CIE Biology Paper-4: Specimen Questions 60 - 60 of 120

Question number: 60

Essay Question▾

Describe in Detail

What is the role of phosphorous in animals and plants. Explain phosphorous cycle.


Phosphorus cycle-It is found in water, soil and sediments. It cannot be found in air in the gas form. So phosphorous is usually a liquid at standard temperatures and pressures. It is the slowest cycle.

The role of phosphorous in animals and plants-

  • Phosphorous is major nutrients for the growth of development of animals and plants
  • It plays a critical role in cell development and is a key component of molecules that store energy, such as ATP (adenosine triphosphate), DNA and lipids
  • Insufficient phosphorus in the soil can result in a decreased crop yield

The phosphorous cycle- Phosphorus moves in a cycle by rocks, water, soil, sediments, and organisms.

Steps of phosphorous cycle is as follows-

1. Over time weathering cause rocks to release phosphate ions and other minerals. These inorganic phosphates is then distributed in soils as well as in water.

2. Plants take up inorganic phosphate from the soil. Animals may then consume plants. Once in the plant or animal, the phosphate is incorporated into organic molecules such as DNA.

3. When the plant or animal dies, it decays, and the organic phosphate is returned to the soil.

4. Phosphorus in soil can end up in waterways and eventually oceans. Once there, it can be incorporated into sediments over time.

Image shows phosphorous cycle

Image shows phosphorous cycle

Image shows phosphorous cycle which is found in water, soil and sediments

Most phosphorus is unavailable to plants since most of our phosphorus is locked up in sediments and rocks; it is not available for plants to use. A lot of the phosphorus in soils is also unavailable to plants.

Soil phosphorus becomes unavailable to plants through several routes:

Bacteria: Bacteria convert plant-available phosphate into organic forms that are then not available to plants.

Adsorption: Inorganic and available phosphorus can be chemically bound (adsorbed) to soil particles, making it unavailable to plants.

pH: Soils are less than pH 4 or greater than pH 8, the phosphorus starts to become tied up with other compounds, making it less available to plants.