Diatoms b/w Diatoms

Diatoms

Diatoms, a type of algae, are found floating in the phytoplankton of the seas. Their cell walls contain silica. When diatoms die they sink to the sea floor. Their soft parts decay and the silica remains. Over time the pressure of the seawater pushes the silica together to form one large layer. This silica is mined and used as an ingredient in abrasives and polishes.

Nitrogen cycle

  • Nitrogen cycle - Step 1

    1/6 Plants absorb the nitrate ions by diffusion and active transport. The plants need the nitrogen for the synthesis of proteins and other compounds. The nitrogen compounds are passed through the food chain as other organisms feed on the plants and each other.

    Waste products (undigested food, urine and faeces) and dead organisms which contain nitrogen compounds are added to the soil.

  • Nitrogen cycle - Step 2

    2/6 Ammonification (decay)

    The decomposers - certain soil bacteria and fungi e.g. ammonifying bacteria - break down proteins in dead organisms and animal wastes, releasing ammonium ions.

  • Nitrogen cycle - Step 3

    3/6 Nitrification

    Nitrification is a two-step process that is carried out by the nitrifying bacteria. Ammonia or ammonium ions are oxidized first to nitrites and then to nitrates.

  • Nitrogen cycle - Step 4

    4/6 Denitrification

    Nitrates are reduced to nitrogen gas by denitrifying bacteria.

  • Nitrogen cycle - Step 5

    5/6 Nitrogen fixation - this is the first step in the synthesis of virtually all nitrogenous compounds.

    1. Atmospheric fixation - this occurs spontaneously due to lightning. Only a small amount is fixed this way.
    2. Industrial fixation - the Haber process is used to make nitrogen fertilizers. This is very energy inefficient.
  • Nitrogen cycle - Step 6

    6/6 Biological fixation - Nitrogen fixing bacteria fix 60% of nitrogen gas.

    • Some nitrogen fixing bacteria e.g. Rhizobium live in the root nodules of legumes.
    • Some nitrogen fixing bacteria e.g. Azotobacter are free living in the soil.

Nitrogen in the air

Nitrogen is required by all living organisms for the synthesis of proteins, nucleic acids and other nitrogen containing compounds. The Earth’s atmosphere contains almost 80 % nitrogen gas. It cannot be used in this form by most living organisms until it has been fixed, that is reduced (combined with hydrogen), to ammonia.

The nitrogen cycle is a series of processes that convert nitrogen gas to organic substances and back to nitrogen in nature. It is a continuous cycle that is maintained by the decomposers and nitrogen bacteria. The nitrogen cycle can be broken down into four types of reaction and micro-organisms play roles in all of these.