Page 309 - ABCTE Study Guide_Neat

Nitrogen fixation is the conversion of N 2 to ammonia (NH 3) by several species of aquatic or terrestrial
bacteria, either symbiotic with plants or living in the soil. NH 3 can dissolve quickly in water and is
converted to ammonium, which is then taken into the tissues of plants, such as soybeans, as well as
organisms feeding on these plants in a process called assimilation. Nitrogen is also released through
waste products when these organisms die and decompose. Thus, without the essential nitrogen-fixing
organisms, modern agriculture often must add sufficient nitrogen to soils in order for food crops to grow.

Ammonification occurs when bacteria convert nitrogen from wastes or decomposition to NH 3, which
again is dissolved to form ammonium. Nitrification is the process of converting some of the ammonium in
the soil into nitrates or nitrites that can then be assimilated by plants or animals. Denitrification occurs
when other bacteria convert nitrates back to N 2, which is then released into the atmosphere. Click here to
see a NASA image of the nitrogen cycle.

Question

In the nitrogen cycle, assimilation describes

A the conversion of nitrogen by bacteria.

B the conversion of nitrates back to N 2.

C the intake of ammonium by plants and the animals that eat them.

D the conversion of N 2 to ammonia.

Answer

The correct answer is C. Choices A, B, and D also describe different parts of the nitrogen cycle.
Assimilation, however, refers to the taking in of nutrient material into the tissue of a living organism.

Energy

If an ecosystem is composed of many interactions between organisms and their environment, the most
essential of these interactions is the flow of energy throughout the system.

As in the nitrogen cycle, sunlight is abundant on earth but available for use only by producers such as
green plants and some bacteria and algae. Thus, while humans are the most evolved of species
physiologically, we depend upon the actions of often very simple organisms for our survival.

Food Chains and Food Webs

The flow of energy from the sun to producers, consumers, and decomposers is often represented by a
food chain, though the actual flow of energy is much more complex and is better represented by a still
oversimplified food web.