Question

the flow of the energy is unidirected.why??

Answer 1

Some biological molecules, such as those in visual or photosynthetic systems, have evolved to efficiently convert energy from one form to another. How do these molecules channel energy rapidly and efficiently so that useful work can be performed without this energy being dissipated ineffectively into the surroundings? Dissipation of molecular vibrational excitation energy typically takes place on picosecond time scales, so biological molecules must be able to channel energy rapidly and efficiently if they are to be able to direct it in a useful manner. In biological systems excited by light, the nonradiative electronic transitions can occur on time scales (≪ 10−12 ps) that are even faster than vibrational energy dissipation (1–3), hinting at how nature solves the problem of directing energy flow. On page 1006 of this issue, Kukura et al. (4) take an important step forward in defining the process of directed energy flow in the visual pigment rhodopsin.

Answer 2

The flow of energy is unidirectional this can be understand by taking an example take a food chain of grass goat lion in this food chain grass take energy from Sun goat take energy from grass lion take energy from goat and now lion can't back its energy to the goat this is one reason that secondary animals or primary animals or producers can't back it energy and another reason it that lion cannot back energy to the sun this is another reason that energy flow uni-directional.