GENERAL BIOLOGY II (BLY 102) - 3 CreditsFUNGI AND THEIR STRUCTURE

Dr Olayinka O. Elutade

Microbiology Programme

Bowen University, Iwo,

Osun State, Nigeria

INTRODUCTION

• Fungi:

Are eukaryotic unicellular and multicellular organisms

The cells contain a membrane-bound nucleus, mitochondria and a complex system of internal membranes, including the endoplasmic reticulum and Golgi apparatus

Cells do not have chloroplasts or chlorophyll

Like plants, they have thick cell walls

However, their cell walls contain complex polysaccharides called chitin; while cell walls in plants contain cellulose

chitin gives structural strength to the cell walls of fungi

There are two forms of body structures in fungi:

Unicellular forms

Multicellular forms

Unicellular forms of fungi

• These are made up of single cells called yeasts

Examples include:

Saccharomyces cerevisiae (baker’s yeast)

Candida species (the agents of thrush, a common fungal infection)

• Yeasts often inhabit moist environments, including plant sap and animal tissues, where there is a ready supply of soluble nutrients, such as sugars and amino acids.

Figure 1: Yeast cells (Jin et al., 2018)

Multicellular forms of Fungi

• The multicellular body structure consists of filaments (end-to-end arrangements) of cells

• Two stages of the morphological multicellular forms are known:

Vegetative:

consists of a tangle of slender thread-like structures called hyphae (singular, hypha),

Reproductive:

Which can be more conspicuousFigure 2: Filamentous arrangement of cells (Jinet al., 2018)

THE MYCELIUM

• Mycelium:

Is a mass of hyphae

Can grow on a surface, in soil or decaying material, in a liquid, or even on living tissue

Can consist of three types:

Septate hyphae

Non-septate hyphae

Specialized hyphae

Fig. 1: The mycelium (Preparmy.com)

• Septate hypha:

Are hyphae with separate cells which have endwalls called septa (singular, septum)

The tiny holes in the septa allow for the rapid flow of nutrients and small molecules from cell to cell along the hypha

Are also called perforated septa

Examples include Aspergillus species, Fusarium species

Non-septate hypha:

Are hyphae with large cells containing many nuclei,

Do not have endwalls or septa

Are also called coenocytic hyphae or aseptate hyphae

Examples include Rhizopus species

Septate and Non-septate Hypha

Fig. 2: Septate and Non-septate hypha (Studyblue.com)

Specialized Hyphae• Haustoria:

Is a specialized hyphae

Which penetrate the tissues of the host, release digestive enzymes within the host's body, and absorb the digested nutrients

Haustoria (Biology-pictures.blogspot.com)

Specialized Hyphae (Cont’d)• Arbuscules:

Are specialized hyphae

That are in use to exchangenutrients with their plant hosts

Such mutually beneficialrelationships between fungi andplant roots are calledmycorrhizae (the term means“fungus roots”)

Almost all vascular plants havemycorrhizae and rely on theirfungal partners for essentialnutrients.

This is because the vast mycelialnetworks of the fungi are moreefficient than the plants’ roots atacquiring these minerals fromthe soil.

Arbuscules (Valent USA.com)

Reproduction in Fungi• Asexual Reproduction:

Some fungi only can reproduce asexually through mitosis

May occur through such methods as budding, fragmentationand spore formation

o Budding:

Yeast cells reproduce asexually by budding

A new cell develops while attached to the parent cell; theplasma membrane pinches off to partially separate the newindividual from the parent cell

o Fragmentation:

Occurs when the mycelium of a fungus is physically brokenapart or fragmented

Can occur when an animal digs in the soil where a fungus isgrowing

When the fragments of mycelia land in a location with suitablegrowing conditions, the hyphae will grow into new mycelia

Reproduction in Fungi (Cont’d)

oSpore Production:

A spore is a reproductivehaploid cell with a hard outercoat that develops into a neworganism without the fusion ofgametes.

Spores produce new hyphaethat eventually form a mycelium.

Some fungal spores are thin-walled and germinate quickly;others are thick-walled and takelonger periods of time togerminate.

In sexual reproduction, thefungi’s diploid, reproductivestructure produces haploidspores by meiosis. Thesespores form the next generation,which grows into new mycelia.

Spore formation and germination in Rhizopus (Bread Mould) [learninsta.com]

Reproduction in Fungi (Cont’d)

oSpore Production (Cont’d):

Most fungi produce large quantities of spores as an adaptationfor survival

This adaption ensures that at least a small percentage ofspores will land in suitable locations and begin to grow,producing the next generation

The spores are small and lightweight that wind and even thesmallest animals such as insets, can disperse them.

Each spore has a cell wall which is often tough and water-proofso that the spore can survive extreme temperature andmoisture

The Diversity of Fungi

• Phylum Zygomycota:

A common example is a common mould that grows on bread and other foods called Rhizopus stolonifera.

o Has a type of hyphae called stolon that spreads across the surface of the food and another type of hyphae called rhizoids which anchors the mycelium, penetrates the food, produces enzymes and absorbs the nutrients

o Reproduces asexually when sporangia form at the upright tips of sporangiophores

o Reproduces sexually, under unfavourableconditions, when haploid hyphae from two compactible mating strains fuse to form a gametangium.

Importance of Fungi

• Are symbiotic partners in mycorrhizae

Fungi supply essential nutrients to plants and are enormouslyimportant in natural ecosystems and agriculture.

• Are essential decomposers in ecosystems

Fungi, along with bacteria, break down organic matter and restockthe environment with vital nutrients essential for plant growth

• Production of foods and beverages

Yeasts are important in the production of alcoholic beverages andbread

• Fungi have a number of culinary uses

Some mushrooms are cooked and eaten as human foods

• Production of medicine

For example, the antibiotic, penicillin, is produced from the fungiPenicillium species