Fungi .docx

Fungi Fungi are complex organisms that form a variety of different structures. They are classed as Eukaryotic organisms for the following reasons: - They have a membrane-bound nucleus - They have membrane-bound organelles e.g., mitochondria Most Fungi are saprophytic, meaning they obtain their nouri...

Fungi Fungi are complex organisms that form a variety of different structures. They are classed as Eukaryotic organisms for the following reasons: - They have a membrane-bound nucleus - They have membrane-bound organelles e.g., mitochondria Most Fungi are saprophytic, meaning they obtain their nourishment by breaking down plant and animal material. For example, Rhizopus. Some Fungi are parasitic, meaning they cause harm to the organism which they feed off and live on. For example, Athlete’s Foot. Some fungi are extremely poisonous e.g., The destroying Angel (Amanita phalloides). Far more fungi, such as the field mushroom (Psalliota campestris), are edible and completely harmless. Nutrition Fungi normally exist as microscopically thin threads called hypha that permeate through the substance they grow on. This substance is called the substrate. A collection of hyphae is called a mycelium. Fungi obtain their nourishment by extracellular digestion. This involves three steps: 1. The cells of the hyphae exude digestive enzymes 2. These enzymes break down any dead plant materials around them 3. The products of this digestion are then absorbed by the hyphae, but they are also available to any plants whose roots are in the area to use as nutrients. Rhizopus The black bread mould, Rhizopus, is a mould that commonly appears on stale bread and rotting fruit. It is a haploid (n). This means that it only has one set of chromosomes. Structure - Stolon: Aerial hyphae growing horizontally. They spread over the surface, turning downward at intervals, to create rhizoids - Rhizoids: Branched hyphae which penetrate the food source anchoring the fungus. Rhizoid release digestive enzymes into food and absorb digested food. - Sporangiophore: These are hyphae that arise unbranched from rhizoid tufts. They enlarge at the tip to form spherical sporangia. - Sporangium: The swelling at the tip of sporangiophore containing spores. - Columella: A cross wall which pushes up into sporangium from below. They separate spores from the sporangiophore. - Spores: Small, black, light, asexual reproductive structures. Spores can be transported by wind (7) Asexual Reproduction Asexual reproduction is the most common type of reproduction in Rhizopus. The tip of the sporangiophore swells to form a columella and this is cut off from the sporangiophore by a structure called the apophysis. The columella produces numerous haploid spores by mitosis, and these are held in the sporangium. When mature, the wall of the sporangium turns black. The sporangium dries out and bursts, releasing hundreds of spores that are carried by wind. When these spores land on a suitable substrate, they germinate, producing a hypha that grows through the substrate forming a new product. Sexual Reproduction Sexual reproduction occurs when hyphae of two different strains of Rhizopus grow side by side. There is no structural difference between the strains, so one is designated plus and the other minus. The process of sexual reproduction is as follows - Swellings, called progametangia, form from each hypha, opposite each other - Once they meet, the tips of each progametangia are cut off from the stalk and are now gametangia - The walls separating the two gametangia break down and fertilisation occurs when the nuclei from each of the gametangia fuse to form diploid zygotes - The wall thickens and forms a resistant zygospore - The zygospore is able to withstand adverse conditions. The zygospore will now be disperses and will germinate once it finds suitable conditions - The zygote divides by meiosis and a haploid sporangiophore grows vertically upwards - A sporangium forms at the top and asexual reproduction begins, spreading the fungus through the substrate (8) Yeast Yeast (Saccharomyces cerevisiae) is a unicellular fungus. Nutrition The most common yeast is baker’s yeast (Saccharomyces cerevisiae). However, there are many other types. For example, Thrush (Candida albicans), a parasite, is a common infection of the mouth and vagina. Baker’s yeast is a saprophyte, obtaining its minerals and requirements from sugar-rich solutions e.g., rotting fruit. Respiration Yeast obtains its energy by anaerobic respiration of sugars. Yeast breaks down glucose according to the equation C6H12O6 = 2 C2H5OH + 2 CO2 Glucose Ethanol Carbon Dioxide It produces an enzyme called zymase which is used to speed up the reaction. The reaction produces only a small amount of energy. Reproduction Asexual reproduction carried out by budding. A bud forms at one end of the and the nucleus and the vacuole of the parent cell split in two. One part of each moves into the bud. The bud is cut off from the parent cell, forming two separate cells. If food is plentiful, buds can form at both ends of a yeast cell, and buds can form on buds. This is known as pseudomycellium. (9) Economic Importance of Fungi Benefits - Yeast used to produce Alcohol - The mould (Penicillium Roquefort) is used to flavour cheese - The mould (Penicillium notatum) led to the production of penicillin Disadvantages - Parasitic fungi can cause disease (Athlete’s Foot, Thrush, Ringworm) - The potato-blight fungus (Phytophthora infestans) led to The Great Famine by causing potatoes to rot Mandatory Practical – Investigate the growth of leaf yeast using agar plants and controls - Obtain an ash leaf by cutting with scissors and tweezers - Put the leaf into a plastic bag and bring it back to the lab - Wash the bench with alcohol and sterilising solution to prevent contamination - Take two sterile nutrient malt agar plates - Seal one plate and label it ‘control’. - Keeping the lid of the Petri dish facing down, attach one leaflet with the upper surface facing up to the lid using petroleum jelly - Replace the lid and seal with the tape again to prevent contamination - Leave the petri dish the right way up for one day at room temperature - Invert the petri dish and leave it at room temperature for three days - Examine both plates - Replicate the experiment to verify results - When finished, soak the petri dishes in sterilising fluid to kill any microorganisms that have grown. Dispose in the rubbish bin - There should be shiny pink colonies on the agar plate containing the leaf, and they should shine form the shape of the leaflet - No colonies should be present on the empty dish Precautions when working with Microorganisms Aseptic technique includes all the steps taken to prevent contamination. This is very important as many microorganisms are pathogenic and can cause serious harm to humans and the environment. Sterile means that there are no microorganisms present. The following procedures should be followed to achieve asepsis; - Flame equipment by passing through a Bunsen flame - Keep all experiments in sealed containers - Minimise all openings in terms of size and duration - After use, soak everything in disinfectant - Dispose of all sterilised material in the waste bin