# Behavior of particles in water

## Main Question or Discussion Point

I am looking for help on how to determine sedimentation differences of particles of different densities in a aqueous environment. Will particles of different densities sediment at different speeds, and how can that be calculated. Anotherwords something with a denisty of less that one will clearly float up, while higher densities objects will sink until they hit the bottom. Is there way to determine their speed of movement and how it would change with changes in relative gravity. I suppose at 0 gravity a life preserver will not float and rock will not sink. Thanks in advance for your help.

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Drakkith
Staff Emeritus
Andy Resnick
Sometimes the unit 'svedberg' (or sedimentation coefficient') is used in centrifugation:

http://en.wikipedia.org/wiki/Sedimentation_coefficient

It's an odd unit, but may meet your needs. I don't think it can be predicted for anything but simple particles.

Thank you for your links I appreciate it and they were helpful. Let me back up a bit and explain why I am asking. I am a biologist, so this physics stuff is killing me. I had a undergrad working in my lab and she wanted to do a different experiment. She decided she was going to test the growth limits of yeast, S. cerevisiae at high artificial gravity. It took sometime, but she was able to use a swinging bucket centrifuge to maintain petri plates at constant speed. After 2 days of spinning she would check the yeast for growth. I expected them to stop growing at 50-100 g, but that was not the case. Indeed they formed colonies on agar and grew at normal rates all the way up to 2000g. We would have gone higher, but we were limited by the speed of our centrifuge, cracking plates and media that failed to hold together. Either way it was impressive. To follow that up we ran our set of 4800 yeast mutants that are all each deleted for one gene, to identify genes that might confer sensitivity when knocked out. She pulled out 24 genes like this and 13 were part of the mitochondrial large subunit ribosome, which is a 50S particle and part of the whole 74S mitochondrial ribosome. Statistically, the p-value of her pulling these out at random is improbable. Turns out the mt-ribosome has a high density 1.64g/ml and high s-value. Most proteins are in the 1.2 g/ml range. Our best hypothesis is that the mutants lead to uncoupling of the subunits. Free subunits could be acting like projectiles or maybe battering rams would be a better analogy.
Any one have any comments or ideas?

Andy Resnick
I'm interested in the topic of gravity-related physiology, but don't work in it directly. Try these groups-although I didn't see yeast specifically, I'm sure it's used as a model system:

http://www.nasa.gov/mission_pages/station/research/experiments/Genara-A.html
http://gateway.nlm.nih.gov/MeetingAbstracts/ma?f=102234727.html
http://www.aeroi.org/iss_epp/files/Hammon.pdf [Broken]
http://www.biomedsearch.com/nih/Hippocampal-gene-expression-modulated-by/14984417.html

48 h @ 2000x g is impressive! Is there a reason you can't grow them directly on/in agar-filled tubes? I think you can determine the differential "force" between the two subunits based on the (equivalent) masses 50S/75S and the acceleration (2000xg), but according to my "magic book" (Wilson & Walker's "Principles& Techniques of Biochemistry and Molecular Biology", it appears that ribosomes don't separate out until 100,000 x g and higher.

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Yeast has not been much used as a model system for gravity related physiology. There has been some studies looking at resistants to very high g forces for short periods of time, but never anything to look at the limits of actual growth and division. We don't use tubes, since it would limit us to one strain per tube. We actually use rectangular microtiter dishes that have 1 well. These allow us to plate using robotics 384 different strains on a single plate. We are able to screen the entire collection of yeast deletion strains with only 20 plates.

The standard protocol for ribosome purification is through differential centrifugation and involves pelleting ribosomes at 100,000Xg for 4 hrs or 160,000Xg for 90 minutes. My guess is that in our situation (at 2000Xg) many of the ribosomes will be pressed against the mitochondrial inner membrane. I would like to be able to calculate the force these ribosomes are pushing on that membrane to see if it would be enough to punch a hole in it. This would theoretically kill the cells.

In any case, the force would be $\Delta\rho V a$, where $\Delta\rho$ is the difference in density between ribosome and the surrounding fluid, V is the volume of the ribosome and 'a' would be the acceleration: 2000x g.