Before assessing the two given cases, an assumption needs to be made about the digestion of the two types of food items, corn and beef. Let us assume that both get digested by expending same amount of energy by the human body. Let the amount of energy at producer level be an arbitrary X.
Case 1: Corn to human:
Since corn is at the producer level, it will have X amount of energy. The transfer of energy from corn to human will take place with a loss of 90% energy at the producer level. Hence the humans will receive 10% of X amount of energy.
Case 2: Corn to cow to human:
Since corn has X amount of energy, only 10% will pass to the next level, i.e. cow. So cow has 10% of X. Cow transfers only 10% of the energy that it has. Hence humans receive only 10% of 10% of X amount of energy.
It can be seen that humans receive more energy in case 1 than case 2. Hence we can say that it is more efficient for humans to feed on corn.
Cellular respiration, on the other hand, is the process by which living things convert oxygen and glucose to carbon dioxide and water, thereby yielding energy. It does not require the presence of sunlight and is always occurring in living organisms. Cellular respiration takes place in the mitochondria of cells.
Answer:
Our atmosphere (air) is comprised of 78% of nitrogen. However, this nitrogen is in its elementary state (N₂) and therefore is quite inactive, so the plants can not use it directly. Changing the elementary nitrogen into more reactive forms (NH₃, NO₂, HNO₃) is occurring through the process called nitrogen fixation. It is happening via two processes.
First one is physical nitrogen fixation. When some type of atmospheric physical energy (lightning bolt, for example) reacts with N₂, it splits it in two very reactive N atoms, enabling them to react with oxygen in order to form NO₂.
Second process is biological nitrogen fixation. It's done by bacteria, special bacteria living in the soil, able to use elementary nitrogen for its metabolical pathways resulting in production of ammonia, which can be used by plants. It's noteworthy that bacteria are responsible for about 90% of all nitrogen fixation.
