Biology Reference
In-Depth Information
is available, the internal cell environment is thus fully hydrated because of movement of water
from outside (high water potential) to inside the cells (low water potential) and the cells appear
fully turgid. However, water potential is governed by two factors. The fi rst is the concentration of
solutes that creates the solute- or osmotic potential and the pressure exerted by the pressure- or
turgor potential. The osmotic potential is generally lower (more negative) than the water potential
and the turgor potential is the difference between them. The soluble proteins and other molecules
interact with water by their polar and charged groups that are often positioned around their external
surfaces. This results in the formation of a highly ordered structure known as 'hydration shell' or
'water shell'. But in conditions of drought (a water-defi cit stress), heat (at high temperature) or cold
(due to chilling and freezing) and high salinity the water potential of the environment decreases
as a result water fl ows out of the cells and the cells face water-defi cit stress. As a result of this, the
protective hydration shell around the macromolecules is disturbed leading to their denaturation and
consequent loss of activity. In order to combat these different stress conditions, the microbial (/plant)
cells show some common and some variable metabolic responses. Water-defi cit in the environment
leads to drought conditions and not all organisms can face this challenge but some of them do so by
developing tolerance to drought. They do so by not desiccating themselves but by taking recourse
to store water (as in desert cacti) or synthesize water (as in rodents) to cater to future needs. It is
diffi cult to set internal limits of water content common to all organisms as especially it is diffi cult to
identify a specifi c feature or trait common to them. The limits and frontiers of desiccation tolerant
life and constraints of such tolerance have been reviewed (Alpert, 2005, 2006). Desiccation tolerance
in prokaryotes has received greater attention quite early and the cyanobacteria have emerged as
good experimental systems to study desiccation (Potts, 1994, 1999, 2001). A detailed account on
desiccation has been presented later at the end of this chapter.
3) Oxidative stress:
Atmospheric oxygen is generally non-reactive to most of the organic molecules
because it possesses two electrons that have parallel spins, whereas the organic molecules have
paired electrons with opposite spins. Oxygen can react with a divalent reductant provided it has
two unpaired electrons that spin opposite to those of oxygen, Due to the spin restriction, oxygen
can not be reduced in biochemical reactions but if oxygen is activated it can participate in reactions
with organic molecules. This activation of oxygen may take place by two different mechanisms.
The fi rst is by the formation of singlet oxygen species (
1
O
2
*). This is possible by the absorption
of suffi cient energy to reverse the spin of one of the unpaired electrons to generate
1
O
2
* species.
During PSI and PSII and respiration as well as during chemical or environmental stress conditions,
the
1
O
2
* species is generated in plants and cyanobacteria.
1
O
2
* is more reactive towards organic
molecules than oxygen and can transfer its excitation energy to other biological molecules or can
react with them to produce endoperoxides or hydroperoxides (Haliwell and Gutteridge, 1989).
In the second mechanism, the
1
O
2
* species is reduced step-wise to produce superoxide radicals
(O
2
•-
), hydrogen peroxide (H
2
O
2
) and hydroxyl radicals (OH·). All these (
1
O
2
*, O
2
•-
, H
2
O
2
, OH·) are
powerful oxidizing agents and are collectively known as ROS. Both O
2
•-
and OH· have an unpaired
electron that makes them highly reactive with organic molecules. The
1
O
2
* species has a very short
half-life in cells (Gorman and Rodgers, 1992) and it can last for nearly 4 µs in water and 100 µs in a
non-polar environment (Foyer and Harbinson, 1994). The O
2
•-
anion is moderately reactive and has
a half-life of 2-4 µs. Because of its negative charge it cannot diffuse through biological membranes
and so dismutated readily to H
2
O
2
. It oxidizes the [4 Fe-4S]
2+
clusters to [3 Fe-4S]
1+
releasing iron
(Fe
2+
). Thus the activity of quinones and metal-containing enzyme complexes (Fe
3+
and Cu
2+
) gets
affected. Further, O
2
•-
can cause the formation of hydroperoxy radicals (HO
•
2
) by protonation that
