Spores of fungus, Candida albicans

The long strands are the tubular filaments (hyphae) that have developed from the fungal spores. Yeast cells (rounded, yellow) are budding from the ends of the hyphae (red). Candida albicans causes the infection known as candidiasis which affects the moist mucous membranes of the body, such as skin folds, mouth, respiratory tract and vagina. Oral and vaginal conditions are known as thrush.

More about fungi

Nitrogen cycle

  1. 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.

  2. 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.

  3. 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.

  4. Denitrification

    Nitrates are reduced to nitrogen gas by denitrifying bacteria.

  5. Nitrogen fixation

    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.
  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.

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