If you have just eaten, your body will secrete insulin. Insulin is the hormone that allows glucose to enter cells. That way they will have the proper energy for their normal functions.
If you have not eaten for a while, your body is running out of glucose to feed the cells so it needs to secrete another hormone called glucagon. This hormone is the complete opposite of insulin because it breaks down glycogen, proteins and fats into glucose.
What ecosystem are you talking about ?
Energy is lost at each trophic level.
Dissolved O2 conc. in flask with algae will be higher since photosynthesis produces O2.
Decomposers eat dead remains of organisms that are quaternary consumer (no one eats them while alive). So, they break these down and therefore turn into nutrients in the soil for plants to use.
