Chapter 10: Fungi And Their Economic Importance
In the late 1850s, in the town of Lille in northern France, a distiller was in trouble. His vats of beetroot juice were meant to turn into alcohol. Instead, batch after batch was turning sour and worthless, and he could not say why. He asked a young chemist at the local Faculty of Sciences to look into it. The chemist was Louis Pasteur.
Pasteur did something the distiller had not thought to do. He took a drop from a vat that was working well and a drop from a vat that had gone sour, and he put both under a microscope.
The good vat was crowded with plump, rounded bodies, many of them with small buds swelling out of one side. The sour vat had almost none of these. In their place were far smaller rod-shaped specks, and the liquid around them was acid rather than alcohol.
That single comparison changed how people understood a craft they had practised for thousands of years. Bread, wine, beer and cheese had never been the work of chemistry alone. Something alive was doing the work — and if a different living thing got in, you got a different product.
The rounded budding bodies were yeast: tiny, one-celled fungi. This chapter is about them and their relatives, and about how much of what you eat, drink and take as medicine passes through the hands of a fungus first.
Diagram
Fig. 10.0-1 — What Pasteur Saw In The Two Vats Two circular microscope fields side by side, drawn as simple line art. Left field labelled ‘alcohol-producing vat’: several oval yeast cells, some with a small rounded bud attached at one side. Right field labelled ‘soured vat’: many much smaller rod-shaped bodies scattered across the field, drawn clearly smaller than the yeast cells. Label ‘yeast cell’, ‘bud’, ‘rod-shaped bacteria’. Add a scale note showing that both fields are at the same magnification.
10.1 Fungi — General Features
You have met fungi far more often than you may realise. The grey fluff on stale bread, the soft rot spreading through a forgotten orange, the umbrella-shaped growths that push out of damp soil after the first heavy rain — all of these are fungi.
Before we look at what fungi do for us, and to us, it helps to know what kind of living thing we are dealing with. That is what this section sets out.
10.1.1 What A Fungus Is Like
A fungus is a living thing that has no chlorophyll and takes its food ready-made from other organisms, living or dead. The plural is ‘fungi’. Chlorophyll is the green pigment that lets a plant trap sunlight and build its own food, and a fungus has none of it. This one absence decides almost everything else about how a fungus lives.
Look closely at a mushroom or a patch of mould and you will find no roots, no stem, no leaves and no flowers. The fungal body is not divided into such parts at all. It has nothing to hold up to the light, because it has nothing to do with light.
Made Of Cells With A True Nucleus
Fungal cells are eukaryotic — each cell has a proper nucleus, enclosed in its own membrane, with the cell’s genetic material packed inside it. Bacteria are not built this way. A bacterial cell has no membrane-bound nucleus, and its genetic material lies loose in the cytoplasm.
So although fungi and bacteria are often spoken of together as micro-organisms that spoil food and cause disease, they are not close relatives. A fungal cell is organised much more like the cells in your own body than like a bacterial cell.
Most fungi are made of many cells joined together. A few are not. Yeast, which you will study in section 10.3, is a single cell living on its own, and it is a fungus all the same.
Getting Food Ready-Made
Because it cannot manufacture food, every fungus must obtain it from something else. Nutrition of this kind is called heterotrophic nutrition — food is taken in ready-made rather than built from carbon dioxide and water.
Fungi do this in two main ways. A saprophyte feeds on dead and decaying matter, such as bread, fallen leaves, dung or leather. A parasite feeds on a living host, and the host is harmed in the process. The bread mould of the next section is a saprophyte; the fungus that causes ringworm on human skin is a parasite.
10.1.2 Why Fungi Are Not Plants
For a long time, fungi were bundled together with plants. They are fixed in one place, they spread over a surface as a plant spreads over soil, and they were easy to file alongside mosses and algae.
That arrangement was eventually dropped, and for a good reason. The whole point of being a plant is making your own food from sunlight. A fungus does the opposite: it takes food that some other organism made. Grouping the two together puts organisms with opposite ways of feeding into one box.
Under the five kingdom classification, which sorts all living things into five large groups, fungi are therefore given a kingdom of their own — Kingdom Fungi. Its members are set apart by having no chlorophyll and by feeding heterotrophically. The five kingdoms are set out in FC 10.1.
Flowchart
FC 10.1 — The Five Kingdoms All living organisms → Monera (single-celled, no true nucleus) / Protista (single-celled, true nucleus) / Fungi (no chlorophyll, heterotrophic) / Plantae (chlorophyll, make their own food) / Animalia (no chlorophyll, take in solid food)
The differences that separate fungi from green plants are drawn together in Table 10.1.
10.2 Moulds
The commonest fungi you are likely to meet are the moulds — fungi that spread as a fine, soft, often coloured growth over the surface of something else. They appear on food, on bread, on fruit, on leather bags and shoes, and on the bark of trees.
A mould does not look like much to the eye. Under a hand lens, though, a patch of mould turns out to be a dense tangle of fine threads, and every one of those threads is doing a job.
10.2.1 Rhizopus, The Bread Mould
The fungus most often studied as an example of a mould is Rhizopus, the common bread mould. The species usually found on bread is Rhizopus stolonifer.
Bread is not its only home. Rhizopus also grows on fruits, on animal dung and on leather goods. What it needs is a warm, humid climate and something organic to feed on. This is why bread keeps for days in a cool dry tin and spoils within two days in a damp kitchen during the monsoon.
The material a fungus grows on and feeds from is called its substratum. For the mould in the picture, the substratum is the slice of bread. Its structure is shown in Fig. 10.2-1.
Diagram
Fig. 10.2-1 — Rhizopus Growing On Bread Line drawing of a slice of bread, surface view, with a mould colony spreading across it. Show the tangled network of fine threads over and into the bread; show horizontal threads running along the surface and short branched threads plunging into the bread at intervals; show two or three upright stalks rising from the surface, each ending in a rounded swollen head. Label mycelium, hypha, stolon, rhizoids, sporangiophore, sporangium, substratum (bread).
10.2.2 Hyphae And Mycelium
Each fine thread in the tangle is called a hypha (plural, hyphae). A hypha is transparent, thread-like and only a few cells thick, and it grows by lengthening at its tip. The whole mass of hyphae taken together is the mycelium — the body of the fungus.
The mycelium is not a shapeless mat. Different hyphae in it are shaped for different jobs, and once you know which is which, the drawing in Fig. 10.2-1 reads easily.
Three Kinds Of Hyphae
Some hyphae run horizontally across the surface of the bread. These are the stolons, and they carry the colony sideways to fresh territory.
At intervals along a stolon, a tuft of short branched hyphae plunges down into the bread. These are the rhizoids. They anchor the mould to its substratum and are the threads through which food is taken in.
Other hyphae grow straight upward, away from the bread and into the air. These are the reproductive stalks, and section 10.2.4 follows what happens at their tips.
10.2.3 Feeding By Extracellular Digestion
A mould has no mouth, and a hypha is far too fine to take in a crumb of bread. Yet the bread beneath a mould patch clearly disappears, turning soft and discoloured. How is it being eaten?
The answer is that the digestion happens outside the fungus. The rhizoids secrete enzymes into the bread around them. These enzymes break down the starch and other complex food in the bread into simple, soluble substances. The hyphae then absorb those dissolved substances straight through their walls.
Digestion of this kind — carried out outside the body, on the substratum itself, before anything is taken in — is called extracellular digestion.
Notice how neatly this fits the life of a saprophyte. Your own digestion happens inside a gut, so you must first swallow the food. A mould cannot swallow anything, so it sends its enzymes out to the food instead. The result is the same: complex food is made simple and soluble before it is absorbed.
10.2.4 Reproduction In Bread Mould
Bread mould reproduces in two ways — asexually and sexually. The asexual method is by far the more common, and it is the one that puts mould on your bread.
Asexual Reproduction By Spores
Within a few days of the mycelium establishing itself, some hyphae stop spreading sideways and grow erect. Each such upright stalk is a sporangiophore — literally, a stalk that carries a sporangium.
The tip of each sporangiophore swells into a rounded case called a sporangium. Inside it, hundreds of tiny reproductive bodies are formed. Each of these is a spore, a single reproductive cell that can grow into a whole new fungus by itself. As the spores ripen, the sporangium darkens, which is why an old mould patch looks black and pin-headed. A mature sporangium and a bursting one are shown in Fig. 10.2-2.
Diagram
Fig. 10.2-2 — A Sporangium Before And After Bursting Two highly magnified line drawings side by side. Left: a mature sporangium — a rounded dark case at the tip of an upright stalk, cut open to show the mass of spores packed inside around a central dome. Right: the same sporangium after rupture, its wall split and spores dispersing outward. Label sporangiophore, sporangium wall, spores.
When ripe, the sporangium bursts and releases its spores. Being extremely light, the spores are carried away by air currents. Most land where nothing can grow. A few land on a suitable substratum, and if there is enough warmth and moisture there, each such spore germinates and grows into a new mycelium. The sequence is set out in FC 10.2.
Flowchart
FC 10.2 — Asexual Reproduction In Bread Mould Mycelium on bread → some hyphae grow erect as sporangiophores → tip of each swells into a sporangium → spores form inside and the sporangium darkens → sporangium bursts → spores scattered by wind → a spore lands on a warm, damp substratum → spore germinates → new mycelium (cycle returns to the start)
Sexual Reproduction
Bread mould also reproduces sexually, though far less often. Hyphae from two mycelia of different strains grow towards each other and their gametes fuse. The product of that fusion is a thick-walled zygospore, which can survive harsh conditions and later germinates to produce a new mycelium.
10.3 Yeast
Not every fungus builds a mycelium. Yeast is a fungus whose whole body is a single cell, and that one cell does everything — feeds, respires and reproduces.
Yeasts belong to the genus Saccharomyces, which includes several species. They are usually found as separate rounded or oval cells. Sometimes the cells stay joined after reproducing, forming short chains.
10.3.1 Occurrence Of Yeast
Yeast cells drift freely in the air around you. Given a suitable place to settle, they grow readily wherever there is sugar in solution.
You will find them on the skins of ripe grapes, in the nectar of flowers, in sugar cane juice, and in the sweet fluids that ooze from plants. Anything sugary and left standing is, sooner or later, colonised by yeast.
This is worth holding on to, because it explains something in section 10.5. Crushed grapes begin fermenting on their own, without anyone adding yeast to them. The yeast was already sitting on the fruit.
10.3.2 Structure Of A Yeast Cell
A yeast cell is ovoid — egg-shaped rather than perfectly round. It is bounded by a distinct cell wall, and inside the cell wall lies the cytoplasm.
A single well-defined nucleus sits within the cytoplasm, as it must in any eukaryote. The cytoplasm also holds one or more vacuoles, along with stored food in the form of glycogen granules and fat. A yeast cell is shown in Fig. 10.3-1.
Diagram
Fig. 10.3-1 — A Single Yeast Cell Diagrammatic line drawing of one ovoid yeast cell, highly magnified. Label cell wall, cytoplasm, nucleus, vacuole, fat vacuole, glycogen granule. Labels on straight, non-crossing lead lines.
Reproduction By Budding
Yeast multiplies by budding. A small bulge appears on the surface of the parent cell and slowly grows. The nucleus of the parent divides, and one of the daughter nuclei passes into the bulge.
The bud then either breaks away as an independent yeast cell, or stays attached and begins budding itself. When several remain attached, the result is the short branching chain of cells shown in Fig. 10.3-2.
Diagram
Fig. 10.3-2 — Budding In Yeast Three stages drawn in a row, highly magnified. Stage 1: a single ovoid cell with a small bulge on one side. Stage 2: the bulge enlarged, with the parent nucleus divided and one daughter nucleus passing into it. Stage 3: a short chain of four cells still attached to one another, each with its own nucleus. Label parent cell, bud, nucleus, budding chain.
10.3.3 Nutrition In Yeast
Yeast lives on sugar. A simple sugar such as glucose is small enough to pass straight through the cell wall and membrane, so the cell can absorb it directly.
Cane sugar is not so simple. Sucrose, the sugar in cane juice, is a larger molecule made of two simple sugars joined together. Before it can be absorbed, one of the yeast’s enzymes must split it into simple sugars.
So a yeast cell in cane juice does a little digestion first and absorbs afterwards — the same order you met with the bread mould, for the same reason.
10.3.4 Respiration In Yeast
All living cells release energy from food by respiration. What is unusual about yeast is that it can do this without oxygen at all.
Respiration carried out in the absence of oxygen is called anaerobic respiration. In yeast, a molecule of glucose is broken down into ethanol and carbon dioxide, and a small amount of energy is released:
glucose → ethanol + carbon dioxide + energy
\[C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2 + 2ATP\]
The ATP in that equation stands for adenosine triphosphate, the substance in which a cell stores the energy it has just released for immediate use. Two molecules of it is a small yield. Respiration using oxygen releases far more energy from the same glucose molecule, which is why anaerobic respiration is, for most organisms, a poor second best.
For yeast it is enough. And the two waste products it leaves behind — ethanol and carbon dioxide — are exactly the two substances that make yeast valuable to us.
10.3.5 Economic Importance Of Yeast
Yeast matters to us above all for one property: alcoholic fermentation, the breakdown of sugar into ethanol and carbon dioxide by yeast in the absence of oxygen. Every use in section 10.5 rests on it.
The ethanol produced is of great commercial value. Where it is made for beverages, its quality is not fixed. It depends both on the material being fermented — grape, barley, cane — and on the particular kind of yeast employed.
10.4 Useful And Harmful Fungi
Fungi affect human life in two opposite ways, and often the same group of fungi does both. Some give us food, flavour and medicine. Others destroy our crops and stores, and a few cause disease.
It is worth being careful with the words ‘useful’ and ‘harmful’ here. They describe a fungus from our point of view, not its own. A mould rotting an orange is doing exactly what a mould does; it is only harmful because the orange was ours.
10.4.1 Useful Fungi
Several species of Penicillium are used in processing flavoured cheeses. The mould is introduced into the curd, grows through it as it ripens, and gives the finished cheese its characteristic flavour and its blue or greenish veining. How cheese is made is taken up in section 10.5.4.
The same genus gave medicine something far more important. One of its species, Penicillium chrysogenum, is the mould from which the antibiotic penicillin is manufactured on a large scale.
Diagram
Fig. 10.4-1 — Two Common Moulds That Grow On Fruit Two highly magnified line drawings side by side, each showing an upright stalk rising from a branched hypha. Left, Penicillium: the stalk divides near its top into several short branches, each ending in a chain of rounded spores, so the whole head looks like a fine brush. Right, Aspergillus: the stalk ends in a rounded swollen head from which chains of spores radiate outward in all directions, like a sunburst. Label Penicillium, Aspergillus, stalk, spore chains, swollen head.
The two moulds are told apart by the shape of their spore-bearing heads, as Table 10.2 sets out.
10.4.2 Antibiotics — The First Wonder Drug
An antibiotic is a substance, produced by one micro-organism, that kills or stops the growth of other micro-organisms. Antibiotics are the medicines we rely on to treat bacterial infections.
The very first of them was penicillin, and it came not from a laboratory chemist but from a mould.
Penicillin is why an infected wound that would once have been fatal is now treated with a course of tablets. It was the first of a long line of antibiotics, several of which are still obtained from fungi.
10.4.3 Harmful Fungi
Fungi are also responsible for a great deal of damage. Their chief offence is spoiling food. A saprophytic fungus does not distinguish between a fallen log and a sack of grain, and stored food is a rich, undefended substratum.
Penicillium and Aspergillus, the two moulds in Fig. 10.4-1, are among the commonest food spoilers. Both grow readily on oranges, lemons and other citrus fruits, softening the flesh beneath a spreading patch of green. Fig. 10.4-2 shows the damage.
Diagram
Fig. 10.4-2 — Penicillium Growing On An Orange Photograph of an orange with a circular patch of mould on its skin: a soft sunken area with a white outer ring of young mould and a powdery blue-green centre where the spores have matured. Caption should point out the white advancing edge and the green sporing centre.
Fungi also attack living plants. Many species cause serious damage to forest trees, breaking down the wood and weakening the trunk.
A smaller number are parasites of animals, including human beings.
Finally, a warning that follows from all of this. The greenish fungus you see on an overripe orange is indeed a species of Penicillium. It is not medicine, it has not been purified, and it may carry substances that make you ill. No mould growing on spoiled food should ever be eaten or applied to a wound.
10.5 Fungi In Industry
Fungi are put to work on an enormous scale. Some of the practices that use them are thousands of years old and were perfected long before anyone knew a fungus was involved. Others are recent and highly controlled.
Four of them are taken up here: the making of wine, of bread, and of cheese, and the process that underlies the first two.
10.5.1 Fermentation
Everything in this section depends on one process. Fermentation is the breaking down of carbohydrates by micro-organisms, without oxygen, into simpler products.
The micro-organisms are yeast and certain bacteria. The carbohydrate is usually a sugar. The product depends on which micro-organism is at work: yeast yields ethanol, while some bacteria yield lactic acid instead.
Note the condition at the end. Without oxygen is not an incidental detail; it is what makes the process fermentation rather than ordinary respiration.
10.5.2 Wine And Other Alcoholic Beverages
Wine, beer and similar drinks are all products of fermentation. What differs between them is the raw material. Wine is made from grape juice; beer is made from barley maltose. In both, the micro-organism employed is yeast, Saccharomyces cerevisiae.
The reaction is the one you met in section 10.3.4, with the sugar of the fruit as the starting material:
glucose → ethanol + carbon dioxide
\[C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2\]
Grapes are crushed and the juice is fermented in large vessels. Yeast cells may be added, or the process may begin on its own, because yeast is already present on the grape skins.
Fermentation does not run on for ever. Ethanol builds up in the vessel, and at a concentration of about 12 per cent it begins to kill the yeast cells themselves. The yeast is stopped by its own waste product, and what remains in the vessel is wine. The steps are set out in FC 10.3.
Flowchart
FC 10.3 — Making Wine Ripe grapes → crushed to release juice → juice run into large fermentation vessels → yeast added, or yeast already present on the skins → sugar fermented without oxygen → ethanol and carbon dioxide produced → ethanol reaches about 12 per cent → yeast cells begin to die and fermentation stops → wine
Wine is not pure alcohol. It contains water, ethanol, dissolved carbon dioxide, some remaining sugar and a number of other substances in small amounts, and the proportions vary from wine to wine.
Spirits such as whisky and gin have a much higher alcohol content than wine or beer. They are not fermented further — no yeast could survive it. They are obtained by distilling the fermented mixture, which separates the alcohol from most of the water and concentrates it.
10.5.3 Baking
Bread begins as a dough — flour, whether atta or maida, mixed with some fat, salt and water. Left to itself, that dough would bake into a dense, heavy slab.
Yeast changes it. Enzymes convert some of the starch in the flour into sugar, and the yeast ferments that sugar. The carbon dioxide released cannot escape from the sticky dough, so it collects as thousands of tiny bubbles trapped throughout it.
As the bubbles grow, the whole dough swells. This rising of the dough is called leavening, and it can take the dough to about three times its original volume.
Baking then does two things. The heat makes the trapped gas expand further, pushing the bread up and open, and it sets the dough around those spaces. The ethanol produced along with the carbon dioxide evaporates in the oven. What is left is the light, spongy texture of a well-made loaf.
10.5.4 Cheese Processing
Cheese is made from milk, and people have been making it for centuries. It is a concentrated food: rich in fat and protein, and a good source of calcium, phosphorus and vitamin A, with some vitamin B as well.
Its manufacture has four major steps, shown in FC 10.4.
Flowchart
FC 10.4 — Making Cheese Milk → curdling by lactic acid bacteria (Lactobacillus) → curd separated from whey → curd processed to remove moisture (cottage cheese) → salting → ripening at controlled temperature and humidity → cheese with its characteristic flavour
Curdling And Separation
Lactic acid bacteria, Lactobacillus, are added to the milk. They ferment the milk sugar into lactic acid, and the acid makes the milk proteins clot into a soft solid called curd. The watery liquid left behind after curdling is the whey, and the curd is separated from it.
Processing And Salting
The curd is then processed to drive out more moisture. At this stage it is known as cottage cheese.
Salting follows. Salt removes still more moisture from the curd, and it also prevents undesirable micro-organisms from growing in the cheese while it matures.
Ripening
Finally the cheese is ripened. It is kept at a suitable temperature and humidity, sometimes for months.
During this time the micro-organisms in it go on working. Those added at the curdling stage, along with any mould introduced deliberately, break down fats and proteins in the curd and give each variety of cheese its particular flavour. A cheese takes its character from which micro-organisms were used and how long they were allowed to work.
10.6 Mushroom Cultivation
Mushrooms and toadstools are a familiar sight in the warm, rainy season, pushing up overnight from damp soil, dung and rotting wood. Most of them are wild, and a great many species grow in nature without any human help.
A few kinds are grown deliberately, indoors, as a crop. That is the subject of this section.
10.6.1 Mushrooms In Nature
Mushrooms come in a wide range of shapes and sizes. Many have the familiar form of a stalk carrying a cap; in some varieties the stalk is absent altogether and the cap grows directly from the substratum.
It helps to be clear about what a mushroom actually is. The mushroom you see is not the whole fungus. It is the reproductive part — the spore-producing body — pushed up into the air by a mycelium that is spread through the soil or the wood below. Spores are made on the underside of the cap, and being carried by air currents they are far better dispersed from a raised cap than from ground level.
Diagram
Fig. 10.6-1 — Structure Of A Mushroom Line drawing of a whole mushroom, cut vertically down one side so that the interior is visible. Show the mycelium spreading through the substratum below, a stalk rising from it, and a rounded cap. Under the cap show the closely packed gills radiating from the stalk to the rim. Label mycelium, substratum, stalk, cap, gills (spore-bearing surface).
Not All Mushrooms Are Edible
This is the point on which everything else in the section rests. Most wild mushrooms are mildly or severely poisonous, and only a small number are fit to eat.
The edible kinds have been gathered and eaten by people since ancient times. Agaricus bisporus, the white button mushroom, is the commonest edible mushroom, and it is eaten practically all over the world. Two other edible fungi, the morel and the puffball, are shown alongside it in Fig. 10.6-2.
Diagram
Fig. 10.6-2 — Three Edible Fungi Three photographs in a row. First, cultivated white button mushrooms — a dense bed of pale rounded caps on short stalks. Second, a morel — an upright fungus with a tall, deeply pitted and ridged honeycomb-like cap on a pale stalk. Third, a puffball — a rounded body releasing a cloud of spores from an opening at its top. Caption each photograph with the common name.
10.6.2 Kinds Of Mushrooms Cultivated In India
Three kinds of mushroom are widely cultivated in India and abroad:
- White button mushroom (Agaricus bisporus)
- Paddy straw mushroom (Volvariella)
- Oyster mushroom (Pleurotus)
Mushroom cultivation is carried out indoors, so very little land is needed for it. This is a real advantage in a country where farmland is limited.
The mushrooms are grown on a substrate, or compost — decayed organic matter used for fertilising land — prepared from various agricultural wastes. Straw and other waste that would otherwise be burnt or discarded is recycled into a crop.
10.6.4 Preservation Of Mushrooms
Mushrooms are highly perishable. Freshly harvested, they hold a great deal of water and spoil within a day or two, so preservation is part of the crop, not an afterthought.
Their shelf life is increased by a variety of processes:
- vacuum cooling
- giving gamma radiation and storing at 15 °C
- freeze drying — the sublimation of ice from frozen mushrooms at a reduced pressure — in a solution of citric acid, ascorbic acid and brine
Each of these works by attacking the same two conditions that let spoilage organisms grow: warmth and water. Cooling removes the warmth; freeze drying removes the water directly, turning the ice in the frozen mushroom into vapour without letting it melt.
10.6.5 Nutritive Value Of Mushrooms
Mushrooms are an excellent source of vitamins and minerals. They contain a good amount of niacin, pantothenic acid and biotin, all of them B-group vitamins.
An unusual point in their favour is that these vitamins are well retained during cooking, canning and dehydration. Many foods lose much of their vitamin content in processing; mushrooms lose comparatively little.
10.7 Summary
- A fungus is a living organism that has no chlorophyll and cannot make its own food.
- The fungal body is not divided into root, stem, leaves and flowers.
- Fungal cells are eukaryotic — each has a true nucleus enclosed in a membrane. Bacterial cells do not.
- Most fungi are multicellular, but some, such as yeast, consist of a single cell.
- Fungi are heterotrophic. A saprophyte feeds on dead and decaying matter; a parasite feeds on a living host and harms it.
- In the five kingdom classification, fungi are placed in a kingdom of their own, separate from plants, because they lack chlorophyll and feed heterotrophically.
- Moulds are the commonest fungi. Rhizopus, the bread mould, grows on bread, fruit, animal dung and leather goods in warm, humid conditions.
- The body of a mould is a mycelium — a mass of fine transparent threads called hyphae.
- Stolons run horizontally over the substratum, rhizoids anchor the mould and absorb food, and erect hyphae carry the reproductive structures.
- Moulds feed by extracellular digestion: enzymes are secreted onto the substratum, and the digested products are then absorbed.
- Bread mould reproduces asexually by spores. A sporangiophore grows erect, its tip swells into a sporangium, the sporangium darkens and bursts, and wind-scattered spores germinate on a warm, moist substratum to form new mycelia.
- It also reproduces sexually. Gametes from two mycelia of different strains fuse to form a thick-walled zygospore.
- Yeast is a one-celled fungus of the genus Saccharomyces, found in air and in all kinds of sugary solutions.
- A yeast cell is ovoid, with a cell wall, cytoplasm, a nucleus, one or more vacuoles, and stored glycogen and fat. It multiplies by budding.
- Yeast absorbs simple sugar directly, but sucrose must first be split into simple sugars by an enzyme.
- Yeast respires anaerobically, breaking glucose down into ethanol and carbon dioxide with a small release of energy: \(C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2 + 2ATP\).
- Fermentation is the breakdown of carbohydrates by yeast and bacteria, in the absence of oxygen, into simpler products — ethanol or lactic acid.
- Wine is made by fermenting grape juice with yeast, Saccharomyces cerevisiae; beer is made from barley maltose. Fermentation stops at about 12 per cent ethanol, which kills the yeast. Spirits are obtained by distilling the fermented mixture.
- In baking, yeast ferments sugar formed from the flour’s starch. Trapped carbon dioxide leavens the dough to about three times its volume, and baking expands the bubbles to give bread its spongy texture.
- Cheese is made from milk in four steps: curdling by lactic acid bacteria (Lactobacillus) and separation of curd from whey, processing to cottage cheese, salting, and ripening, during which micro-organisms give each cheese its flavour.
- Several species of Penicillium are used in processing flavoured cheeses, and Penicillium chrysogenum is the source of the antibiotic penicillin, the first antibiotic to be discovered.
- Penicillium and Aspergillus spoil oranges, lemons and other citrus fruit; other fungi damage forest trees; and some are parasites that cause disease.
- A mushroom is the spore-producing body of a fungus whose mycelium lies in the substratum. Most wild mushrooms are poisonous, and only some are edible.
- Three kinds are widely cultivated: white button mushroom (Agaricus bisporus), paddy straw mushroom (Volvariella) and oyster mushroom (Pleurotus).
- Cultivation is done indoors on compost made from agricultural wastes, so very little land is required.
- The white button mushroom is grown in five steps: composting, spawning, casing, cropping and harvesting, and preservation.
- Mushrooms are highly perishable and are preserved by vacuum cooling, gamma radiation with storage at 15 °C, and freeze drying.
- Mushrooms are rich in vitamins and minerals, particularly niacin, pantothenic acid and biotin, and these vitamins survive cooking, canning and dehydration well.
Flowchart
FC 10.6 — Fungi And Human Life At A Glance Fungi (no chlorophyll, heterotrophic) → branch 1, useful: yeast → fermentation → wine and beer, and leavened bread / Penicillium → flavoured cheeses and penicillin / cultivated species → mushrooms as food; branch 2, harmful: saprophytes → spoilage of food and damage to forest trees / parasites → diseases of plants and animals
10.8 Key Terms
- Alcoholic fermentation — the breakdown of sugar by yeast, without oxygen, into ethanol and carbon dioxide.
- Anaerobic respiration — the release of energy from food by a cell in the absence of oxygen.
- Antibiotic — a substance produced by one micro-organism that kills or stops the growth of other micro-organisms.
- Budding — reproduction in which a small outgrowth of a parent cell receives a nucleus and grows into a new individual.
- Compost — decayed organic matter used for fertilising land, and used as the bed on which mushrooms are grown.
- Eukaryotic — describing a cell that has a true nucleus enclosed in a membrane.
- Extracellular digestion — the breakdown of food outside the body by secreted enzymes, followed by absorption of the digested products.
- Fermentation — the breakdown of carbohydrates by micro-organisms, in the absence of oxygen, into simpler products such as ethanol or lactic acid.
- Five kingdom classification — the scheme that sorts all living organisms into five kingdoms, one of which is Fungi.
- Fungus — an organism without chlorophyll that takes its food ready-made from living or dead matter.
- Heterotrophic nutrition — a mode of nutrition in which food is taken ready-made instead of being made by the organism itself.
- Hypha — one of the fine transparent threads that make up the body of a fungus.
- Leavening — the rising of dough caused by carbon dioxide trapped in it during fermentation.
- Mould — a fungus that grows as a soft, often coloured surface growth on food and other organic matter.
- Mycelium — the whole mass of hyphae forming the body of a fungus.
- Parasite — an organism that takes its food from a living host and harms it in doing so.
- Penicillin — the first antibiotic to be discovered, obtained from the mould Penicillium.
- Rhizoid — a short branched hypha that anchors a mould to its substratum and absorbs food from it.
- Saprophyte — an organism that takes its food from dead and decaying matter.
- Spawn — mycelium of a selected mushroom, introduced into compost to start the crop; also called ‘mushroom seed’.
- Sporangiophore — an erect hypha that bears a sporangium at its tip.
- Sporangium — the swollen case at the tip of a sporangiophore, inside which spores are formed.
- Spore — a single reproductive cell that can grow into a new fungus without fusing with another cell.
- Stolon — a hypha that runs horizontally over the substratum and spreads the colony sideways.
- Substratum — the material on which a fungus grows and from which it obtains its food.
- Yeast — a one-celled fungus of the genus Saccharomyces that ferments sugar to ethanol and carbon dioxide.
- Zygospore — the thick-walled resting structure formed when gametes from two mycelia of different strains fuse.
10.9 Points To Ponder
Yeast stops its own fermentation by poisoning itself with the ethanol it produces. What does this suggest about the risks of releasing waste products into the very place you live in? Where else in nature might the same problem arise?
A fungus digests its food outside its body; you digest yours inside a gut. Each arrangement suits its owner. What can an animal do, because its digestion is internal, that a mould cannot?
Almost every method of preserving food — drying, salting, refrigerating, vacuum cooling — removes either water or warmth. Why should these two conditions matter so much to fungi in particular?
A mushroom releases spores from the underside of a raised cap; a bread mould releases them from the tip of an erect stalk. Both fungi have pushed their spores up into the air. What does that shared solution tell you about the problem they both face?
Bread, wine and cheese were being made for thousands of years before anyone knew that micro-organisms existed. If the process worked without the explanation, what did an explanation actually add?
Fungi break down dead wood, fallen leaves, dung and leather. Suppose every fungus on Earth disappeared tomorrow. Trace what would begin to accumulate, and what would stop being returned to the soil.
10.10 Objective Questions
- Choose the correct option:
- The mass of hyphae that forms the body of a mould is called: (1 mark)
- sporangium
- mycelium
- stolon
- spawn
- Fungi are placed in a kingdom separate from plants mainly because they: (1 mark)
- are fixed in one place
- have no true nucleus
- have no chlorophyll and feed heterotrophically
- reproduce by spores
- The hyphae that anchor a bread mould and absorb food from the bread are the: (1 mark)
- stolons
- rhizoids
- sporangiophores
- gills
- Fermentation of grape juice stops when the ethanol concentration reaches about: (1 mark)
- 2 per cent
- 6 per cent
- 12 per cent
- 25 per cent
- In cheese-making, the curd is separated from the: (1 mark)
- whey
- brine
- compost
- spawn
- The ‘mushroom seed’ introduced into compost actually consists of: (1 mark)
- spores of the mushroom
- mycelium of the selected mushroom
- dried pin heads
- lactic acid bacteria
- The mass of hyphae that forms the body of a mould is called: (1 mark)
- For each pair below, choose from: (a) both A and R are true and R is the correct explanation of A; (b) both A and R are true but R is not the correct explanation of A; (c) A is true but R is false; (d) A is false but R is true. (3 marks)
- Assertion (A): A mould can feed on bread without taking any part of it inside its body. Reason (R): Rhizoids secrete enzymes onto the substratum and the hyphae absorb the digested products.
- Assertion (A): Mushrooms can be cultivated indoors, in the dark. Reason (R): Mushrooms trap light using a pigment other than chlorophyll.
- Assertion (A): Dough rises before it is baked. Reason (R): Yeast ferments sugar and releases carbon dioxide, which is trapped in the dough.
- Match each item in column A with the correct item in column B. (5 marks)
| Column A | Column B |
|---|---|
| a. Volvariella | i. lactic acid bacterium used in curdling milk |
| b. Lactobacillus | ii. source of the antibiotic penicillin |
| c. Saccharomyces cerevisiae | iii. paddy straw mushroom |
| d. Penicillium chrysogenum | iv. common bread mould |
| e. Rhizopus stolonifer | v. yeast used in making wine |
- Complete each sentence with one suitable word. (5 marks)
- The material on which a fungus grows and feeds is called its ______.
- The swollen case at the tip of an erect hypha, in which spores are formed, is the ______.
- Yeast multiplies by a process called ______.
- Spreading a thin layer of soil over the compost in mushroom cultivation is the step called ______.
- The rising of dough due to trapped carbon dioxide is known as ______.
- State whether each statement is true or false. (4 marks)
- All fungi are multicellular.
- Yeast can respire in the absence of oxygen.
- The greenish mould growing on a rotting orange can be used as penicillin.
- Freeze drying removes water from mushrooms without letting the ice melt.
- Name the following. (5 marks)
- The horizontal hypha that spreads a mould colony sideways over its substratum.
- The thick-walled structure formed when gametes from two mycelia of different strains fuse.
- The stage in cheese-making at which the processed curd is called cottage cheese.
- The genus of mould whose spore-bearing stalk ends in a rounded swollen head.
- The scientific name of the white button mushroom.
- Pick the odd one out in each group and give a reason for your choice. (3 marks)
- Stolon, rhizoid, sporangiophore, whey
- Composting, spawning, casing, distillation
- Agaricus, Volvariella, Pleurotus, Aspergillus
10.11 Check Your Concepts
Define extracellular digestion. (2 marks)
State two conditions a Rhizopus spore needs before it can germinate on a slice of bread. (2 marks)
Name the two products formed when yeast ferments glucose, and state which of them makes bread rise. (2 marks)
Give a reason for each of the following: (3 marks)
- Bread kept in a damp cupboard spoils faster than bread kept in a dry tin.
- Salt is added to the curd during cheese-making.
- Mushroom cultivation needs very little land.
Differentiate between the following pairs, stating the basis of difference in each case: (3 marks)
- Saprophyte and parasite
- Sporangium and sporangiophore
- Hypha and mycelium
Why is casing described as a very important step in growing the white button mushroom? State any three things it does. (3 marks)
Explain, with the help of a word equation, what happens inside a yeast cell during anaerobic respiration, and state why the energy released is small. (4 marks)
Describe the four major steps in the manufacture of cheese, naming the micro-organism used at the first step. (4 marks)
Describe asexual reproduction in bread mould, from the growth of the erect hypha to the formation of a new mycelium. (5 marks)
‘A fungus is not a plant.’ Justify this statement with four points of difference between fungi and green plants, and state one way in which fungi are nevertheless useful to human beings. (5 marks)
10.12 Figure-Based Questions
- Study Fig. 10.2-1. (4 marks)
- Name the fungus shown and the substratum on which it is growing.
- Identify the part that runs horizontally over the surface of the bread.
- State one function of the branched threads that penetrate the bread.
- Name the part that grows upright, away from the substratum.
- Study Fig. 10.2-2. (3 marks)
- Name the process that the two drawings, taken together, represent.
- Name the structures released by the ruptured case.
- State how these structures reach a fresh substratum.
- Study Fig. 10.3-1 and Fig. 10.3-2. (4 marks)
- Name the organism shown, and state whether it is unicellular or multicellular.
- Name any two structures visible inside the cell in Fig. 10.3-1.
- Name the method of reproduction shown in Fig. 10.3-2.
- Redraw the first stage of Fig. 10.3-2 and label the parent cell, the bud and the nucleus, using straight, non-crossing lead lines.
- Study Fig. 10.4-1. (3 marks)
- Identify the two moulds shown, using the shape of their spore-bearing heads.
- State one difference between the two heads as drawn.
- Name one fruit on which both these moulds are commonly seen.
- Study Fig. 10.6-1. (4 marks)
- Name the part of the fungus that lies within the substratum.
- Name the closely packed structures on the underside of the cap.
- State the function of these structures.
- Redraw the mushroom and label the cap, the stalk and the mycelium, using straight, non-crossing lead lines.