Answer:
Changes can occur to physical properties of a substance, but the chemical composition of the substance remains the same.
Selectively permeable means that cells only allow certain molecules to enter or exit a cell.
Heterocyst-forming cyanobacteria differentiate highly specialized cells to provide fixed nitrogen to the vegetative cells in a filament. In the presence of a source of combined nitrogen such as nitrate or ammonium, Anabaena PCC 7120 grows as long filaments containing hundreds of photosynthetic vegetative cells.
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The answer is; desertification
Topsoil is usually the riches soil in terms of nutrient is important in support of plant vegetation. It is rich because it contains humus and other decomposed materials that recycle nutrients. When the topsoil is washed away, the local region becomes barren in that it cannot support vegetation. This causes the area to turn into a desert.
