Flower Wallpaper Tumblr Hd Black and White Widescreen Download for Mobile images designs: Flowers Wallpaper Desktop

Thursday 19 September 2013

Flowers Wallpaper Desktop

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In the florets of P. selloum, this is achieved by diffusion through a network of tiny intercellular gas spaces that permeate the tissue to the center. Oxygen demand is so high, that the oxygen partial pressure at the center of the floret drops to about one-quarter of atmospheric, but remains just above the critical level where oxygen uptake becomes diffusion-limited (Seymour, 2001).

Heat production occurs by rapid respiration in the thermogenic cells of the flowers. In most thermogenic species studied so far, the substrate for respiration is carbohydrate, often imported from other parts of the plant, but in P. selloum, the substrate is lipid that is stored in the florets prior to blooming (Seymour et al., 1984). Analysis of heat production by direct calorimetry and respirometry show that all of the energy in the substrates ends up as heat in P. selloum (Seymour et al., 1983) and in the lotus, Nelumbo nucifera (Lamprecht et al., 1998). Although there is the possibility of some energy going into synthesis of floral structures, this appears to be negligible.

Heat production in thermogenic plants has been generally thought to be associated with an increase in the activity of the cyanide-resistant electron transport pathway in mitochondria (McIntosh, 1994). This pathway is mediated by the “alternative oxidase” (AOX) that accepts electrons from the ubiquinone pool and uses them to reduce oxygen to water. The free energy by the flow of electrons from ubiquinol to AOX does not generate ATP but instead is lost as heat. The AOX pathway seems to be present in all plants at variable capacity, but it is particularly active in thermogenic species (see Web Topic 11.3). For instance, a recent study on thermogenic receptacles of sacred lotus (N. nucifera) has revealed that an increase in respiration through the AOX is responsible for the heat production (Watling et al., 2006). On the contrary, in the case of mammals, uncoupling proteins (UCPs) have been shown to play a crucial role in non-shivering heat-production (Nicholls and Locke, 1984). UCPs reside in the mitochondrial inner membrane, across which they dissipate energy from the proton gradient that is built up by the respiratory chain, and this leads to heat production. In the skunk cabbage (Symplocarpus renifolius), thermogenic cells surrounding the stamens in the florets (Figure 2) appear to possess activities for both AOX and UCP, and this functional coexpression seems to be the molecular basis of heat production (Onda et al., 2008).

A few species of the most powerfully thermogenic flowers also exhibit temperature regulation, which is the maintenance of a relatively constant temperature in the flower, regardless of external air temperature. Rather precise thermoregulation has been discovered in Philodendron (Nagy et al., 1972), Symplocarpus (Knutson, 1974, Ito et al., 2004) and Nelumbo (Seymour and Schultze-Motel, 1996). In these cases, the respiratory rate increases almost linearly as the ambient temperature drops below 30°C, and the mean temperature of the flower is almost constant (Figure 3). Flower temperature varies only 6°C while ambient temperature varies 35°C.

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