Ahmed Abdullah said:
Another question, some form of life like seeds are able to remain inert for years but can become alive (i.e they germinate) when given suitable conditions. I am not sure whether seeds are alive or death but they are not certainly death ... because death thing can never become alive. In that sense seeds are living thing that can live exceptionally long time without nutrient supply. How they escape the inevitable equillibrium?
I'm not biologist, but I've studied yeast cells in brewing...
As far as I know from say yeast cells, when the exit the active reproduction cycle they go dormant, and can sometimes also sporulate. This usually happens when the growth conditions are not met. During this time there is still a basal metabolic activity, although it's at minimum. During this time the cell feeds on internal energy reserves, typically glycogen and similar carbohydrates.
When times are bad, the cells prepares for dormancy, but accumulating carbohydrate reserves etc - it's a survival trick.
During storage the glycogen level of a yeast cell drops.
Then when the growth conditions are restored in the environment the cells has a startup procedure to "start up the engines", manufacture new enzymes and transport proteins etc. For this process, normally the internal carbohydrate reserves are used. So a very low energy cell can be a slow starter. Sometimes, but usually inefficiently passive diffusion of nutrients through the cellmembrane can also take place.
One has found yeast cells in beer bootles in old sunken ships which carried beer and wine, that has been recultured that was almost 100 years old. I think that is unusual though. Usually in old beer bottles, the best survivors are certain type of baceteria rather than yeast cells.
Like has been said the life of a cell probably mainly has the purpose of survival in order to reproduce itself. So whenever growth conditions are met, the cell machinery are activated. This is driven by free energy utilisation like all other process. Yeast get their energy mainly from sugars, they respire if they can, otherwise then can do anaerobic growth for about 4-5 generations without oxygen. The limiting factor is not energy production, it's the cellmembrane that is depleted of unsaturated fats and sterols, so the cellmembrane which is of outermost importance to a unicellular organisms simply doens't work properly - this is the number one practial limiting factor for yeast-dosage for brewers. Since beer making is anaerobic fermentation (continous O2 inclustion would destroy the beer), then the pitching rate is calculate to give 3-4 generations of yeast growth before the sugar in the wort is out.
During fermentation there is an energy balance in the cell, from all the energy produced, some of course goes to growth and biosynthesis (building new cells, by budding), but in a stressful beerfermentaiton ALOT of energy goes to stress handling and cell maintenance. When more energy to maintain cell integrity is needed, than what the sugar-transport and fermentation system can supply, then the cell starts to prepare for exiting the active reproduction cycle, and is accumulates glycogen and trehaolse. This is the mechanis by which a cell can go dormant and stop eating, although there is technically food left. Eating that food is too expensive in the high stress enviruonemnt. Here brewers see a "stuck fermentation".
/Fredrik
