Fungi

Introduction

  • All fungi are heterotrophic.
  • They can besingle-celled (e.g. Yeasts) or multicellular (e.g. mushrooms).
  • They do not contain chlorophyll.
  • All fungi are eukaryotic – meaning they have a membrane-bound nucleus and membrane-bound organelles.
  • All fungi have cell walls made of chitin.

Nutrition: way in which organisms obtains and uses food.

  • There are two types of heterotrophic nutrition in fungi:
    • Saprophytic
    • Parasitic.
  • Saprophytic fungi: obtain their food from dead organic matter; e.g. fungi of decay.
  • Parasitic fungi: obtain their food from living organisms; e.g. Athlete’s foot

Yeast (Saccharomyces cerevisiae)

Structure

  • Single-celled
  • Cell wall made of chitin
  • Granular cytoplasm
Yeast cells

Reproduction

  • Asexual by mitosis in a process known as ‘budding’ – a small swelling forms on the cell; it fills with cytoplasm and the nucleus divides by mitosis with one of the resulting nuclei moving into the ‘bud’.

Rhizopus (Common bread mould)

Structure

  • Multicellular
  • Cell wall made of chitin
  • Hyphae – thin, microscopic, thread-like tubules.
  • Sporangia – structures that hold spores.
Structure of Rhizopus

Reproduction

  • Asexual – by means of formation of spores, a process known as sporulation.
  • Sexual – by means of formation of a diploid zygospore.

Laboratory procedures when handling microorganisms:

  • Use aseptic technique: procedure where contact with, or contamination by, microorganisms is avoided.
  • Always wear a lab coat.
  • Wash your hands before and after the experiment.
  • Wear protective gloves where appropriate.
  • Wear safety glasses where appropriate.
  • Keep your hands away from your face at all times in the laboratory.
  • Clean the bench thoroughly before and after use and swab with disinfectant, such as 70% ethanol or Milton.
  • Clean and sterilise all glassware involved in the experiment before and after use by placing in an autoclave or a pressure cooker for 15 minutes.
  • When using Petri dishes and containers to grow microorganisms, only open very slightly and for the shortest possible time to avoid contamination.
  • If using forceps or an inoculating loop, use a Bunsen flame to sterilise before and after use.

Practical activity: to investigate the growth of leaf yeast using agar plates and controls.

Apparatus/Equipment:

  • Malt
  • Agar powder
  • Beakers
  • Hotplate/ heating equipment
  • Disinfectant
  • Petri dishes
  • Ash leaves
  • Ethanol
  • Petroleum jelly
  • Incubator
  • Lab coat and safety goggles and gloves

Method:

  • Follow aseptic technique as described above.
  • Make up a 1.5% solution of malt agar (1.5 g agar in 100 ml distilled water along with a teaspoon of malt). [NOTE: Leaf yeast will not grow without a source of maltose]
  • Sterilise by boiling the agar solution.
  • Carefully pour the agar into three Petri dishes and allow to set solid (~10 minutes)
  • Obtain old Ash leaves from your local park. [NOTE: September/October is the best time of year to do this activity, as the leaf yeast have had the spring and summer months to reproduce on the leaves).
  • Disinfect (by rubbing with alcohol or sterilising fluid) one of the leaves (this acts as a control).
  • Attach this leaf to the inside of the lid of one Petri dish using some petroleum jelly and ensuring the underside of the leaf is facing the agar.
  • Attach the test leaf (not sterilised) to the inside of the lid of the other Petri dish.
  • Ensure neither leaf is touching the agar.
  • Leave the third Petri dish closed – this acts as a negative control.
  • Seal the dishes shut using parafilm.
  • Leave the dishes upside down in the incubator set at 25˚C for 24 hours and then turn them right side up for the remaining few days (4-6 days).

Result:

  • Pink colonies form on the agar of the test.
  • The controls showed no growth of leaf yeast.
Pink leaf yeast growing on malt agar

Conclusion:

  • Leaf yeast grow best on the underside of a leaf – as they are not washed off by rain or damaged by UV light.
  • Leaf yeast live in larger numbers on older leaves as they have had the spring and summer months to grow and multiply.

Fungi notes page

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