Living Beings and Their Structure

Are all the cells in all living things the same? Do they all do the same job? No: a nerve cell, a muscle cell and a leaf cell look different and work differently. But they share a basic plan.

The cell is the basic unit of life. The body of every organism, whether microscopic or gigantic, is made of one or many cells. That makes the cell the structural unit of life: the building block.

The cell is also where life’s work happens. Digestion, respiration, reproduction, transport of materials and photosynthesis all take place inside cells, or are done by cells, through chemical reactions and through movements such as the contraction and relaxation of muscle cells. That makes the cell the functional unit of life as well. So the cell is both the structural and functional unit of life.

The branch of biology that studies cells is called cytology (from the Greek kytos, a hollow vessel or cell).

3.1.1 Shape and size of cells

Cells come in many shapes, and a cell’s shape usually suits its job.

• Animal cells are generally irregular in shape. They may be oval, round (spherical), rod-like, long, cube-shaped or column-shaped.
• Nerve cells are thin and long, like threads, with branches at the ends: good for carrying messages over long distances. One nerve cell can run from your spine to your big toe, about a meter.
• Muscle cells are mostly spindle-shaped: long, fat in the middle and pointed at both ends. They pull by getting shorter.
• Plant cells are usually many-sided, often six-sided (hexagonal), and fit together like bricks in a wall; in an onion peel they are long and six-sided. Their stiff walls keep that shape.
• Bacteria have four main shapes: spherical (round), rod-like, filamentous (in long threads) and spiral.

Most cells are far too small to see with the naked eye. But some are big:

• The smallest known cells are those of Mycoplasma, a kind of bacterium, only about 0.2 micrometers (0.0002 mm) across.
• The biggest cell is the egg cell of an ostrich. The whole egg is about 15–18 cm long; the cell itself is its yolk (see below).

To get a feel for how small cells are, zoom in step by step.

Structure of a cell

A cell has several parts. To make them easy to study, we can put them into four groups:

1. the cell covering (cell wall and cell membrane)
2. the cytoplasm
3. the cell components (organelles and inclusions)
4. the nucleus

You can see real cells yourself, in an onion and in your own mouth.

Parts of a cell

Now the four groups, one at a time.

1. The covering of the cell

The covering is the outermost layer of a cell. There are two kinds: the cell wall and the cell membrane.

The cell wall is found only in the cells of plants, fungi and bacteria. Animal cells and most protozoa (single-celled animal-like organisms) have none. It is tough, rigid, a little elastic, and non-living. In plants it is made mainly of cellulose, which is also what cotton and paper are made of.

The cell membrane, also called the plasma membrane, is found in every cell. In animals and most protozoa it is the outermost layer of the cell. In plants, fungi and bacteria it lies just inside the cell wall. It is much thinner, smoother and more elastic than the wall, and it is living.

Functions of the cell wall and the cell membrane:

1. The cell wall gives support and a fixed shape to plant, bacterial and fungal cells.
2. The cell wall protects the cell, for example from bursting and from attack by some germs.
3. In the root hairs of plants, the cell wall lets water and dissolved minerals from the soil pass through to the membrane, which takes them in.
4. The cell membrane controls what goes into and out of the cell: water, food and oxygen in, wastes out. It lets some substances through and stops others, so it is called semi-permeable (selectively permeable).
5. In single-celled protozoa such as Amoeba, the cell membrane pushes out pseudopodia (“false feet”) to move and to catch food, helps get rid of wastes, and protects the cell from its surroundings.

2. Cytoplasm

The cytoplasm is a clear, sticky, jelly-like fluid that fills the cell between the cell membrane and the nucleus. It is mostly water, with dissolved salts, minerals, sugars, proteins, vitamins and more. The cell’s components, such as ribosomes, plastids, mitochondria, Golgi bodies, endoplasmic reticulum and vacuoles, float in it.

Functions of the cytoplasm:

1. Most of the cell’s chemical reactions, its metabolism, take place in the cytoplasm.
2. It keeps the right balance of water and minerals in the cell.
3. It holds the cell components (mitochondria, Golgi bodies, plastids and others) and lets materials move between them.

3. Cell components

The working parts inside a cell are called cell components. There are two types: cell organelles and cell inclusions.

• Cell organelles (“little organs”) are the living parts. Each one does a particular job that life needs, just as the organs of your body do. Most organelles are found in both plant and animal cells; a few only in one.
• Cell inclusions are non-living substances in the cytoplasm: droplets of water, crystals of salts and minerals, stored food such as starch grains and fat droplets, pigments, and wastes. They are raw materials, stores of energy and leftovers of the organelles’ work.

Here are the main organelles, one by one.

Mitochondria (singular: mitochondrion)

A cell has many mitochondria, from a few to thousands. They are rod-shaped or thread-like, with a double membrane: a smooth outer one and a highly folded inner one. The folds give a huge surface where cellular respiration happens: glucose is broken down with oxygen, giving carbon dioxide and water and releasing energy.

$$\text{glucose} + \text{oxygen} \longrightarrow \text{carbon dioxide} + \text{water} + \text{energy}$$

Part of this energy is stored in molecules of ATP (adenosine triphosphate), which carry it to every part of the cell that needs it; the rest escapes as heat, which keeps you warm. The carbon dioxide and water are passed out of the cell. Because mitochondria release the energy that every other part of the cell runs on, they are called the power-houses of the cell. Without them, the cell can’t survive.

Functions:

1. They release energy from food (glucose) by cellular respiration, and store it in ATP.
2. They contain a little DNA of their own, which is passed on to the offspring (in humans, only from the mother).

Plastids

Plastids are organelles found only in plants, algae and a few protists such as Euglena. Many are colored. By their color, there are three types:

A. Chloroplasts are the green plastids. They contain the green pigment chlorophyll. They are usually round or oval, but in the alga Spirogyra they are long spiral ribbons. Each has a double membrane, and inside, stacks of disc-like membranes (grana) holding the chlorophyll, in a fluid called the stroma.

Functions of chloroplasts:

1. They make plants green.
2. In sunlight they carry out photosynthesis: they make carbohydrates (glucose, then starch) from carbon dioxide and water. Carbohydrates are the basic food of almost all living things, so green plants are called the producers of an ecosystem.
3. They also help make fatty acids and amino acids (the building blocks of fats and proteins).
4. Like mitochondria, they contain their own DNA, which is passed on to the offspring.

B. Chromoplasts are the colored plastids other than green: red, orange and yellow. They are found in the colored parts of plants: flowers, ripe fruits, some seeds, carrots and some leaves.

Functions of chromoplasts:

1. They make flowers colorful, and the colors attract insects and birds, which carry pollen: pollination.
2. They color ripe fruits and seeds, which attract birds and other animals that eat them and scatter the seeds: seed dispersal.

C. Leucoplasts are colorless (white) plastids, usually oval. They are found in the parts of a plant that get no light: roots, underground stems, the white flesh of fruits, and seeds (their cotyledons or endosperm).

Function of leucoplasts: they store food, such as starch (in a potato or a grain of rice), proteins and fats (oils), for later use.

Ribosomes

Ribosomes are the smallest organelles: tiny grains made of RNA and protein, with no membrane around them. Some float free in the cytoplasm; many are stuck on the surface of the endoplasmic reticulum. Mitochondria and chloroplasts have small ribosomes of their own.

Functions:

1. They join amino acids together to make proteins, following instructions copied from the DNA in the nucleus. So they are called the protein factories of the cell.
2. This is how a cell’s genes are put to work, or expressed: a gene in the DNA is copied into a message (RNA), and the ribosome reads the message and builds the protein.

Endoplasmic reticulum (ER)

The endoplasmic reticulum, or ER for short, is a network of folded membranes forming narrow canals and flat sacs, running from the nucleus out towards the cell membrane. Where its outer surface is covered with ribosomes, it looks grainy and is called rough ER; where it has no ribosomes, it is smooth ER.

Functions:

1. It forms a framework that supports the cell, so it is sometimes called the cell’s skeleton (endoskeleton).
2. It contains enzymes that help in the cell’s metabolism: rough ER helps make proteins, smooth ER makes fats (lipids) and breaks down some poisons.
3. It makes new membranes, which the cell needs when it grows and divides, and it helps form the Golgi body.
4. It is the cell’s transport system: materials move through its canals from one part of the cell to another.

Golgi body

The Golgi body was discovered by the Italian scientist Camillo Golgi in 1898 and is named after him. It is a stack of curved, flattened, membrane-bound sacs called cisternae (like a pile of puris), with tiny bubbles called vesicles budding off its edges. It is also called the Golgi apparatus or Golgi complex.

Functions:

1. It sorts, packs and sends out the substances a cell makes. Gland cells use it to secrete mucus, saliva, enzymes, hormones and, in plants, gums. So it is called the cell’s post office.
2. During cell division it helps make the new cell membrane (in animal cells) and the new cell plate, which becomes the wall between the two new cells (in plant cells).
3. It helps in transport inside the cell and in sorting proteins to where they are needed.

Lysosomes

A lysosome is a very small, round organelle with a single membrane, full of digestive enzymes. Lysosomes are found in most animal cells, and in large numbers in white blood cells and in the cells of the liver, kidneys, pancreas and spleen. (Plant cells do this kind of digesting mainly in their vacuole.)

Functions:

1. Their enzymes digest proteins, fats and carbohydrates such as glycogen, so food taken into the cell can be used.
2. They digest worn-out parts of the cell, dead cells and germs swallowed by white blood cells: they are the cell’s clean-up crew. The broken-down materials are reused.
3. When a cell is badly damaged or old, its lysosomes may burst and their enzymes digest the cell itself. For this reason lysosomes are called suicide bags (or suicidal bags).
4. In germinating seeds, similar digestive enzymes break down stored food to feed the growing embryo.

Vacuoles

A vacuole is not really a working organelle but a space, a sac surrounded by a membrane called the tonoplast. It is filled with a watery liquid called cell sap: water with dissolved salts, sugars, enzymes and wastes. In animal cells, vacuoles are small, or there are none. In a mature plant cell, one large vacuole fills most of the cell.

Some special vacuoles:

• Some water-dwelling bacteria have air (gas) vacuoles that help them float near the surface.
• Freshwater protozoa such as Amoeba and Paramecium have contractile vacuoles that fill with the extra water that keeps seeping in, and squirt it out, so that the cell doesn’t burst.
• Amoeba has food vacuoles, in which it stores and digests the food it has swallowed; the undigested waste is passed out.

Functions:

1. Vacuoles keep the balance of water and salts in the cytoplasm, and a plant cell’s large vacuole keeps the cell firm.
2. They collect wastes before these are passed out of the cell, and store water, food and pigments. (The purple of an onion’s skin and the red of a beetroot are pigments in vacuoles.)

Now explore the two kinds of cell yourself.

4. Nucleus

The nucleus is the central part of the cell. It is usually round and is surrounded by a double membrane with pores (holes) in it. It is found in the cells of all eukaryotes: plants, animals, protozoa and fungi. (A few kinds of cell lose it as they mature, such as your red blood cells.) Bacteria are prokaryotes: they have no proper nucleus, only genetic material (DNA) lying loose in the cytoplasm, with no membrane around it.

The nucleus controls and coordinates the work of all the other parts of the cell, so it is called the brain (or control center) of the cell. It has four main parts:

1. nuclear membrane
2. nucleolus
3. nucleoplasm
4. chromatin fibers

Nuclear membrane. The nuclear membrane is the outer covering of the nucleus: two layers of fat (lipid) and protein, joined at many places to leave pores. It is elastic and protects the nucleus, and through its pores it lets substances pass into and out of the nucleus: materials from the cytoplasm come in, and the messages copied from the DNA go out to the ribosomes.

Nucleolus. The nucleolus is a dark, round body inside the nucleus, with no membrane. It is made of protein and RNA, and it makes ribosomes, which then pass out through the pores into the cytoplasm. A cell that makes a lot of protein has a big nucleolus. It disappears during cell division and appears again afterwards.

Nucleoplasm. The nucleoplasm is the liquid that fills the nucleus: water with dissolved amino acids, minerals, the building blocks of DNA and RNA, and enzymes. The nucleolus and the chromatin fibers lie in it. It provides the materials and enzymes for making new DNA and RNA, and it gets them from the cytoplasm through the pores of the nuclear membrane.

Chromatin fibers. Chromatin is a tangled network of very long, extremely thin threads, too thin to see with an ordinary microscope. It is the genetic material: the instructions that control all the cell’s work. Chromatin is made of DNA wound around proteins called histones. Each species has a fixed number of these threads: every body cell of a human has 46, in 23 pairs. Through them, the characters of the parents pass to their offspring.

When a cell is about to divide, the chromatin threads coil up tightly into short, thick, visible bodies called chromosomes. After the division they unwind and become thin, invisible chromatin again.

Functions of the nucleus:

1. It controls and coordinates all the cell’s activities: its metabolism, its growth and the movement of materials in it.
2. It carries the genetic characters and passes them from parents to offspring through reproduction.
3. It takes part in cell division: first the nucleus divides into two, then the rest of the cell divides, so that each new cell gets a nucleus. This is how the body grows and repairs itself.
4. In single-celled organisms such as Amoeba, the division of the nucleus is the start of reproduction itself: one cell becomes two new organisms.

Comparison between plant and animal cells

How is an organ like the stomach built? Do cells help it work? And how is the whole human body put together?

The human body is very complex. It has about 200 different kinds of cells, around 30 trillion cells in all, and they are organized in levels, each built from the one before:

1. Cells of the same kind (or of a few kinds) group together to form a tissue: muscle tissue, nerve tissue, the lining tissue of the stomach.
2. Different tissues join, at a particular place in the body, to form an organ, which does one or more particular jobs: the stomach, the heart, the lungs.
3. Organs that work together, directly connected and coordinated, form an organ system, which carries out one major task: the digestive system, the respiratory system.
4. All the organ systems, working together, make up the whole organism: the body.

$$\text{cell} \rightarrow \text{tissue} \rightarrow \text{organ} \rightarrow \text{organ system} \rightarrow \text{organism}$$

Digestion, breathing, reproduction, transport, the building of new body material (assimilation) and the removal of wastes (excretion): every one of these begins with the work of one or many cells. It is cells that make every tissue, and that start every chemical and physical process of the body.

Take digestion. In the walls of the food pipe, stomach and intestine, the middle layer is muscle tissue, made of long muscle cells (muscle fibers). This muscle squeezes and relaxes in waves, one after another, pushing the food along: this is called peristalsis. The inner wall of the stomach and intestine is lined with glandular tissue, whose cells make digestive juices with enzymes that break down the different nutrients in food. So muscle tissue and gland tissue, each made of cells, together make the stomach work; the stomach, intestine and other organs together make the digestive system work; and the digestive system feeds every cell of the body.

So cells, tissues, organs and organ systems depend on one another to build the body and keep it working.

1. Select the correct option

d. What is marked R in this figure?

2. Differentiate between

3. Give reasons

4. Answer these questions

There are millions of kinds of living things on Earth. Each kind has its own features that make it different from the others. They differ in their habitat (where they live), their mode of nutrition (how they get food), how they reproduce, and their body structure.

But look carefully and you also find likenesses. A bat flies and a rabbit runs, yet both give birth to live young and feed them on milk until they are old enough to eat solid food. Both have hair. So bats and rabbits are put in the same group, the mammals (class Mammalia). In the same way, all known organisms are sorted into groups and smaller groups.

Sorting living things into groups by their likenesses and differences is called classification. Its aims:

1. It makes the study of millions of organisms easy and systematic: study one member of a group and you know a lot about all of them.
2. It shows how organisms are related, and so gives an idea of how life has evolved on Earth.

Living things can be classified in different ways, but there are two main systems: the two-kingdom system and the five-kingdom system.

3.3.1 Two-kingdom system of classification

The two-kingdom system was proposed by the Swedish biologist Carl Linnaeus in the 1700s (in his books Systema Naturae, 1735, and Species Plantarum, 1753). He sorted all living things into just two kingdoms: Plantae (plants) and Animalia (animals).

Basis of the two-kingdom system. Linnaeus looked at nutrition, movement, growth and reproduction:

• Organisms that move about, take in ready-made food (heterotrophic nutrition) and stop growing after a certain age were put in the animal kingdom.
• Organisms that don’t move about, make their own food (autotrophic nutrition) and keep growing all their lives were put in the plant kingdom.

Features of the two-kingdom system:

1. All living things are divided into two kingdoms, plants and animals.
2. It was the first scientific system of classification.
3. It is simple: plants and animals are easy to tell apart.

Drawbacks of the two-kingdom system:

1. Organisms that can’t make their own food were put in the plant kingdom with those that can: fungi, which have no chlorophyll, were put with algae, which do photosynthesis.
2. Prokaryotes (bacteria, with no proper nucleus) and eukaryotes (such as algae, with a nucleus) were put in the same kingdom.
3. It doesn’t separate unicellular (one-celled) from multicellular organisms.
4. Some organisms fit in neither kingdom. A lichen is a fungus and an alga living together as one; Euglena makes food like a plant but swims and has no cell wall, like an animal.

Five-kingdom system of classification

To remove these drawbacks, the American ecologist Robert H. Whittaker proposed the five-kingdom system in 1969, more than 200 years after Linnaeus. His five kingdoms are Monera, Protista, Fungi (also called Mycota), Plantae and Animalia.

Basis of the five-kingdom system. Whittaker sorted organisms by:

1. type of cell: prokaryotic (no proper nucleus) or eukaryotic (with a nucleus);
2. body structure: unicellular or multicellular;
3. mode of nutrition: autotrophic (making their own food) or heterotrophic (taking food from others);
4. role in the ecosystem: producer, consumer or decomposer;
5. phylogenetic relationship: the line of evolution, from the simplest to the most developed.

So:

Kingdom	Who is in it	Examples
Monera	all unicellular prokaryotes	bacteria, blue-green algae (cyanobacteria), mycoplasma, archaea
Protista	unicellular eukaryotes (except fungi such as yeast)	Amoeba, Paramecium, Euglena, Plasmodium
Fungi (Mycota)	eukaryotes with a cell wall and no chlorophyll that absorb food from dead matter (saprophytes) or from living hosts (parasites), digesting it outside their bodies	mushroom, yeast, Mucor, yarsagumba
Plantae	multicellular green autotrophs: producers	moss, fern, pine, mustard
Animalia	multicellular eukaryotic consumers with no cell wall	earthworm, tortoise, bat, human

Features of the five-kingdom system:

1. Unicellular and multicellular organisms are put in separate kingdoms.
2. Prokaryotes and eukaryotes are separated.
3. Autotrophs (Plantae) are separated from heterotrophs (Animalia, which take in food and digest it inside, and Fungi, which absorb food).
4. The kingdoms are arranged in order from the simplest to the most developed, which shows the path of evolution.

Try sorting here first.

There are many kinds of living things around us that are too small to see with the naked eye. They are found in air, water and soil, in food left uncovered, in dirt, and on and inside the bodies of living, dead and decaying organisms. These are called microorganisms (micro, small). They affect our daily lives in many ways, directly and indirectly. The rotting of the broken egg is their work.

Microorganisms include bacteria, some fungi (yeast, molds), some algae (Chlamydomonas, Chlorella), protozoa (Amoeba, Paramecium, Plasmodium, Vorticella), and viruses (which are not quite living, as you will see). Some small worms, like the eggs and young of the blood fluke Schistosoma, are microscopic too. Microorganisms are commonly called germs or microbes. The branch of biology that studies them is microbiology.

(A) Virus

Viruses are the smallest germs. They can infect plants, animals, bacteria, fungi, and every other kind of organism. The word virus is Latin for poison.

A virus is not a cell: it is acellular. Its body is only some genetic material, DNA or RNA, wrapped in a coat of protein called a capsid. Some viruses, such as the coronavirus and the flu virus, have an extra, fatty outer covering called an envelope. Viruses come in many shapes and sizes.

Viruses show features of both living and non-living things, so they are called a link (or bridge) between the living and the non-living:

• Outside a living cell, a virus is like a non-living particle. It doesn’t eat, drink, breathe or grow, and has no metabolism of its own. Some can even be crystallized like salt.
• Inside a living host cell, it takes over the cell and uses the cell’s own materials and machinery to make hundreds of copies of itself, which usually damages or kills the cell. Because it can multiply only inside a living host, a virus is called an obligate parasite.

Because a virus is not a cell and has no metabolism, it is not counted as an organism, and it belongs to none of the five kingdoms. Viruses are found everywhere: in air, water and soil, and in every kind of living thing.

Examples: rhinovirus (the common cold), coronaviruses (including SARS-CoV-2, which causes COVID-19), the tobacco mosaic virus (a disease of tobacco and tomato leaves), bacteriophages (viruses that infect bacteria) and even virophages, viruses that infect other viruses.

(B) Bacteria

Bacteria (singular: bacterium) are unicellular prokaryotes: single cells with no proper nucleus, so they belong to the kingdom Monera. Their cells may be round, rod-shaped, thread-like (filamentous) or spiral (Figure 3.2). They live everywhere: in air, water and soil, and on and inside other organisms. Some live in boiling hot springs, in ice, or in very salty water.

Almost every bacterium has a cell wall (tiny Mycoplasma is one of the few without), and many have a slimy capsule outside it, which protects them. Some can turn into tough spores that survive heat, cold and dryness for years, waiting for better conditions.

Some bacteria make their own food (they are autotrophic), like the green cyanobacteria. But most are saprophytic: they feed on dead matter, which makes them the great decomposers of every ecosystem. Some are parasitic and cause diseases in plants and animals.

Examples: cyanobacteria (blue-green algae), Rhizobium, Azotobacter, Salmonella, Pseudomonas, Lactobacillus.

(C) Amoeba

Amoeba is a unicellular eukaryote (a protozoan), in the kingdom Protista. It lives in water, damp soil and decaying matter, and some kinds live inside other animals.

It has no fixed shape: it can push out pseudopodia (“false feet”) from any point of its cell membrane. With these temporary bulges it moves, and it catches food by flowing around it and enclosing it in a food vacuole. Its single cell does everything: digestion, excretion, reproduction (by splitting into two) and the rest.

There are many kinds of amoebae:

• Entamoeba histolytica lives in water and food contaminated with feces. If it gets into the intestine, it causes amoebic dysentery (amoebiasis): stomach pain and diarrhea with blood and mucus. Boiling or filtering drinking water, and washing hands, prevents it.
• Naegleria fowleri lives freely in warm fresh water, such as warm ponds and poorly cleaned pools. Drinking such water is not what harms you: it is dangerous only if water goes up the nose, for example while diving or swimming. From there it can reach the brain and cause a rare but usually deadly brain infection.

(D) Fungi

Fungi (singular: fungus) are eukaryotes, unicellular or multicellular, that have a cell wall but no chlorophyll. They form the kingdom Fungi (Mycota). Most grow on dead and decaying matter and absorb their food from it, so they are called saprophytes. Some are parasites: they infect plants and animals and cause fungal diseases, such as rust in wheat and ringworm and athlete’s foot in people.

A fungus’s body is usually made of very thin threads called hyphae; the whole network of threads is called the mycelium (plural mycelia). Yeast is an exception: it is a single cell.

Examples: Mucor (bread mold), mushrooms, yeast, yarsagumba (Cordyceps, which grows out of a caterpillar), Penicillium, and Puccinia, the fungus that causes rust disease in wheat.

3.4.1 Effects of microorganisms

Microorganisms are often thought of only as “germs”, but most of them are harmless, and many are essential to us.

Positive effects

1. Amoebae and other protozoa feed on bacteria. In soil, water and the tanks that clean sewage, they keep bacteria in check and help clean the water.
2. Most bacteria are decomposers. They break down the excreta and dead bodies of organisms, turning complex organic substances back into simple minerals, which makes the soil fertile again. Without them, the world would be piled high with the dead. Bacteria such as Clostridium decompose dead matter; others, such as Nitrosomonas and Nitrobacter, then turn the ammonia that is released into nitrates, which plants can use. In this way bacteria keep the ecosystem in balance.
3. Rhizobium bacteria live in the root nodules of leguminous plants (beans, peas, lentils, gram). They take nitrogen from the air and turn it into compounds the plant can use, which makes the soil more fertile and increases crop yields.
4. Lactobacillus and Lactococcus bacteria turn milk into curd (dahi) and yoghurt; Lactococcus lactis (also called Streptococcus lactis) is used to make cheese from milk.
5. Bacteria and fungi are used to make medicines: antibiotics (penicillin comes from the fungus Penicillium), vaccines, vitamins, and even human insulin, which is now made by bacteria and yeast given the human insulin gene.
6. Yeast, a fungus, turns the sugar in foods into alcohol and carbon dioxide. This is fermentation. Factories use yeast to make alcohol and, with the help of other bacteria, vinegar (acetic acid). The carbon dioxide it makes is what makes bread and sel roti dough rise.
7. In Nepal, microbes have been used at home for centuries: yeast and molds in marcha to make jaand and raksi, and lactic acid bacteria to ferment gundruk, sinki and pickles.
8. Bacteriophages kill bacteria. They are used in some countries to treat bacterial infections that antibiotics can no longer cure (such as some kinds of dysentery and diarrhea), and in cleaning water and protecting food.
9. In genetic engineering, harmless viruses are used to carry genes into cells: to add, replace or repair genes, to develop high-yielding plants and animals, and to treat some diseases.
10. Viruses lie on the border between living and non-living, so studying them helps us understand how life may have begun and how it has evolved.

Negative effects

1. Microorganisms cause most communicable diseases (diseases that spread from one to another) in people, animals and plants. Diseases lower the quality and amount of production in every kind of farming: crops, fish farming, silkworm farming (sericulture), bee-keeping (apiculture), fruit growing (horticulture), animal husbandry and nurseries. Human diseases include tuberculosis (bacteria), AIDS (a virus, HIV), pneumonia (bacteria or viruses), typhoid (bacteria, Salmonella Typhi) and COVID-19 (a virus). Plant diseases include tobacco mosaic (a virus), blight and root rot (fungi and bacteria).
2. Fruit, vegetables and cooked food left uncovered for hours or days start to smell and rot. Microbes such as Clostridium get into the food from air, water, hands or flies, multiply, and make poisons (toxins). Eating such food causes food poisoning: diarrhea, vomiting, fever, loss of water from the body (dehydration), a dry mouth (xerostomia) and headache. Food poisoning can be caused by Clostridium, Salmonella, norovirus and Giardia; molds such as Mucor spoil food, and some molds make poisons of their own.
3. Pimples, boils and pus in wounds are caused by infection with harmful microbes. If the germs or their poisons get into the blood, they can cause sepsis, in which organs fail one after another, and it can kill. To stay safe, keep wounds and pimples clean with gauze or cotton soaked in a suitable antiseptic (such as Dettol, Betadine or Savlon), dress wounds on time, take antibiotics only as a doctor prescribes, and keep yourself clean.
4. Microbes spoil stored grain, vegetables and other food, and farmers and shopkeepers lose a great deal.
5. At rubbish dumps, microbes acting on organic waste give off foul smells that pollute the air around.

3.4.2 Methods of food preservation

We keep food for a long time by preserving it. Preserving does two things: it keeps the food’s nutrients from breaking down, and it keeps microorganisms from spoiling it. To do this, the food is given physical and chemical conditions in which microbes can’t grow and the food doesn’t decay: dryness, cold, salt, sugar, acid, or no air.

Protecting food from microorganisms while keeping its nutritional value is called food preservation.

Microbes need four things to grow: food, water, warmth and (most of them) air. Every method of preserving food takes away at least one of these, or adds something they can’t stand, like strong salt or acid.

1. Dry preservation

Drying food until very little moisture is left, before storing it, is called dry preservation. Without water, microbes can’t grow, and with only a little moisture left the food keeps its nutrients. It can then be stored at room temperature for months or even years. It is very easy and cheap.

In Nepal it has been used for centuries for grains, dried meat (sukuti), dried fish, dried fruits, nuts, dried vegetables and fermented vegetables (gundruk, sinki). Food is dried for some days in moderate warmth (about 20–30 °C), in the sun or in an electric dryer, until it is completely dry outside and has very little moisture inside.

2. Wet preservation

Keeping food dipped in a liquid in which microbes can’t grow is called wet preservation. The common liquids are a strong salt solution (brine), citric acid (the juice of lemons and other citrus fruits), vinegar, and mustard oil or olive oil.

The food must be free of germs to start with, so it is washed or boiled and then dried well before it goes into the liquid. The jar must be closed airtight, so that no germs or mold spores get in from the air. In Nepal, pickles (achar) of lemon, chilli, ginger, garlic, fenugreek (methi), radish and bamboo shoot (tama) have been made this way since ancient times.

3. Cold preservation

Keeping food in closed rooms or boxes at a low temperature, usually between about −18 °C and 4 °C, is called cold preservation. At these temperatures most microorganisms become inactive: they grow very slowly or not at all. Examples: keeping raw or cooked food in a refrigerator (about 4 °C) or freezer (about −18 °C), and storing potatoes, fruits and vegetables in cold stores for months.

An old way of keeping farm produce cool is still used in Nepal: potatoes, ginger, turmeric, squash (iskus) and fresh paddy are kept in a dry, well-covered pit in the ground, where it stays cool all year.

See how much difference the place you keep food makes.

1. Select the best answer

2. Differentiate between

3. Give reasons

4. Answer these questions