A seed is a living system with a low water content and metabolism reduced to the minimum. It contains genetic information which enables the life of a new plant.
The most critical period for a seed is imbibition and the beginning of germination, which represents a shifting of the system from a latent state steady state. Seed storage ageing factor induces qualitative and quantitative changes, which could have as a consequence loss of viability. If deterioration is not significant, the system results in a new plant by rehydration and substance allocation present in processes of hydrolysis and biosynthesis.
During its lifetime cycle, a plant dissipates energy gradients from the point of growth and development. Environmental stresses increase the internal entropy of a plant, moving it closer to equilibrium. In response, plants employ repair systems, requiring additional energy for the recovery processes, having as a consequence a lowering of the energy available for work and an increase in entropy.
A seed is a biological system in the state of anhydrobiosis with living processes reduced to the minimum tomaintain the germination ability viability - the crucial biological aspect. Anhydrobiosis is a highly stable state of suspended energy due to desiccation pending recovery by rehydration. This state seems always to be characterized by the cessation of measurable metabolism. Bryant et al.
Glasses exhibit temperature-dependent transitions during which they pass from a glassy mechanical solid to a state with a markedly decreased viscosity.
This can be detected by a change in the heat capacity or by direct measurement of mechanical relaxation of viscosity Walters et al. Water is of great importance for living systems; either it is a reaction medium or a reactant. The main characteristic of seed is low water content, which could vary depending on plant species, environment and seed condition Beardmore et al.
Seed water consists of two components, bound and bulk water Krishnan et al. Ageing is a characteristic of all living systems, seeds included. Irrespective to fact that vitrification presents a conservation state for seed systems close to a steady state , silent metabolic processes are present, with a lower ability to counteract developed injuries. This means that vitrification has a double nature: conservation low entropy and enthalpy with a low ability of recovery, as opposed to the hydrated state, where the entropy and enthalpy are large with high mobility and high recovery ability.
From the point of glass stability, water and temperature play significant roles in determining the storage longevity of orthodox seeds. Some models have demonstrated that the effects of water and temperature on seed aging are interdependent Beardmore et al. On the other hand, Walters et al. Furthermore, Vertucci established that an increase of seed moisture over 0. This gives a more complex view to the maintenance of seed viability. Desiccation of plant tissues presents a shift of the water from the liquid to the vapour phase Sun, Temperature influences evaporation, as well as the partial water vapour pressure in the air and the energy status of water in plant tissue, both in dry and hydrated plant tissue.
An increase in temperature results in a decrease in the equilibrium water content at a given relative humidity water activity or an increase in the equilibrium water activity for a given tissue water content Fig. The temperature dependence of the isotherm shift is described by the Clausius-Clapeyron Equation:.
The structural changes observed during the process of seed ageing consider disturbances of the glass structure Hoekstra et al. Based on thermodynamics, the change of internal energy of a system represents the maximal work which could be achieved.
Analysis of the water sorption isotherms. The difference between these two curves shows hysteresis, indicating the irreversibility of water sorption in the tissues during dehydration and rehydration. The sigmoid shape of sorption curves is presumably due to the existence of three types of water-binding sites in tissues strong I , weak II and multilayer molecular sorption sites III.
Data from Sun Molecular mobility was found to be inversely correlated with storage stability. With decreasing water content, the molecular mobility reached a minimum, but increased again at very low water contents. This correlation suggests that storage stability might be at least partially controlled by molecular mobility Buitink et al. Krishnan et al. The thermodynamic properties showed a critical upper limit, with tolerant species having higher values than susceptible species.
In general, the values of the critical limits of the thermodynamic parameters decreased with increasing temperature. The differential enthalpy and entropy increased in seeds with period of storage and became asymptotic as the seeds lost their viability. The importance of temperature, as a seed deterioration factor was also emphasised by Dragicevic , with the increased values of the differential free energy found during accelerated ageing of susceptible sugary genotypes and tolerant dent genotypes maize seeds Fig.
It should be mentioned that the observed research data were calculated using temperature sums, which have a significant function in plant development. Parallel to the results of Krishnan et al. Meanwhile, the radical decrease in germination corresponds with the trend of enthalpy decrease, with values present on the negative part of the scale, indicating a shift of the system from a relatively ordered state to a random state Davies ; Sun, The observed results are in accordance with the data of Krishnan et al.
Life depends on a balance between entropy and enthalpy. For plants, the required energy for maintaining an ordered state is achieved by oxidation respiration of photosynthesized substances. Mitochondrial respiration provides energy for biosynthesis and its balance with photosynthesis determines the rate of plant biomass accumulation Millar et al.
The result of oxidation is an overall reducing environment in cells. During oxidation, photo-oxidative stress and photorespiration, the production of reactive oxygen species ROS is an unavoidable consequence Brosche et al.
The free energy of different reactive oxygen species. Sets of redox couples can be independent from other sets if the activation energies for the reactions are high and there are no enzymatic systems to link them kinetically, which is commonly the case in seeds. The reduction potential can be described as the voltage reducing capacity present in the number of available electrons. The reducing capacity could be estimated by determining the concentration of the reduced species in a redox couple using the Nernst equation:.
The Nernst equation has a wide range of applications in biology because many biochemical reactions in living organisms involve electron transfer reactions. These reactions are responsible for energy production. The voltage of an electrochemical cell is directly related to the change in the Gibbs energy:. Using Eq. The reduction potential of various redox couples in the cell could be viewed as triggers to activate a cellular switchboard that move the cell through different physiological phases: from proliferation Fig.
With the exception of stress situations, this phenomenon is connected with planned dismissing of individual parts of an organism, which lose functionality programmed cell death , such as necrosis of aleurone and seed rest, after shifting to autonomic nutrition Mrva et al. This approach represents the first step into a new area of quantitative biology. Reduction potential-driven nano-switches move cells through different biological stages. The redox environment of a cell changes throughout its life cycle.
A The switches for proliferation are fully on. B When E hc becomes more positive, the differentiation switches can be turned on while proliferation decreases. C The more positive the E hc becomes, the more differentiation switches are turned on until they reach a maximum, when nearly all cells are differentiating. D Cells that are not terminally differentiated could undergo proliferation with an appropriate signal and associated redox environment.
Exothermic reactions are good examples of this type of reaction because they release heat. The next time you do laundry, put some laundry detergent in your hand and add a small amount of water. Do you feel the heat? This is a safe and simple example of an exothermic and thus exergonic reaction. A more spectacular exergonic reaction is produced by dropping a small piece of an alkali metal in water. For example, lithium metal in water burns and produces a pink flame.
A glow stick is an excellent example of a reaction that is exergonic, yet not exothermic. The chemical reaction releases energy in the form of light, yet it doesn't produce heat. Actively scan device characteristics for identification.
Use precise geolocation data. Select personalised content. Create a personalised content profile. Measure ad performance. Select basic ads. Create a personalised ads profile. Select personalised ads. Apply market research to generate audience insights. This type of graph is called a reaction coordinate diagram. In the case of an exergonic reaction, depicted below, the chart indicates two key things: 1 the difference between the free energy of the reactants and products is negative and 2 the progress of the reaction requires some input of free energy shown as an energy hill.
This graph does not tell us how the energy in the system was redistributed, only that the difference between enthalpy and entropy is negative. These reactions are said to occur spontaneously. Understanding which chemical reactions are spontaneous is extremely useful for biologists that are trying to understand whether a reaction is likely to "go" or not.
It is important to note that the term spontaneous - in the context of thermodynamics - does NOT imply anything about how fast the reaction proceeds. The change in free energy only describes the difference between beginning and end states NOT how fast that transition takes. This is somewhat contrary to the everyday use of the term which usually carries the implicit understanding that something happens quickly.
However, an iron nail exposed to air does not rust instantly - it may take years. These chemical reactions are called endergonic reactions , and they are NOT spontaneous. An endergonic reaction will not take place on its own without the transfer of energy into the reaction or increase of entropy somewhere else. Exergonic and endergonic reactions result in changes in Gibbs free energy. In exergonic reaction the free energy of the products is lower than that of the reactants; meanwhile in endergonic the free energy of the products is higher than that of the reactants.
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