considerably. For average sized bacteria, traffic times exist in the range of 1 second, but again, if they are much bigger, the chances of molecules meeting their partners goes down dramatically.
If the bacterium grows too big, the diffusion rate, mixing time, and traffic time can become too long to permit survival. Therefore, size limitations seem to be set for bacteria. However, some bacteria just have to be rule breakers. There are two excellent examples of bacteria that have evolved ways to overcome the diffusion problems associated with increased size, and we'll start to look at them next week.
Schulz, H., & Jørgensen, B. (2001). Big Bacteria Annual Review of Microbiology, 55 (1), 105-137 DOI: 10.1146/annurev.micro.55.1.105
For more information on numbers in nature, diffusion, and cytoskeleton, as well as web-based activities and experiments, go to:
Cell size and volume:
scaling in nature: