Fungi - Morphology, Reproduction, Physiology and Importance in Simple Words

By Momina Shehzadi | BS Zoology Graduate, Pakistan

In this article, we will learn Fungi - Morphology, Reproduction, Physiology and Importance - in simple words with diagram. This topic is very important for BS Zoology and Microbiology students.

Fungi are found everywhere around us. They are present in soil, in water, in air, on dead plants, on dead animals, on our food, on our skin and even inside our body. When bread becomes old, green or black spots appear on it. These spots are fungi. When we see mushroom growing after rain, that is also a fungus. When yeast is used to make bread soft, that is also a fungus.

Fungi are very different from bacteria and plants. They are eukaryotic organisms. It means their cells have a true nucleus and other membrane bound organelles. They do not have chlorophyll, so they cannot make their own food like plants. They get their food from other living or dead things. That is why they are called heterotrophic organisms.

The study of fungi is called mycology. A scientist who studies fungi is called a mycologist. Mycology is very important in microbiology, medicine, agriculture and industry because many fungi are useful and many are harmful.

WHAT ARE FUNGI

Fungi are eukaryotic, heterotrophic organisms that have no chlorophyll. The body of a fungus is called thallus. In unicellular fungi like yeast, the whole body is made of a single cell. In multicellular fungi like molds, the body is made of many long filaments.

Fungi need organic compounds for nutrition. When they feed on dead organic matter, they are known as saprophytes. When they live in or on another organism and get food from it, they are known as parasites. Some fungi live in a useful relationship with other organisms.

Fungi are larger than most bacteria. Yeast cells are generally larger than bacteria. They vary considerably in size and shape, ranging from 1 to 5 micrometers in width and from 5 to 30 micrometers or more in length. Each species has a characteristic shape. Some are spherical and some are elongated. Yeasts have no flagella or other organelles of locomotion.

MORPHOLOGY AND PHYSIOLOGY OF FUNGI

Morphology means the study of shape and structure. Physiology means the study of functions and growth requirements. Both are important to identify fungi in laboratory.

1. Size and Shape

Yeast cells are larger than most bacteria. They are usually oval, round or elongated in shape. Their size depends on age and environment. Young cells are small and old cells are large. In general, yeast cells are 1 to 5 micrometers in width and 5 to 30 micrometers or more in length. Each species has a characteristic shape, so shape helps in identification.

2. Thallus

The thallus of a mold consists essentially of two parts, the mycelium and the hyphae. The mycelium is a complex of several filaments called hyphae. Hyphae generally arise from a spore which on germination puts out a germ tube or tubes. These germ tubes elongate and branch to form hyphae.

3. Hyphae

Each hypha is about 5 to 10 micrometers wide, as compared with a bacterial cell which is usually 1 micrometer in diameter. Hyphae are composed of an outer tube like wall surrounding a cavity, the lumen. It is filled with protoplasm. Between the protoplasm and the wall is the plasma membrane. It surrounds the protoplasm.

The hyphal wall consists of microfibrils composed for the most part of hemicelluloses or chitin. True cellulose occurs only in the walls of lower fungi. Wall matrix in which the microfibrils are embedded consists of proteins, lipids, and other substances.

Growth of a hypha is distal, near the tip. The major region of elongation takes place in the region just behind the tip. The young hypha divides into cells by cross walls. These are formed by centripetal invagination from the existing cell wall. These cross walls constrict the plasmalemma and grow inward to form an incomplete septum. It has a central pore. It allows for protoplasmic streaming. Even nuclei may migrate from cell to cell in the hypha.

Forms of Hyphae

Hyphae occur in three forms:

1. Nonseptate or coenocytic, such hyphae have no septa.

2. Septate with uninucleate cells. Each cell has one nucleus in each compartment.

3. Septate with multinucleate cells. Each cell has more than one nucleus.

4. Vegetative and Reproductive Mycelia

Mycelia are either vegetative or reproductive. Some hyphae of the vegetative mycelium penetrate into the medium in order to obtain nutrients. Soluble nutrients are absorbed through the walls. Insoluble nutrients are first digested externally by secreted enzymes. Reproductive mycelia are responsible for spore formation.

The mycelium of a mold may be a loosely woven network or it may be an organized, compact structure, as in mushrooms. It usually extends from the medium into the air.

5. Fruiting Bodies

Asexual and sexual spores may be surrounded by highly organized protective structures called fruiting bodies. Asexual fruiting bodies have names such as acervulus and pycnidium. Sexual fruiting bodies have names such as perithecium and apothecium.

PHYSIOLOGY OF FUNGI

Fungi are better able to withstand certain extreme environmental conditions than most other microorganisms. For example, yeasts and molds generally can tolerate more acidic conditions than most other microbes. Also, yeasts and molds can grow in a substrate or medium containing concentrations of sugars that inhibit most bacteria. This is why culture media for fungi usually contain peptone, a hydrolyzed protein product.

Fungi are heterotrophic. Unlike some bacteria, they cannot use inorganic carbon compounds, such as carbon dioxide, as their sole carbon source. Carbon must come from an organic source, such as glucose. However, they are capable of using a wide variety of materials for nutrition. Some species can use inorganic compounds of nitrogen, such as ammonium salts. But all fungi can use organic nitrogen.

Some yeasts are facultative. They can grow under both aerobic and anaerobic conditions. Molds and many yeasts are usually aerobic microorganisms. Fungi grow over a wide range of temperature, with the optimum for most saprophytic species from 22 to 30 degree C. Pathogenic species have a higher temperature optimum, generally 30 to 37 degree C. Some fungi will grow at or near 0 degree C and thus can cause spoilage of meat and vegetables in cold storage.

CULTIVATION OF FUNGI

Molds and yeasts are studied by the same general cultural methods used for bacteria. Nearly all of them grow aerobically on the usual bacteriological media at temperatures ranging from 20 to 30 degree C. Most of them grow more slowly than bacteria, so that media which support bacteria as well as fungi are to be isolated. It is good practice to use a medium that favors their growth but is not optimal for the growth of bacteria.

Acidic media with pH 5.6 are inhibitory to many bacteria. A relatively high concentration of sugar in culture media for fungi is usually used to favor fungi. This is why culture media for fungi usually contain peptone, a hydrolyzed protein product.

Table: Comparative Physiology of Fungi and Bacteria

Characteristic: Fungi - Cell type Eukaryotic, Optimum pH 3.8 to 5.6, Optimum temperature 22 to 30 degree C for saprophytes and 30 to 37 degree C for parasites, Oxygen requirement Strictly aerobic for molds and facultative for some yeasts, Light requirement None, Sugar concentration in laboratory media 4 to 5 percent, Carbon requirement Organic, Cell wall structural components Chitin, cellulose, or hemicellulose, Antibiotic susceptibility Resistant to penicillin and Tetracyclines, sensitive to griseofulvin.

Characteristic: Bacteria - Cell type Prokaryotic, Optimum pH 6.5 to 7.5, Optimum temperature 20 to 37 degree C for mesophiles, Oxygen requirement Aerobic to anaerobic, Light requirement Some photosynthetic groups occur, Sugar concentration in laboratory media 0.5 to 1 percent, Carbon requirement Inorganic or organic, Cell wall structural components Peptidoglycan, Antibiotic susceptibility Resistant to griseofulvin, sensitive to penicillin, Tetracyclines and chloramphenicol.

One of the best known and oldest media for the growth of fungi was devised by Sabouraud. It contains maltose and peptone as its principal ingredients. The most common modification used in America now contains glucose and peptone. This medium is especially useful for growing pathogenic fungi from infected body fluids and exudates. Its partial selective action is due to the high sugar concentration and low pH.

REPRODUCTION IN FUNGI

Fungi reproduce naturally by a variety of means. It does not involve the union of nuclei, sex cells, or sex organs. It is accomplished by:

1. Fission of somatic cells, yielding two similar daughter cells.

2. Budding of somatic cells or spores, each bud a small outgrowth of the parent cell developing into a new individual.

3. Fragmentation or disjointing of the hyphal cells, each fragment becoming a new organism.

4. Spore formation.

Some fungi are dimorphic. That is, they exist in two forms. In animals and other hosts, they form a yeast like form, but when growing saprophytically in soil or on a laboratory medium they have a filamentous mold form. Thus a fungal colony may be a mass of yeast cells or it may be a filamentous mat of mold.

I) ASEXUAL REPRODUCTION

Asexual reproduction does not involve the union of nuclei, sex cells, or sex organs.

Kinds of Asexual Spores

Asexual spores, whose function is to disseminate the species, are produced in large numbers. There are many kinds of asexual spores.

1. Sporangiospores

These single celled spores are formed within sacs called sporangia at the end of special hyphae called sporangiophores. Aplanospores are nonmotile sporangiospores. Zoospores are motile sporangiospores. Their motility is due to the presence of flagella.

2. Conidiospores or Conidia

Small, single celled conidia are called microconidia. Large, multicelled conidia are called macroconidia. Conidia are formed at the tip or side of a hypha.

3. Oidia or Arthrospores

These single celled spores are formed by disjointing of hyphal cells.

4. Chlamydospores

These thick walled, single celled spores are highly resistant to adverse conditions. They are formed from cells of the vegetative hypha.

5. Blastospores

These are the spores formed by budding.

II) SEXUAL REPRODUCTION

Sexual reproduction is carried out by fusion of the compatible nuclei of two parent cells.

1. The process of sexual reproduction begins with the joining of two cells and fusion of their protoplasts, called plasmogamy.

2. After this the two haploid nuclei of two mating types fuse together, called karyogamy, to form a diploid nucleus.

3. This is followed by meiosis, which again reduces the number of chromosomes to the haploid number.

Sex Organs

The sex organs of fungi, if they are present, are called gametangia. They form differentiated sex cells called gametes or may contain instead one or more gamete nuclei. If the male and female gametangia are morphologically different, the male gametangium is called the antheridium and the female gametangium is called the oogonium.

Various Methods of Sexual Reproduction

1. Gametic Copulation - Fusion of naked gametes, one or both of which are motile.

2. Gamete-Gametangial Copulation - Two gametangia come into contact but do not fuse. The male nucleus migrates through a pore or fertilization tube into the female gametangium.

3. Gametangial Copulation - Two gametangia or their protoplasts fuse and give rise to a zygote that develops into a resting spore.

4. Somatic Copulation - Fusion of somatic or vegetative cells.

5. Spermatization - Union of a special male structure called a spermatium with a female receptive structure. The spermatium empties its contents into the latter during plasmogamy.

Sexual Spores

Sexual spores, which are produced by the fusion of two nuclei, occur less frequently, later, and in smaller numbers than do asexual spores. There are several types of sexual spores:

1. Ascospores - These single celled spores are produced in a sac called an ascus. There are usually eight ascospores in each ascus.

2. Basidiospores - These single celled spores are borne on a club shaped structure called a basidium.

3. Zygospores - Zygospores are large, thick walled spores. These are formed when the tips of two sexually compatible hyphae, or gametangia, of certain fungi fuse together.

4. Oospores - These are formed within a special female structure, the oogonium. Fertilization of the eggs, or oospheres, by male gametes formed in an antheridium gives rise to oospores. There are one or more oospheres in each oogonium.

IMPORTANCE OF FUNGI

1. As Saprophytes

Saprophytes decompose complex plant and animal remains, breaking them down into simpler chemical substances that are returned to the soil, thereby increasing its fertility. Thus they can be quite beneficial to humans. They are undesirable when they decompose timber, textiles, food, and other materials.

Saprophytic fungi are also important in industrial fermentations. For example, the brewing of beer, the making of wine, the leavening of dough and the production of antibiotics such as penicillin. The leavening of dough depends on fungal activity. The ripening of some cheeses also depends on fungal activity.

2. As Parasites

As parasites, when living in or on another organism, fungi cause diseases in plants, humans, and other animals. Although fungal diseases are less commonly encountered than bacterial or virus diseases in humans and other animals, they are of great importance in causing diseases of plants.

3. As a Tool for the Study of Biological Processes

The science of fungi is called mycology. Fungi have also become tools for the study of some biological processes. They find them highly suitable for the study of physiologist, biophysicist, geneticist, and biochemist.

DIFFERENCE BETWEEN FUNGI AND BACTERIA

Fungi are eukaryotic, larger in size, have true nucleus, have chitin in cell wall, reproduce by spores and budding, and grow slowly. Bacteria are prokaryotic, smaller in size, have no true nucleus, have peptidoglycan in cell wall, reproduce by binary fission, and grow rapidly. Fungi prefer acidic pH and high sugar. Bacteria prefer neutral pH and low sugar. This difference helps in laboratory to separate fungi from bacteria.

CONCLUSION

Fungi are eukaryotic, heterotrophic organisms without chlorophyll. Their body is made of hyphae and mycelium or single yeast cell. They reproduce by asexual spores like conidia and sporangiospores and by sexual spores like ascospores, basidiospores, zygospores and oospores. They are important as decomposers, in industry, in medicine and as pathogens. For BS Zoology students, it is essential to understand morphology and reproduction of fungi because it is asked in theory and practical exams.

FAQs for Students - Fungi

Q1: What are fungi and why are they called heterotrophic?

Ans: Fungi are eukaryotic organisms without chlorophyll. They are called heterotrophic because they cannot make their own food and they depend on organic matter from dead or living organisms for nutrition.

Q2: What is difference between hypha and mycelium?

Ans: Hypha is a single long filament or thread of a fungus. Mycelium is a complex or network of many hyphae together. Mycelium forms the main body of a mold.

Q3: What is difference between yeast and mold?

Ans: Yeast is unicellular fungus which reproduces mainly by budding. Mold is multicellular fungus which is made of hyphae and mycelium and reproduces by spores. Yeast colonies are smooth and creamy, mold colonies are filamentous and cottony.

Q4: What are asexual spores in fungi?

Ans: Asexual spores are produced without fusion of nuclei. Examples are sporangiospores, conidia, oidia, chlamydospores and blastospores. They are produced in large numbers for quick spread.

Q5: What are sexual spores in fungi?

Ans: Sexual spores are produced after fusion of two nuclei and meiosis. Examples are ascospores in ascus, basidiospores on basidium, zygospores after fusion of gametangia, and oospores in oogonium.

Q6: Why fungi grow better in acidic media?

Ans: Fungi can tolerate acidic pH around 5.6 and high sugar concentration. Bacteria are inhibited in acidic and high sugar media. So acidic media like Sabouraud agar is used to isolate fungi and to prevent bacterial contamination.

Also Read: Bacteriological Media - Types and Preparation

Written by Momina Shehzadi, BS Zoology Graduate from Pakistan.