I dunno, if you were to eat that cucumber, you would absorb part of the energy from the cucumber and convert it into energy of your own, which is then used to power your body which can be used on a treadmill for example or operating machinery.
Thus the sunlight absorbed by the plant during its growth is transferred from plant to human, and eventually through waste back into the soil and plant again. Of course it's not a closed system, and entropy strikes along the way so that only a small percentage of the energy that the plant absorbed makes it back into the ground at the end of the process.
A better example really would be rapeseed, that absorbs energy from the sun and is converted into rapeseed oil which is converted into biofuel which is then burnt for energy.
It's all about releasing the energy stored in objects in a manner which is as waste-free as possible and with minimal environmental side-effects.
